Spring Framework

Reference Documentation

(Work in progress)

3.0.M3

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Table of Contents

Preface
1. Introduction
1.1. Dependency Injection
1.2. Modules
1.2.1. Core Container
1.2.2. Data Access/Integration
1.2.3. Web
1.2.4. AOP and Instrumentation
1.2.5. Test
1.3. Usage scenarios
2. What's new in Spring 3.0?
2.1. Java 5
2.2. Improved documentation
2.3. New module organization and build system
2.4. Overview of new features
2.4.1. Core APIs updated for Java 5
2.4.2. Spring Expression Language
2.4.3. The Inversion of Control (IoC) container
2.4.3.1. Java based bean metadata
2.4.3.2. Defining bean metadata within components
2.4.4. The Data Tier
2.4.5. The Web Tier
2.4.5.1. Comprehensive REST support
2.4.5.2. @MVC additions
2.4.6. Declarative model validation
2.4.7. Early support for Java EE 6
3. Getting started with Spring
3.1. Creating an ApplicationContext
3.2. The Data Access Object
3.3. The Business Layer
3.4. The Web UI
I. Core Technologies
4. The IoC container
4.1. Introduction
4.2. Basics - containers and beans
4.2.1. The container
4.2.1.1. Configuration metadata
4.2.2. Instantiating a container
4.2.2.1. Composing XML-based configuration metadata
4.2.3. The beans
4.2.3.1. Naming beans
4.2.3.2. Instantiating beans
4.2.4. Using the container
4.3. Dependencies
4.3.1. Injecting dependencies
4.3.1.1. Constructor Injection
4.3.1.2. Setter Injection
4.3.1.3. Some examples
4.3.2. Dependencies and configuration in detail
4.3.2.1. Straight values (primitives, Strings, etc.)
4.3.2.2. References to other beans (collaborators)
4.3.2.3. Inner beans
4.3.2.4. Collections
4.3.2.5. Nulls
4.3.2.6. Shortcuts and other convenience options for XML-based configuration metadata
4.3.2.7. Compound property names
4.3.3. Using depends-on
4.3.4. Lazily-instantiated beans
4.3.5. Autowiring collaborators
4.3.5.1. Excluding a bean from being available for autowiring
4.3.6. Checking for dependencies
4.3.7. Method Injection
4.3.7.1. Lookup method injection
4.3.7.2. Arbitrary method replacement
4.4. Bean scopes
4.4.1. The singleton scope
4.4.2. The prototype scope
4.4.3. Singleton beans with prototype-bean dependencies
4.4.4. The other scopes
4.4.4.1. Initial web configuration
4.4.4.2. The request scope
4.4.4.3. The session scope
4.4.4.4. The global session scope
4.4.4.5. Scoped beans as dependencies
4.4.5. Custom scopes
4.4.5.1. Creating your own custom scope
4.4.5.2. Using a custom scope
4.5. Customizing the nature of a bean
4.5.1. Lifecycle callbacks
4.5.1.1. Initialization callbacks
4.5.1.2. Destruction callbacks
4.5.1.3. Default initialization & destroy methods
4.5.1.4. Combining lifecycle mechanisms
4.5.1.5. Shutting down the Spring IoC container gracefully in non-web applications
4.5.2. Knowing who you are
4.5.2.1. BeanFactoryAware
4.5.2.2. BeanNameAware
4.6. Bean definition inheritance
4.7. Container extension points
4.7.1. Customizing beans using BeanPostProcessors
4.7.1.1. Example: Hello World, BeanPostProcessor-style
4.7.1.2. Example: The RequiredAnnotationBeanPostProcessor
4.7.2. Customizing configuration metadata with BeanFactoryPostProcessors
4.7.2.1. Example: the PropertyPlaceholderConfigurer
4.7.2.2. Example: the PropertyOverrideConfigurer
4.7.3. Customizing instantiation logic using FactoryBeans
4.8. The ApplicationContext
4.8.1. BeanFactory or ApplicationContext?
4.8.2. Internationalization using MessageSources
4.8.3. Events
4.8.4. Convenient access to low-level resources
4.8.5. Convenient ApplicationContext instantiation for web applications
4.9. Glue code and the evil singleton
4.10. Deploying a Spring ApplicationContext as a J2EE RAR file
4.11. Annotation-based configuration
4.11.1. @Required
4.11.2. @Autowired
4.11.3. Fine-tuning annotation-based autowiring with qualifiers
4.11.4. CustomAutowireConfigurer
4.11.5. @Resource
4.11.6. @PostConstruct and @PreDestroy
4.12. Classpath scanning, managed components and writing configurations using Java
4.12.1. @Component and further stereotype annotations
4.12.2. Auto-detecting components
4.12.3. Using filters to customize scanning
4.12.4. Using the @Configuration annotation
4.12.5. Using the @Bean annotation
4.12.5.1. Declaring a bean
4.12.5.2. Injecting dependencies
4.12.5.3. Receiving lifecycle callbacks
4.12.5.4. Specifying bean scope
4.12.5.5. Customizing bean naming
4.12.6. Defining bean metadata within components
4.12.7. Naming autodetected components
4.12.8. Providing a scope for autodetected components
4.12.9. Providing qualifier metadata with annotations
4.13. Registering a LoadTimeWeaver
5. Resources
5.1. Introduction
5.2. The Resource interface
5.3. Built-in Resource implementations
5.3.1. UrlResource
5.3.2. ClassPathResource
5.3.3. FileSystemResource
5.3.4. ServletContextResource
5.3.5. InputStreamResource
5.3.6. ByteArrayResource
5.4. The ResourceLoader
5.5. The ResourceLoaderAware interface
5.6. Resources as dependencies
5.7. Application contexts and Resource paths
5.7.1. Constructing application contexts
5.7.1.1. Constructing ClassPathXmlApplicationContext instances - shortcuts
5.7.2. Wildcards in application context constructor resource paths
5.7.2.1. Ant-style Patterns
5.7.2.2. The classpath*: prefix
5.7.2.3. Other notes relating to wildcards
5.7.3. FileSystemResource caveats
6. Validation, Data-binding, the BeanWrapper, and PropertyEditors
6.1. Introduction
6.2. Validation using Spring's Validator interface
6.3. Resolving codes to error messages
6.4. Bean manipulation and the BeanWrapper
6.4.1. Setting and getting basic and nested properties
6.4.2. Built-in PropertyEditor implementations
6.4.2.1. Registering additional custom PropertyEditors
7. Spring Expression Language (SpEL)
7.1. Introduction
7.2. Feature Overview
7.3. Expression Evaluation using Spring's Expression Interface
7.3.1. The EvaluationContext interface
7.3.1.1. Type Conversion
7.4. Expression support for defining bean definitions
7.4.1. XML based configuration
7.4.2. Annotation-based configuration
7.5. Language Reference
7.5.1. Literal expressions
7.5.2. Properties, Arrays, Lists, Maps, Indexers
7.5.3. Methods
7.5.4. Operators
7.5.4.1. Relational operators
7.5.4.2. Logical operators
7.5.4.3. Mathematical operators
7.5.5. Assignment
7.5.6. Types
7.5.7. Constructors
7.5.8. Variables
7.5.8.1. The #this variable
7.5.9. Functions
7.5.10. Ternary Operator (If-Then-Else)
7.5.11. Collection Selection
7.5.12. Collection Projection
7.5.13. Expression templating
7.6. Classes used in the examples
8. Aspect Oriented Programming with Spring
8.1. Introduction
8.1.1. AOP concepts
8.1.2. Spring AOP capabilities and goals
8.1.3. AOP Proxies
8.2. @AspectJ support
8.2.1. Enabling @AspectJ Support
8.2.2. Declaring an aspect
8.2.3. Declaring a pointcut
8.2.3.1. Supported Pointcut Designators
8.2.3.2. Combining pointcut expressions
8.2.3.3. Sharing common pointcut definitions
8.2.3.4. Examples
8.2.4. Declaring advice
8.2.4.1. Before advice
8.2.4.2. After returning advice
8.2.4.3. After throwing advice
8.2.4.4. After (finally) advice
8.2.4.5. Around advice
8.2.4.6. Advice parameters
8.2.4.7. Advice ordering
8.2.5. Introductions
8.2.6. Aspect instantiation models
8.2.7. Example
8.3. Schema-based AOP support
8.3.1. Declaring an aspect
8.3.2. Declaring a pointcut
8.3.3. Declaring advice
8.3.3.1. Before advice
8.3.3.2. After returning advice
8.3.3.3. After throwing advice
8.3.3.4. After (finally) advice
8.3.3.5. Around advice
8.3.3.6. Advice parameters
8.3.3.7. Advice ordering
8.3.4. Introductions
8.3.5. Aspect instantiation models
8.3.6. Advisors
8.3.7. Example
8.4. Choosing which AOP declaration style to use
8.4.1. Spring AOP or full AspectJ?
8.4.2. @AspectJ or XML for Spring AOP?
8.5. Mixing aspect types
8.6. Proxying mechanisms
8.6.1. Understanding AOP proxies
8.7. Programmatic creation of @AspectJ Proxies
8.8. Using AspectJ with Spring applications
8.8.1. Using AspectJ to dependency inject domain objects with Spring
8.8.1.1. Unit testing @Configurable objects
8.8.1.2. Working with multiple application contexts
8.8.2. Other Spring aspects for AspectJ
8.8.3. Configuring AspectJ aspects using Spring IoC
8.8.4. Load-time weaving with AspectJ in the Spring Framework
8.8.4.1. A first example
8.8.4.2. Aspects
8.8.4.3. 'META-INF/aop.xml'
8.8.4.4. Required libraries (JARS)
8.8.4.5. Spring configuration
8.8.4.6. Environment-specific configuration
8.9. Further Resources
9. Spring AOP APIs
9.1. Introduction
9.2. Pointcut API in Spring
9.2.1. Concepts
9.2.2. Operations on pointcuts
9.2.3. AspectJ expression pointcuts
9.2.4. Convenience pointcut implementations
9.2.4.1. Static pointcuts
9.2.4.2. Dynamic pointcuts
9.2.5. Pointcut superclasses
9.2.6. Custom pointcuts
9.3. Advice API in Spring
9.3.1. Advice lifecycles
9.3.2. Advice types in Spring
9.3.2.1. Interception around advice
9.3.2.2. Before advice
9.3.2.3. Throws advice
9.3.2.4. After Returning advice
9.3.2.5. Introduction advice
9.4. Advisor API in Spring
9.5. Using the ProxyFactoryBean to create AOP proxies
9.5.1. Basics
9.5.2. JavaBean properties
9.5.3. JDK- and CGLIB-based proxies
9.5.4. Proxying interfaces
9.5.5. Proxying classes
9.5.6. Using 'global' advisors
9.6. Concise proxy definitions
9.7. Creating AOP proxies programmatically with the ProxyFactory
9.8. Manipulating advised objects
9.9. Using the "autoproxy" facility
9.9.1. Autoproxy bean definitions
9.9.1.1. BeanNameAutoProxyCreator
9.9.1.2. DefaultAdvisorAutoProxyCreator
9.9.1.3. AbstractAdvisorAutoProxyCreator
9.9.2. Using metadata-driven auto-proxying
9.10. Using TargetSources
9.10.1. Hot swappable target sources
9.10.2. Pooling target sources
9.10.3. Prototype target sources
9.10.4. ThreadLocal target sources
9.11. Defining new Advice types
9.12. Further resources
10. Testing
10.1. Introduction
10.2. Unit testing
10.2.1. Mock objects
10.2.1.1. JNDI
10.2.1.2. Servlet API
10.2.1.3. Portlet API
10.2.2. Unit testing support classes
10.2.2.1. General utilities
10.2.2.2. Spring MVC
10.3. Integration testing
10.3.1. Overview
10.3.2. Goals
10.3.2.1. Context management and caching
10.3.2.2. Dependency Injection of test fixtures
10.3.2.3. Transaction management
10.3.2.4. Integration testing support classes
10.3.3. JDBC testing support
10.3.4. Annotations
10.3.5. Spring TestContext Framework
10.3.5.1. Key abstractions
10.3.5.2. Context management and caching
10.3.5.3. Dependency Injection of test fixtures
10.3.5.4. Transaction management
10.3.5.5. TestContext support classes
10.3.6. PetClinic example
10.4. Further Resources
II. Data Access
11. Transaction management
11.1. Introduction
11.2. Motivations
11.3. Key abstractions
11.4. Resource synchronization with transactions
11.4.1. High-level approach
11.4.2. Low-level approach
11.4.3. TransactionAwareDataSourceProxy
11.5. Declarative transaction management
11.5.1. Understanding the Spring Framework's declarative transaction implementation
11.5.2. A first example
11.5.3. Rolling back
11.5.4. Configuring different transactional semantics for different beans
11.5.5. <tx:advice/> settings
11.5.6. Using @Transactional
11.5.6.1. @Transactional settings
11.5.7. Transaction propagation
11.5.7.1. Required
11.5.7.2. RequiresNew
11.5.7.3. Nested
11.5.8. Advising transactional operations
11.5.9. Using @Transactional with AspectJ
11.6. Programmatic transaction management
11.6.1. Using the TransactionTemplate
11.6.1.1. Specifying transaction settings
11.6.2. Using the PlatformTransactionManager
11.7. Choosing between programmatic and declarative transaction management
11.8. Application server-specific integration
11.8.1. IBM WebSphere
11.8.2. BEA WebLogic
11.8.3. Oracle OC4J
11.9. Solutions to common problems
11.9.1. Use of the wrong transaction manager for a specific DataSource
11.10. Further Resources
12. DAO support
12.1. Introduction
12.2. Consistent exception hierarchy
12.3. Consistent abstract classes for DAO support
13. Data access using JDBC
13.1. Introduction
13.1.1. Choosing a style
13.1.2. The package hierarchy
13.2. Using the JDBC Core classes to control basic JDBC processing and error handling
13.2.1. JdbcTemplate
13.2.1.1. Examples
13.2.1.2. JdbcTemplate idioms (best practices)
13.2.2. NamedParameterJdbcTemplate
13.2.3. SimpleJdbcTemplate
13.2.4. DataSource
13.2.5. SQLExceptionTranslator
13.2.6. Executing statements
13.2.7. Running Queries
13.2.8. Updating the database
13.2.9. Retrieving auto-generated keys
13.3. Controlling database connections
13.3.1. DataSourceUtils
13.3.2. SmartDataSource
13.3.3. AbstractDataSource
13.3.4. SingleConnectionDataSource
13.3.5. DriverManagerDataSource
13.3.6. TransactionAwareDataSourceProxy
13.3.7. DataSourceTransactionManager
13.3.8. NativeJdbcExtractor
13.4. JDBC batch operations
13.4.1. Batch operations with the JdbcTemplate
13.4.2. Batch operations with the SimpleJdbcTemplate
13.5. Simplifying JDBC operations with the SimpleJdbc classes
13.5.1. Inserting data using SimpleJdbcInsert
13.5.2. Retrieving auto-generated keys using SimpleJdbcInsert
13.5.3. Specifying the columns to use for a SimpleJdbcInsert
13.5.4. Using SqlParameterSource to provide parameter values
13.5.5. Calling a stored procedure using SimpleJdbcCall
13.5.6. Declaring parameters to use for a SimpleJdbcCall
13.5.7. How to define SqlParameters
13.5.8. Calling a stored function using SimpleJdbcCall
13.5.9. Returning ResultSet/REF Cursor from a SimpleJdbcCall
13.6. Modeling JDBC operations as Java objects
13.6.1. SqlQuery
13.6.2. MappingSqlQuery
13.6.3. SqlUpdate
13.6.4. StoredProcedure
13.6.5. SqlFunction
13.7. Common issues with parameter and data value handling
13.7.1. Providing SQL type information for parameters
13.7.2. Handling BLOB and CLOB objects
13.7.3. Passing in lists of values for IN clause
13.7.4. Handling complex types for stored procedure calls
14. Object Relational Mapping (ORM) data access
14.1. Introduction
14.2. Hibernate
14.2.1. Resource management
14.2.2. SessionFactory setup in a Spring container
14.2.3. The HibernateTemplate
14.2.4. Implementing Spring-based DAOs without callbacks
14.2.5. Implementing DAOs based on plain Hibernate 3 API
14.2.6. Programmatic transaction demarcation
14.2.7. Declarative transaction demarcation
14.2.8. Transaction management strategies
14.2.9. Container resources versus local resources
14.2.10. Spurious application server warnings when using Hibernate
14.3. JDO
14.3.1. PersistenceManagerFactory setup
14.3.2. JdoTemplate and JdoDaoSupport
14.3.3. Implementing DAOs based on the plain JDO API
14.3.4. Transaction management
14.3.5. JdoDialect
14.4. Oracle TopLink
14.4.1. SessionFactory abstraction
14.4.2. TopLinkTemplate and TopLinkDaoSupport
14.4.3. Implementing DAOs based on plain TopLink API
14.4.4. Transaction management
14.5. iBATIS SQL Maps
14.5.1. Setting up the SqlMapClient
14.5.2. Using SqlMapClientTemplate and SqlMapClientDaoSupport
14.5.3. Implementing DAOs based on plain iBATIS API
14.6. JPA
14.6.1. JPA setup in a Spring environment
14.6.1.1. LocalEntityManagerFactoryBean
14.6.1.2. Obtaining an EntityManagerFactory from JNDI
14.6.1.3. LocalContainerEntityManagerFactoryBean
14.6.1.4. Dealing with multiple persistence units
14.6.2. JpaTemplate and JpaDaoSupport
14.6.3. Implementing DAOs based on plain JPA
14.6.4. Exception Translation
14.7. Transaction Management
14.8. JpaDialect
15. Marshalling XML using O/X Mappers
15.1. Introduction
15.2. Marshaller and Unmarshaller
15.2.1. Marshaller
15.2.2. Unmarshaller
15.2.3. XmlMappingException
15.3. Using Marshaller and Unmarshaller
15.4. XML Schema-based Configuration
15.5. JAXB
15.5.1. Jaxb2Marshaller
15.5.1.1. XML Schema-based Configuration
15.6. Castor
15.6.1. CastorMarshaller
15.6.2. Mapping
15.7. XMLBeans
15.7.1. XmlBeansMarshaller
15.7.1.1. XML Schema-based Configuration
15.8. JiBX
15.8.1. JibxMarshaller
15.8.1.1. XML Schema-based Configuration
15.9. XStream
15.9.1. XStreamMarshaller
III. The Web
16. Web MVC framework
16.1. Introduction
16.1.1. Pluggability of other MVC implementations
16.1.2. Features of Spring Web MVC
16.2. The DispatcherServlet
16.3. Controllers
16.3.1. AbstractController and WebContentGenerator
16.3.2. Other simple controllers
16.3.3. The MultiActionController
16.3.4. Command controllers
16.4. Handler mappings
16.4.1. BeanNameUrlHandlerMapping
16.4.2. SimpleUrlHandlerMapping
16.4.3. Intercepting requests - the HandlerInterceptor interface
16.5. Views and resolving them
16.5.1. Resolving views - the ViewResolver interface
16.5.2. Chaining ViewResolvers
16.5.3. Redirecting to views
16.5.3.1. RedirectView
16.5.3.2. The redirect: prefix
16.5.3.3. The forward: prefix
16.6. Using locales
16.6.1. AcceptHeaderLocaleResolver
16.6.2. CookieLocaleResolver
16.6.3. SessionLocaleResolver
16.6.4. LocaleChangeInterceptor
16.7. Using themes
16.7.1. Introduction
16.7.2. Defining themes
16.7.3. Theme resolvers
16.8. Spring's multipart (fileupload) support
16.8.1. Introduction
16.8.2. Using the MultipartResolver
16.8.3. Handling a file upload in a form
16.9. Handling exceptions
16.10. Convention over configuration
16.10.1. The Controller - ControllerClassNameHandlerMapping
16.10.2. The Model - ModelMap (ModelAndView)
16.10.3. The View - RequestToViewNameTranslator
16.11. Annotation-based controller configuration
16.11.1. Setting up the dispatcher for annotation support
16.11.2. Defining a controller with @Controller
16.11.3. Mapping requests with @RequestMapping
16.11.3.1. Advanced @RequestMapping options
16.11.4. Supported handler method arguments and return types
16.11.5. Binding request parameters to method parameters with @RequestParam
16.11.6. Providing a link to data from the model with @ModelAttribute
16.11.7. Specifying attributes to store in a Session with @SessionAttributes
16.11.8. Mapping cookie values with the @CookieValue annotation
16.11.9. Mapping request header attributes with the @RequestHeader annotation
16.11.10. Customizing WebDataBinder initialization
16.11.10.1. Customizing data binding with @InitBinder
16.11.10.2. Configuring a custom WebBindingInitializer
16.12. Further Resources
17. View technologies
17.1. Introduction
17.2. JSP & JSTL
17.2.1. View resolvers
17.2.2. 'Plain-old' JSPs versus JSTL
17.2.3. Additional tags facilitating development
17.2.4. Using Spring's form tag library
17.2.4.1. Configuration
17.2.4.2. The form tag
17.2.4.3. The input tag
17.2.4.4. The checkbox tag
17.2.4.5. The checkboxes tag
17.2.4.6. The radiobutton tag
17.2.4.7. The radiobuttons tag
17.2.4.8. The password tag
17.2.4.9. The select tag
17.2.4.10. The option tag
17.2.4.11. The options tag
17.2.4.12. The textarea tag
17.2.4.13. The hidden tag
17.2.4.14. The errors tag
17.3. Tiles
17.3.1. Dependencies
17.3.2. How to integrate Tiles
17.3.2.1. UrlBasedViewResolver
17.3.2.2. ResourceBundleViewResolver
17.3.2.3. SimpleSpringPreparerFactory and SpringBeanPreparerFactory
17.4. Velocity & FreeMarker
17.4.1. Dependencies
17.4.2. Context configuration
17.4.3. Creating templates
17.4.4. Advanced configuration
17.4.4.1. velocity.properties
17.4.4.2. FreeMarker
17.4.5. Bind support and form handling
17.4.5.1. The bind macros
17.4.5.2. Simple binding
17.4.5.3. Form input generation macros
17.4.5.4. HTML escaping and XHTML compliance
17.5. XSLT
17.5.1. My First Words
17.5.1.1. Bean definitions
17.5.1.2. Standard MVC controller code
17.5.1.3. Convert the model data to XML
17.5.1.4. Defining the view properties
17.5.1.5. Document transformation
17.5.2. Summary
17.6. Document views (PDF/Excel)
17.6.1. Introduction
17.6.2. Configuration and setup
17.6.2.1. Document view definitions
17.6.2.2. Controller code
17.6.2.3. Subclassing for Excel views
17.6.2.4. Subclassing for PDF views
17.7. JasperReports
17.7.1. Dependencies
17.7.2. Configuration
17.7.2.1. Configuring the ViewResolver
17.7.2.2. Configuring the Views
17.7.2.3. About Report Files
17.7.2.4. Using JasperReportsMultiFormatView
17.7.3. Populating the ModelAndView
17.7.4. Working with Sub-Reports
17.7.4.1. Configuring Sub-Report Files
17.7.4.2. Configuring Sub-Report Data Sources
17.7.5. Configuring Exporter Parameters
18. REST support
18.1. Introduction
18.2. Creating RESTful services
18.2.1. URI Templates
18.2.1.1. Mapping RESTful URLs with the @PathVariable annotation
18.2.1.2. Mapping the request body with the @RequestBody annotation
18.2.2. Returning multiple representations
18.2.3. Views
18.2.3.1. Feed Views
18.2.3.2. XML Marshalling View
18.2.4. HTTP Method Conversion
18.2.4.1. Supporting Spring form tags
18.2.5. ETag support
18.2.6. Exception Handling
18.3. Accessing RESTful services on the Client
18.3.1. RestTemplate
18.3.2. HTTP Message Conversion
18.3.2.1. StringHttpMessageConverter
18.3.2.2. FormHttpMessageConverter
18.3.2.3. ByteArrayMessageConverter
18.3.2.4. MarshallingHttpMessageConverter
18.3.2.5. SourceHttpMessageConverter
19. Integrating with other web frameworks
19.1. Introduction
19.2. Common configuration
19.3. JavaServer Faces 1.1 and 1.2
19.3.1. DelegatingVariableResolver (JSF 1.1/1.2)
19.3.2. SpringBeanVariableResolver (JSF 1.1/1.2)
19.3.3. SpringBeanFacesELResolver (JSF 1.2+)
19.3.4. FacesContextUtils
19.4. Apache Struts 1.x and 2.x
19.4.1. ContextLoaderPlugin
19.4.1.1. DelegatingRequestProcessor
19.4.1.2. DelegatingActionProxy
19.4.2. ActionSupport Classes
19.5. WebWork 2.x
19.6. Tapestry 3.x and 4.x
19.6.1. Injecting Spring-managed beans
19.6.1.1. Dependency Injecting Spring Beans into Tapestry pages
19.6.1.2. Component definition files
19.6.1.3. Adding abstract accessors
19.6.1.4. Dependency Injecting Spring Beans into Tapestry pages - Tapestry 4.x style
19.7. Further Resources
20. Portlet MVC Framework
20.1. Introduction
20.1.1. Controllers - The C in MVC
20.1.2. Views - The V in MVC
20.1.3. Web-scoped beans
20.2. The DispatcherPortlet
20.3. The ViewRendererServlet
20.4. Controllers
20.4.1. AbstractController and PortletContentGenerator
20.4.2. Other simple controllers
20.4.3. Command Controllers
20.4.4. PortletWrappingController
20.5. Handler mappings
20.5.1. PortletModeHandlerMapping
20.5.2. ParameterHandlerMapping
20.5.3. PortletModeParameterHandlerMapping
20.5.4. Adding HandlerInterceptors
20.5.5. HandlerInterceptorAdapter
20.5.6. ParameterMappingInterceptor
20.6. Views and resolving them
20.7. Multipart (file upload) support
20.7.1. Using the PortletMultipartResolver
20.7.2. Handling a file upload in a form
20.8. Handling exceptions
20.9. Annotation-based controller configuration
20.9.1. Setting up the dispatcher for annotation support
20.9.2. Defining a controller with @Controller
20.9.3. Mapping requests with @RequestMapping
20.9.4. Supported handler method arguments
20.9.5. Binding request parameters to method parameters with @RequestParam
20.9.6. Providing a link to data from the model with @ModelAttribute
20.9.7. Specifying attributes to store in a Session with @SessionAttributes
20.9.8. Customizing WebDataBinder initialization
20.9.8.1. Customizing data binding with @InitBinder
20.9.8.2. Configuring a custom WebBindingInitializer
20.10. Portlet application deployment
IV. Integration
21. Remoting and web services using Spring
21.1. Introduction
21.2. Exposing services using RMI
21.2.1. Exporting the service using the RmiServiceExporter
21.2.2. Linking in the service at the client
21.3. Using Hessian or Burlap to remotely call services via HTTP
21.3.1. Wiring up the DispatcherServlet for Hessian and co.
21.3.2. Exposing your beans by using the HessianServiceExporter
21.3.3. Linking in the service on the client
21.3.4. Using Burlap
21.3.5. Applying HTTP basic authentication to a service exposed through Hessian or Burlap
21.4. Exposing services using HTTP invokers
21.4.1. Exposing the service object
21.4.2. Linking in the service at the client
21.5. Web services
21.5.1. Exposing servlet-based web services using JAX-RPC
21.5.2. Accessing web services using JAX-RPC
21.5.3. Registering JAX-RPC Bean Mappings
21.5.4. Registering your own JAX-RPC Handler
21.5.5. Exposing servlet-based web services using JAX-WS
21.5.6. Exporting standalone web services using JAX-WS
21.5.7. Exporting web services using the JAX-WS RI's Spring support
21.5.8. Accessing web services using JAX-WS
21.5.9. Exposing web services using XFire
21.6. JMS
21.6.1. Server-side configuration
21.6.2. Client-side configuration
21.7. Auto-detection is not implemented for remote interfaces
21.8. Considerations when choosing a technology
22. Enterprise Java Beans (EJB) integration
22.1. Introduction
22.2. Accessing EJBs
22.2.1. Concepts
22.2.2. Accessing local SLSBs
22.2.3. Accessing remote SLSBs
22.2.4. Accessing EJB 2.x SLSBs versus EJB 3 SLSBs
22.3. Using Spring's EJB implementation support classes
22.3.1. EJB 2.x base classes
22.3.2. EJB 3 injection interceptor
23. JMS (Java Message Service)
23.1. Introduction
23.2. Using Spring JMS
23.2.1. JmsTemplate
23.2.2. Connections
23.2.3. Destination Management
23.2.4. Message Listener Containers
23.2.4.1. SimpleMessageListenerContainer
23.2.4.2. DefaultMessageListenerContainer
23.2.4.3. ServerSessionMessageListenerContainer
23.2.5. Transaction management
23.3. Sending a Message
23.3.1. Using Message Converters
23.3.2. SessionCallback and ProducerCallback
23.4. Receiving a message
23.4.1. Synchronous Reception
23.4.2. Asynchronous Reception - Message-Driven POJOs
23.4.3. The SessionAwareMessageListener interface
23.4.4. The MessageListenerAdapter
23.4.5. Processing messages within transactions
23.5. Support for JCA Message Endpoints
23.6. JMS Namespace Support
24. JMX
24.1. Introduction
24.2. Exporting your beans to JMX
24.2.1. Creating an MBeanServer
24.2.2. Reusing an existing MBeanServer
24.2.3. Lazy-initialized MBeans
24.2.4. Automatic registration of MBeans
24.2.5. Controlling the registration behavior
24.3. Controlling the management interface of your beans
24.3.1. The MBeanInfoAssembler Interface
24.3.2. Using source-Level metadata
24.3.3. Using JDK 5.0 Annotations
24.3.4. Source-Level Metadata Types
24.3.5. The AutodetectCapableMBeanInfoAssembler interface
24.3.6. Defining management interfaces using Java interfaces
24.3.7. Using MethodNameBasedMBeanInfoAssembler
24.4. Controlling the ObjectNames for your beans
24.4.1. Reading ObjectNames from Properties
24.4.2. Using the MetadataNamingStrategy
24.4.3. The <context:mbean-export/> element
24.5. JSR-160 Connectors
24.5.1. Server-side Connectors
24.5.2. Client-side Connectors
24.5.3. JMX over Burlap/Hessian/SOAP
24.6. Accessing MBeans via Proxies
24.7. Notifications
24.7.1. Registering Listeners for Notifications
24.7.2. Publishing Notifications
24.8. Further Resources
25. JCA CCI
25.1. Introduction
25.2. Configuring CCI
25.2.1. Connector configuration
25.2.2. ConnectionFactory configuration in Spring
25.2.3. Configuring CCI connections
25.2.4. Using a single CCI connection
25.3. Using Spring's CCI access support
25.3.1. Record conversion
25.3.2. The CciTemplate
25.3.3. DAO support
25.3.4. Automatic output record generation
25.3.5. Summary
25.3.6. Using a CCI Connection and Interaction directly
25.3.7. Example for CciTemplate usage
25.4. Modeling CCI access as operation objects
25.4.1. MappingRecordOperation
25.4.2. MappingCommAreaOperation
25.4.3. Automatic output record generation
25.4.4. Summary
25.4.5. Example for MappingRecordOperation usage
25.4.6. Example for MappingCommAreaOperation usage
25.5. Transactions
26. Email
26.1. Introduction
26.2. Usage
26.2.1. Basic MailSender and SimpleMailMessage usage
26.2.2. Using the JavaMailSender and the MimeMessagePreparator
26.3. Using the JavaMail MimeMessageHelper
26.3.1. Sending attachments and inline resources
26.3.1.1. Attachments
26.3.1.2. Inline resources
26.3.2. Creating email content using a templating library
26.3.2.1. A Velocity-based example
27. Scheduling and Thread Pooling
27.1. Introduction
27.2. Using the OpenSymphony Quartz Scheduler
27.2.1. Using the JobDetailBean
27.2.2. Using the MethodInvokingJobDetailFactoryBean
27.2.3. Wiring up jobs using triggers and the SchedulerFactoryBean
27.3. Using JDK Timer support
27.3.1. Creating custom timers
27.3.2. Using the MethodInvokingTimerTaskFactoryBean
27.3.3. Wrapping up: setting up the tasks using the TimerFactoryBean
27.4. The Spring TaskExecutor abstraction
27.4.1. TaskExecutor types
27.4.2. Using a TaskExecutor
28. Dynamic language support
28.1. Introduction
28.2. A first example
28.3. Defining beans that are backed by dynamic languages
28.3.1. Common concepts
28.3.1.1. The <lang:language/> element
28.3.1.2. Refreshable beans
28.3.1.3. Inline dynamic language source files
28.3.1.4. Understanding Constructor Injection in the context of dynamic-language-backed beans
28.3.2. JRuby beans
28.3.3. Groovy beans
28.3.3.1. Customising Groovy objects via a callback
28.3.4. BeanShell beans
28.4. Scenarios
28.4.1. Scripted Spring MVC Controllers
28.4.2. Scripted Validators
28.5. Bits and bobs
28.5.1. AOP - advising scripted beans
28.5.2. Scoping
28.6. Further Resources
29. Annotations and Source Level Metadata Support
29.1. Introduction
29.2. Spring's metadata support
29.3. Annotations
29.3.1. @Required
29.3.2. Other @Annotations in Spring
29.4. Integration with Jakarta Commons Attributes
29.5. Metadata and Spring AOP autoproxying
29.5.1. Fundamentals
29.5.2. Declarative transaction management
A. XML Schema-based configuration
A.1. Introduction
A.2. XML Schema-based configuration
A.2.1. Referencing the schemas
A.2.2. The util schema
A.2.2.1. <util:constant/>
A.2.2.2. <util:property-path/>
A.2.2.3. <util:properties/>
A.2.2.4. <util:list/>
A.2.2.5. <util:map/>
A.2.2.6. <util:set/>
A.2.3. The jee schema
A.2.3.1. <jee:jndi-lookup/> (simple)
A.2.3.2. <jee:jndi-lookup/> (with single JNDI environment setting)
A.2.3.3. <jee:jndi-lookup/> (with multiple JNDI environment settings)
A.2.3.4. <jee:jndi-lookup/> (complex)
A.2.3.5. <jee:local-slsb/> (simple)
A.2.3.6. <jee:local-slsb/> (complex)
A.2.3.7. <jee:remote-slsb/>
A.2.4. The lang schema
A.2.5. The jms schema
A.2.6. The tx (transaction) schema
A.2.7. The aop schema
A.2.8. The context schema
A.2.8.1. <property-placeholder/>
A.2.8.2. <annotation-config/>
A.2.8.3. <component-scan/>
A.2.8.4. <load-time-weaver/>
A.2.8.5. <spring-configured/>
A.2.8.6. <mbean-export/>
A.2.9. The tool schema
A.2.10. The beans schema
A.3. Setting up your IDE
A.3.1. Setting up Eclipse
A.3.2. Setting up IntelliJ IDEA
A.3.3. Integration issues
A.3.3.1. XML parsing errors in the Resin v.3 application server
B. Extensible XML authoring
B.1. Introduction
B.2. Authoring the schema
B.3. Coding a NamespaceHandler
B.4. Coding a BeanDefinitionParser
B.5. Registering the handler and the schema
B.5.1. 'META-INF/spring.handlers'
B.5.2. 'META-INF/spring.schemas'
B.6. Using a custom extension in your Spring XML configuration
B.7. Meatier examples
B.7.1. Nesting custom tags within custom tags
B.7.2. Custom attributes on 'normal' elements
B.8. Further Resources
C. spring-beans-2.0.dtd
D. spring.tld
D.1. Introduction
D.2. The bind tag
D.3. The escapeBody tag
D.4. The hasBindErrors tag
D.5. The htmlEscape tag
D.6. The message tag
D.7. The nestedPath tag
D.8. The theme tag
D.9. The transform tag
E. spring-form.tld
E.1. Introduction
E.2. The checkbox tag
E.3. The checkboxes tag
E.4. The errors tag
E.5. The form tag
E.6. The hidden tag
E.7. The input tag
E.8. The label tag
E.9. The option tag
E.10. The options tag
E.11. The password tag
E.12. The radiobutton tag
E.13. The radiobuttons tag
E.14. The select tag
E.15. The textarea tag

Preface

Developing software applications is hard enough even with good tools and technologies. Implementing applications using platforms which promise everything but turn out to be heavy-weight, hard to control and not very efficient during the development cycle makes it even harder. Spring provides a light-weight solution for building enterprise-ready applications, while still supporting the possibility of using declarative transaction management, remote access to your logic using RMI or web services, and various options for persisting your data to a database. Spring provides a full-featured MVC framework, and transparent ways of integrating AOP into your software.

Spring could potentially be a one-stop-shop for all your enterprise applications; however, Spring is modular, allowing you to use just those parts of it that you need, without having to bring in the rest. You can use the IoC container, with Struts on top, but you could also choose to use just the Hibernate integration code or the JDBC abstraction layer

Spring has been (and continues to be) designed to be non-intrusive, meaning dependencies, from your domain logic code, on the framework itself are generally none. For your integration layer like the data access layer there will of course be some dependencies on the data access technology in use and also on the Spring libraries, but these dependencies should be easy to isolate from the rest of your code base.

This document provides a reference guide to Spring's features. Since this document is still to be considered very much work-in-progress, if you have any requests or comments, please post them on the user mailing list or on the support forums at http://forum.springsource.org/.

1. Introduction

Fundamentally, what is Spring? We think of it as a Platform for your Java code. It provides comprehensive infrastructural support for developing Java applications. Spring deals with the plumbing so you can focus on solving the domain problem

Spring as a platform allows applications to be built from “plain old Java objects” (POJOs). This is true for the Java SE programming model as well as within a number of other environments including full and partial Java EE. Spring allows enterprise services to be applied to POJOs in a non-invasive way

Examples of Spring as a platform:

  • Make a Java method execute in a database transaction; without the implementer dealing with transaction APIs

  • Make a local Java method a remote-procedure; without the implementer dealing with remoting APIs

  • Make a local Java method a management operation; without the implementer dealing with JMX APIs

  • Make a local Java method a message handler; without the implementer dealing with JMS APIs

1.1 Dependency Injection

Java applications (a loose term which runs the gamut from constrained applets to full-fledged n-tier server-side enterprise applications) typically are composed of a number of objects that collaborate with one another to form the application proper. The objects in an application can thus be said to have dependencies between themselves.

The Java language and platform provides a wealth of functionality for architecting and building applications, ranging all the way from the very basic building blocks of primitive types and classes (and the means to define new classes), to rich full-featured application servers and web frameworks. One area that is decidedly conspicuous by its absence is any means of taking the basic building blocks and composing them into a coherent whole; this area has typically been left to the purvey of the architects and developers tasked with building an application (or applications). Now to be fair, there are a number of design patterns devoted to the business of composing the various classes and object instances that makeup an all-singing, all-dancing application. Design patterns such as Factory, Abstract Factory, Builder, Decorator, and Service Locator (to name but a few) have widespread recognition and acceptance within the software development industry (presumably that is why these patterns have been formalized as patterns in the first place). This is all very well, but these patterns are just that: best practices given a name, typically together with a description of what the pattern does, where the pattern is typically best applied, the problems that the application of the pattern addresses, and so forth. Notice that the last paragraph used the phrase “... a description of what the pattern does...”; pattern books and wikis are typically listings of such formalized best practice that you can certainly take away, mull over, and then implement yourself in your application.

The IoC component of the Spring Framework addresses the enterprise concern of taking the classes, objects, and services that are to compose an application, by providing a formalized means of composing these various disparate components into a fully working application ready for use. The Spring Framework takes best practices that have been proven over the years in numerous applications and formalized as design patterns, and actually codifies these patterns as first class objects that you as an architect and developer can take away and integrate into your own application(s). This is a Very Good Thing Indeed as attested to by the numerous organizations and institutions that have used the Spring Framework to engineer robust, maintainable applications.

1.2 Modules

The Spring Framework contains a lot of features, which are well-organized in ab out twenty modules. These modules can be grouped together based on their primary features into Core Container, Data Access/Integration, Web, AOP (Aspect Oriented Programming), Instrumentation and Test. These groups are shown in the diagram below.

Overview of the Spring Framework

1.2.1 Core Container

The Core Container consists of the Core, Beans, Context and Expression modules.

The Core and Beans modules provide the most fundamental parts of the framework and provides the IoC and Dependency Injection features. The basic concept here is the BeanFactory, which provides a sophisticated implementation of the factory pattern which removes the need for programmatic singletons and allows you to decouple the configuration and specification of dependencies from your actual program logic.

The Context module build on the solid base provided by the Core and Beans modules: it provides a way to access objects in a framework-style manner in a fashion somewhat reminiscent of a JNDI-registry. The Context module inherits its features from the Beans module and adds support for internationalization (I18N) (using for example resource bundles), event-propagation, resource-loading, and the transparent creation of contexts by, for example, a servlet container. The Context module also contains support for some Java EE features like EJB, JMX and basic remoting support.

The Expression Language module provides a powerful expression language for querying and manipulating an object graph at runtime. It can be seen as an extension of the unified expression language (unified EL) as specified in the JSP 2.1 specification. The language supports setting and getting of property values, property assignment, method invocation, accessing the context of arrays, collections and indexers, logical and arithmetic operators, named variables, and retrieval of objects by name from Spring's IoC container. It also supports list projection and selection, as well as common list aggregators.

1.2.2 Data Access/Integration

The Data Access/Integration layer consists of the JDBC, ORM, OXM, JMS and Transaction modules.

The JDBC module provides a JDBC-abstraction layer that removes the need to do tedious JDBC coding and parsing of database-vendor specific error codes.

The ORM module provides integration layers for popular object-relational mapping APIs, including JPA, JDO, Hibernate, and iBatis. Using the ORM package you can use all those O/R-mappers in combination with all the other features Spring offers, such as the simple declarative transaction management feature mentioned previously.

The OXM module provides an abstraction layer for using a number of Object/XML mapping implementations. Supported technologies include JAXB, Castor, XMLBeans, JiBX and XStream.

The JMS module provides Spring's support for the Java Messaging Service. It contains features for both producing and consuming messages.

The Transaction module provides a way to do programmatic as well as declarative transaction management, not only for classes implementing special interfaces, but for all your POJOs (plain old Java objects).

1.2.3 Web

The Web layer consists of the Web, Web-Servlet and Web-Portlet modules.

Spring's Web module provides basic web-oriented integration features, such as multipart file-upload functionality, the initialization of the IoC container using servlet listeners and a web-oriented application context. It also contains the web related parts of Spring's remoting support.

The Web-Servlet module provides Spring's Model-View-Controller (MVC) implementation for web-applications. Spring's MVC framework is not just any old implementation; it provides a clean separation between domain model code and web forms, and allows you to use all the other features of the Spring Framework.

The Web-Portlet module provides the MVC implementation to be used in a portlet environment and mirrors what is provided in the Web-Servlet module.

1.2.4 AOP and Instrumentation

Spring's AOP module provides an AOP Alliance-compliant aspect-oriented programming implementation allowing you to define, for example, method-interceptors and pointcuts to cleanly decouple code implementing functionality that should logically speaking be separated. Using source-level metadata functionality you can also incorporate all kinds of behavioral information into your code, in a manner similar to that of .NET attributes.

There is also a separate Aspects module that provides integration with AspectJ.

The Instrumentation module provides class instrumentation support and classloader implementations to be used in certain application servers.

1.2.5 Test

The Test module contains the Test Framework that supports testing Spring components using JUnit or TestNG. It provides consistent loading of Spring ApplicationContexts and caching of those contexts. It also contains a number of Mock objects that are usful in many testing scenarios to test your code in isolation.

1.3 Usage scenarios

With the building blocks described above you can use Spring in all sorts of scenarios, from applets up to fully-fledged enterprise applications using Spring's transaction management functionality and web framework integration.

Typical full-fledged Spring web application

By using Spring's declarative transaction management features the web application is fully transactional, just as it would be when using container managed transactions as provided by Enterprise JavaBeans. All your custom business logic can be implemented using simple POJOs, managed by Spring's IoC container. Additional services include support for sending email, and validation that is independent of the web layer enabling you to choose where to execute validation rules. Spring's ORM support is integrated with JPA, Hibernate, JDO and iBatis; for example, when using Hibernate, you can continue to use your existing mapping files and standard Hibernate SessionFactory configuration. Form controllers seamlessly integrate the web-layer with the domain model, removing the need for ActionForms or other classes that transform HTTP parameters to values for your domain model.

Spring middle-tier using a third-party web framework

Sometimes the current circumstances do not allow you to completely switch to a different framework. The Spring Framework does not force you to use everything within it; it is not an all-or-nothing solution. Existing front-ends built using WebWork, Struts, Tapestry, or other UI frameworks can be integrated perfectly well with a Spring-based middle-tier, allowing you to use the transaction features that Spring offers. The only thing you need to do is wire up your business logic using an ApplicationContext and integrate your web layer using a WebApplicationContext.

Remoting usage scenario

When you need to access existing code via web services, you can use Spring's Hessian-, Burlap-, Rmi- or JaxRpcProxyFactory classes. Enabling remote access to existing applications suddenly is not that hard anymore.

EJBs - Wrapping existing POJOs

The Spring Framework also provides an access- and abstraction- layer for Enterprise JavaBeans, enabling you to reuse your existing POJOs and wrap them in Stateless Session Beans, for use in scalable, failsafe web applications that might need declarative security.

2. What's new in Spring 3.0?

If you have been using the Spring Framework for some time, you will be aware that Spring has undergone two major revisions: Spring 2.0, released in October 2006, and Spring 2.5, released in November 2007. It is now time for a third overhaul resulting in Spring 3.0.

2.1 Java 5

The entire framework code has been revised to take advantage of Java 5 features like generics, varargs and other language improvements. We have done our best to still keep the code backwards compatible. We now have consistent use of generic Collections and Maps, consistent use of generified FactoryBeans, and also consistent resolution of bridge methods in the Spring AOP API. Generified ApplicationListeners automatically receive specific event types only. All callback interfaces such as TransactionCallback and HibernateCallback declare a generic result value now. Overall, the Spring core codebase is now freshly revised and optimized for Java 5.

Spring's TaskExecutor abstraction has been updated for close integration with Java 5's java.util.concurrent facilities. We provide first-class support for Callables and Futures now, as well as ExecutorService adapters, ThreadFactory integration, etc. This has been aligned with JSR-236 (Concurrency Utilities for Java EE 6) as far as possible. Furthermore, we provide support for asynchronous method invocations through the use of the new @Async annotation (or EJB 3.1's @Asynchronous annotation).

2.2 Improved documentation

The Spring reference documentation has also substantially been updated to reflect all of the changes and new features for Spring 3.0. While every effort has been made to ensure that there are no errors in this documentation, some errors may nevertheless have crept in. If you do spot any typos or even more serious errors, and you can spare a few cycles during lunch, please do bring the error to the attention of the Spring team by raising an issue.

2.3 New module organization and build system

The framework modules have been revised and are now managed separately with one source-tree per module jar:

  • org.springframework.aop

  • org.springframework.beans

  • org.springframework.context

  • org.springframework.context.support

  • org.springframework.expression

  • org.springframework.instrument

  • org.springframework.jdbc

  • org.springframework.jms

  • org.springframework.orm

  • org.springframework.oxm

  • org.springframework.test

  • org.springframework.transaction

  • org.springframework.web

  • org.springframework.web.portlet

  • org.springframework.web.servlet

We are now using a new Spring build system as known from Spring Web Flow 2.0. This gives us:

  • Ivy-based "Spring Build" system

  • consistent deployment procedure

  • consistent dependency management

  • consistent generation of OSGi manifests

2.4 Overview of new features

This is a list of new features for Spring 3.0. We will cover these features in more detail later in this section.

  • Spring Expression Language

  • IoC enhancements/Java based bean metadata

  • Object to XML mapping functionality (OXM) moved from Spring Web Services project

  • Comprehensive REST support

  • @MVC additions

  • Declarative model validation

  • Early support for Java EE 6

2.4.1 Core APIs updated for Java 5

BeanFactory interface returns typed bean instances as far as possible:

  • T getBean(Stringname, Class<T> requiredType)

  • Map<String, T> getBeansOfType(Class<T> type)

Spring's TaskExecutor interface now extends java.util.concurrent.Executor:

  • extended AsyncTaskExecutor supports standard Callables with Futures

New Java 5 based converter API and SPI:

  • stateless ConversionService and Converters

  • superseding standard JDK PropertyEditors

Typed ApplicationListener<E>

2.4.2 Spring Expression Language

Spring introduces an expression language which is similar to Unified EL in its syntax but offers significantly more features. The expression language can be used when defining XML and Annotation based bean definitions and also serves as the foundation for expression language support across the Spring portfolio. Details of this new functionality can be found in the chapter Spring Expression Language (SpEL).

The Spring Expression Language was created to provide the Spring community a single, well supported expression language that can be used across all the products in the Spring portfolio. Its language features are driven by the requirements of the projects in the Spring portfolio, including tooling requirements for code completion support within the Eclipse based SpringSource Tool Suite.

The following is an example of how the Expression Language can be used to configure some properties of a database setup

<bean class="mycompany.RewardsTestDatabase">
    <property name="databaseName"
        value="#{systemProperties.databaseName}"/>
    <property name="keyGenerator"
        value="#{strategyBean.databaseKeyGenerator}"/>
</bean>

This functionality is also available if you prefer to configure your components using annotations:

@Repository 
public class RewardsTestDatabase {

    @Value("#{systemProperties.databaseName}")
    public void setDatabaseName(String dbName) { … }

    @Value("#{strategyBean.databaseKeyGenerator}")
    public voidsetKeyGenerator(KeyGenerator kg) { … }
}

2.4.3 The Inversion of Control (IoC) container

2.4.3.1 Java based bean metadata

Some core features from the JavaConfig project have been added to the Spring Framework now. This means that the following annotations are now directly supported:

  • @Configuration

  • @Bean

  • @Primary

  • @Lazy

  • @Import

  • @Value

Here is an example of a Java class providing basic configuration using the new JavaConfig features:

@Configuration
public class AppConfig{
    private @Value("#{jdbcProperties.url}") String jdbcUrl;
    private @Value("#{jdbcProperties.username}") String username;
    private @Value("#{jdbcProperties.password}") String password;

    @Bean
    public FooService fooService() {
        return new FooServiceImpl(fooRepository());
    }

    @Bean
    public FooRepository fooRepository() {
        return new HibernateFooRepository(sessionFactory());
    }

    @Bean
    public SessionFactory sessionFactory() {
        // wire up a session factory using
        // AnnotationSessionFactoryBean
        asFactoryBean.setDataSource(dataSource());
        return (SessionFactory) asFactoryBean.getObject();
    }

    @Bean
    public DataSource dataSource() { 
        return new DriverManagerDataSource(jdbcUrl, username, password);
    }
}

To get this to work you need to add the following component scanning entry in your minimal application context XML file.

<context:component-scan 
    base-package="com.myco.config"/>

2.4.3.2 Defining bean metadata within components

@Bean annotated methods are also supported inside Spring components. They contribute a factory bean definition to the container. See Defining bean metadata within components for more information

2.4.4 The Data Tier

Object to XML mapping functionality (OXM) from the Spring Web Services project has been moved to the core Spring Framework now. The functionality is found in the org.springframework.oxm package. More information on the use of the OXM module can be found in the Marshalling XML using O/X Mappers chapter.

2.4.5 The Web Tier

The most exciting new feature for the Web Tier is the support for building RESTful web services and web applications. There are also some new annotations that can be used in any web application.

2.4.5.1 Comprehensive REST support

Server-side support for building RESTful applications has been provided as an extension of the existing annotation driven MVC web framework. Client-side support is provided by the RestTemplate class in the spirit of other template classes such as JdbcTemplate and JmsTemplate. Both server and client side REST functionality make use of HttpConverters to facilitate the conversion between objects and their representation in HTTP request and replies.

The MarhsallingHttpMessageConverter uses the Object to XML mapping functionality mentioned earlier.

Refer to the section on REST support for more information.

2.4.5.2 @MVC additions

Additional annotations such as @CookieValue and @RequestHeaders have been added. See Mapping cookie values with the @CookieValue annotation and Mapping request header attributes with the @RequestHeader annotation for more information.

2.4.6 Declarative model validation

Hibernate Validator, JSR 303

Work in progress... not part of the Spring 3.0 M3 release.

2.4.7 Early support for Java EE 6

We provide support for asynchronous method invocations through the use of the new @Async annotation (or EJB 3.1's @Asynchronous annotation).

JSF 2.0, JPA 2.0, etc

Work in progress... not part of the Spring 3.0 M3 release.

3. Getting started with Spring

This chapter will give you a quick introduction and serve as a guide for how to get started using the Spring Framework for your Java development. We can of course only cover a tiny subset of the available features in this chapter. You will have to turn to the rest of this reference document for more detailed coverage of all features.

3.1 Creating an ApplicationContext

We ...

3.2 The Data Access Object

We ...

3.3 The Business Layer

We ...

3.4 The Web UI

We ...

Part I. Core Technologies

This initial part of the reference documentation covers all of those technologies that are absolutely integral to the Spring Framework.

Foremost amongst these is the Spring Framework's Inversion of Control (IoC) container. A thorough treatment of the Spring Framework's IoC container is closely followed by comprehensive coverage of Spring's Aspect-Oriented Programming (AOP) technologies. The Spring Framework has its own AOP framework, which is conceptually easy to understand, and which successfully addresses the 80% sweet spot of AOP requirements in Java enterprise programming.

Coverage of Spring's integration with AspectJ (currently the richest - in terms of features - and certainly most mature AOP implementation in the Java enterprise space) is also provided.

Finally, the adoption of the test-driven-development (TDD) approach to software development is certainly advocated by the Spring team, and so coverage of Spring's support for integration testing is covered (alongside best practices for unit testing). The Spring team have found that the correct use of IoC certainly does make both unit and integration testing easier (in that the presence of setter methods and appropriate constructors on classes makes them easier to wire together on a test without having to set up service locator registries and suchlike)... the chapter dedicated solely to testing will hopefully convince you of this as well.

4. The IoC container

4.1 Introduction

This chapter covers the Spring Framework's implementation of the Inversion of Control (IoC) [1] principle.

The org.springframework.beans and org.springframework.context packages provide the basis for the Spring Framework's IoC container. The BeanFactory interface provides an advanced configuration mechanism capable of managing objects of any nature. The ApplicationContext interface builds on top of the BeanFactory (it is a sub-interface) and adds other functionality such as easier integration with Spring's AOP features, message resource handling (for use in internationalization), event propagation, and application-layer specific contexts such as the WebApplicationContext for use in web applications.

In short, the BeanFactory provides the configuration framework and basic functionality, while the ApplicationContext adds more enterprise-centric functionality to it. The ApplicationContext is a complete superset of the BeanFactory, and any description of BeanFactory capabilities and behavior is to be considered to apply to the ApplicationContext as well.

This chapter is divided into two parts, with the first part covering the basic principles that apply to both the BeanFactory and ApplicationContext, and with the second part covering those features that apply only to the ApplicationContext interface.

4.2 Basics - containers and beans

In Spring, those objects that form the backbone of your application and that are managed by the Spring IoC container are referred to as beans. A bean is simply an object that is instantiated, assembled and otherwise managed by a Spring IoC container; other than that, there is nothing special about a bean (it is in all other respects one of probably many objects in your application). These beans, and the dependencies between them, are reflected in the configuration metadata used by a container.

4.2.1 The container

The org.springframework.beans.factory.BeanFactory is the actual representation of the Spring IoC container that is responsible for containing and otherwise managing the aforementioned beans.

The BeanFactory interface is the central IoC container interface in Spring. Its responsibilities include instantiating or sourcing application objects, configuring such objects, and assembling the dependencies between these objects.

There are a number of implementations of the BeanFactory interface that come supplied straight out-of-the-box with Spring. The most commonly used BeanFactory implementation is the XmlBeanFactory class. This implementation allows you to express the objects that compose your application, and the doubtless rich interdependencies between such objects, in terms of XML. The XmlBeanFactory takes this XML configuration metadata and uses it to create a fully configured system or application.

The Spring IoC container

4.2.1.1 Configuration metadata

As can be seen in the above image, the Spring IoC container consumes some form of configuration metadata; this configuration metadata is nothing more than how you (as an application developer) inform the Spring container as to how to instantiate, configure, and assemble [the objects in your application]”. This configuration metadata is typically supplied in a simple and intuitive XML format. When using XML-based configuration metadata, you write bean definitions for those beans that you want the Spring IoC container to manage, and then let the container do its stuff.

[Note]Note

XML-based metadata is by far the most commonly used form of configuration metadata. It is not however the only form of configuration metadata that is allowed. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. The XML-based configuration metadata format really is simple though, and so the majority of this chapter will use the XML format to convey key concepts and features of the Spring IoC container.

You can find details of another form of metadata that the Spring container can consume in the section entitled Section 4.11, “Annotation-based configuration”

In the vast majority of application scenarios, explicit user code is not required to instantiate one or more instances of a Spring IoC container. For example, in a web application scenario, a simple eight (or so) lines of boilerplate J2EE web descriptor XML in the web.xml file of the application will typically suffice (see Section 4.8.5, “Convenient ApplicationContext instantiation for web applications”).

Spring configuration consists of at least one bean definition that the container must manage, but typically there will be more than one bean definition. When using XML-based configuration metadata, these beans are configured as <bean/> elements inside a top-level <beans/> element.

These bean definitions correspond to the actual objects that make up your application. Typically you will have bean definitions for your service layer objects, your data access objects (DAOs), presentation objects such as Struts Action instances, infrastructure objects such as Hibernate SessionFactories, JMS Queues, and so forth. Typically one does not configure fine-grained domain objects in the container, because it is usually the responsibility of DAOs and business logic to create/load domain objects.

Find below an example of the basic structure of XML-based configuration metadata.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

  <bean id="..." class="...">
    <!-- collaborators and configuration for this bean go here -->
  </bean>

  <bean id="..." class="...">
    <!-- collaborators and configuration for this bean go here -->
  </bean>

  <!-- more bean definitions go here -->

</beans>

4.2.2 Instantiating a container

Instantiating a Spring IoC container is straightforward.

ApplicationContext context = new ClassPathXmlApplicationContext(
        new String[] {"services.xml", "daos.xml"});

// an ApplicationContext is also a BeanFactory (via inheritance)
BeanFactory factory = context;

4.2.2.1 Composing XML-based configuration metadata

It can often be useful to split up container definitions into multiple XML files. One way to then load an application context which is configured from all these XML fragments is to use the application context constructor which takes multiple Resource locations. With a bean factory, a bean definition reader can be used multiple times to read definitions from each file in turn.

Generally, the Spring team prefers the above approach, since it keeps container configuration files unaware of the fact that they are being combined with others. An alternate approach is to use one or more occurrences of the <import/> element to load bean definitions from another file (or files). Let's look at a sample:

<beans>

    <import resource="services.xml"/>
    <import resource="resources/messageSource.xml"/>
    <import resource="/resources/themeSource.xml"/>

    <bean id="bean1" class="..."/>
    <bean id="bean2" class="..."/>

</beans>

In this example, external bean definitions are being loaded from 3 files, services.xml, messageSource.xml, and themeSource.xml. All location paths are considered relative to the definition file doing the importing, so services.xml in this case must be in the same directory or classpath location as the file doing the importing, while messageSource.xml and themeSource.xml must be in a resources location below the location of the importing file. As you can see, a leading slash is actually ignored, but given that these are considered relative paths, it is probably better form not to use the slash at all. The contents of the files being imported must be valid XML bean definition files according to the Spring Schema or DTD, including the top level <beans/> element.

[Note]Note

It is possible to reference files in parent directories using a relative "../" path. However, this is not recommended because it creates a dependency on a file that is outside the current application. This is in particular not recommended for "classpath:" URLs (e.g. "classpath:../services.xml") where the runtime resolution process will pick the "nearest" classpath root and then look into its parent directory. This is fragile since classpath configuration changes may lead to a different directory being picked.

Note that you can always use fully qualified resource locations instead of relative paths: e.g. "file:C:/config/services.xml" or "classpath:/config/services.xml". However, be aware that you are coupling your application's configuration to specific absolute locations then. It is generally preferable to keep an indirection for such absolute locations, e.g. through "${...}" placeholders that are resolved against JVM system properties at runtime.

4.2.3 The beans

A Spring IoC container manages one or more beans. These beans are created using the configuration metadata that has been supplied to the container (typically in the form of XML <bean/> definitions).

Within the container itself, these bean definitions are represented as BeanDefinition objects, which contain (among other information) the following metadata:

  • a package-qualified class name: typically this is the actual implementation class of the bean being defined.

  • bean behavioral configuration elements, which state how the bean should behave in the container (scope, lifecycle callbacks, and so forth).

  • references to other beans which are needed for the bean to do its work; these references are also called collaborators or dependencies.

  • other configuration settings to set in the newly created object. An example would be the number of connections to use in a bean that manages a connection pool, or the size limit of the pool.

The concepts listed above directly translate to a set of properties that each bean definition consists of. Some of these properties are listed below, along with a link to further documentation about each of them.


Besides bean definitions which contain information on how to create a specific bean, certain BeanFactory implementations also permit the registration of existing objects that have been created outside the factory (by user code). The DefaultListableBeanFactory class supports this through the registerSingleton(..) method. (Typical applications solely work with beans defined through metadata bean definitions though.)

4.2.3.1 Naming beans

Every bean has one or more ids (also called identifiers, or names; these terms refer to the same thing). These ids must be unique within the container the bean is hosted in. A bean will almost always have only one id, but if a bean has more than one id, the extra ones can essentially be considered aliases.

When using XML-based configuration metadata, you use the 'id' or 'name' attributes to specify the bean identifier(s). The 'id' attribute allows you to specify exactly one id, and as it is a real XML element ID attribute, the XML parser is able to do some extra validation when other elements reference the id; as such, it is the preferred way to specify a bean id. However, the XML specification does limit the characters which are legal in XML IDs. This is usually not a constraint, but if you have a need to use one of these special XML characters, or want to introduce other aliases to the bean, you may also or instead specify one or more bean ids, separated by a comma (,), semicolon (;), or whitespace in the 'name' attribute.

Please note that you are not required to supply a name for a bean. If no name is supplied explicitly, the container will generate a unique name for that bean. The motivations for not supplying a name for a bean will be discussed later (one use case is inner beans).

Aliasing beans

In a bean definition itself, you may supply more than one name for the bean, by using a combination of up to one name specified via the id attribute, and any number of other names via the name attribute. All these names can be considered equivalent aliases to the same bean, and are useful for some situations, such as allowing each component used in an application to refer to a common dependency using a bean name that is specific to that component itself.

Having to specify all aliases when the bean is actually defined is not always adequate however. It is sometimes desirable to introduce an alias for a bean which is defined elsewhere. In XML-based configuration metadata this may be accomplished via the use of the <alias/> element.

<alias name="fromName" alias="toName"/>

In this case, a bean in the same container which is named 'fromName', may also after the use of this alias definition, be referred to as 'toName'.

As a concrete example, consider the case where component A defines a DataSource bean called componentA-dataSource, in its XML fragment. Component B would however like to refer to the DataSource as componentB-dataSource in its XML fragment. And the main application, MyApp, defines its own XML fragment and assembles the final application context from all three fragments, and would like to refer to the DataSource as myApp-dataSource. This scenario can be easily handled by adding to the MyApp XML fragment the following standalone aliases:

<alias name="componentA-dataSource" alias="componentB-dataSource"/>
<alias name="componentA-dataSource" alias="myApp-dataSource" />

Now each component and the main application can refer to the dataSource via a name that is unique and guaranteed not to clash with any other definition (effectively there is a namespace), yet they refer to the same bean.

4.2.3.2 Instantiating beans

A bean definition essentially is a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.

If you are using XML-based configuration metadata, you can specify the type (or class) of object that is to be instantiated using the 'class' attribute of the <bean/> element. This 'class' attribute (which internally eventually boils down to being a Class property on a BeanDefinition instance) is normally mandatory (see the section called “Instantiation using an instance factory method” and Section 4.6, “Bean definition inheritance” for the two exceptions) and is used for one of two purposes. The class property specifies the class of the bean to be constructed in the common case where the container itself directly creates the bean by calling its constructor reflectively (somewhat equivalent to Java code using the 'new' operator). In the less common case where the container invokes a static, factory method on a class to create the bean, the class property specifies the actual class containing the static factory method that is to be invoked to create the object (the type of the object returned from the invocation of the static factory method may be the same class or another class entirely, it doesn't matter).

Instantiation using a constructor

When creating a bean using the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being created does not need to implement any specific interfaces or be coded in a specific fashion. Just specifying the bean class should be enough. However, depending on what type of IoC you are going to use for that specific bean, you may need a default (empty) constructor.

Additionally, the Spring IoC container isn't limited to just managing true JavaBeans, it is also able to manage virtually any class you want it to manage. Most people using Spring prefer to have actual JavaBeans (having just a default (no-argument) constructor and appropriate setters and getters modeled after the properties) in the container, but it is also possible to have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.

When using XML-based configuration metadata you can specify your bean class like so:

<bean id="exampleBean" class="examples.ExampleBean"/>

<bean name="anotherExample" class="examples.ExampleBeanTwo"/>

The mechanism for supplying arguments to the constructor (if required), or setting properties of the object instance after it has been constructed, is described shortly.

Instantiation using a static factory method

When defining a bean which is to be created using a static factory method, along with the class attribute which specifies the class containing the static factory method, another attribute named factory-method is needed to specify the name of the factory method itself. Spring expects to be able to call this method (with an optional list of arguments as described later) and get back a live object, which from that point on is treated as if it had been created normally via a constructor. One use for such a bean definition is to call static factories in legacy code.

The following example shows a bean definition which specifies that the bean is to be created by calling a factory-method. Note that the definition does not specify the type (class) of the returned object, only the class containing the factory method. In this example, the createInstance() method must be a static method.

<bean id="exampleBean"
      class="examples.ExampleBean2"
      factory-method="createInstance"/>

The mechanism for supplying (optional) arguments to the factory method, or setting properties of the object instance after it has been returned from the factory, will be described shortly.

Instantiation using an instance factory method

In a fashion similar to instantiation via a static factory method, instantiation using an instance factory method is where a non-static method of an existing bean from the container is invoked to create a new bean. To use this mechanism, the 'class' attribute must be left empty, and the 'factory-bean' attribute must specify the name of a bean in the current (or parent/ancestor) container that contains the instance method that is to be invoked to create the object. The name of the factory method itself must be set using the 'factory-method' attribute.

<!-- the factory bean, which contains a method called createInstance() -->
<bean id="serviceLocator" class="com.foo.DefaultServiceLocator">
  <!-- inject any dependencies required by this locator bean -->
</bean>

<!-- the bean to be created via the factory bean -->
<bean id="exampleBean"
      factory-bean="serviceLocator"
      factory-method="createInstance"/>

Although the mechanisms for setting bean properties are still to be discussed, one implication of this approach is that the factory bean itself can be managed and configured via DI.

[Note]Note

When the Spring documentation makes mention of a 'factory bean', this will be a reference to a bean that is configured in the Spring container that will create objects via an instance or static factory method. When the documentation mentions a FactoryBean (notice the capitalization) this is a reference to a Spring-specific FactoryBean .

4.2.4 Using the container

A BeanFactory is essentially nothing more than the interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. The BeanFactory enables you to read bean definitions and access them using the bean factory. When using just the BeanFactory you would create one and read in some bean definitions in the XML format as follows:

Resource res = new FileSystemResource("beans.xml");
BeanFactory factory = new XmlBeanFactory(res);

Basically that is all there is to it. Using getBean(String) you can retrieve instances of your beans; the client-side view of the BeanFactory is simple. The BeanFactory interface has just a few other methods, but ideally your application code should never use them... indeed, your application code should have no calls to the getBean(String) method at all, and thus no dependency on Spring APIs at all.

4.3 Dependencies

Your typical enterprise application is not made up of a single object (or bean in the Spring parlance). Even the simplest of applications will no doubt have at least a handful of objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand-alone, each to themselves, to a fully realized application where objects work (or collaborate) together to achieve some goal (usually an application that does what the end-user wants).

4.3.1 Injecting dependencies

The basic principle behind Dependency Injection (DI) is that objects define their dependencies (that is to say the other objects they work with) only through constructor arguments, arguments to a factory method, or properties which are set on the object instance after it has been constructed or returned from a factory method. Then, it is the job of the container to actually inject those dependencies when it creates the bean. This is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself being in control of instantiating or locating its dependencies on its own using direct construction of classes, or something like the Service Locator pattern.

It becomes evident upon usage that code gets much cleaner when the DI principle is applied, and reaching a higher grade of decoupling is much easier when objects do not look up their dependencies, but are provided with them (and additionally do not even know where the dependencies are located and of what concrete class they are). DI exists in two major variants, namely Constructor Injection and Setter Injection.

4.3.1.1 Constructor Injection

Constructor-based DI is effected by invoking a constructor with a number of arguments, each representing a dependency. Additionally, calling a static factory method with specific arguments to construct the bean, can be considered almost equivalent, and the rest of this text will consider arguments to a constructor and arguments to a static factory method similarly. Find below an example of a class that could only be dependency injected using constructor injection. Notice that there is nothing special about this class.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on a MovieFinder
    private MovieFinder movieFinder;

    // a constructor so that the Spring container can 'inject' a MovieFinder
    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    
    // business logic that actually 'uses' the injected MovieFinder is omitted...
}
Constructor Argument Resolution

Constructor argument resolution matching occurs using the argument's type. If there is no potential for ambiguity in the constructor arguments of a bean definition, then the order in which the constructor arguments are defined in a bean definition is the order in which those arguments will be supplied to the appropriate constructor when it is being instantiated. Consider the following class:

package x.y;

public class Foo {

    public Foo(Bar bar, Baz baz) {
        // ...
    }
}

There is no potential for ambiguity here (assuming of course that Bar and Baz classes are not related in an inheritance hierarchy). Thus the following configuration will work just fine, and you do not need to specify the constructor argument indexes and / or types explicitly.

<beans>
    <bean name="foo" class="x.y.Foo">
        <constructor-arg>
            <bean class="x.y.Bar"/>
        </constructor-arg>
        <constructor-arg>
            <bean class="x.y.Baz"/>
        </constructor-arg>
    </bean>
</beans>

When another bean is referenced, the type is known, and matching can occur (as was the case with the preceding example). When a simple type is used, such as <value>true<value>, Spring cannot determine the type of the value, and so cannot match by type without help. Consider the following class:

package examples;

public class ExampleBean {

    // No. of years to the calculate the Ultimate Answer
    private int years;

    // The Answer to Life, the Universe, and Everything
    private String ultimateAnswer;

    public ExampleBean(int years, String ultimateAnswer) {
        this.years = years;
        this.ultimateAnswer = ultimateAnswer;
    }
}
Constructor Argument Type Matching

The above scenario can use type matching with simple types by explicitly specifying the type of the constructor argument using the 'type' attribute. For example:

<bean id="exampleBean" class="examples.ExampleBean">
  <constructor-arg type="int" value="7500000"/>
  <constructor-arg type="java.lang.String" value="42"/>
</bean>
Constructor Argument Index

Constructor arguments can have their index specified explicitly by use of the index attribute. For example:

<bean id="exampleBean" class="examples.ExampleBean">
  <constructor-arg index="0" value="7500000"/>
  <constructor-arg index="1" value="42"/>
</bean>

As well as solving the ambiguity problem of multiple simple values, specifying an index also solves the problem of ambiguity where a constructor may have two arguments of the same type. Note that the index is 0 based.

4.3.1.2 Setter Injection

Setter-based DI is realized by calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Find below an example of a class that can only be dependency injected using pure setter injection. Note that there is nothing special about this class... it is plain old Java.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on the MovieFinder
    private MovieFinder movieFinder;

    // a setter method so that the Spring container can 'inject' a MovieFinder
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // business logic that actually 'uses' the injected MovieFinder is omitted...
}

The BeanFactory supports both of these variants for injecting dependencies into beans it manages. (It in fact also supports injecting setter-based dependencies after some dependencies have already been supplied via the constructor approach.) The configuration for the dependencies comes in the form of a BeanDefinition, which is used together with PropertyEditor instances to know how to convert properties from one format to another. However, most users of Spring will not be dealing with these classes directly (that is programmatically), but rather with an XML definition file which will be converted internally into instances of these classes, and used to load an entire Spring IoC container instance.

Bean dependency resolution generally happens as follows:

  1. The BeanFactory is created and initialized with a configuration which describes all the beans. (Most Spring users use a BeanFactory or ApplicationContext implementation that supports XML format configuration files.)

  2. Each bean has dependencies expressed in the form of properties, constructor arguments, or arguments to the static-factory method when that is used instead of a normal constructor. These dependencies will be provided to the bean, when the bean is actually created.

  3. Each property or constructor argument is either an actual definition of the value to set, or a reference to another bean in the container.

  4. Each property or constructor argument which is a value must be able to be converted from whatever format it was specified in, to the actual type of that property or constructor argument. By default Spring can convert a value supplied in string format to all built-in types, such as int, long, String, boolean, etc.

The Spring container validates the configuration of each bean as the container is created, including the validation that properties which are bean references are actually referring to valid beans. However, the bean properties themselves are not set until the bean is actually created. For those beans that are singleton-scoped and set to be pre-instantiated (such as singleton beans in an ApplicationContext), creation happens at the time that the container is created, but otherwise this is only when the bean is requested. When a bean actually has to be created, this will potentially cause a graph of other beans to be created, as its dependencies and its dependencies' dependencies (and so on) are created and assigned.

You can generally trust Spring to do the right thing. It will detect misconfiguration issues, such as references to non-existent beans and circular dependencies, at container load-time. It will actually set properties and resolve dependencies as late as possible, which is when the bean is actually created. This means that a Spring container which has loaded correctly can later generate an exception when you request a bean if there is a problem creating that bean or one of its dependencies. This could happen if the bean throws an exception as a result of a missing or invalid property, for example. This potentially delayed visibility of some configuration issues is why ApplicationContext implementations by default pre-instantiate singleton beans. At the cost of some upfront time and memory to create these beans before they are actually needed, you find out about configuration issues when the ApplicationContext is created, not later. If you wish, you can still override this default behavior and set any of these singleton beans to lazy-initialize (that is not be pre-instantiated).

If no circular dependencies are involved (see sidebar for a discussion of circular dependencies), when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being passed (via one of the DI flavors) to the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container will totally configure bean B prior to invoking the setter method on bean A; you can read 'totally configure' to mean that the bean will be instantiated (if not a pre-instantiated singleton), all of its dependencies will be set, and the relevant lifecycle methods (such as a configured init method or the IntializingBean callback method) will all be invoked.

4.3.1.3 Some examples

First, an example of using XML-based configuration metadata for setter-based DI. Find below a small part of a Spring XML configuration file specifying some bean definitions.

<bean id="exampleBean" class="examples.ExampleBean">

  <!-- setter injection using the nested <ref/> element -->
  <property name="beanOne"><ref bean="anotherExampleBean"/></property>

  <!-- setter injection using the neater 'ref' attribute -->
  <property name="beanTwo" ref="yetAnotherBean"/>
  <property name="integerProperty" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    private AnotherBean beanOne;
    private YetAnotherBean beanTwo;
    private int i;

    public void setBeanOne(AnotherBean beanOne) {
        this.beanOne = beanOne;
    }

    public void setBeanTwo(YetAnotherBean beanTwo) {
        this.beanTwo = beanTwo;
    }

    public void setIntegerProperty(int i) {
        this.i = i;
    }    
}

As you can see, setters have been declared to match against the properties specified in the XML file. Find below an example of using constructor-based DI.

<bean id="exampleBean" class="examples.ExampleBean">

  <!-- constructor injection using the nested <ref/> element -->
  <constructor-arg>
    <ref bean="anotherExampleBean"/>
  </constructor-arg>
  
  <!-- constructor injection using the neater 'ref' attribute -->
  <constructor-arg ref="yetAnotherBean"/>
  
  <constructor-arg type="int" value="1"/>
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    private AnotherBean beanOne;
    private YetAnotherBean beanTwo;
    private int i;
    
    public ExampleBean(
        AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {
        this.beanOne = anotherBean;
        this.beanTwo = yetAnotherBean;
        this.i = i;
    }
}

As you can see, the constructor arguments specified in the bean definition will be used to pass in as arguments to the constructor of the ExampleBean.

Now consider a variant of this where instead of using a constructor, Spring is told to call a static factory method to return an instance of the object:

<bean id="exampleBean" class="examples.ExampleBean"
      factory-method="createInstance">
  <constructor-arg ref="anotherExampleBean"/>
  <constructor-arg ref="yetAnotherBean"/>
  <constructor-arg value="1"/> 
</bean>

<bean id="anotherExampleBean" class="examples.AnotherBean"/>
<bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean {

    // a private constructor
    private ExampleBean(...) {
      ...
    }
    
    // a static factory method; the arguments to this method can be
    // considered the dependencies of the bean that is returned,
    // regardless of how those arguments are actually used.
    public static ExampleBean createInstance (
            AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) {

        ExampleBean eb = new ExampleBean (...);
        // some other operations...
        return eb;
    }
}

Note that arguments to the static factory method are supplied via <constructor-arg/> elements, exactly the same as if a constructor had actually been used. Also, it is important to realize that the type of the class being returned by the factory method does not have to be of the same type as the class which contains the static factory method, although in this example it is. An instance (non-static) factory method would be used in an essentially identical fashion (aside from the use of the factory-bean attribute instead of the class attribute), so details will not be discussed here.

4.3.2 Dependencies and configuration in detail

As mentioned in the previous section, bean properties and constructor arguments can be defined as either references to other managed beans (collaborators), or values defined inline. Spring's XML-based configuration metadata supports a number of sub-element types within its <property/> and <constructor-arg/> elements for just this purpose.

4.3.2.1 Straight values (primitives, Strings, etc.)

The <value/> element specifies a property or constructor argument as a human-readable string representation. As mentioned previously, JavaBeans PropertyEditors are used to convert these string values from a String to the actual type of the property or argument.

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
  
  <!-- results in a setDriverClassName(String) call -->
  <property name="driverClassName">
    <value>com.mysql.jdbc.Driver</value>
  </property>
  <property name="url">
    <value>jdbc:mysql://localhost:3306/mydb</value>
  </property>
  <property name="username">
    <value>root</value>
  </property>
  <property name="password">
    <value>masterkaoli</value>
  </property>
</bean>

The <property/> and <constructor-arg/> elements also support the use of the 'value' attribute, which can lead to much more succinct configuration. When using the 'value' attribute, the above bean definition reads like so:

<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
  
  <!-- results in a setDriverClassName(String) call -->
  <property name="driverClassName" value="com.mysql.jdbc.Driver"/>
  <property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
  <property name="username" value="root"/>
  <property name="password" value="masterkaoli"/>
</bean>

The Spring team generally prefer the attribute style over the use of nested <value/> elements. If you are reading this reference manual straight through from top to bottom (wow!) then we are getting slightly ahead of ourselves here, but you can also configure a java.util.Properties instance like so:

<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
            
   <!-- typed as a java.util.Properties -->
   <property name="properties">
      <value>
         jdbc.driver.className=com.mysql.jdbc.Driver
         jdbc.url=jdbc:mysql://localhost:3306/mydb
      </value>
   </property>
</bean>

Can you see what is happening? The Spring container is converting the text inside the <value/> element into a java.util.Properties instance using the JavaBeans PropertyEditor mechanism. This is a nice shortcut, and is one of a few places where the Spring team do favor the use of the nested <value/> element over the 'value' attribute style.

The idref element

The idref element is simply an error-proof way to pass the id of another bean in the container (to a <constructor-arg/> or <property/> element).

<bean id="theTargetBean" class="..."/>

<bean id="theClientBean" class="...">
    <property name="targetName">
        <idref bean="theTargetBean" />
    </property>
</bean>

The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:

<bean id="theTargetBean" class="..." />

<bean id="client" class="...">
    <property name="targetName" value="theTargetBean" />
</bean>

The main reason the first form is preferable to the second is that using the idref tag allows the container to validate at deployment time that the referenced, named bean actually exists. In the second variation, no validation is performed on the value that is passed to the 'targetName' property of the 'client' bean. Any typo will only be discovered (with most likely fatal results) when the 'client' bean is actually instantiated. If the 'client' bean is a prototype bean, this typo (and the resulting exception) may only be discovered long after the container is actually deployed.

Additionally, if the bean being referred to is in the same XML unit, and the bean name is the bean id, the 'local' attribute may be used, which allows the XML parser itself to validate the bean id even earlier, at XML document parse time.

<property name="targetName">
   <!-- a bean with an id of 'theTargetBean' must exist; otherwise an XML exception will be thrown -->
   <idref local="theTargetBean"/>
</property>

By way of an example, one common place (at least in pre-Spring 2.0 configuration) where the <idref/> element brings value is in the configuration of AOP interceptors in a ProxyFactoryBean bean definition. If you use <idref/> elements when specifying the interceptor names, there is no chance of inadvertently misspelling an interceptor id.

4.3.2.2 References to other beans (collaborators)

The ref element is the final element allowed inside a <constructor-arg/> or <property/> definition element. It is used to set the value of the specified property to be a reference to another bean managed by the container (a collaborator). As mentioned in a previous section, the referred-to bean is considered to be a dependency of the bean who's property is being set, and will be initialized on demand as needed (if it is a singleton bean it may have already been initialized by the container) before the property is set. All references are ultimately just a reference to another object, but there are 3 variations on how the id/name of the other object may be specified, which determines how scoping and validation is handled.

Specifying the target bean by using the bean attribute of the <ref/> tag is the most general form, and will allow creating a reference to any bean in the same container (whether or not in the same XML file), or parent container. The value of the 'bean' attribute may be the same as either the 'id' attribute of the target bean, or one of the values in the 'name' attribute of the target bean.

<ref bean="someBean"/>

Specifying the target bean by using the local attribute leverages the ability of the XML parser to validate XML id references within the same file. The value of the local attribute must be the same as the id attribute of the target bean. The XML parser will issue an error if no matching element is found in the same file. As such, using the local variant is the best choice (in order to know about errors as early as possible) if the target bean is in the same XML file.

<ref local="someBean"/>

Specifying the target bean by using the 'parent' attribute allows a reference to be created to a bean which is in a parent container of the current container. The value of the 'parent' attribute may be the same as either the 'id' attribute of the target bean, or one of the values in the 'name' attribute of the target bean, and the target bean must be in a parent container to the current one. The main use of this bean reference variant is when you have a hierarchy of containers and you want to wrap an existing bean in a parent container with some sort of proxy which will have the same name as the parent bean.

<!-- in the parent context -->
<bean id="accountService" class="com.foo.SimpleAccountService">
    <!-- insert dependencies as required as here -->
</bean>
<!-- in the child (descendant) context -->
<bean id="accountService"  <-- notice that the name of this bean is the same as the name of the 'parent' bean
      class="org.springframework.aop.framework.ProxyFactoryBean">
      <property name="target">
          <ref parent="accountService"/>  <-- notice how we refer to the parent bean
      </property>
    <!-- insert other configuration and dependencies as required as here -->
</bean>

4.3.2.3 Inner beans

A <bean/> element inside the <property/> or <constructor-arg/> elements is used to define a so-called inner bean. An inner bean definition does not need to have any id or name defined, and it is best not to even specify any id or name value because the id or name value simply will be ignored by the container.

<bean id="outer" class="...">
  <!-- instead of using a reference to a target bean, simply define the target bean inline -->
  <property name="target">
    <bean class="com.example.Person"> <!-- this is the inner bean -->
      <property name="name" value="Fiona Apple"/>
      <property name="age" value="25"/>
    </bean>
  </property>
</bean>

Note that in the specific case of inner beans, the 'scope' flag and any 'id' or 'name' attribute are effectively ignored. Inner beans are always anonymous and they are always scoped as prototypes. Please also note that it is not possible to inject inner beans into collaborating beans other than the enclosing bean.

4.3.2.4 Collections

The <list/>, <set/>, <map/>, and <props/> elements allow properties and arguments of the Java Collection type List, Set, Map, and Properties, respectively, to be defined and set.

<bean id="moreComplexObject" class="example.ComplexObject">
  <!-- results in a setAdminEmails(java.util.Properties) call -->
  <property name="adminEmails">
    <props>
        <prop key="administrator">[email protected]</prop>
        <prop key="support">[email protected]</prop>
        <prop key="development">[email protected]</prop>
    </props>
  </property>
  <!-- results in a setSomeList(java.util.List) call -->
  <property name="someList">
    <list>
        <value>a list element followed by a reference</value>
        <ref bean="myDataSource" />
    </list>
  </property>
  <!-- results in a setSomeMap(java.util.Map) call -->
  <property name="someMap">
    <map>
        <entry>
            <key>
                <value>an entry</value>
            </key>
            <value>just some string</value>
        </entry>
        <entry>
            <key>
                <value>a ref</value>
            </key>
            <ref bean="myDataSource" />
        </entry>
    </map>
  </property>
  <!-- results in a setSomeSet(java.util.Set) call -->
  <property name="someSet">
    <set>
        <value>just some string</value>
        <ref bean="myDataSource" />
    </set>
  </property>
</bean>
[Note]Note

The nested element style used this initial example tends to become quite verbose. Fortunately, there are attribute shortcuts for most elements, which you can read about in Section 4.3.2.6, “Shortcuts and other convenience options for XML-based configuration metadata”.

Note that the value of a map key or value, or a set value, can also again be any of the following elements:

bean | ref | idref | list | set | map | props | value | null
Collection merging

As of Spring 2.0, the container also supports the merging of collections. This allows an application developer to define a parent-style <list/>, <map/>, <set/> or <props/> element, and have child-style <list/>, <map/>, <set/> or <props/> elements inherit and override values from the parent collection; that is to say the child collection's values will be the result obtained from the merging of the elements of the parent and child collections, with the child's collection elements overriding values specified in the parent collection.

Please note that this section on merging makes use of the parent-child bean mechanism. This concept has not yet been introduced, so readers unfamiliar with the concept of parent and child bean definitions may wish to read the relevant section before continuing.

Find below an example of the collection merging feature:

<beans>
<bean id="parent" abstract="true" class="example.ComplexObject">
    <property name="adminEmails">
        <props>
            <prop key="administrator">[email protected]</prop>
            <prop key="support">[email protected]</prop>
        </props>
    </property>
</bean>
<bean id="child" parent="parent">
    <property name="adminEmails">
        <!-- the merge is specified on the *child* collection definition -->
        <props merge="true">
            <prop key="sales">[email protected]</prop>
            <prop key="support">[email protected]</prop>
        </props>
    </property>
</bean>
<beans>

Notice the use of the merge=true attribute on the <props/> element of the adminEmails property of the child bean definition. When the child bean is actually resolved and instantiated by the container, the resulting instance will have an adminEmails Properties collection that contains the result of the merging of the child's adminEmails collection with the parent's adminEmails collection.

[email protected]
[email protected]
[email protected]

Notice how the child Properties collection's value set will have inherited all the property elements from the parent <props/>. Notice also how the child's value for the support value overrides the value in the parent collection.

This merging behavior applies similarly to the <list/>, <map/>, and <set/> collection types. In the specific case of the <list/> element, the semantics associated with the List collection type, that is the notion of an ordered collection of values, is maintained; the parent's values will precede all of the child list's values. In the case of the Map, Set, and Properties collection types, there is no notion of ordering and hence no ordering semantics are in effect for the collection types that underlie the associated Map, Set and Properties implementation types used internally by the container.

Finally, some minor notes about the merging support are in order; you cannot merge different collection types (e.g. a Map and a List), and if you do attempt to do so an appropriate Exception will be thrown; and in case it is not immediately obvious, the 'merge' attribute must be specified on the lower level, inherited, child definition; specifying the 'merge' attribute on a parent collection definition is redundant and will not result in the desired merging; and (lastly), please note that this merging feature is only available in Spring 2.0 (and later versions).

Strongly-typed collection (Java 5+ only)

If you are using Java 5 or Java 6, you will be aware that it is possible to have strongly typed collections (using generic types). That is, it is possible to declare a Collection type such that it can only contain String elements (for example). If you are using Spring to dependency inject a strongly-typed Collection into a bean, you can take advantage of Spring's type-conversion support such that the elements of your strongly-typed Collection instances will be converted to the appropriate type prior to being added to the Collection.

public class Foo {
                
    private Map<String, Float> accounts;
    
    public void setAccounts(Map<String, Float> accounts) {
        this.accounts = accounts;
    }
}
<beans>
    <bean id="foo" class="x.y.Foo">
        <property name="accounts">
            <map>
                <entry key="one" value="9.99"/>
                <entry key="two" value="2.75"/>
                <entry key="six" value="3.99"/>
            </map>
        </property>
    </bean>
</beans>

When the 'accounts' property of the 'foo' bean is being prepared for injection, the generics information about the element type of the strongly-typed Map<String, Float> is actually available via reflection, and so Spring's type conversion infrastructure will actually recognize the various value elements as being of type Float and so the string values '9.99', '2.75', and '3.99' will be converted into an actual Float type.

4.3.2.5 Nulls

The <null/> element is used to handle null values. Spring treats empty arguments for properties and the like as empty Strings. The following XML-based configuration metadata snippet results in the email property being set to the empty String value ("")

<bean class="ExampleBean">
  <property name="email"><value/></property>
</bean>

This is equivalent to the following Java code: exampleBean.setEmail(""). The special <null> element may be used to indicate a null value. For example:

<bean class="ExampleBean">
  <property name="email"><null/></property>
</bean>

The above configuration is equivalent to the following Java code: exampleBean.setEmail(null).

4.3.2.6 Shortcuts and other convenience options for XML-based configuration metadata

The configuration metadata shown so far is a tad verbose. That is why there are several options available for you to limit the amount of XML you have to write to configure your components. The first is a shortcut to define values and references to other beans as part of a <property/> definition. The second is slightly different format of specifying properties altogether.

XML-based configuration metadata shortcuts

The <property/>, <constructor-arg/>, and <entry/> elements all support a 'value' attribute which may be used instead of embedding a full <value/> element. Therefore, the following:

<property name="myProperty">
  <value>hello</value>
</property>
<constructor-arg>
  <value>hello</value>
</constructor-arg>
<entry key="myKey">
  <value>hello</value>
</entry>

are equivalent to:

<property name="myProperty" value="hello"/>
<constructor-arg value="hello"/>
<entry key="myKey" value="hello"/>

The <property/> and <constructor-arg/> elements support a similar shortcut 'ref' attribute which may be used instead of a full nested <ref/> element. Therefore, the following:

<property name="myProperty">
  <ref bean="myBean">
</property>
<constructor-arg>
  <ref bean="myBean">
</constructor-arg>

... are equivalent to:

<property name="myProperty" ref="myBean"/>
<constructor-arg ref="myBean"/>

Note however that the shortcut form is equivalent to a <ref bean="xxx"> element; there is no shortcut for <ref local="xxx">. To enforce a strict local reference, you must use the long form.

Finally, the entry element allows a shortcut form to specify the key and/or value of the map, in the form of the 'key' / 'key-ref' and 'value' / 'value-ref' attributes. Therefore, the following:

<entry>
  <key>
    <ref bean="myKeyBean" />
  </key>
  <ref bean="myValueBean" />
</entry>

is equivalent to:

<entry key-ref="myKeyBean" value-ref="myValueBean"/>

Again, the shortcut form is equivalent to a <ref bean="xxx"> element; there is no shortcut for <ref local="xxx">.

The p-namespace and how to use it to configure properties

The second option you have to limit the amount of XML you have to write to configure your components is to use the special "p-namespace". Spring 2.0 and later features support for extensible configuration formats using namespaces. Those namespaces are all based on an XML Schema definition. In fact, the beans configuration format that you've been reading about is defined in an XML Schema document.

One special namespace is not defined in an XSD file, and only exists in the core of Spring itself. The so-called p-namespace doesn't need a schema definition and is an alternative way of configuring your properties differently than the way you have seen so far. Instead of using nested <property/> elements, using the p-namespace you can use attributes as part of the bean element that describe your property values. The values of the attributes will be taken as the values for your properties.

The following two XML snippets boil down to the same thing in the end: the first is using the standard XML format whereas the second example is using the p-namespace.

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
    <bean name="classic" class="com.example.ExampleBean">
        <property name="email" value="[email protected]/>
    </bean>
    
    <bean name="p-namespace" class="com.example.ExampleBean"
          p:email="[email protected]"/>
</beans>

As you can see, we are including an attribute in the p-namespace called email in the bean definition - this is telling Spring that it should include a property declaration. As previously mentioned, the p-namespace doesn't have a schema definition, so the name of the attribute can be set to whatever name your property has.

This next example includes two more bean definitions that both have a reference to another bean:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    
    <bean name="john-classic" class="com.example.Person">
        <property name="name" value="John Doe"/>
        <property name="spouse" ref="jane"/>
    </bean>

    <bean name="john-modern" 
        class="com.example.Person"
        p:name="John Doe"
        p:spouse-ref="jane"/>

    <bean name="jane" class="com.example.Person">
        <property name="name" value="Jane Doe"/>
    </bean>
</beans>

As you can see, this example doesn't only include a property value using the p-namespace, but also uses a special format to declare property references. Whereas the first bean definition uses <property name="spouse" ref="jane"/> to create a reference from bean john to bean jane, the second bean definition uses p:spouse-ref="jane" as an attribute to do the exact same thing. In this case 'spouse' is the property name whereas the '-ref' part indicates that this is not a straight value but rather a reference to another bean.

[Note]Note

Please note that the p-namespace is not quite as flexible as the standard XML format - for example particular, the 'special' format used to declare property references will clash with properties that end in 'Ref', whereas the standard XML format would have no problem there. We recommend that you choose carefully which approach you are going to use in your projects. You should also communicate this to your team members so you won't end up with XML documents using all three approaches at the same time. This will prevent people from not understanding the application because of different ways of configuring it, and will add to the overall consistency of your codebase.

4.3.2.7 Compound property names

Compound or nested property names are perfectly legal when setting bean properties, as long as all components of the path except the final property name are not null. Consider the following bean definition...

<bean id="foo" class="foo.Bar">
  <property name="fred.bob.sammy" value="123" />
</bean>

The foo bean has a fred property which has a bob property, which has a sammy property, and that final sammy property is being set to the value 123. In order for this to work, the fred property of foo, and the bob property of fred must not be null be non-null after the bean is constructed, or a NullPointerException will be thrown.

4.3.3 Using depends-on

For most situations, the fact that a bean is a dependency of another is expressed by the fact that one bean is set as a property of another. This is typically accomplished with the <ref/> element in XML-based configuration metadata. For the relatively infrequent situations where dependencies between beans are less direct (for example, when a static initializer in a class needs to be triggered, such as database driver registration), the 'depends-on' attribute may be used to explicitly force one or more beans to be initialized before the bean using this element is initialized. Find below an example of using the 'depends-on' attribute to express a dependency on a single bean.

<bean id="beanOne" class="ExampleBean" depends-on="manager"/>

<bean id="manager" class="ManagerBean" />

If you need to express a dependency on multiple beans, you can supply a list of bean names as the value of the 'depends-on' attribute, with commas, whitespace and semicolons all valid delimiters, like so:

<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
  <property name="manager" ref="manager" />
</bean>

<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
[Note]Note

The 'depends-on' attribute at the bean definition level is used not only to specify an initialization time dependency, but also to specify the corresponding destroy time dependency (in the case of singleton beans only). Dependent beans that define a 'depends-on' relationship with a given bean will be destroyed first - prior to the given bean itself being destroyed. As a consequence, 'depends-on' may be used to control shutdown order too.

4.3.4 Lazily-instantiated beans

The default behavior for ApplicationContext implementations is to eagerly pre-instantiate all singleton beans at startup. Pre-instantiation means that an ApplicationContext will eagerly create and configure all of its singleton beans as part of its initialization process. Generally this is a good thing, because it means that any errors in the configuration or in the surrounding environment will be discovered immediately (as opposed to possibly hours or even days down the line).

However, there are times when this behavior is not what is wanted. If you do not want a singleton bean to be pre-instantiated when using an ApplicationContext, you can selectively control this by marking a bean definition as lazy-initialized. A lazily-initialized bean indicates to the IoC container whether or not a bean instance should be created at startup or when it is first requested.

When configuring beans via XML, this lazy loading is controlled by the 'lazy-init' attribute on the <bean/> element; for example:

<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/>

<bean name="not.lazy" class="com.foo.AnotherBean"/>

When the above configuration is consumed by an ApplicationContext, the bean named 'lazy' will not be eagerly pre-instantiated when the ApplicationContext is starting up, whereas the 'not.lazy' bean will be eagerly pre-instantiated.

One thing to understand about lazy-initialization is that even though a bean definition may be marked up as being lazy-initialized, if the lazy-initialized bean is the dependency of a singleton bean that is not lazy-initialized, when the ApplicationContext is eagerly pre-instantiating the singleton, it will have to satisfy all of the singletons dependencies, one of which will be the lazy-initialized bean! So don't be confused if the IoC container creates one of the beans that you have explicitly configured as lazy-initialized at startup; all that means is that the lazy-initialized bean is being injected into a non-lazy-initialized singleton bean elsewhere.

It is also possible to control lazy-initialization at the container level by using the 'default-lazy-init' attribute on the <beans/> element; for example:

<beans default-lazy-init="true">
    <!-- no beans will be pre-instantiated... -->
</beans>

4.3.5 Autowiring collaborators

The Spring container is able to autowire relationships between collaborating beans. This means that it is possible to automatically let Spring resolve collaborators (other beans) for your bean by inspecting the contents of the BeanFactory. The autowiring functionality has five modes. Autowiring is specified per bean and can thus be enabled for some beans, while other beans will not be autowired. Using autowiring, it is possible to reduce or eliminate the need to specify properties or constructor arguments, thus saving a significant amount of typing. [2] When using XML-based configuration metadata, the autowire mode for a bean definition is specified by using the autowire attribute of the <bean/> element. The following values are allowed:

Table 4.2. Autowiring modes

ModeExplanation
no

No autowiring at all. Bean references must be defined via a ref element. This is the default, and changing this is discouraged for larger deployments, since explicitly specifying collaborators gives greater control and clarity. To some extent, it is a form of documentation about the structure of a system.

byName

Autowiring by property name. This option will inspect the container and look for a bean named exactly the same as the property which needs to be autowired. For example, if you have a bean definition which is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring will look for a bean definition named master, and use it to set the property.

byType

Allows a property to be autowired if there is exactly one bean of the property type in the container. If there is more than one, a fatal exception is thrown, and this indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. If this is not desirable, setting the dependency-check="objects" attribute value specifies that an error should be thrown in this case.

constructor

This is analogous to byType, but applies to constructor arguments. If there isn't exactly one bean of the constructor argument type in the container, a fatal error is raised.

autodetect

Chooses constructor or byType through introspection of the bean class. If a default constructor is found, the byType mode will be applied.


Note that explicit dependencies in property and constructor-arg settings always override autowiring. Please also note that it is not currently possible to autowire so-called simple properties such as primitives, Strings, and Classes (and arrays of such simple properties). (This is by-design and should be considered a feature.) When using either the byType or constructor autowiring mode, it is possible to wire arrays and typed-collections. In such cases all autowire candidates within the container that match the expected type will be provided to satisfy the dependency. Strongly-typed Maps can even be autowired if the expected key type is String. An autowired Map's values will consist of all bean instances that match the expected type, and the Map's keys will contain the corresponding bean names.

Autowire behavior can be combined with dependency checking, which will be performed after all autowiring has been completed.

It is important to understand the various advantages and disadvantages of autowiring. Some advantages of autowiring include:

  • Autowiring can significantly reduce the volume of configuration required. However, mechanisms such as the use of a bean template (discussed elsewhere in this chapter) are also valuable in this regard.

  • Autowiring can cause configuration to keep itself up to date as your objects evolve. For example, if you need to add an additional dependency to a class, that dependency can be satisfied automatically without the need to modify configuration. Thus there may be a strong case for autowiring during development, without ruling out the option of switching to explicit wiring when the code base becomes more stable.

Some disadvantages of autowiring:

  • Autowiring is more magical than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity which might have unexpected results, the relationships between your Spring-managed objects are no longer documented explicitly.

  • Wiring information may not be available to tools that may generate documentation from a Spring container.

Another issue to consider when autowiring by type is that multiple bean definitions within the container may match the type specified by the setter method or constructor argument to be autowired. For arrays, collections, or Maps, this is not necessarily a problem. However for dependencies that expect a single value, this ambiguity will not be arbitrarily resolved. Instead, if no unique bean definition is available, an Exception will be thrown. You do have several options when confronted with this scenario. First, you may abandon autowiring in favor of explicit wiring. Second, you may designate that certain bean definitions are never to be considered as candidates by setting their 'autowire-candidate' attributes to 'false' as described in the next section. Third, you may designate a single bean definition as the primary candidate by setting the 'primary' attribute of its <bean/> element to 'true'. Finally, if you are using at least Java 5, you may be interested in exploring the more fine-grained control available with annotation-based configuration as described in the section entitled Section 4.11, “Annotation-based configuration”.

When deciding whether to use autowiring, there is no wrong or right answer in all cases. A degree of consistency across a project is best though; for example, if autowiring is not used in general, it might be confusing to developers to use it just to wire one or two bean definitions.

4.3.5.1 Excluding a bean from being available for autowiring

You can also (on a per-bean basis) totally exclude a bean from being an autowire candidate. When configuring beans using Spring's XML format, the 'autowire-candidate' attribute of the <bean/> element can be set to 'false'; this has the effect of making the container totally exclude that specific bean definition from being available to the autowiring infrastructure.

Another option is to limit autowire candidates based on pattern-matching against bean names. The top-level <beans/> element accepts one or more patterns within its 'default-autowire-candidates' attribute. For example, to limit autowire candidate status to any bean whose name ends with 'Repository', provide a value of '*Repository'. To provide multiple patterns, define them in a comma-separated list. Note that an explicit value of 'true' or 'false' for a bean definition's 'autowire-candidate' attribute always takes precedence, and for such beans, the pattern matching rules will not apply.

These techniques can be useful when you have one or more beans that you absolutely never ever want to have injected into other beans via autowiring. It does not mean that an excluded bean cannot itself be configured using autowiring... it can, it is rather that it itself will not be considered as a candidate for autowiring other beans.

4.3.6 Checking for dependencies

The Spring IoC container also has the ability to check for the existence of unresolved dependencies of a bean deployed into the container. These are JavaBeans properties of the bean, which do not have actual values set for them in the bean definition, or alternately provided automatically by the autowiring feature.

This feature is sometimes useful when you want to ensure that all properties (or all properties of a certain type) are set on a bean. Of course, in many cases a bean class will have default values for many properties, or some properties do not apply to all usage scenarios, so this feature is of limited use. Dependency checking can also be enabled and disabled per bean, just as with the autowiring functionality. The default is to not check dependencies. Dependency checking can be handled in several different modes. When using XML-based configuration metadata, this is specified via the 'dependency-check' attribute in a bean definition, which may have the following values.

Table 4.3. Dependency checking modes

ModeExplanation
none

No dependency checking. Properties of the bean which have no value specified for them are simply not set.

simple

Dependency checking is performed for primitive types and collections (everything except collaborators).

object

Dependency checking is performed for collaborators only.

all

Dependency checking is done for collaborators, primitive types and collections.


If you are using Java 5 and thus have access to source-level annotations, you may find the section entitled Section 29.3.1, “@Required” to be of interest.

4.3.7 Method Injection

For most application scenarios, the majority of the beans in the container will be singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, the typical and common approach of handling this dependency by defining one bean to be a property of the other is quite adequate. There is a problem when the bean lifecycles are different. Consider a singleton bean A which needs to use a non-singleton (prototype) bean B, perhaps on each method invocation on A. The container will only create the singleton bean A once, and thus only get the opportunity to set the properties once. There is no opportunity for the container to provide bean A with a new instance of bean B every time one is needed.

One solution to this issue is to forego some inversion of control. Bean A can be made aware of the container by implementing the BeanFactoryAware interface, and use programmatic means to ask the container via a getBean("B") call for (a typically new) bean B instance every time it needs it. Find below an admittedly somewhat contrived example of this approach:

// a class that uses a stateful Command-style class to perform some processing
package fiona.apple;

// lots of Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;

public class CommandManager implements BeanFactoryAware {

   private BeanFactory beanFactory;

   public Object process(Map commandState) {
      // grab a new instance of the appropriate Command
      Command command = createCommand();
      // set the state on the (hopefully brand new) Command instance
      command.setState(commandState);
      return command.execute();
   }

   // the Command returned here could be an implementation that executes asynchronously, or whatever
   protected Command createCommand() {
      return (Command) this.beanFactory.getBean("command"); // notice the Spring API dependency
   }

   public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
      this.beanFactory = beanFactory;
   }
}

The above example is generally not a desirable solution since the business code is then aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this use case to be handled in a clean fashion.

4.3.7.1 Lookup method injection

Lookup method injection refers to the ability of the container to override methods on container managed beans, to return the result of looking up another named bean in the container. The lookup will typically be of a prototype bean as in the scenario described above. The Spring Framework implements this method injection by dynamically generating a subclass overriding the method, using bytecode generation via the CGLIB library.

So if you look at the code from previous code snippet (the CommandManager class), the Spring container is going to dynamically override the implementation of the createCommand() method. Your CommandManager class is not going to have any Spring dependencies, as can be seen in this reworked example below:

package fiona.apple;

// no more Spring imports! 

public abstract class CommandManager {

   public Object process(Object commandState) {
      // grab a new instance of the appropriate Command interface
      Command command = createCommand();
      // set the state on the (hopefully brand new) Command instance
      command.setState(commandState);
      return command.execute();
   }

    // okay... but where is the implementation of this method?
   protected abstract Command createCommand();
}

In the client class containing the method to be injected (the CommandManager in this case), the method that is to be 'injected' must have a signature of the following form:

<public|protected> [abstract] <return-type> theMethodName(no-arguments);

If the method is abstract, the dynamically-generated subclass will implement the method. Otherwise, the dynamically-generated subclass will override the concrete method defined in the original class. Let's look at an example:

<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="command" class="fiona.apple.AsyncCommand" scope="prototype">
  <!-- inject dependencies here as required -->
</bean>

<!-- commandProcessor uses statefulCommandHelper -->
<bean id="commandManager" class="fiona.apple.CommandManager">
  <lookup-method name="createCommand" bean="command"/>
</bean>

The bean identified as commandManager will call its own method createCommand() whenever it needs a new instance of the command bean. It is important to note that the person deploying the beans must be careful to deploy the command bean as a prototype (if that is actually what is needed). If it is deployed as a singleton, the same instance of the command bean will be returned each time!

Please be aware that in order for this dynamic subclassing to work, you will need to have the CGLIB jar(s) on your classpath. Additionally, the class that the Spring container is going to subclass cannot be final, and the method that is being overridden cannot be final either. Also, testing a class that has an abstract method can be somewhat odd in that you will have to subclass the class yourself and supply a stub implementation of the abstract method. Finally, objects that have been the target of method injection cannot be serialized.

[Tip]Tip

The interested reader may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be of use; the approach is similar to that of the ObjectFactoryCreatingFactoryBean, but it allows you to specify your own lookup interface as opposed to having to use a Spring-specific lookup interface such as the ObjectFactory. Consult the (copious) Javadoc for the ServiceLocatorFactoryBean for a full treatment of this alternative approach (that does reduce the coupling to Spring).

4.3.7.2 Arbitrary method replacement

A less commonly useful form of method injection than Lookup Method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section (which describes this somewhat advanced feature), until this functionality is actually needed.

When using XML-based configuration metadata, the replaced-method element may be used to replace an existing method implementation with another, for a deployed bean. Consider the following class, with a method computeValue, which we want to override:

public class MyValueCalculator {

  public String computeValue(String input) {
    // some real code...
  }

  // some other methods...

}

A class implementing the org.springframework.beans.factory.support.MethodReplacer interface provides the new method definition.

/** meant to be used to override the existing computeValue(String)
    implementation in MyValueCalculator
  */
public class ReplacementComputeValue implements MethodReplacer {

    public Object reimplement(Object o, Method m, Object[] args) throws Throwable {
        // get the input value, work with it, and return a computed result
        String input = (String) args[0];
        ... 
        return ...;
    }
}

The bean definition to deploy the original class and specify the method override would look like this:

<bean id="myValueCalculator class="x.y.z.MyValueCalculator">
  <!-- arbitrary method replacement -->
  <replaced-method name="computeValue" replacer="replacementComputeValue">
    <arg-type>String</arg-type>
  </replaced-method>
</bean>

<bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>

One or more contained <arg-type/> elements within the <replaced-method/> element may be used to indicate the method signature of the method being overridden. Note that the signature for the arguments is actually only needed in the case that the method is actually overloaded and there are multiple variants within the class. For convenience, the type string for an argument may be a substring of the fully qualified type name. For example, all the following would match java.lang.String.

    java.lang.String
    String
    Str

Since the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by allowing you to type just the shortest string that will match an argument type.

4.4 Bean scopes

When you create a bean definition what you are actually creating is a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, just like a class, you can potentially have many object instances created from a single recipe.

You can control not only the various dependencies and configuration values that are to be plugged into an object that is created from a particular bean definition, but also the scope of the objects created from a particular bean definition. This approach is very powerful and gives you the flexibility to choose the scope of the objects you create through configuration instead of having to 'bake in' the scope of an object at the Java class level. Beans can be defined to be deployed in one of a number of scopes: out of the box, the Spring Framework supports exactly five scopes (of which three are available only if you are using a web-aware ApplicationContext).

The scopes supported out of the box are listed below:

Table 4.4. Bean scopes

ScopeDescription

singleton

Scopes a single bean definition to a single object instance per Spring IoC container.

prototype

Scopes a single bean definition to any number of object instances.

request

Scopes a single bean definition to the lifecycle of a single HTTP request; that is each and every HTTP request will have its own instance of a bean created off the back of a single bean definition. Only valid in the context of a web-aware Spring ApplicationContext.

session

Scopes a single bean definition to the lifecycle of a HTTP Session. Only valid in the context of a web-aware Spring ApplicationContext.

global session

Scopes a single bean definition to the lifecycle of a global HTTP Session. Typically only valid when used in a portlet context. Only valid in the context of a web-aware Spring ApplicationContext.


4.4.1 The singleton scope

When a bean is a singleton, only one shared instance of the bean will be managed, and all requests for beans with an id or ids matching that bean definition will result in that one specific bean instance being returned by the Spring container.

To put it another way, when you define a bean definition and it is scoped as a singleton, then the Spring IoC container will create exactly one instance of the object defined by that bean definition. This single instance will be stored in a cache of such singleton beans, and all subsequent requests and references for that named bean will result in the cached object being returned.

Please be aware that Spring's concept of a singleton bean is quite different from the Singleton pattern as defined in the seminal Gang of Four (GoF) patterns book. The GoF Singleton hard codes the scope of an object such that one and only one instance of a particular class will ever be created per ClassLoader. The scope of the Spring singleton is best described as per container and per bean. This means that if you define one bean for a particular class in a single Spring container, then the Spring container will create one and only one instance of the class defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as a singleton in XML, you would write configuration like so:

<bean id="accountService" class="com.foo.DefaultAccountService"/>

<!-- the following is equivalent, though redundant (singleton scope is the default); using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>

<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="true"/>

4.4.2 The prototype scope

The non-singleton, prototype scope of bean deployment results in the creation of a new bean instance every time a request for that specific bean is made (that is, it is injected into another bean or it is requested via a programmatic getBean() method call on the container). As a rule of thumb, you should use the prototype scope for all beans that are stateful, while the singleton scope should be used for stateless beans.

The following diagram illustrates the Spring prototype scope. Please note that a DAO would not typically be configured as a prototype, since a typical DAO would not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.

To define a bean as a prototype in XML, you would write configuration like so:

<!-- using spring-beans-2.0.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>

<!-- the following is equivalent and preserved for backward compatibility in spring-beans.dtd -->
<bean id="accountService" class="com.foo.DefaultAccountService" singleton="false"/>

There is one quite important thing to be aware of when deploying a bean in the prototype scope, in that the lifecycle of the bean changes slightly. Spring does not manage the complete lifecycle of a prototype bean: the container instantiates, configures, decorates and otherwise assembles a prototype object, hands it to the client and then has no further knowledge of that prototype instance. This means that while initialization lifecycle callback methods will be called on all objects regardless of scope, in the case of prototypes, any configured destruction lifecycle callbacks will not be called. It is the responsibility of the client code to clean up prototype scoped objects and release any expensive resources that the prototype bean(s) are holding onto. (One possible way to get the Spring container to release resources used by prototype-scoped beans is through the use of a custom bean post-processor which would hold a reference to the beans that need to be cleaned up.)

In some respects, you can think of the Spring containers role when talking about a prototype-scoped bean as somewhat of a replacement for the Java 'new' operator. All lifecycle aspects past that point have to be handled by the client. (The lifecycle of a bean in the Spring container is further described in the section entitled Section 4.5.1, “Lifecycle callbacks”.)

4.4.3 Singleton beans with prototype-bean dependencies

When using singleton-scoped beans that have dependencies on beans that are scoped as prototypes, please be aware that dependencies are resolved at instantiation time. This means that if you dependency inject a prototype-scoped bean into a singleton-scoped bean, a brand new prototype bean will be instantiated and then dependency injected into the singleton bean... but that is all. That exact same prototype instance will be the sole instance that is ever supplied to the singleton-scoped bean, which is fine if that is what you want.

However, sometimes what you actually want is for the singleton-scoped bean to be able to acquire a brand new instance of the prototype-scoped bean again and again and again at runtime. In that case it is no use just dependency injecting a prototype-scoped bean into your singleton bean, because as explained above, that only happens once when the Spring container is instantiating the singleton bean and resolving and injecting its dependencies. If you are in the scenario where you need to get a brand new instance of a (prototype) bean again and again and again at runtime, you are referred to the section entitled Section 4.3.7, “Method Injection”

[Note]Backwards compatibility note: specifying the lifecycle scope in XML

If you are referencing the 'spring-beans.dtd' DTD in a bean definition file(s), and you are being explicit about the lifecycle scope of your beans you must use the "singleton" attribute to express the lifecycle scope (remembering that the singleton lifecycle scope is the default). If you are referencing the 'spring-beans-2.0.dtd' DTD or the Spring 2.0 XSD schema, then you will need to use the "scope" attribute (because the "singleton" attribute was removed from the definition of the new DTD and XSD files in favor of the "scope" attribute).

To be totally clear about this, this means that if you use the "singleton" attribute in an XML bean definition then you must be referencing the 'spring-beans.dtd' DTD in that file. If you are using the "scope" attribute then you must be referencing either the 'spring-beans-2.0.dtd' DTD or the 'spring-beans-2.5.xsd' XSD in that file.

4.4.4 The other scopes

The other scopes, namely request, session, and global session are for use only in web-based applications (and can be used irrespective of which particular web application framework you are using, if indeed any). In the interest of keeping related concepts together in one place in the reference documentation, these scopes are described here.

[Note]Note

The scopes that are described in the following paragraphs are only available if you are using a web-aware Spring ApplicationContext implementation (such as XmlWebApplicationContext). If you try using these next scopes with regular Spring IoC containers such as the XmlBeanFactory or ClassPathXmlApplicationContext, you will get an IllegalStateException complaining about an unknown bean scope.

4.4.4.1 Initial web configuration

In order to support the scoping of beans at the request, session, and global session levels (web-scoped beans), some minor initial configuration is required before you can set about defining your bean definitions. Please note that this extra setup is not required if you just want to use the 'standard' scopes (namely singleton and prototype).

Now as things stand, there are a couple of ways to effect this initial setup depending on your particular Servlet environment...

If you are accessing scoped beans within Spring Web MVC, i.e. within a request that is processed by the Spring DispatcherServlet, or DispatcherPortlet, then no special setup is necessary: DispatcherServlet and DispatcherPortlet already expose all relevant state.

When using a Servlet 2.4+ web container, with requests processed outside of Spring's DispatcherServlet (e.g. when using JSF or Struts), you need to add the following javax.servlet.ServletRequestListener to the declarations in your web application's 'web.xml' file.

<web-app>
  ...
  <listener>
    <listener-class>org.springframework.web.context.request.RequestContextListener</listener-class>
  </listener>
  ...
</web-app>

If you are using an older web container (Servlet 2.3), you will need to use the provided javax.servlet.Filter implementation. Find below a snippet of XML configuration that has to be included in the 'web.xml' file of your web application if you want to have access to web-scoped beans in requests outside of Spring's DispatcherServlet on a Servlet 2.3 container. (The filter mapping depends on the surrounding web application configuration and so you will have to change it as appropriate.)

<web-app>
  ..
  <filter> 
    <filter-name>requestContextFilter</filter-name> 
    <filter-class>org.springframework.web.filter.RequestContextFilter</filter-class>
  </filter> 
  <filter-mapping> 
    <filter-name>requestContextFilter</filter-name> 
    <url-pattern>/*</url-pattern>
  </filter-mapping>
  ...
</web-app>

That's it. DispatcherServlet, RequestContextListener and RequestContextFilter all do exactly the same thing, namely bind the HTTP request object to the Thread that is servicing that request. This makes beans that are request- and session-scoped available further down the call chain.

4.4.4.2 The request scope

Consider the following bean definition:

<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>

With the above bean definition in place, the Spring container will create a brand new instance of the LoginAction bean using the 'loginAction' bean definition for each and every HTTP request. That is, the 'loginAction' bean will be effectively scoped at the HTTP request level. You can change or dirty the internal state of the instance that is created as much as you want, safe in the knowledge that other requests that are also using instances created off the back of the same 'loginAction' bean definition will not be seeing these changes in state since they are particular to an individual request. When the request is finished processing, the bean that is scoped to the request will be discarded.

4.4.4.3 The session scope

Consider the following bean definition:

<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>

With the above bean definition in place, the Spring container will create a brand new instance of the UserPreferences bean using the 'userPreferences' bean definition for the lifetime of a single HTTP Session. In other words, the 'userPreferences' bean will be effectively scoped at the HTTP Session level. Just like request-scoped beans, you can change the internal state of the instance that is created as much as you want, safe in the knowledge that other HTTP Session instances that are also using instances created off the back of the same 'userPreferences' bean definition will not be seeing these changes in state since they are particular to an individual HTTP Session. When the HTTP Session is eventually discarded, the bean that is scoped to that particular HTTP Session will also be discarded.

4.4.4.4 The global session scope

Consider the following bean definition:

<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>

The global session scope is similar to the standard HTTP Session scope (described immediately above), and really only makes sense in the context of portlet-based web applications. The portlet specification defines the notion of a global Session that is shared amongst all of the various portlets that make up a single portlet web application. Beans defined at the global session scope are scoped (or bound) to the lifetime of the global portlet Session.

Please note that if you are writing a standard Servlet-based web application and you define one or more beans as having global session scope, the standard HTTP Session scope will be used, and no error will be raised.

4.4.4.5 Scoped beans as dependencies

Being able to define a bean scoped to a HTTP request or Session (or indeed a custom scope of your own devising) is all very well, but one of the main value-adds of the Spring IoC container is that it manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject a (for example) HTTP request scoped bean into another bean, you will need to inject an AOP proxy in place of the scoped bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped object, but that is smart enough to be able to retrieve the real, target object from the relevant scope (for example a HTTP request) and delegate method calls onto the real object.

[Note]Note

You do not need to use the <aop:scoped-proxy/> in conjunction with beans that are scoped as singletons or prototypes. It is an error to try to create a scoped proxy for a singleton bean (and the resulting BeanCreationException will certainly set you straight in this regard).

Let's look at the configuration that is required to effect this; the configuration is not hugely complex (it takes just one line), but it is important to understand the “why” as well as the “how” behind it.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/aop 
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">

    <!-- a HTTP Session-scoped bean exposed as a proxy -->
    <bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
          
          <!-- this next element effects the proxying of the surrounding bean -->
          <aop:scoped-proxy/>
    </bean>
    
    <!-- a singleton-scoped bean injected with a proxy to the above bean -->
    <bean id="userService" class="com.foo.SimpleUserService">
    
        <!-- a reference to the proxied 'userPreferences' bean -->
        <property name="userPreferences" ref="userPreferences"/>

    </bean>
</beans>

To create such a proxy, you need only to insert a child <aop:scoped-proxy/> element into a scoped bean definition (you may also need the CGLIB library on your classpath so that the container can effect class-based proxying; you will also need to be using Appendix A, XML Schema-based configuration). So, just why do you need this <aop:scoped-proxy/> element in the definition of beans scoped at the request, session, globalSession and 'insert your custom scope here' level? The reason is best explained by picking apart the following bean definition (please note that the following 'userPreferences' bean definition as it stands is incomplete):

<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>

<bean id="userManager" class="com.foo.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>

From the above configuration it is evident that the singleton bean 'userManager' is being injected with a reference to the HTTP Session-scoped bean 'userPreferences'. The salient point here is that the 'userManager' bean is a singleton... it will be instantiated exactly once per container, and its dependencies (in this case only one, the 'userPreferences' bean) will also only be injected (once!). This means that the 'userManager' will (conceptually) only ever operate on the exact same 'userPreferences' object, that is the one that it was originally injected with. This is not what you want when you inject a HTTP Session-scoped bean as a dependency into a collaborating object (typically). Rather, what we do want is a single 'userManager' object, and then, for the lifetime of a HTTP Session, we want to see and use a 'userPreferences' object that is specific to said HTTP Session.

Rather what you need then is to inject some sort of object that exposes the exact same public interface as the UserPreferences class (ideally an object that is a UserPreferences instance) and that is smart enough to be able to go off and fetch the real UserPreferences object from whatever underlying scoping mechanism we have chosen (HTTP request, Session, etc.). We can then safely inject this proxy object into the 'userManager' bean, which will be blissfully unaware that the UserPreferences reference that it is holding onto is a proxy. In the case of this example, when a UserManager instance invokes a method on the dependency-injected UserPreferences object, it is really invoking a method on the proxy... the proxy will then go off and fetch the real UserPreferences object from (in this case) the HTTP Session, and delegate the method invocation onto the retrieved real UserPreferences object.

That is why you need the following, correct and complete, configuration when injecting request-, session-, and globalSession-scoped beans into collaborating objects:

<bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
    <aop:scoped-proxy/>
</bean>

<bean id="userManager" class="com.foo.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>
Choosing the type of proxy created

By default, when the Spring container is creating a proxy for a bean that is marked up with the <aop:scoped-proxy/> element, a CGLIB-based class proxy will be created. This means that you need to have the CGLIB library on the classpath of your application.

Note: CGLIB proxies will only intercept public method calls! Do not call non-public methods on such a proxy; they will not be delegated to the scoped target object.

You can choose to have the Spring container create 'standard' JDK interface-based proxies for such scoped beans by specifying 'false' for the value of the 'proxy-target-class' attribute of the <aop:scoped-proxy/> element. Using JDK interface-based proxies does mean that you don't need any additional libraries on your application's classpath to effect such proxying, but it does mean that the class of the scoped bean must implement at least one interface, and all of the collaborators into which the scoped bean is injected must be referencing the bean via one of its interfaces.

<!-- DefaultUserPreferences implements the UserPreferences interface -->
<bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session">
    <aop:scoped-proxy proxy-target-class="false"/>
</bean>

<bean id="userManager" class="com.foo.UserManager">
    <property name="userPreferences" ref="userPreferences"/>
</bean>

The section entitled Section 8.6, “Proxying mechanisms” may also be of some interest with regard to understanding the nuances of choosing whether class-based or interface-based proxying is right for you.

4.4.5 Custom scopes

As of Spring 2.0, the bean scoping mechanism in Spring is extensible. This means that you are not limited to just the bean scopes that Spring provides out of the box; you can define your own scopes, or even redefine the existing scopes (although that last one would probably be considered bad practice - please note that you cannot override the built-in singleton and prototype scopes).

4.4.5.1 Creating your own custom scope

Scopes are defined by the org.springframework.beans.factory.config.Scope interface. This is the interface that you will need to implement in order to integrate your own custom scope(s) into the Spring container, and is described in detail below. You may wish to look at the Scope implementations that are supplied with the Spring Framework itself for an idea of how to go about implementing your own. The Scope Javadoc explains the main class to implement when you need your own scope in more detail too.

The Scope interface has four methods dealing with getting objects from the scope, removing them from the scope and allowing them to be 'destroyed' if needed.

The first method should return the object from the underlying scope. The session scope implementation for example will return the session-scoped bean (and if it does not exist, return a new instance of the bean, after having bound it to the session for future reference).

Object get(String name, ObjectFactory objectFactory)

The second method should remove the object from the underlying scope. The session scope implementation for example, removes the session-scoped bean from the underlying session. The object should be returned (you are allowed to return null if the object with the specified name wasn't found)

Object remove(String name)

The third method is used to register callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed. Please refer to the Javadoc or a Spring scope implementation for more information on destruction callbacks.

void registerDestructionCallback(String name, Runnable destructionCallback)

The last method deals with obtaining the conversation identifier for the underlying scope. This identifier is different for each scope. For a session for example, this can be the session identifier.

String getConversationId()

4.4.5.2 Using a custom scope

After you have written and tested one or more custom Scope implementations, you then need to make the Spring container aware of your new scope(s). The central method to register a new Scope with the Spring container is declared on the ConfigurableBeanFactory interface (implemented by most of the concrete BeanFactory implementations that ship with Spring); this central method is displayed below:

void registerScope(String scopeName, Scope scope);

The first argument to the registerScope(..) method is the unique name associated with a scope; examples of such names in the Spring container itself are 'singleton' and 'prototype'. The second argument to the registerScope(..) method is an actual instance of the custom Scope implementation that you wish to register and use.

Let's assume that you have written your own custom Scope implementation, and you have registered it like so:

// note: the ThreadScope class does not ship with the Spring Framework
Scope customScope = new ThreadScope();
beanFactory.registerScope("thread", customScope);

You can then create bean definitions that adhere to the scoping rules of your custom Scope like so:

<bean id="..." class="..." scope="thread"/>

If you have your own custom Scope implementation(s), you are not just limited to only programmatic registration of the custom scope(s). You can also do the Scope registration declaratively, using the CustomScopeConfigurer class.

The declarative registration of custom Scope implementations using the CustomScopeConfigurer class is shown below:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/aop 
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">

    <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
        <property name="scopes">
            <map>
                <entry key="thread">
                    <bean class="com.foo.ThreadScope"/>
                </entry>
            </map>
        </property>
    </bean>

    <bean id="bar" class="x.y.Bar" scope="thread">
        <property name="name" value="Rick"/>
        <aop:scoped-proxy/>
    </bean>

    <bean id="foo" class="x.y.Foo">
        <property name="bar" ref="bar"/>
    </bean>

</beans>
[Note]Note

Note that, when placing a <aop:scoped-proxy/> in a FactoryBean implementation, it is the factory bean itself that is scoped, not the object returned from getObject().

4.5 Customizing the nature of a bean

4.5.1 Lifecycle callbacks

The Spring Framework provides several callback interfaces to change the behavior of your bean in the container; they include InitializingBean and DisposableBean. Implementing these interfaces will result in the container calling afterPropertiesSet() for the former and destroy() for the latter to allow the bean to perform certain actions upon initialization and destruction.

Internally, the Spring Framework uses BeanPostProcessor implementations to process any callback interfaces it can find and call the appropriate methods. If you need custom features or other lifecycle behavior Spring doesn't offer out-of-the-box, you can implement a BeanPostProcessor yourself. More information about this can be found in the section entitled Section 4.7, “Container extension points”.

All the different lifecycle callback interfaces are described below. In one of the appendices, you can find diagrams that show how Spring manages beans, how those lifecycle features change the nature of your beans, and how they are managed.

4.5.1.1 Initialization callbacks

Implementing the org.springframework.beans.factory.InitializingBean interface allows a bean to perform initialization work after all necessary properties on the bean have been set by the container. The InitializingBean interface specifies exactly one method:

void afterPropertiesSet() throws Exception;

Generally, the use of the InitializingBean interface can be avoided and is actually discouraged since it unnecessarily couples the code to Spring. As an alternative, bean definitions provide support for a generic initialization method to be specified. In the case of XML-based configuration metadata, this is done using the 'init-method' attribute. For example, the following definition:

<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean {
    
    public void init() {
        // do some initialization work
    }
}

...is exactly the same as...

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean {
    
    public void afterPropertiesSet() {
        // do some initialization work
    }
}

... but does not couple the code to Spring.

4.5.1.2 Destruction callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface allows a bean to get a callback when the container containing it is destroyed. The DisposableBean interface specifies a single method:

void destroy() throws Exception;

Generally, the use of the DisposableBean callback interface can be avoided and is actually discouraged since it unnecessarily couples the code to Spring. As an alternative, bean definitions provide support for a generic destroy method to be specified. When using XML-based configuration metadata this is done via the 'destroy-method' attribute on the <bean/>. For example, the following definition:

<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean {

    public void cleanup() {
        // do some destruction work (like releasing pooled connections)
    }
}

...is exactly the same as...

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean {

    public void destroy() {
        // do some destruction work (like releasing pooled connections)
    }
}

... but does not couple the code to Spring.

4.5.1.3 Default initialization & destroy methods

When writing initialization and destroy method callbacks that do not use the Spring-specific InitializingBean and DisposableBean callback interfaces, one typically finds oneself writing methods with names such as init(), initialize(), dispose(), etc. The names of such lifecycle callback methods are (hopefully!) standardized across a project so that all developers on a team use the same method names and thus ensure some level of consistency.

The Spring container can be configured to 'look' for named initialization and destroy callback method names on every bean. This means that you, as an application developer, can simply write your application classes, use a convention of having an initialization callback called init(), and then (without having to configure each and every bean with, in the case of XML-based configuration, an 'init-method="init"' attribute) be safe in the knowledge that the Spring IoC container will call that method when the bean is being created (and in accordance with the standard lifecycle callback contract described previously).

Let's look at an example to make the use of this feature completely clear. For the sake of the example, let us say that one of the coding conventions on a project is that all initialization callback methods are to be named init() and that destroy callback methods are to be called destroy(). This leads to classes like so...

public class DefaultBlogService implements BlogService {

    private BlogDao blogDao;

    public void setBlogDao(BlogDao blogDao) {
        this.blogDao = blogDao;
    }

    // this is (unsurprisingly) the initialization callback method
    public void init() {
        if (this.blogDao == null) {
            throw new IllegalStateException("The [blogDao] property must be set.");
        }
    }
}
<beans default-init-method="init">

    <bean id="blogService" class="com.foo.DefaultBlogService">
        <property name="blogDao" ref="blogDao" />
    </bean>

</beans>

Notice the use of the 'default-init-method' attribute on the top-level <beans/> element. The presence of this attribute means that the Spring IoC container will recognize a method called 'init' on beans as being the initialization method callback, and when a bean is being created and assembled, if the bean's class has such a method, it will be invoked at the appropriate time.

Destroy method callbacks are configured similarly (in XML that is) using the 'default-destroy-method' attribute on the top-level <beans/> element.

The use of this feature can save you the (small) housekeeping chore of specifying an initialization and destroy method callback on each and every bean, and it is great for enforcing a consistent naming convention for initialization and destroy method callbacks, as consistency is something that should always be aimed for.

Consider the case where you have some existing beans where the underlying classes already have initialization callback methods that are named at variance with the convention. You can always override the default by specifying (in XML that is) the method name using the 'init-method' and 'destroy-method' attributes on the <bean/> element itself.

Finally, please be aware that the Spring container guarantees that a configured initialization callback is called immediately after a bean has been supplied with all of its dependencies. This means that the initialization callback will be called on the raw bean reference, which means that any AOP interceptors or suchlike that will ultimately be applied to the bean will not yet be in place. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. Note that, if the target bean and the proxy are defined separately, your code can even interact with the raw target bean, bypassing the proxy. Hence, it would be very inconsistent to apply the interceptors to the init method, since that would couple the lifecycle of the target bean with its proxy/interceptors and leave strange semantics when talking to the raw target bean directly.

4.5.1.4 Combining lifecycle mechanisms

As of Spring 2.5, there are three options for controlling bean lifecycle behavior: the InitializingBean and DisposableBean callback interfaces; custom init() and destroy() methods; and the @PostConstruct and @PreDestroy annotations.

When combining different lifecycle mechanisms - for example, in a class hierarchy in which various lifecycle mechanisms are in use - developers should be aware of the order in which these mechanisms are applied. The following is the ordering for initialization methods:

  • Methods annotated with @PostConstruct

  • afterPropertiesSet() as defined by the InitializingBean callback interface

  • A custom configured init() method

Destroy methods are called in the same order:

  • Methods annotated with @PreDestroy

  • destroy() as defined by the DisposableBean callback interface

  • A custom configured destroy() method

[Note]Note

If multiple lifecycle mechanisms are configured for a given bean, and each mechanism is configured with a different method name, then each configured method will be executed in the order listed above; however, if the same method name is configured - for example, init() for an initialization method - for more than one of the aforementioned lifecycle mechanisms, that method will only be executed once.

4.5.1.5 Shutting down the Spring IoC container gracefully in non-web applications

[Note]Note

This next section does not apply to web applications (in case the title of this section did not make that abundantly clear). Spring's web-based ApplicationContext implementations already have code in place to handle shutting down the Spring IoC container gracefully when the relevant web application is being shutdown.

If you are using Spring's IoC container in a non-web application environment, for example in a rich client desktop environment, and you want the container to shutdown gracefully and call the relevant destroy callbacks on your singleton beans, you will need to register a shutdown hook with the JVM. This is quite easy to do (see below), and will ensure that your Spring IoC container shuts down gracefully and that all resources held by your singletons are released. Of course it is still up to you to both configure the destroy callbacks for your singletons and implement such destroy callbacks correctly.

So to register a shutdown hook that enables the graceful shutdown of the relevant Spring IoC container, you simply need to call the registerShutdownHook() method that is declared on the AbstractApplicationContext class. To wit...

import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Boot {

    public static void main(final String[] args) throws Exception {
        AbstractApplicationContext ctx
            = new ClassPathXmlApplicationContext(new String []{"beans.xml"});

        // add a shutdown hook for the above context... 
        ctx.registerShutdownHook();

        // app runs here...

        // main method exits, hook is called prior to the app shutting down...
    }
}

4.5.2 Knowing who you are

4.5.2.1 BeanFactoryAware

A class which implements the org.springframework.beans.factory.BeanFactoryAware interface is provided with a reference to the BeanFactory that created it, when it is created by that BeanFactory.

public interface BeanFactoryAware {

    void setBeanFactory(BeanFactory beanFactory) throws BeansException;
}

This allows beans to manipulate the BeanFactory that created them programmatically, through the BeanFactory interface, or by casting the reference to a known subclass of this which exposes additional functionality. Primarily this would consist of programmatic retrieval of other beans. While there are cases when this capability is useful, it should generally be avoided, since it couples the code to Spring and does not follow the Inversion of Control style, where collaborators are provided to beans as properties.

An alternative option that is equivalent in effect to the BeanFactoryAware-based approach is to use the org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean. (It should be noted that this approach still does not reduce the coupling to Spring, but it does not violate the central principle of IoC as much as the BeanFactoryAware-based approach.)

The ObjectFactoryCreatingFactoryBean is a FactoryBean implementation that returns a reference to an object (factory) that can in turn be used to effect a bean lookup. The ObjectFactoryCreatingFactoryBean class does itself implement the BeanFactoryAware interface; what client beans are actually injected with is an instance of the ObjectFactory interface. This is a Spring-specific interface (and hence there is still no total decoupling from Spring), but clients can then use the ObjectFactory's getObject() method to effect the bean lookup (under the hood the ObjectFactory implementation instance that is returned simply delegates down to a BeanFactory to actually lookup a bean by name). All that you need to do is supply the ObjectFactoryCreatingFactoryBean with the name of the bean that is to be looked up. Let's look at an example:

package x.y;

public class NewsFeed {
    
    private String news;

    public void setNews(String news) {
        this.news = news;
    }

    public String getNews() {
        return this.toString() + ": '" + news + "'";
    }
}
package x.y;

import org.springframework.beans.factory.ObjectFactory;

public class NewsFeedManager {

    private ObjectFactory factory;

    public void setFactory(ObjectFactory factory) {
        this.factory = factory;
    }

    public void printNews() {
        // here is where the lookup is performed; note that there is no
        // need to hard code the name of the bean that is being looked up...
        NewsFeed news = (NewsFeed) factory.getObject();
        System.out.println(news.getNews());
    }
}

Find below the XML configuration to wire together the above classes using the ObjectFactoryCreatingFactoryBean approach.

<beans>
    <bean id="newsFeedManager" class="x.y.NewsFeedManager">
        <property name="factory">
            <bean
class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">
                <property name="targetBeanName">
                    <idref local="newsFeed" />
                </property>
            </bean>
        </property>
    </bean>
    <bean id="newsFeed" class="x.y.NewsFeed" scope="prototype">
        <property name="news" value="... that's fit to print!" />
    </bean>
</beans>

And here is a small driver program to test the fact that new (prototype) instances of the newsFeed bean are actually being returned for each call to the injected ObjectFactory inside the NewsFeedManager's printNews() method.

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import x.y.NewsFeedManager;

public class Main {

    public static void main(String[] args) throws Exception {

        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        NewsFeedManager manager = (NewsFeedManager) ctx.getBean("newsFeedManager");
        manager.printNews();
        manager.printNews();
    }
}

The output from running the above program will look like so (results will of course vary on your machine).

[email protected]: '... that's fit to print!'
[email protected]: '... that's fit to print!'

As of Spring 2.5, you can rely upon autowiring of the BeanFactory as yet another alternative to implementing the BeanFactoryAware interface. The "traditional" constructor and byType autowiring modes (as described in the section entitled Section 4.3.5, “Autowiring collaborators”) are now capable of providing a dependency of type BeanFactory for either a constructor argument or setter method parameter respectively. For more flexibility (including the ability to autowire fields and multiple parameter methods), consider using the new annotation-based autowiring features. In that case, the BeanFactory will be autowired into a field, constructor argument, or method parameter that is expecting the BeanFactory type as long as the field, constructor, or method in question carries the @Autowired annotation. For more information, see the section entitled Section 4.11.2, “@Autowired”.

4.5.2.2 BeanNameAware

If a bean implements the org.springframework.beans.factory.BeanNameAware interface and is deployed in a BeanFactory, the BeanFactory will call the bean through this interface to inform the bean of the name it was deployed under. The callback will be invoked after population of normal bean properties but before an initialization callback like InitializingBean's afterPropertiesSet or a custom init-method.

4.6 Bean definition inheritance

A bean definition potentially contains a large amount of configuration information, including container specific information (for example initialization method, static factory method name, and so forth) and constructor arguments and property values. A child bean definition is a bean definition that inherits configuration data from a parent definition. It is then able to override some values, or add others, as needed. Using parent and child bean definitions can potentially save a lot of typing. Effectively, this is a form of templating.

When working with a BeanFactory programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users will never work with them on this level, instead configuring bean definitions declaratively in something like the XmlBeanFactory. When using XML-based configuration metadata a child bean definition is indicated simply by using the 'parent' attribute, specifying the parent bean as the value of this attribute.

<bean id="inheritedTestBean" abstract="true"
    class="org.springframework.beans.TestBean">
  <property name="name" value="parent"/>
  <property name="age" value="1"/>
</bean>

<bean id="inheritsWithDifferentClass"
      class="org.springframework.beans.DerivedTestBean"
      parent="inheritedTestBean" init-method="initialize">
    
  <property name="name" value="override"/>
  <!-- the age property value of 1 will be inherited from  parent -->

</bean>

A child bean definition will use the bean class from the parent definition if none is specified, but can also override it. In the latter case, the child bean class must be compatible with the parent, that is it must accept the parent's property values.

A child bean definition will inherit constructor argument values, property values and method overrides from the parent, with the option to add new values. If any init-method, destroy-method and/or static factory method settings are specified, they will override the corresponding parent settings.

The remaining settings will always be taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.

Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute. In the case that the parent definition does not specify a class, and so explicitly marking the parent bean definition as abstract is required:

<bean id="inheritedTestBeanWithoutClass" abstract="true">
    <property name="name" value="parent"/>
    <property name="age" value="1"/>
</bean>

<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
    parent="inheritedTestBeanWithoutClass" init-method="initialize">
  <property name="name" value="override"/>
  <!-- age will inherit the value of 1 from the parent bean definition-->
</bean>

The parent bean cannot get instantiated on its own since it is incomplete, and it is also explicitly marked as abstract. When a definition is defined to be abstract like this, it is usable only as a pure template bean definition that will serve as a parent definition for child definitions. Trying to use such an abstract parent bean on its own (by referring to it as a ref property of another bean, or doing an explicit getBean() call with the parent bean id), will result in an error. Similarly, the container's internal preInstantiateSingletons() method will completely ignore bean definitions which are defined as abstract.

[Note]Note

ApplicationContexts (but not BeanFactories) will by default pre-instantiate all singletons. Therefore it is important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the 'abstract' attribute to 'true', otherwise the application context will actually (attempt to) pre-instantiate the abstract bean.

4.7 Container extension points

The IoC component of the Spring Framework has been designed for extension. There is typically no need for an application developer to subclass any of the various BeanFactory or ApplicationContext implementation classes. The Spring IoC container can be infinitely extended by plugging in implementations of special integration interfaces. The next few sections are devoted to detailing all of these various integration interfaces.

4.7.1 Customizing beans using BeanPostProcessors

The first extension point that we will look at is the BeanPostProcessor interface. This interface defines a number of callback methods that you as an application developer can implement in order to provide your own (or override the containers default) instantiation logic, dependency-resolution logic, and so forth. If you want to do some custom logic after the Spring container has finished instantiating, configuring and otherwise initializing a bean, you can plug in one or more BeanPostProcessor implementations.

You can configure multiple BeanPostProcessors if you wish. You can control the order in which these BeanPostProcessors execute by setting the 'order' property (you can only set this property if the BeanPostProcessor implements the Ordered interface; if you write your own BeanPostProcessor you should consider implementing the Ordered interface too); consult the Javadoc for the BeanPostProcessor and Ordered interfaces for more details.

[Note]Note

BeanPostProcessors operate on bean (or object) instances; that is to say, the Spring IoC container will have instantiated a bean instance for you, and then BeanPostProcessors get a chance to do their stuff.

If you want to change the actual bean definition (that is the recipe that defines the bean), then you rather need to use a BeanFactoryPostProcessor (described below in the section entitled Section 4.7.2, “Customizing configuration metadata with BeanFactoryPostProcessors”.

Also, BeanPostProcessors are scoped per-container. This is only relevant if you are using container hierarchies. If you define a BeanPostProcessor in one container, it will only do its stuff on the beans in that container. Beans that are defined in another container will not be post-processed by BeanPostProcessors in another container, even if both containers are part of the same hierarchy.

The org.springframework.beans.factory.config.BeanPostProcessor interface consists of exactly two callback methods. When such a class is registered as a post-processor with the container (see below for how this registration is effected), for each bean instance that is created by the container, the post-processor will get a callback from the container both before any container initialization methods (such as afterPropertiesSet and any declared init method) are called, and also afterwards. The post-processor is free to do what it wishes with the bean instance, including ignoring the callback completely. A bean post-processor will typically check for callback interfaces, or do something such as wrap a bean with a proxy; some of the Spring AOP infrastructure classes are implemented as bean post-processors and they do this proxy-wrapping logic.

It is important to know that a BeanFactory treats bean post-processors slightly differently than an ApplicationContext. An ApplicationContext will automatically detect any beans which are defined in the configuration metadata which is supplied to it that implement the BeanPostProcessor interface, and register them as post-processors, to be then called appropriately by the container on bean creation. Nothing else needs to be done other than deploying the post-processors in a similar fashion to any other bean. On the other hand, when using a BeanFactory implementation, bean post-processors explicitly have to be registered, with code like this:

ConfigurableBeanFactory factory = new XmlBeanFactory(...);
            
// now register any needed BeanPostProcessor instances
MyBeanPostProcessor postProcessor = new MyBeanPostProcessor();
factory.addBeanPostProcessor(postProcessor);

// now start using the factory

This explicit registration step is not convenient, and this is one of the reasons why the various ApplicationContext implementations are preferred above plain BeanFactory implementations in the vast majority of Spring-backed applications, especially when using BeanPostProcessors.

[Note]BeanPostProcessors and AOP auto-proxying

Classes that implement the BeanPostProcessor interface are special, and so they are treated differently by the container. All BeanPostProcessors and their directly referenced beans will be instantiated on startup, as part of the special startup phase of the ApplicationContext, then all those BeanPostProcessors will be registered in a sorted fashion - and applied to all further beans. Since AOP auto-proxying is implemented as a BeanPostProcessor itself, no BeanPostProcessors or directly referenced beans are eligible for auto-proxying (and thus will not have aspects 'woven' into them.

For any such bean, you should see an info log message: Bean 'foo' is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)”.

Find below some examples of how to write, register, and use BeanPostProcessors in the context of an ApplicationContext.

4.7.1.1 Example: Hello World, BeanPostProcessor-style

This first example is hardly compelling, but serves to illustrate basic usage. All we are going to do is code a custom BeanPostProcessor implementation that simply invokes the toString() method of each bean as it is created by the container and prints the resulting string to the system console. Yes, it is not hugely useful, but serves to get the basic concepts across before we move into the second example which is actually useful.

Find below the custom BeanPostProcessor implementation class definition:

package scripting;

import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.beans.BeansException;

public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor {

    // simply return the instantiated bean as-is
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean; // we could potentially return any object reference here...
    }

    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        System.out.println("Bean '" + beanName + "' created : " + bean.toString());
        return bean;
    }
}
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:lang="http://www.springframework.org/schema/lang"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/lang 
           http://www.springframework.org/schema/lang/spring-lang-3.0.xsd">

    <lang:groovy id="messenger"
          script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy">
        <lang:property name="message" value="Fiona Apple Is Just So Dreamy."/> 
    </lang:groovy>
    
    <!-- 
        when the above bean ('messenger') is instantiated, this custom
        BeanPostProcessor implementation will output the fact to the system console
     -->
    <bean class="scripting.InstantiationTracingBeanPostProcessor"/>

</beans>

Notice how the InstantiationTracingBeanPostProcessor is simply defined; it doesn't even have a name, and because it is a bean it can be dependency injected just like any other bean. (The above configuration also just so happens to define a bean that is backed by a Groovy script. The Spring 2.0 dynamic language support is detailed in the chapter entitled Chapter 28, Dynamic language support.)

Find below a small driver script to exercise the above code and configuration;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;

public final class Boot {

    public static void main(final String[] args) throws Exception {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml");
        Messenger messenger = (Messenger) ctx.getBean("messenger");
        System.out.println(messenger);
    }
}

The output of executing the above program will be (something like) this:

Bean 'messenger' created : [email protected]
[email protected]

4.7.1.2 Example: The RequiredAnnotationBeanPostProcessor

Using callback interfaces or annotations in conjunction with a custom BeanPostProcessor implementation is a common means of extending the Spring IoC container. This next example is a bit of a cop-out, in that you are directed to the section entitled Section 29.3.1, “@Required” which demonstrates the usage of a custom BeanPostProcessor implementation that ships with the Spring distribution which ensures that JavaBean properties on beans that are marked with an (arbitrary) annotation are actually (configured to be) dependency-injected with a value.

4.7.2 Customizing configuration metadata with BeanFactoryPostProcessors

The next extension point that we will look at is the org.springframework.beans.factory.config.BeanFactoryPostProcessor. The semantics of this interface are similar to the BeanPostProcessor, with one major difference: BeanFactoryPostProcessors operate on the bean configuration metadata; that is, the Spring IoC container will allow BeanFactoryPostProcessors to read the configuration metadata and potentially change it before the container has actually instantiated any other beans.

You can configure multiple BeanFactoryPostProcessors if you wish. You can control the order in which these BeanFactoryPostProcessors execute by setting the 'order' property (you can only set this property if the BeanFactoryPostProcessor implements the Ordered interface; if you write your own BeanFactoryPostProcessor you should consider implementing the Ordered interface too); consult the Javadoc for the BeanFactoryPostProcessor and Ordered interfaces for more details.

[Note]Note

If you want to change the actual bean instances (the objects that are created from the configuration metadata), then you rather need to use a BeanPostProcessor (described above in the section entitled Section 4.7.1, “Customizing beans using BeanPostProcessors”.

Also, BeanFactoryPostProcessors are scoped per-container. This is only relevant if you are using container hierarchies. If you define a BeanFactoryPostProcessor in one container, it will only do its stuff on the bean definitions in that container. Bean definitions in another container will not be post-processed by BeanFactoryPostProcessors in another container, even if both containers are part of the same hierarchy.

A bean factory post-processor is executed manually (in the case of a BeanFactory) or automatically (in the case of an ApplicationContext) to apply changes of some sort to the configuration metadata that defines a container. Spring includes a number of pre-existing bean factory post-processors, such as PropertyOverrideConfigurer and PropertyPlaceholderConfigurer, both described below. A custom BeanFactoryPostProcessor can also be used to register custom property editors, for example.

In a BeanFactory, the process of applying a BeanFactoryPostProcessor is manual, and will be similar to this:

XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));

// bring in some property values from a Properties file
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setLocation(new FileSystemResource("jdbc.properties"));

// now actually do the replacement
cfg.postProcessBeanFactory(factory);

This explicit registration step is not convenient, and this is one of the reasons why the various ApplicationContext implementations are preferred above plain BeanFactory implementations in the vast majority of Spring-backed applications, especially when using BeanFactoryPostProcessors.

An ApplicationContext will detect any beans which are deployed into it which implement the BeanFactoryPostProcessor interface, and automatically use them as bean factory post-processors, at the appropriate time. Nothing else needs to be done other than deploying these post-processor in a similar fashion to any other bean.

[Note]Note

Just as in the case of BeanPostProcessors, you typically don't want to have BeanFactoryPostProcessors marked as being lazily-initialized. If they are marked as such, then the Spring container will never instantiate them, and thus they won't get a chance to apply their custom logic. If you are using the 'default-lazy-init' attribute on the declaration of your <beans/> element, be sure to mark your various BeanFactoryPostProcessor bean definitions with 'lazy-init="false"'.

4.7.2.1 Example: the PropertyPlaceholderConfigurer

The PropertyPlaceholderConfigurer is used to externalize property values from a BeanFactory definition, into another separate file in the standard Java Properties format. This is useful to allow the person deploying an application to customize environment-specific properties (for example database URLs, usernames and passwords), without the complexity or risk of modifying the main XML definition file or files for the container.

Consider the following XML-based configuration metadata fragment, where a DataSource with placeholder values is defined. We will configure some properties from an external Properties file, and at runtime, we will apply a PropertyPlaceholderConfigurer to the metadata which will replace some properties of the DataSource:

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations">
        <value>classpath:com/foo/jdbc.properties</value>
    </property>
</bean>

<bean id="dataSource" destroy-method="close"
      class="org.apache.commons.dbcp.BasicDataSource">
    <property name="driverClassName" value="${jdbc.driverClassName}"/>
    <property name="url" value="${jdbc.url}"/>
    <property name="username" value="${jdbc.username}"/>
    <property name="password" value="${jdbc.password}"/>
</bean>

The actual values come from another file in the standard Java Properties format:

jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root

With the context namespace introduced in Spring 2.5, it is possible to configure property placeholders with a dedicated configuration element. Multiple locations may be provided as a comma-separated list for the location attribute.

<context:property-placeholder location="classpath:com/foo/jdbc.properties"/>

The PropertyPlaceholderConfigurer doesn't only look for properties in the Properties file you specify, but also checks against the Java System properties if it cannot find a property you are trying to use. This behavior can be customized by setting the systemPropertiesMode property of the configurer. It has three values, one to tell the configurer to always override, one to let it never override and one to let it override only if the property cannot be found in the properties file specified. Please consult the Javadoc for the PropertyPlaceholderConfigurer for more information.

[Tip]Class name substitution

The PropertyPlaceholderConfigurer can be used to substitute class names, which is sometimes useful when you have to pick a particular implementation class at runtime. For example:

<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations">
        <value>classpath:com/foo/strategy.properties</value>
    </property>
    <property name="properties">
        <value>custom.strategy.class=com.foo.DefaultStrategy</value>
    </property>
</bean>

<bean id="serviceStrategy" class="${custom.strategy.class}"/>

If the class is unable to be resolved at runtime to a valid class, resolution of the bean will fail once it is about to be created (which is during the preInstantiateSingletons() phase of an ApplicationContext for a non-lazy-init bean.)

4.7.2.2 Example: the PropertyOverrideConfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, is similar to the PropertyPlaceholderConfigurer, but in contrast to the latter, the original definitions can have default values or no values at all for bean properties. If an overriding Properties file does not have an entry for a certain bean property, the default context definition is used.

Note that the bean factory definition is not aware of being overridden, so it is not immediately obvious when looking at the XML definition file that the override configurer is being used. In case that there are multiple PropertyOverrideConfigurer instances that define different values for the same bean property, the last one will win (due to the overriding mechanism).

Properties file configuration lines are expected to be in the format:

beanName.property=value

An example properties file might look like this:

dataSource.driverClassName=com.mysql.jdbc.Driver
dataSource.url=jdbc:mysql:mydb

This example file would be usable against a container definition which contains a bean called dataSource, which has driver and url properties.

Note that compound property names are also supported, as long as every component of the path except the final property being overridden is already non-null (presumably initialized by the constructors). In this example...

foo.fred.bob.sammy=123

... the sammy property of the bob property of the fred property of the foo bean is being set to the scalar value 123.

Note: Specified override values are always literal values; they are not translated into bean references. This also applies when the original value in the XML bean definition specifies a bean reference

With the context namespace introduced in Spring 2.5, it is possible to configure property overriding with a dedicated configuration element:

<context:property-override location="classpath:override.properties"/>

4.7.3 Customizing instantiation logic using FactoryBeans

The org.springframework.beans.factory.FactoryBean interface is to be implemented by objects that are themselves factories.

The FactoryBean interface is a point of pluggability into the Spring IoC containers instantiation logic. If you have some complex initialization code that is better expressed in Java as opposed to a (potentially) verbose amount of XML, you can create your own FactoryBean, write the complex initialization inside that class, and then plug your custom FactoryBean into the container.

The FactoryBean interface provides three methods:

  • Object getObject(): has to return an instance of the object this factory creates. The instance can possibly be shared (depending on whether this factory returns singletons or prototypes).

  • boolean isSingleton(): has to return true if this FactoryBean returns singletons, false otherwise

  • Class getObjectType(): has to return either the object type returned by the getObject() method or null if the type isn't known in advance

The FactoryBean concept and interface is used in a number of places within the Spring Framework; at the time of writing there are over 50 implementations of the FactoryBean interface that ship with Spring itself.

Finally, there is sometimes a need to ask a container for an actual FactoryBean instance itself, not the bean it produces. This may be achieved by prepending the bean id with '&' (sans quotes) when calling the getBean method of the BeanFactory (including ApplicationContext). So for a given FactoryBean with an id of myBean, invoking getBean("myBean") on the container will return the product of the FactoryBean, but invoking getBean("&myBean") will return the FactoryBean instance itself.

4.8 The ApplicationContext

While the beans package provides basic functionality for managing and manipulating beans, including in a programmatic way, the context package adds the ApplicationContext interface, which enhances BeanFactory functionality in a more framework-oriented style. Many users will use ApplicationContext in a completely declarative fashion, not even having to create it manually, but instead relying on support classes such as ContextLoader to automatically instantiate an ApplicationContext as part of the normal startup process of a J2EE web-app. (Of course, it is still possible to create an ApplicationContext programmatically.)

The basis for the context package is the ApplicationContext interface, located in the org.springframework.context package. Deriving from the BeanFactory interface, it provides all the functionality of BeanFactory. To allow working in a more framework-oriented fashion, using layering and hierarchical contexts, the context package also provides the following functionality:

  • MessageSource, providing access to messages in i18n-style.

  • Access to resources, such as URLs and files.

  • Event propagation to beans implementing the ApplicationListener interface.

  • Loading of multiple (hierarchical) contexts, allowing each to be focused on one particular layer, for example the web layer of an application.

4.8.1 BeanFactory or ApplicationContext?

Short version: use an ApplicationContext unless you have a really good reason for not doing so. For those of you that are looking for slightly more depth as to the 'but why' of the above recommendation, keep reading.

As the ApplicationContext includes all functionality of the BeanFactory, it is generally recommended that it be used in preference to the BeanFactory, except for a few limited situations such as in an Applet, where memory consumption might be critical and a few extra kilobytes might make a difference. However, for most 'typical' enterprise applications and systems, the ApplicationContext is what you will want to use. Versions of Spring 2.0 and above make heavy use of the BeanPostProcessor extension point (to effect proxying and suchlike), and if you are using just a plain BeanFactory then a fair amount of support such as transactions and AOP will not take effect (at least not without some extra steps on your part), which could be confusing because nothing will actually be wrong with the configuration.

Find below a feature matrix that lists what features are provided by the BeanFactory and ApplicationContext interfaces (and attendant implementations). (The following sections describe functionality that ApplicationContext adds to the basic BeanFactory capabilities in a lot more depth than the said feature matrix.)

Table 4.5. Feature Matrix

FeatureBeanFactoryApplicationContext

Bean instantiation/wiring

Yes

Yes

Automatic BeanPostProcessor registration

No

Yes

Automatic BeanFactoryPostProcessor registration

No

Yes

Convenient MessageSource access (for i18n)

No

Yes

ApplicationEvent publication

No

Yes


4.8.2 Internationalization using MessageSources

The ApplicationContext interface extends an interface called MessageSource, and therefore provides messaging (i18n or internationalization) functionality. Together with the HierarchicalMessageSource, capable of resolving hierarchical messages, these are the basic interfaces Spring provides to do message resolution. Let's quickly review the methods defined there:

  • String getMessage(String code, Object[] args, String default, Locale loc): the basic method used to retrieve a message from the MessageSource. When no message is found for the specified locale, the default message is used. Any arguments passed in are used as replacement values, using the MessageFormat functionality provided by the standard library.

  • String getMessage(String code, Object[] args, Locale loc): essentially the same as the previous method, but with one difference: no default message can be specified; if the message cannot be found, a NoSuchMessageException is thrown.

  • String getMessage(MessageSourceResolvable resolvable, Locale locale): all properties used in the methods above are also wrapped in a class named MessageSourceResolvable, which you can use via this method.

When an ApplicationContext gets loaded, it automatically searches for a MessageSource bean defined in the context. The bean has to have the name 'messageSource'. If such a bean is found, all calls to the methods described above will be delegated to the message source that was found. If no message source was found, the ApplicationContext attempts to see if it has a parent containing a bean with the same name. If so, it uses that bean as the MessageSource. If it can't find any source for messages, an empty DelegatingMessageSource will be instantiated in order to be able to accept calls to the methods defined above.

Spring currently provides two MessageSource implementations. These are the ResourceBundleMessageSource and the StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested messaging. The StaticMessageSource is hardly ever used but provides programmatic ways to add messages to the source. The ResourceBundleMessageSource is more interesting and is the one we will provide an example for:

<beans>
  <bean id="messageSource"
        class="org.springframework.context.support.ResourceBundleMessageSource">
    <property name="basenames">
      <list>
        <value>format</value>
        <value>exceptions</value>
        <value>windows</value>
      </list>
    </property>
  </bean>
</beans>

This assumes you have three resource bundles defined on your classpath called format, exceptions and windows. Using the JDK standard way of resolving messages through ResourceBundles, any request to resolve a message will be handled. For the purposes of the example, lets assume the contents of two of the above resource bundle files are...

# in 'format.properties'
message=Alligators rock!
# in 'exceptions.properties'
argument.required=The '{0}' argument is required.

Some (admittedly trivial) driver code to exercise the MessageSource functionality can be found below. Remember that all ApplicationContext implementations are also MessageSource implementations and so can be cast to the MessageSource interface.

public static void main(String[] args) {
    MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
    String message = resources.getMessage("message", null, "Default", null);
    System.out.println(message);
}

The resulting output from the above program will be...

Alligators rock!

So to summarize, the MessageSource is defined in a file called 'beans.xml' (this file exists at the root of your classpath). The 'messageSource' bean definition refers to a number of resource bundles via its basenames property; the three files that are passed in the list to the basenames property exist as files at the root of your classpath (and are called format.properties, exceptions.properties, and windows.properties respectively).

Lets look at another example, and this time we will look at passing arguments to the message lookup; these arguments will be converted into Strings and inserted into placeholders in the lookup message. This is perhaps best explained with an example:

<beans>

    <!-- this MessageSource is being used in a web application -->
    <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource">
        <property name="basename" value="test-messages"/>
    </bean>
    
    <!-- let's inject the above MessageSource into this POJO -->
    <bean id="example" class="com.foo.Example">
        <property name="messages" ref="messageSource"/>
    </bean>

</beans>
public class Example {

    private MessageSource messages;

    public void setMessages(MessageSource messages) {
        this.messages = messages;
    }

    public void execute() {
        String message = this.messages.getMessage("argument.required",
            new Object [] {"userDao"}, "Required", null);
        System.out.println(message);
    }

}

The resulting output from the invocation of the execute() method will be...

The 'userDao' argument is required.

With regard to internationalization (i18n), Spring's various MessageResource implementations follow the same locale resolution and fallback rules as the standard JDK ResourceBundle. In short, and continuing with the example 'messageSource' defined previously, if you want to resolve messages against the British (en-GB) locale, you would create files called format_en_GB.properties, exceptions_en_GB.properties, and windows_en_GB.properties respectively.

Locale resolution is typically going to be managed by the surrounding environment of the application. For the purpose of this example though, we'll just manually specify the locale that we want to resolve our (British) messages against.

# in 'exceptions_en_GB.properties'
argument.required=Ebagum lad, the '{0}' argument is required, I say, required.
public static void main(final String[] args) {
    MessageSource resources = new ClassPathXmlApplicationContext("beans.xml");
    String message = resources.getMessage("argument.required",
        new Object [] {"userDao"}, "Required", Locale.UK);
    System.out.println(message);
}

The resulting output from the running of the above program will be...

Ebagum lad, the 'userDao' argument is required, I say, required.

The MessageSourceAware interface can also be used to acquire a reference to any MessageSource that has been defined. Any bean that is defined in an ApplicationContext that implements the MessageSourceAware interface will be injected with the application context's MessageSource when it (the bean) is being created and configured.

Note: As an alternative to ResourceBundleMessageSource, Spring also provides a ReloadableResourceBundleMessageSource class. This variant supports the same bundle file format but is more flexible than the standard JDK based ResourceBundleMessageSource implementation. In particular, it allows for reading files from any Spring resource location (not just from the classpath) and supports hot reloading of bundle property files (while efficiently caching them in between). Check out the ReloadableResourceBundleMessageSource javadoc for details.

4.8.3 Events

Event handling in the ApplicationContext is provided through the ApplicationEvent class and ApplicationListener interface. If a bean which implements the ApplicationListener interface is deployed into the context, every time an ApplicationEvent gets published to the ApplicationContext, that bean will be notified. Essentially, this is the standard Observer design pattern. Spring provides the following standard events:

Table 4.6. Built-in Events

EventExplanation
ContextRefreshedEventPublished when the ApplicationContext is initialized or refreshed, e.g. using the refresh() method on the ConfigurableApplicationContext interface. "Initialized" here means that all beans are loaded, post-processor beans are detected and activated, singletons are pre-instantiated, and the ApplicationContext object is ready for use. A refresh may be triggered multiple times, as long as the context hasn't been closed - provided that the chosen ApplicationContext actually supports such "hot" refreshes (which e.g. XmlWebApplicationContext does but GenericApplicationContext doesn't).
ContextStartedEventPublished when the ApplicationContext is started, using the start() method on the ConfigurableApplicationContext interface. "Started" here means that all Lifecycle beans will receive an explicit start signal. This will typically be used for restarting after an explicit stop, but may also be used for starting components that haven't been configured for autostart (e.g. haven't started on initialization already).
ContextStoppedEventPublished when the ApplicationContext is stopped, using the stop() method on the ConfigurableApplicationContext interface. "Stopped" here means that all Lifecycle beans will receive an explicit stop signal. A stopped context may be restarted through a start() call.
ContextClosedEventPublished when the ApplicationContext is closed, using the close() method on the ConfigurableApplicationContext interface. "Closed" here means that all singleton beans are destroyed. A closed context has reached its end of life; it cannot be refreshed or restarted.
RequestHandledEventA web-specific event telling all beans that an HTTP request has been serviced (this will be published after the request has been finished). Note that this event is only applicable for web applications using Spring's DispatcherServlet.

Implementing custom events can be done as well. Simply call the publishEvent() method on the ApplicationContext, specifying a parameter which is an instance of your custom event class implementing ApplicationEvent. Event listeners receive events synchronously. This means the publishEvent() method blocks until all listeners have finished processing the event (it is possible to supply an alternate event publishing strategy via a ApplicationEventMulticaster implementation). Furthermore, when a listener receives an event it operates inside the transaction context of the publisher, if a transaction context is available.

Let's look at an example. First, the ApplicationContext:

<bean id="emailer" class="example.EmailBean">
  <property name="blackList">
    <list>
      <value>[email protected]</value>
      <value>[email protected]</value>
      <value>[email protected]</value>
    </list>
  </property>
</bean>

<bean id="blackListListener" class="example.BlackListNotifier">
  <property name="notificationAddress" value="[email protected]"/>
</bean>

Now, let's look at the actual classes:

public class EmailBean implements ApplicationContextAware {

    private List blackList;
    private ApplicationContext ctx;

    public void setBlackList(List blackList) {
        this.blackList = blackList;
    }

    public void setApplicationContext(ApplicationContext ctx) {
        this.ctx = ctx;
    }

    public void sendEmail(String address, String text) {
        if (blackList.contains(address)) {
            BlackListEvent event = new BlackListEvent(address, text);
            ctx.publishEvent(event);
            return;
        }
        // send email...
    }
}
public class BlackListNotifier implements ApplicationListener {

    private String notificationAddress;
    
    public void setNotificationAddress(String notificationAddress) {
        this.notificationAddress = notificationAddress;
    }

    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof BlackListEvent) {
            // notify appropriate person...
        }
    }
}

Of course, this particular example could probably be implemented in better ways (perhaps by using AOP features), but it should be sufficient to illustrate the basic event mechanism.

4.8.4 Convenient access to low-level resources

For optimal usage and understanding of application contexts, users should generally familiarize themselves with Spring's Resource abstraction, as described in the chapter entitled Chapter 5, Resources.

An application context is a ResourceLoader, able to be used to load Resources. A Resource is essentially a java.net.URL on steroids (in fact, it just wraps and uses a URL where appropriate), which can be used to obtain low-level resources from almost any location in a transparent fashion, including from the classpath, a filesystem location, anywhere describable with a standard URL, and some other variations. If the resource location string is a simple path without any special prefixes, where those resources come from is specific and appropriate to the actual application context type.

A bean deployed into the application context may implement the special callback interface, ResourceLoaderAware, to be automatically called back at initialization time with the application context itself passed in as the ResourceLoader. A bean may also expose properties of type Resource, to be used to access static resources, and expect that they will be injected into it like any other properties. The person deploying the bean may specify those Resource properties as simple String paths, and rely on a special JavaBean PropertyEditor that is automatically registered by the context, to convert those text strings to actual Resource objects.

The location path or paths supplied to an ApplicationContext constructor are actually resource strings, and in simple form are treated appropriately to the specific context implementation ( ClassPathXmlApplicationContext treats a simple location path as a classpath location), but may also be used with special prefixes to force loading of definitions from the classpath or a URL, regardless of the actual context type.

4.8.5 Convenient ApplicationContext instantiation for web applications

As opposed to the BeanFactory, which will often be created programmatically, ApplicationContext instances can be created declaratively using for example a ContextLoader. Of course you can also create ApplicationContext instances programmatically using one of the ApplicationContext implementations. First, let's examine the ContextLoader mechanism and its implementations.

The ContextLoader mechanism comes in two flavors: the ContextLoaderListener and the ContextLoaderServlet. They both have the same functionality but differ in that the listener version cannot be reliably used in Servlet 2.3 containers. Since the Servlet 2.4 specification, servlet context listeners are required to execute immediately after the servlet context for the web application has been created and is available to service the first request (and also when the servlet context is about to be shut down): as such a servlet context listener is an ideal place to initialize the Spring ApplicationContext. It is up to you as to which one you use, but all things being equal you should probably prefer ContextLoaderListener; for more information on compatibility, have a look at the Javadoc for the ContextLoaderServlet.

You can register an ApplicationContext using the ContextLoaderListener as follows:

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value>
</context-param>

<listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

<!-- or use the ContextLoaderServlet instead of the above listener
<servlet>
  <servlet-name>context</servlet-name>
  <servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class>
  <load-on-startup>1</load-on-startup>
</servlet>
-->

The listener inspects the 'contextConfigLocation' parameter. If the parameter does not exist, the listener will use /WEB-INF/applicationContext.xml as a default. When it does exist, it will separate the String using predefined delimiters (comma, semicolon and whitespace) and use the values as locations where application contexts will be searched for. Ant-style path patterns are supported as well: e.g. /WEB-INF/*Context.xml (for all files whose name ends with "Context.xml", residing in the "WEB-INF" directory) or /WEB-INF/**/*Context.xml (for all such files in any subdirectory of "WEB-INF").

The ContextLoaderServlet can be used instead of the ContextLoaderListener. The servlet will use the 'contextConfigLocation' parameter just as the listener does.

4.9 Glue code and the evil singleton

The majority of the code inside an application is best written in a DI style, where that code is served out of a Spring IoC container, has its own dependencies supplied by the container when it is created, and is completely unaware of the container. However, for the small glue layers of code that are sometimes needed to tie other code together, there is sometimes a need for singleton (or quasi-singleton) style access to a Spring IoC container. For example, third party code may try to construct new objects directly (Class.forName() style), without the ability to force it to get these objects out of a Spring IoC container. If the object constructed by the third party code is just a small stub or proxy, which then uses a singleton style access to a Spring IoC container to get a real object to delegate to, then inversion of control has still been achieved for the majority of the code (the object coming out of the container); thus most code is still unaware of the container or how it is accessed, and remains decoupled from other code, with all ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java implementation object, coming out of a Spring IoC container. While the Spring IoC container itself ideally does not have to be a singleton, it may be unrealistic in terms of memory usage or initialization times (when using beans in the Spring IoC container such as a Hibernate SessionFactory) for each bean to use its own, non-singleton Spring IoC container.

As another example, in complex J2EE applications with multiple layers (various JAR files, EJBs, and WAR files packaged as an EAR), with each layer having its own Spring IoC container definition (effectively forming a hierarchy), the preferred approach when there is only one web-app (WAR) in the top hierarchy is to simply create one composite Spring IoC container from the multiple XML definition files from each layer. All of the various Spring IoC container implementations may be constructed from multiple definition files in this fashion. However, if there are multiple sibling web-applications at the root of the hierarchy, it is problematic to create a Spring IoC container for each web-application which consists of mostly identical bean definitions from lower layers, as there may be issues due to increased memory usage, issues with creating multiple copies of beans which take a long time to initialize (for example a Hibernate SessionFactory), and possible issues due to side-effects. As an alternative, classes such as ContextSingletonBeanFactoryLocator or SingletonBeanFactoryLocator may be used to demand-load multiple hierarchical (that is one container is the parent of another) Spring IoC container instances in a singleton fashion, which may then be used as the parents of the web-application Spring IoC container instances. The result is that bean definitions for lower layers are loaded only as needed, and loaded only once.

You can see a detailed example of the usage of these classes by viewing the Javadoc for the SingletonBeanFactoryLocator and ContextSingletonBeanFactoryLocator classes. As mentioned in the chapter on EJBs, the Spring convenience base classes for EJBs normally use a non-singleton BeanFactoryLocator implementation, which is easily replaced by the use of SingletonBeanFactoryLocator and ContextSingletonBeanFactoryLocator.

4.10 Deploying a Spring ApplicationContext as a J2EE RAR file

Since Spring 2.5, it is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the context and all of its required bean classes and library JARs in a J2EE RAR deployment unit. This is the equivalent of bootstrapping a standalone ApplicationContext, just hosted in J2EE environment, being able to access the J2EE server's facilities. RAR deployment is intended as a more 'natural' alternative to the not uncommon scenario of deploying a headless WAR file - i.e. a WAR file without any HTTP entry points, just used for bootstrapping a Spring ApplicationContext in a J2EE environment.

RAR deployment is ideal for application contexts that do not need any HTTP entry points but rather just consist of message endpoints and scheduled jobs etc. Beans in such a context may use application server resources such as the JTA transaction manager and JNDI-bound JDBC DataSources and JMS ConnectionFactory instances, and may also register with the platform's JMX server - all through Spring's standard transaction management and JNDI and JMX support facilities. Application components may also interact with the application's server JCA WorkManager through Spring's TaskExecutor abstraction.

Check out the JavaDoc of the SpringContextResourceAdapter class for the configuration details involved in RAR deployment.

For simple deployment needs, all you need to do is the following: Package all application classes into a RAR file (which is just a standard JAR file with a different file extension), add all required library jars into the root of the RAR archive, add a "META-INF/ra.xml" deployment descriptor (as shown in SpringContextResourceAdapter's JavaDoc) as well as the corresponding Spring XML bean definition file(s) (typically "META-INF/applicationContext.xml"), and drop the resulting RAR file into your application server's deployment directory!

NOTE: Such RAR deployment units are usually self-contained; they do not expose components to the 'outside' world, not even to other modules of the same application. Interaction with a RAR-based ApplicationContext usually happens through JMS destinations that it shares with other modules. A RAR-based ApplicationContext may also - for example - schedule some jobs, reacting to new files in the file system (or the like). If it actually needs to allow for synchronous access from the outside, it could for example export RMI endpoints, which of course may be used by other application modules on the same machine as well.

4.11 Annotation-based configuration

As mentioned in the section entitled Section 4.7.1.2, “Example: The RequiredAnnotationBeanPostProcessor”, using a BeanPostProcessor in conjunction with annotations is a common means of extending the Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing required properties with the @Required annotation. As of Spring 2.5, it is now possible to follow that same general approach to drive Spring's dependency injection. Essentially, the @Autowired annotation provides the same capabilities as described in Section 4.3.5, “Autowiring collaborators” but with more fine-grained control and wider applicability. Spring 2.5 also adds support for JSR-250 annotations such as @Resource, @PostConstruct, and @PreDestroy. Use of these annotations also requires that certain BeanPostProcessors be registered within the Spring container. As always, these can be registered as individual bean definitions, but they can also be implicitly registered by including the following tag in an XML-based Spring configuration (notice the inclusion of the 'context' namespace):

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.0.xsd">
               
     <context:annotation-config/>
     
</beans>

(The implicitly registered post-processors include AutowiredAnnotationBeanPostProcessor, CommonAnnotationBeanPostProcessor, PersistenceAnnotationBeanPostProcessor, as well as the aforementioned RequiredAnnotationBeanPostProcessor.)

[Note]Note

Note that <context:annotation-config/> only looks for annotations on beans in the same application context it is defined in. This means that, if you put <context:annotation-config/> in a WebApplicationContext for a DispatcherServlet, it only checks for @Autowired beans in your controllers, and not your services. See Section 16.2, “The DispatcherServlet” for more information.

4.11.1 @Required

The @Required annotation applies to bean property setter methods, as in the following example:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Required
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}

This annotation simply indicates that the affected bean property must be populated at configuration time: either through an explicit property value in a bean definition or through autowiring. The container will throw an exception if the affected bean property has not been populated; this allows for eager and explicit failure, avoiding NullPointerExceptions or the like later on. Note that it is still recommended to put assertions into the bean class itself (for example into an init method) in order to enforce those required references and values even when using the class outside of a container.

4.11.2 @Autowired

As expected, the @Autowired annotation may be applied to "traditional" setter methods:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}

The annotation may also be applied to methods with arbitrary names and/or multiple arguments:

public class MovieRecommender {

    private MovieCatalog movieCatalog;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

The @Autowired annotation may even be applied on constructors and fields:

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

It is also possible to provide all beans of a particular type from the ApplicationContext by adding the annotation to a field or method that expects an array of that type:

public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogs;

    // ...
}

The same applies for typed collections:

public class MovieRecommender {

    private Set<MovieCatalog> movieCatalogs;
    
    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...
}

Even typed Maps may be autowired as long as the expected key type is String. The Map values will contain all beans of the expected type, and the keys will contain the corresponding bean names:

public class MovieRecommender {

    private Map<String, MovieCatalog> movieCatalogs;
    
    @Autowired
    public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }

    // ...
}

By default, the autowiring will fail whenever zero candidate beans are available; the default behavior is to treat annotated methods, constructors, and fields as indicating required dependencies. This behavior can be changed as demonstrated below.

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired(required=false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}
[Note]Note

Only one annotated constructor per-class may be marked as required, but multiple non-required constructors can be annotated. In that case, each will be considered among the candidates and Spring will use the greediest constructor whose dependencies can be satisfied.

Prefer the use of @Autowired's required attribute over the @Required annotation. The required attribute indicates that the property is not required for autowiring purposes, simply skipping it if it cannot be autowired. @Required, on the other hand, is stronger in that it enforces the property to have been set in any of the container's supported ways; if no value has been injected, a corresponding exception will be raised.

@Autowired may also be used for well-known "resolvable dependencies": the BeanFactory interface, the ApplicationContext interface, the ResourceLoader interface, the ApplicationEventPublisher interface and the MessageSource interface. These interfaces (and their extended interfaces such as ConfigurableApplicationContext or ResourcePatternResolver) will be automatically resolved, with no special setup necessary.

public class MovieRecommender {

    @Autowired
    private ApplicationContext context;

    public MovieRecommender() {
    }

    // ...
}

4.11.3 Fine-tuning annotation-based autowiring with qualifiers

Since autowiring by type may lead to multiple candidates, it is often necessary to have more control over the selection process. One way to accomplish this is with Spring's @Qualifier annotation. This allows for associating qualifier values with specific arguments, narrowing the set of type matches so that a specific bean is chosen for each argument. In the simplest case, this can be a plain descriptive value:

public class MovieRecommender {

    @Autowired
    @Qualifier("main")
    private MovieCatalog movieCatalog;

    // ...
}

The @Qualifier annotation can also be specified on individual constructor arguments or method parameters:

public class MovieRecommender {

    private MovieCatalog movieCatalog;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(@Qualifier("main") MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

The corresponding bean definitions would look like as follows. The bean with qualifier value "main" would be wired with the constructor argument that has been qualified with the same value.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="main"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier value="action"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

For a fallback match, the bean name is considered as a default qualifier value. This means that the bean may be defined with an id "main" instead of the nested qualifier element, leading to the same matching result. However, note that while this can be used to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even when using the bean name fallback, always have narrowing semantics within the set of type matches; they do not semantically express a reference to a unique bean id. Good qualifier values would be "main" or "EMEA" or "persistent", expressing characteristics of a specific component - independent from the bean id (which may be auto-generated in case of an anonymous bean definition like the one above).

Qualifiers also apply to typed collections (as discussed above): e.g. to Set<MovieCatalog>. In such a case, all matching beans according to the declared qualifiers are going to be injected as a collection. This implies that qualifiers do not have to be unique; they rather simply constitute filtering criteria. For example, there could be multiple MovieCatalog beans defined with the same qualifier value "action"; all of which would be injected into a Set<MovieCatalog> annotated with @Qualifier("action").

[Tip]Tip

If you intend to express annotation-driven injection by name, do not primarily use @Autowired - even if is technically capable of referring to a bean name through @Qualifier values. Instead, prefer the JSR-250 @Resource annotation which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.

As a specific consequence of this semantic difference, beans which are themselves defined as a collection or map type cannot be injected via @Autowired since type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection/map bean by unique name.

Note: In contrast to @Autowired which is applicable to fields, constructors and multi-argument methods (allowing for narrowing through qualifier annotations at the parameter level), @Resource is only supported for fields and bean property setter methods with a single argument. As a consequence, stick with qualifiers if your injection target is a constructor or a multi-argument method.

You may create your own custom qualifier annotations as well. Simply define an annotation and provide the @Qualifier annotation within your definition:

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Genre {

    String value();
}

Then you can provide the custom qualifier on autowired fields and parameters:

public class MovieRecommender {

    @Autowired
    @Genre("Action")
    private MovieCatalog actionCatalog;
    
    private MovieCatalog comedyCatalog;
    
    @Autowired
    public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) {
        this.comedyCatalog = comedyCatalog;
    }

    // ...
}

The next step is to provide the information on the candidate bean definitions. You can add <qualifier/> tags as sub-elements of the <bean/> tag and then specify the 'type' and 'value' to match your custom qualifier annotations. The type will be matched against the fully-qualified class name of the annotation, or as a convenience when there is no risk of conflicting names, you may use the 'short' class name. Both are demonstrated in the following example.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="Genre" value="Action"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="example.Genre" value="Comedy"/>
        <!-- inject any dependencies required by this bean -->
    </bean>
    
    <bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>

In the next section, entitled Section 4.12, “Classpath scanning, managed components and writing configurations using Java”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see: Section 4.12.9, “Providing qualifier metadata with annotations”.

In some cases, it may be sufficient to use an annotation without a value. This may be useful when the annotation serves a more generic purpose and could be applied across several different types of dependencies. For example, you may provide an offline catalog that would be searched when no Internet connection is available. First define the simple annotation:

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface Offline {

}

Then add the annotation to the field or property to be autowired:

public class MovieRecommender {

    @Autowired
    @Offline
    private MovieCatalog offlineCatalog;

    // ...
}

Now the bean definition only needs a qualifier 'type':

<bean class="example.SimpleMovieCatalog">
    <qualifier type="Offline"/>
    <!-- inject any dependencies required by this bean -->
</bean>

It is also possible to define custom qualifier annotations that accept named attributes in addition to or instead of the simple 'value' attribute. If multiple attribute values are then specified on a field or parameter to be autowired, a bean definition must match all such attribute values to be considered an autowire candidate. As an example, consider the following annotation definition:

@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MovieQualifier {

    String genre();
    
    Format format();
}

In this case Format is an enum:

public enum Format {
    
    VHS, DVD, BLURAY
}

The fields to be autowired are annotated with the custom qualifier and include values for both attributes: 'genre' and 'format'.

public class MovieRecommender {

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Action")
    private MovieCatalog actionVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.VHS, genre="Comedy")
    private MovieCatalog comedyVhsCatalog;

    @Autowired
    @MovieQualifier(format=Format.DVD, genre="Action")
    private MovieCatalog actionDvdCatalog;

    @Autowired
    @MovieQualifier(format=Format.BLURAY, genre="Comedy")
    private MovieCatalog comedyBluRayCatalog;
   
    // ...
}

Finally, the bean definitions should contain matching qualifier values. This example also demonstrates that bean meta attributes may be used instead of the <qualifier/> sub-elements. If available, the <qualifier/> and its attributes would take precedence, but the autowiring mechanism will fallback on the values provided within the <meta/> tags if no such qualifier is present (see the last 2 bean definitions below).

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config/>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="MovieQualifier">
            <attribute key="format" value="VHS"/>
            <attribute key="genre" value="Action"/>
        </qualifier>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <qualifier type="MovieQualifier">
            <attribute key="format" value="VHS"/>
            <attribute key="genre" value="Comedy"/>
        </qualifier>
        <!-- inject any dependencies required by this bean -->
    </bean>

    <bean class="example.SimpleMovieCatalog">
        <meta key="format" value="DVD"/>
        <meta key="genre" value="Action"/>
        <!-- inject any dependencies required by this bean -->
    </bean>
    
    <bean class="example.SimpleMovieCatalog">
        <meta key="format" value="BLURAY"/>
        <meta key="genre" value="Comedy"/>
        <!-- inject any dependencies required by this bean -->
    </bean>

</beans>

4.11.4 CustomAutowireConfigurer

The CustomAutowireConfigurer is a BeanFactoryPostProcessor that enables further customization of the autowiring process. Specifically, it allows you to register your own custom qualifier annotation types even if they are not themselves annotated with Spring's @Qualifier annotation.

<bean id="customAutowireConfigurer" class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer">
    <property name="customQualifierTypes">
        <set>
            <value>example.CustomQualifier</value>
        </set>
    </property>
</bean>

Note that the particular implementation of AutowireCandidateResolver that will be activated for the application context depends upon the Java version. If running on less than Java 5, the qualifier annotations are not supported, and therefore autowire candidates are solely determined by the 'autowire-candidate' value of each bean definition as well as any 'default-autowire-candidates' pattern(s) available on the <beans/> element. If running on Java 5 or greater, the presence of @Qualifier annotations or any custom annotations registered with the CustomAutowireConfigurer will also play a role.

Regardless of the Java version, the determination of a "primary" candidate (when multiple beans qualify as autowire candidates) is the same: if exactly one bean definition among the candidates has a 'primary' attribute set to 'true', it will be selected.

4.11.5 @Resource

Spring also supports injection using the JSR-250 @Resource annotation on fields or bean property setter methods. This is a common pattern found in Java EE 5 and Java 6 (e.g. in JSF 1.2 managed beans or JAX-WS 2.0 endpoints), which Spring supports for Spring-managed objects as well.

@Resource takes a 'name' attribute, and by default Spring will interpret that value as the bean name to be injected. In other words, it follows by-name semantics as demonstrated in this example:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Resource(name="myMovieFinder")
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}

If no name is specified explicitly, then the default name will be derived from the name of the field or setter method: In case of a field, it will simply be equivalent to the field name; in case of a setter method, it will be equivalent to the bean property name. So the following example is going to have the bean with name "movieFinder" injected into its setter method:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Resource
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}
[Note]Note

The name provided with the annotation will be resolved as a bean name by the BeanFactory of which the CommonAnnotationBeanPostProcessor is aware. Note that the names may be resolved via JNDI if Spring's SimpleJndiBeanFactory is configured explicitly. However, it is recommended to rely on the default behavior and simply use Spring's JNDI lookup capabilities to preserve the level of indirection.

Similar to @Autowired, @Resource may fall back to standard bean type matches (i.e. find a primary type match instead of a specific named bean) as well as resolve well-known "resolvable dependencies": the BeanFactory interface, the ApplicationContext interface, the ResourceLoader interface, the ApplicationEventPublisher interface and the MessageSource interface. Note that this only applies to @Resource usage with no explicit name specified!

So the following example will have its customerPreferenceDao field looking for a bean with name "customerPreferenceDao" first, then falling back to a primary type match for the type CustomerPreferenceDao. The "context" field will simply be injected based on the known resolvable dependency type ApplicationContext.

public class MovieRecommender {

    @Resource
    private CustomerPreferenceDao customerPreferenceDao;

    @Resource
    private ApplicationContext context;

    public MovieRecommender() {
    }

    // ...
}

4.11.6 @PostConstruct and @PreDestroy

The CommonAnnotationBeanPostProcessor not only recognizes the @Resource annotation but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support for these annotations offers yet another alternative to those described in the sections on initialization callbacks and destruction callbacks. Provided that the CommonAnnotationBeanPostProcessor is registered within the Spring ApplicationContext, a method carrying one of these annotations will be invoked at the same point in the lifecycle as the corresponding Spring lifecycle interface's method or explicitly declared callback method. In the example below, the cache will be pre-populated upon initialization and cleared upon destruction.

public class CachingMovieLister {

    @PostConstruct
    public void populateMovieCache() {
        // populates the movie cache upon initialization...
    }
    
    @PreDestroy
    public void clearMovieCache() {
        // clears the movie cache upon destruction...
    }
}
[Note]Note

For details regarding the effects of combining various lifecycle mechanisms, see Section 4.5.1.4, “Combining lifecycle mechanisms”.

4.12 Classpath scanning, managed components and writing configurations using Java

Thus far most of the examples within this chapter have used XML for specifying the configuration metadata that produces each BeanDefinition within the Spring container. The previous section (Section 4.11, “Annotation-based configuration”) demonstrated the possibility of providing a considerable amount of the configuration metadata using source-level annotations. Even in those examples however, the "base" bean definitions were explicitly defined in the XML file while the annotations were driving the dependency injection only. The current section introduces an option for implicitly detecting the candidate components by scanning the classpath and matching against filters.

[Note]Note

Starting with Spring 3.0 many of the features provided by the Spring JavaConfig project have been added to the core Spring Framework. This allows you to define beans using Java rather than using the traditional XML files. Take a look at the @Configuration, @Bean, @Import and @DependsOn annotations for how to use these new features.

4.12.1 @Component and further stereotype annotations

Beginning with Spring 2.0, the @Repository annotation was introduced as a marker for any class that fulfills the role or stereotype of a repository (a.k.a. Data Access Object or DAO). Among the possibilities for leveraging such a marker is the automatic translation of exceptions as described in Section 14.6.4, “Exception Translation”.

Spring 2.5 introduces further stereotype annotations: @Component, @Service and @Controller. @Component serves as a generic stereotype for any Spring-managed component; whereas, @Repository, @Service, and @Controller serve as specializations of @Component for more specific use cases (e.g., in the persistence, service, and presentation layers, respectively). What this means is that you can annotate your component classes with @Component, but by annotating them with @Repository, @Service, or @Controller instead, your classes are more properly suited for processing by tools or associating with aspects. For example, these stereotype annotations make ideal targets for pointcuts. Of course, it is also possible that @Repository, @Service, and @Controller may carry additional semantics in future releases of the Spring Framework. Thus, if you are making a decision between using @Component or @Service for your service layer, @Service is clearly the better choice. Similarly, as stated above, @Repository is already supported as a marker for automatic exception translation in your persistence layer.

4.12.2 Auto-detecting components

Spring provides the capability of automatically detecting 'stereotyped' classes and registering corresponding BeanDefinitions with the ApplicationContext. For example, the following two classes are eligible for such autodetection:

@Service
public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}
@Repository
public class JpaMovieFinder implements MovieFinder {
    // implementation elided for clarity
}

To autodetect these classes and register the corresponding beans requires the inclusion of the following element in XML where 'basePackage' would be a common parent package for the two classes (or alternatively a comma-separated list could be specified that included the parent package of each class).

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.0.xsd">
               
     <context:component-scan base-package="org.example"/>
     
</beans>
[Note]Note

Note that the scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When building jars with Ant, make sure to not activate the files-only switch of the jar task!

Furthermore, the AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor are both included implicitly when using the component-scan element. That means that the two components are autodetected and wired together - all without any bean configuration metadata provided in XML.

[Note]Note

The registration of those post-processors can be disabled by including the annotation-config attribute with a value of 'false'.

4.12.3 Using filters to customize scanning

By default, classes annotated with @Component, @Repository, @Service, or @Controller (or classes annotated with a custom annotation that itself is annotated with @Component) are the only detected candidate components. However it is simple to modify and extend this behavior by applying custom filters. These can be added as either include-filter or exclude-filter sub-elements of the 'component-scan' element. Each filter element requires the 'type' and 'expression' attributes. Five filtering options exist as described below.

Table 4.7. Filter Types

Filter TypeExample ExpressionDescription
annotationorg.example.SomeAnnotationAn annotation to be present at the type level in target components.
assignableorg.example.SomeClassA class (or interface) that the target components are assignable to (extend/implement).
aspectjorg.example..*Service+An AspectJ type expression to be matched by the target components.
regexorg\.example\.Default.*A regex expression to be matched by the target components' class names.
customorg.example.MyCustomTypeFilterA custom implementation of the org.springframework.core.type.TypeFilter interface.

Find below an example of the XML configuration for ignoring all @Repository annotations and using "stub" repositories instead.

<beans ...>

     <context:component-scan base-package="org.example">
        <context:include-filter type="regex" expression=".*Stub.*Repository"/>
        <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Repository"/>
     </context:component-scan>

</beans>
[Note]Note

It is also possible to disable the default filters by providing use-default-filters="false" as an attribute of the <component-scan/> element. This will in effect disable automatic detection of classes annotated with @Component, @Repository, @Service, or @Controller.

4.12.4 Using the @Configuration annotation

The central artifact in Spring's new Java-configuration support is the @Configuration-annotated class. These classes consist principally of @Bean-annotated methods that define instantiation, configuration, and initialization logic for objects that will be managed by the Spring IoC container.

Annotating a class with the @Configuration indicates that the class may be used by the Spring IoC container as a source of bean definitions. The simplest possible @Configuration class would read as follows:

@Configuration
public class AppConfig {

}            

An application may make use of one @Configuration-annotated class, or many. @Configuration is meta-annotated as a @Component, therefore Configuration-classes are candidates for component-scanning and may also take advantage of @Autowired annotations at the field and method level but not at the constructor level. Configuration-classes must also have a default constructor. Externalized values may be wired into Configuration-classes using the @Value annotation.

4.12.5 Using the @Bean annotation

@Bean is a method-level annotation and a direct analog of the XML <bean/> element. The annotation supports some of the attributes offered by <bean/>, such as: init-method, destroy-method, autowiring and name.

You can use the @Bean annotation in a Configuraton-class or in a Component-class.

4.12.5.1 Declaring a bean

To declare a bean, simply annotate a method with the @Bean annotation. Such a method will be used to register a bean definition within a BeanFactory of the type specified as the methods return value. By default, the bean name will be the same as the method name (see bean naming for details on how to customize this behavior). The following is a simple example of a @Bean method declaration:

@Configuration
public class AppConfig {

    @Bean
    public TransferService transferService() {
        return new TransferServiceImpl();
    }

}                

For comparison sake, the configuration above is exactly equivalent to the following Spring XML:

<beans>
    <bean name="transferService" class="com.acme.TransferServiceImpl"/>
</beans>                

Both will result in a bean named transferService being available in the BeanFactory or ApplicationContext, bound to an object instance of type TransferServiceImpl:

transferService -> com.acme.TransferServiceImpl
                

4.12.5.2 Injecting dependencies

When @Beans have dependencies on one another, expressing that dependency is as simple as having one bean method call another:

@Configuration
public class AppConfig {

    @Bean
    public Foo foo() {
        return new Foo(bar());
    }

    @Bean
    public Bar bar() {
        return new Bar();
    }

}                

In the example above, the foo bean recevies a reference to bar via constructor injection.

4.12.5.3 Receiving lifecycle callbacks

Beans created in a Configuration-class supports the regular lifecycle callbacks. Any classes defined with the @Bean annotation can use the @PostConstruct and @PreDestroy annotations from JSR-250, see the section on JSR-250 annotations for further details.

The regular Spring lifecycle callbacks are fully supported as well. If a bean implements InitializingBean, DisposableBean, or Lifecycle, their respective methods will be called by the container.

The standard set of *Aware interfaces such as BeanFactoryAware, BeanNameAware, MessageSourceAware, ApplicationContextAware, etc. are also fully supported.

The @Bean annotation supports specifying arbitrary initialization and destruction callback methods, much like Spring XML's init-method and destroy-method attributes to the bean element:

public class Foo {
    public void init() {
        // initialization logic
    }
}

public class Bar {
    public void cleanup() {
        // destruction logic
    }
}

@Configuration
public class AppConfig {
    @Bean(initMethodName = "init")
    public Foo foo() {
        return new Foo();
    }
    @Bean(destroyMethodName="cleanup")
    public Bar bar() {
        return new Bar();
    }
}

Of course, in the case of Foo above, it would be equally as valid to call the init() method directly during construction:

@Configuration
public class AppConfig {
    @Bean
    public Foo foo() {
        Foo foo = new Foo();
        foo.init();
        return foo;
    }

    // ...
}                    
[Tip]Tip

Remember that because you are working directly in Java, you can do anything you like with your objects, and do not always need to rely on the container!

4.12.5.4 Specifying bean scope

Using the @Scope annotation

You can specify that your beans defined with the @Bean annotation should have a specific scope. You can use any of the standard scopes specified in the Bean Scopes section.

The StandardScopes class provides string constants for each of these four scopes. SINGLETON is the default, and can be overridden by using the @Scope annotation:

@Configuration
public class MyConfiguration {
    @Bean
    @Scope(StandardScopes.PROTOTYPE)
    public Encryptor encryptor() {
        // ...
    }
}                 
@Scope and scoped-proxy

Spring offers a convenient way of working with scoped dependencies through scoped proxies. The easiest way to create such a proxy when using the XML configuration is the <aop:scoped-proxy/> element. Configuring your beans in Java with a @Scope annotation offers equivalent support with the proxyMode attribute. The default is no proxy (ScopedProxyMode.NO) but you can specify ScopedProxyMode.TARGET_CLASS or ScopedProxyMode.INTERFACES.

If we were to port the the XML reference documentation scoped proxy example (see link above) to our @Bean using Java, it would look like the following:

// a HTTP Session-scoped bean exposed as a proxy
@Bean
@Scope(value = StandardScopes.SESSION, proxyMode = ScopedProxyMode.TARGET_CLASS)
public UserPreferences userPreferences() {
   return new UserPreferences();
}

@Bean
public Service userService() {
   UserService service = new SimpleUserService();
   // a reference to the proxied 'userPreferences' bean
   service.seUserPreferences(userPreferences());
   return service;
}                
Lookup method injection

As noted earlier, lookup method injection is an advanced feature that should be comparatively rarely used. It is useful in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this type of configuration provides a natural means for implementing this pattern.

public abstract class CommandManager { 
    public Object process(Object commandState) { 
        // grab a new instance of the appropriate Command interface 
        Command command = createCommand(); 

        // set the state on the (hopefully brand new) Command instance 
        command.setState(commandState); 
        return command.execute(); 
    } 

    // okay... but where is the implementation of this method? 
    protected abstract Command createCommand(); 
}                   

Using Java-configurtion support we can easily create a subclass of CommandManager where the abstract createCommand() is overridden in such a way that it 'looks up' a brand new (prototype) command object:

@Bean
@Scope(StandardScopes.PROTOTYPE)
public AsyncCommand asyncCommand() {
    AsyncCommand command = new AsyncCommand();
    // inject dependencies here as required
    return command;
}

@Bean
public CommandManager commandManager() {
    // return new anonymous implementation of CommandManager with command() overridden
    // to return a new prototype Command object
    return new CommandManager() {
        protected Command command() {
            return asyncCommand();
        }
    }
}                    

4.12.5.5 Customizing bean naming

By default, Configuration-classes uses a @Bean method's name as the name of the resulting bean. This functionality can be overridden, however, using the name attribute.

@Configuration
public class AppConfig {

    @Bean(name = "bar")
    public Foo foo() {
        return new Foo();
    }

}        

4.12.6 Defining bean metadata within components

Spring components can also contribute bean definition metadata to the container. This is done with the same @Bean annotation used to define bean metadata within @Configuration annotated classes. Here is a simple example

@Component
public class FactoryMethodComponent {

  @Bean @Qualifier("public")
  public TestBean publicInstance() {
    return new TestBean("publicInstance");
  }

  public void DoWork() 
  {
    // Component method implementation omitted
  }
}

This class is a Spring component and has application specific code contained in its DoWork method. However, it also contributes a bean definition that has a factory method referring to the method publicInstance. The @Bean annotation identifies the factory method and also other bean definition properties, such as a qualifier value via the @Qualifier annotation. Other method level annotations that can be specified are @Scope, @Lazy, and custom qualifier annotations. Autowired fields and methods are supported as before with the additional support for autowiring of @Bean methods, as shown in the example below

@Component
public class FactoryMethodComponent {

 private static int i;

 @Bean @Qualifier("public")
 public TestBean publicInstance() { 
  return new TestBean("publicInstance");
 }

 // use of a custom qualifier and autowiring of method parameters

 @Bean @BeanAge(1)
 protected TestBean protectedInstance(@Qualifier("public") TestBean spouse, @Value("#{privateInstance.age}") String country) {
  TestBean tb = new TestBean("protectedInstance", 1);
  tb.setSpouse(tb);
  tb.setCountry(country);
  return tb;
 }

 @Bean @Scope(StandardScopes.PROTOTYPE)
 private TestBean privateInstance() {
  return new TestBean("privateInstance", i++);
 }

 @Bean @Scope(value = StandardScopes.SESSION, proxyMode = ScopedProxyMode.TARGET_CLASS)
 public TestBean requestScopedInstance() {
  return new TestBean("requestScopedInstance", 3);
 }
}

Note the use of autowiring of the String method parameter country to the value of the Age property on another bean named privateInstance. A Spring Expression Language element is used to define the value of the property via the notation #{ <expression> }. For @Value annotations, an expression resolver is preconfigured to look for bean names when resolving expression text.

The @Bean methods in a Spring component are processed differently than their counterparts inside a Spring @Configuration class. The difference is that @Component classes are not enhanced with CGLIB to intercept the invocation of methods and fields. CGLIB proxying is the means by which invoking methods or fields within @Configuration classes' @Bean methods create bean metadata references to collaborating objects and do not invoke the method with normal Java semantics. In contrast, calling a method or field within a @Component classes' @Bean method has standard Java semantics.

4.12.7 Naming autodetected components

When a component is autodetected as part of the scanning process, its bean name will be generated by the BeanNameGenerator strategy known to that scanner. By default, any Spring 'stereotype' annotation (@Component, @Repository, @Service, and @Controller) that contains a name value will thereby provide that name to the corresponding bean definition. If such an annotation contains no name value or for any other detected component (such as those discovered due to custom filters), the default bean name generator will return the uncapitalized non-qualified class name. For example, if the following two components were detected, the names would be 'myMovieLister' and 'movieFinderImpl':

@Service("myMovieLister")
public class SimpleMovieLister {
    // ...
}
@Repository
public class MovieFinderImpl implements MovieFinder {
    // ...
}
[Note]Note

If you don't want to rely on the default bean-naming strategy, you may provide a custom bean-naming strategy. First, implement the BeanNameGenerator interface, and be sure to include a default no-arg constructor. Then, provide the fully-qualified class name when configuring the scanner:

<beans ...>
               
     <context:component-scan base-package="org.example"
                             name-generator="org.example.MyNameGenerator" />

</beans>

As a general rule, consider specifying the name with the annotation whenever other components may be making explicit references to it. On the other hand, the auto-generated names are adequate whenever the container is responsible for wiring.

4.12.8 Providing a scope for autodetected components

As with Spring-managed components in general, the default and by far most common scope is 'singleton'. However, there are times when other scopes are needed. Therefore Spring 2.5 introduces a new @Scope annotation as well. Simply provide the name of the scope within the annotation, such as:

@Scope(StandardScopes.PROTOTYPE)
@Repository
public class MovieFinderImpl implements MovieFinder {
    // ...
}
[Note]Note

If you would like to provide a custom strategy for scope resolution rather than relying on the annotation-based approach, implement the ScopeMetadataResolver interface, and be sure to include a default no-arg constructor. Then, provide the fully-qualified class name when configuring the scanner:

<beans ...>
               
     <context:component-scan base-package="org.example"
                             scope-resolver="org.example.MyScopeResolver" />
     
</beans>

When using certain non-singleton scopes, it may be necessary to generate proxies for the scoped objects. The reasoning is described in detail within the section entitled Section 4.4.4.5, “Scoped beans as dependencies”. For this purpose, a scoped-proxy attribute is available on the 'component-scan' element. The three possible values are: 'no', 'interfaces', and 'targetClass'. For example, the following configuration will result in standard JDK dynamic proxies:

<beans ...>
               
     <context:component-scan base-package="org.example"
                             scoped-proxy="interfaces" />
     
</beans>

4.12.9 Providing qualifier metadata with annotations

The @Qualifier annotation was introduced in the section above entitled Section 4.11.3, “Fine-tuning annotation-based autowiring with qualifiers”. The examples in that section demonstrated use of the @Qualifier annotation as well as custom qualifier annotations to provide fine-grained control when resolving autowire candidates. Since those examples were based on XML bean definitions, the qualifier metadata was provided on the candidate bean definitions using the 'qualifier' or 'meta' sub-elements of the 'bean' element in the XML. When relying upon classpath scanning for autodetection of components, then the qualifier metadata may be provided with type-level annotations on the candidate class. The following three examples demonstrate this technique.

@Component
@Qualifier("Action")
public class ActionMovieCatalog implements MovieCatalog {
    // ...
}
@Component
@Genre("Action")
public class ActionMovieCatalog implements MovieCatalog {
    // ...
}
@Component
@Offline
public class CachingMovieCatalog implements MovieCatalog {
    // ...
}
[Note]Note

As with most of the annotation-based alternatives, keep in mind that the annotation metadata is bound to the class definition itself, while the use of XML allows for multiple beans of the same type to provide variations in their qualifier metadata since that metadata is provided per-instance rather than per-class.

4.13 Registering a LoadTimeWeaver

The context namespace introduced in Spring 2.5 provides a load-time-weaver element.

<beans ...>
     
     <context:load-time-weaver/>
     
</beans>

Adding this element to an XML-based Spring configuration file activates a Spring LoadTimeWeaver for the ApplicationContext. Any bean within that ApplicationContext may implement LoadTimeWeaverAware thereby receiving a reference to the load-time weaver instance. This is particularly useful in combination with Spring's JPA support where load-time weaving may be necessary for JPA class transformation. Consult the LocalContainerEntityManagerFactoryBean Javadoc for more detail. For more on AspectJ load-time weaving, see Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”.



[1] See the section entitled Background

5. Resources

5.1 Introduction

Java's standard java.net.URL class and standard handlers for various URL prefixes unfortunately are not quite adequate enough for all access to low-level resources. For example, there is no standardized URL implementation that may be used to access a resource that needs to be obtained from the classpath, or relative to a ServletContext. While it is possible to register new handlers for specialized URL prefixes (similar to existing handlers for prefixes such as http:), this is generally quite complicated, and the URL interface still lacks some desirable functionality, such as a method to check for the existence of the resource being pointed to.

5.2 The Resource interface

Spring's Resource interface is meant to be a more capable interface for abstracting access to low-level resources.

public interface Resource extends InputStreamSource {

    boolean exists();

    boolean isOpen();

    URL getURL() throws IOException;

    File getFile() throws IOException;

    Resource createRelative(String relativePath) throws IOException;

    String getFilename();

    String getDescription();
}
public interface InputStreamSource {

    InputStream getInputStream() throws IOException;
}

Some of the most important methods from the Resource interface are:

  • getInputStream(): locates and opens the resource, returning an InputStream for reading from the resource. It is expected that each invocation returns a fresh InputStream. It is the responsibility of the caller to close the stream.

  • exists(): returns a boolean indicating whether this resource actually exists in physical form.

  • isOpen(): returns a boolean indicating whether this resource represents a handle with an open stream. If true, the InputStream cannot be read multiple times, and must be read once only and then closed to avoid resource leaks. Will be false for all usual resource implementations, with the exception of InputStreamResource.

  • getDescription(): returns a description for this resource, to be used for error output when working with the resource. This is often the fully qualified file name or the actual URL of the resource.

Other methods allow you to obtain an actual URL or File object representing the resource (if the underlying implementation is compatible, and supports that functionality).

The Resource abstraction is used extensively in Spring itself, as an argument type in many method signatures when a resource is needed. Other methods in some Spring APIs (such as the constructors to various ApplicationContext implementations), take a String which in unadorned or simple form is used to create a Resource appropriate to that context implementation, or via special prefixes on the String path, allow the caller to specify that a specific Resource implementation must be created and used.

While the Resource interface is used a lot with Spring and by Spring, it's actually very useful to use as a general utility class by itself in your own code, for access to resources, even when your code doesn't know or care about any other parts of Spring. While this couples your code to Spring, it really only couples it to this small set of utility classes, which are serving as a more capable replacement for URL, and can be considered equivalent to any other library you would use for this purpose.

It is important to note that the Resource abstraction does not replace functionality: it wraps it where possible. For example, a UrlResource wraps a URL, and uses the wrapped URL to do its work.

5.3 Built-in Resource implementations

There are a number of Resource implementations that come supplied straight out of the box in Spring:

5.3.1 UrlResource

The UrlResource wraps a java.net.URL, and may be used to access any object that is normally accessible via a URL, such as files, an HTTP target, an FTP target, etc. All URLs have a standardized String representation, such that appropriate standardized prefixes are used to indicate one URL type from another. This includes file: for accessing filesystem paths, http: for accessing resources via the HTTP protocol, ftp: for accessing resources via FTP, etc.

A UrlResource is created by Java code explicitly using the UrlResource constructor, but will often be created implicitly when you call an API method which takes a String argument which is meant to represent a path. For the latter case, a JavaBeans PropertyEditor will ultimately decide which type of Resource to create. If the path string contains a few well-known (to it, that is) prefixes such as classpath:, it will create an appropriate specialized Resource for that prefix. However, if it doesn't recognize the prefix, it will assume the this is just a standard URL string, and will create a UrlResource.

5.3.2 ClassPathResource

This class represents a resource which should be obtained from the classpath. This uses either the thread context class loader, a given class loader, or a given class for loading resources.

This Resource implementation supports resolution as java.io.File if the class path resource resides in the file system, but not for classpath resources which reside in a jar and have not been expanded (by the servlet engine, or whatever the environment is) to the filesystem. To address this the various Resource implementations always support resolution as a java.net.URL.

A ClassPathResource is created by Java code explicitly using the ClassPathResource constructor, but will often be created implicitly when you call an API method which takes a String argument which is meant to represent a path. For the latter case, a JavaBeans PropertyEditor will recognize the special prefix classpath:on the string path, and create a ClassPathResource in that case.

5.3.3 FileSystemResource

This is a Resource implementation for java.io.File handles. It obviously supports resolution as a File, and as a URL.

5.3.4 ServletContextResource

This is a Resource implementation for ServletContext resources, interpreting relative paths within the relevant web application's root directory.

This always supports stream access and URL access, but only allows java.io.File access when the web application archive is expanded and the resource is physically on the filesystem. Whether or not it's expanded and on the filesystem like this, or accessed directly from the JAR or somewhere else like a DB (it's conceivable) is actually dependent on the Servlet container.

5.3.5 InputStreamResource

A Resource implementation for a given InputStream. This should only be used if no specific Resource implementation is applicable. In particular, prefer ByteArrayResource or any of the file-based Resource implementations where possible.

In contrast to other Resource implementations, this is a descriptor for an already opened resource - therefore returning true from isOpen(). Do not use it if you need to keep the resource descriptor somewhere, or if you need to read a stream multiple times.

5.3.6 ByteArrayResource

This is a Resource implementation for a given byte array. It creates a ByteArrayInputStream for the given byte array.

It's useful for loading content from any given byte array, without having to resort to a single-use InputStreamResource.

5.4 The ResourceLoader

The ResourceLoader interface is meant to be implemented by objects that can return (i.e. load) Resource instances.

public interface ResourceLoader {
    Resource getResource(String location);
}

All application contexts implement the ResourceLoader interface, and therefore all application contexts may be used to obtain Resource instances.

When you call getResource() on a specific application context, and the location path specified doesn't have a specific prefix, you will get back a Resource type that is appropriate to that particular application context. For example, assume the following snippet of code was executed against a ClassPathXmlApplicationContext instance:

Resource template = ctx.getResource("some/resource/path/myTemplate.txt);

What would be returned would be a ClassPathResource; if the same method was executed against a FileSystemXmlApplicationContext instance, you'd get back a FileSystemResource. For a WebApplicationContext, you'd get back a ServletContextResource, and so on.

As such, you can load resources in a fashion appropriate to the particular application context.

On the other hand, you may also force ClassPathResource to be used, regardless of the application context type, by specifying the special classpath: prefix:

Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt);

Similarly, one can force a UrlResource to be used by specifying any of the standard java.net.URL prefixes:

Resource template = ctx.getResource("file:/some/resource/path/myTemplate.txt);
Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt);

The following table summarizes the strategy for converting Strings to Resources:

Table 5.1. Resource strings

PrefixExampleExplanation

classpath:

classpath:com/myapp/config.xml

Loaded from the classpath.

file:

file:/data/config.xml

Loaded as a URL, from the filesystem. [1]

http:

http://myserver/logo.png

Loaded as a URL.

(none)

/data/config.xml

Depends on the underlying ApplicationContext.

[1] But see also the section entitled Section 5.7.3, “FileSystemResource caveats”.


5.5 The ResourceLoaderAware interface

The ResourceLoaderAware interface is a special marker interface, identifying objects that expect to be provided with a ResourceLoader reference.

public interface ResourceLoaderAware {

   void setResourceLoader(ResourceLoader resourceLoader);
}

When a class implements ResourceLoaderAware and is deployed into an application context (as a Spring-managed bean), it is recognized as ResourceLoaderAware by the application context. The application context will then invoke the setResourceLoader(ResourceLoader), supplying itself as the argument (remember, all application contexts in Spring implement the ResourceLoader interface).

Of course, since an ApplicationContext is a ResourceLoader, the bean could also implement the ApplicationContextAware interface and use the supplied application context directly to load resources, but in general, it's better to use the specialized ResourceLoader interface if that's all that's needed. The code would just be coupled to the resource loading interface, which can be considered a utility interface, and not the whole Spring ApplicationContext interface.

As of Spring 2.5, you can rely upon autowiring of the ResourceLoader as an alternative to implementing the ResourceLoaderAware interface. The "traditional" constructor and byType autowiring modes (as described in the section entitled Section 4.3.5, “Autowiring collaborators”) are now capable of providing a dependency of type ResourceLoader for either a constructor argument or setter method parameter respectively. For more flexibility (including the ability to autowire fields and multiple parameter methods), consider using the new annotation-based autowiring features. In that case, the ResourceLoader will be autowired into a field, constructor argument, or method parameter that is expecting the ResourceLoader type as long as the field, constructor, or method in question carries the @Autowired annotation. For more information, see the section entitled Section 4.11.2, “@Autowired”.

5.6 Resources as dependencies

If the bean itself is going to determine and supply the resource path through some sort of dynamic process, it probably makes sense for the bean to use the ResourceLoader interface to load resources. Consider as an example the loading of a template of some sort, where the specific resource that is needed depends on the role of the user. If the resources are static, it makes sense to eliminate the use of the ResourceLoader interface completely, and just have the bean expose the Resource properties it needs, and expect that they will be injected into it.

What makes it trivial to then inject these properties, is that all application contexts register and use a special JavaBeans PropertyEditor which can convert String paths to Resource objects. So if myBean has a template property of type Resource, it can be configured with a simple string for that resource, as follows:

<bean id="myBean" class="...">
  <property name="template" value="some/resource/path/myTemplate.txt"/>
</bean>

Note that the resource path has no prefix, so because the application context itself is going to be used as the ResourceLoader, the resource itself will be loaded via a ClassPathResource, FileSystemResource, or ServletContextResource (as appropriate) depending on the exact type of the context.

If there is a need to force a specific Resource type to be used, then a prefix may be used. The following two examples show how to force a ClassPathResource and a UrlResource (the latter being used to access a filesystem file).

<property name="template" value="classpath:some/resource/path/myTemplate.txt">
<property name="template" value="file:/some/resource/path/myTemplate.txt"/>

5.7 Application contexts and Resource paths

5.7.1 Constructing application contexts

An application context constructor (for a specific application context type) generally takes a string or array of strings as the location path(s) of the resource(s) such as XML files that make up the definition of the context.

When such a location path doesn't have a prefix, the specific Resource type built from that path and used to load the bean definitions, depends on and is appropriate to the specific application context. For example, if you create a ClassPathXmlApplicationContext as follows:

ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");

The bean definitions will be loaded from the classpath, as a ClassPathResource will be used. But if you create a FileSystemXmlApplicationContext as follows:

ApplicationContext ctx =
    new FileSystemXmlApplicationContext("conf/appContext.xml");

The bean definition will be loaded from a filesystem location, in this case relative to the current working directory.

Note that the use of the special classpath prefix or a standard URL prefix on the location path will override the default type of Resource created to load the definition. So this FileSystemXmlApplicationContext...

ApplicationContext ctx =
    new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");

... will actually load its bean definitions from the classpath. However, it is still a FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any unprefixed paths will still be treated as filesystem paths.

5.7.1.1 Constructing ClassPathXmlApplicationContext instances - shortcuts

The ClassPathXmlApplicationContext exposes a number of constructors to enable convenient instantiation. The basic idea is that one supplies merely a string array containing just the filenames of the XML files themselves (without the leading path information), and one also supplies a Class; the ClassPathXmlApplicationContext will derive the path information from the supplied class.

An example will hopefully make this clear. Consider a directory layout that looks like this:

com/
  foo/
    services.xml
    daos.xml
    MessengerService.class

A ClassPathXmlApplicationContext instance composed of the beans defined in the 'services.xml' and 'daos.xml' could be instantiated like so...

ApplicationContext ctx = new ClassPathXmlApplicationContext(
    new String[] {"services.xml", "daos.xml"}, MessengerService.class);

Please do consult the Javadocs for the ClassPathXmlApplicationContext class for details of the various constructors.

5.7.2 Wildcards in application context constructor resource paths

The resource paths in application context constructor values may be a simple path (as shown above) which has a one-to-one mapping to a target Resource, or alternately may contain the special "classpath*:" prefix and/or internal Ant-style regular expressions (matched using Spring's PathMatcher utility). Both of the latter are effectively wildcards

One use for this mechanism is when doing component-style application assembly. All components can 'publish' context definition fragments to a well-known location path, and when the final application context is created using the same path prefixed via classpath*:, all component fragments will be picked up automatically.

Note that this wildcarding is specific to use of resource paths in application context constructors (or when using the PathMatcher utility class hierarchy directly), and is resolved at construction time. It has nothing to do with the Resource type itself. It's not possible to use the classpath*: prefix to construct an actual Resource, as a resource points to just one resource at a time.

5.7.2.1 Ant-style Patterns

When the path location contains an Ant-style pattern, for example:

     /WEB-INF/*-context.xml
     com/mycompany/**/applicationContext.xml
     file:C:/some/path/*-context.xml
     classpath:com/mycompany/**/applicationContext.xml

... the resolver follows a more complex but defined procedure to try to resolve the wildcard. It produces a Resource for the path up to the last non-wildcard segment and obtains a URL from it. If this URL is not a "jar:" URL or container-specific variant (e.g. "zip:" in WebLogic, "wsjar" in WebSphere, etc.), then a java.io.File is obtained from it and used to resolve the wildcard by traversing the filesystem. In the case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or manually parses the jar URL and then traverses the contents of the jar file to resolve the wildcards.

Implications on portability

If the specified path is already a file URL (either explicitly, or implicitly because the base ResourceLoader is a filesystem one, then wildcarding is guaranteed to work in a completely portable fashion.

If the specified path is a classpath location, then the resolver must obtain the last non-wildcard path segment URL via a Classloader.getResource() call. Since this is just a node of the path (not the file at the end) it is actually undefined (in the ClassLoader Javadocs) exactly what sort of a URL is returned in this case. In practice, it is always a java.io.File representing the directory, where the classpath resource resolves to a filesystem location, or a jar URL of some sort, where the classpath resource resolves to a jar location. Still, there is a portability concern on this operation.

If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to get a java.net.JarURLConnection from it, or manually parse the jar URL, to be able to walk the contents of the jar, and resolve the wildcard. This will work in most environments, but will fail in others, and it is strongly recommended that the wildcard resolution of resources coming from jars be thoroughly tested in your specific environment before you rely on it.

5.7.2.2 The classpath*: prefix

When constructing an XML-based application context, a location string may use the special classpath*: prefix:

ApplicationContext ctx =
    new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");

This special prefix specifies that all classpath resources that match the given name must be obtained (internally, this essentially happens via a ClassLoader.getResources(...) call), and then merged to form the final application context definition.

[Note]Classpath*: portability

The wildcard classpath relies on the getResources() method of the underlying classloader. As most application servers nowadays supply their own classloader implementation, the behavior might differ especially when dealing with jar files. A simple test to check if classpath* works is to use the classloader to load a file from within a jar on the classpath: getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try this test with files that have the same name but are placed inside two different locations. In case an inappropriate result is returned, check the application server documentation for settings that might affect the classloader behavior.

The "classpath*:" prefix can also be combined with a PathMatcher pattern in the rest of the location path, for example "classpath*:META-INF/*-beans.xml". In this case, the resolution strategy is fairly simple: a ClassLoader.getResources() call is used on the last non-wildcard path segment to get all the matching resources in the class loader hierarchy, and then off each resource the same PathMatcher resoltion strategy described above is used for the wildcard subpath.

5.7.2.3 Other notes relating to wildcards

Please note that "classpath*:" when combined with Ant-style patterns will only work reliably with at least one root directory before the pattern starts, unless the actual target files reside in the file system. This means that a pattern like "classpath*:*.xml" will not retrieve files from the root of jar files but rather only from the root of expanded directories. This originates from a limitation in the JDK's ClassLoader.getResources() method which only returns file system locations for a passed-in empty string (indicating potential roots to search).

Ant-style patterns with "classpath:" resources are not guaranteed to find matching resources if the root package to search is available in multiple class path locations. This is because a resource such as

    com/mycompany/package1/service-context.xml

may be in only one location, but when a path such as

    classpath:com/mycompany/**/service-context.xml

is used to try to resolve it, the resolver will work off the (first) URL returned by getResource("com/mycompany");. If this base package node exists in multiple classloader locations, the actual end resource may not be underneath. Therefore, preferably, use "classpath*:" with the same Ant-style pattern in such a case, which will search all class path locations that contain the root package.

5.7.3 FileSystemResource caveats

A FileSystemResource that is not attached to a FileSystemApplicationContext (that is, a FileSystemApplicationContext is not the actual ResourceLoader) will treat absolute vs. relative paths as you would expect. Relative paths are relative to the current working directory, while absolute paths are relative to the root of the filesystem.

For backwards compatibility (historical) reasons however, this changes when the FileSystemApplicationContext is the ResourceLoader. The FileSystemApplicationContext simply forces all attached FileSystemResource instances to treat all location paths as relative, whether they start with a leading slash or not. In practice, this means the following are equivalent:

ApplicationContext ctx =
    new FileSystemXmlApplicationContext("conf/context.xml");
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("/conf/context.xml");

As are the following: (Even though it would make sense for them to be different, as one case is relative and the other absolute.)

FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");

In practice, if true absolute filesystem paths are needed, it is better to forgo the use of absolute paths with FileSystemResource / FileSystemXmlApplicationContext, and just force the use of a UrlResource, by using the file: URL prefix.

// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:/some/resource/path/myTemplate.txt");
// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
ApplicationContext ctx =
    new FileSystemXmlApplicationContext("file:/conf/context.xml");

6. Validation, Data-binding, the BeanWrapper, and PropertyEditors

6.1 Introduction

There are pros and cons for considering validation as business logic, and Spring offers a design for validation (and data binding) that does not exclude either one of them. Specifically validation should not be tied to the web tier, should be easy to localize and it should be possible to plug in any validator available. Considering the above, Spring has come up with a Validator interface that is both basic and eminently usable in every layer of an application.

Data binding is useful for allowing user input to be dynamically bound to the domain model of an application (or whatever objects you use to process user input). Spring provides the so-called DataBinder to do exactly that. The Validator and the DataBinder make up the validation package, which is primarily used in but not limited to the MVC framework.

The BeanWrapper is a fundamental concept in the Spring Framework and is used in a lot of places. However, you probably will not ever have the need to use the BeanWrapper directly. Because this is reference documentation however, we felt that some explanation might be in order. We're explaining the BeanWrapper in this chapter since if you were going to use it at all, you would probably do so when trying to bind data to objects, which is strongly related to the BeanWrapper.

Spring uses PropertyEditors all over the place. The concept of a PropertyEditor is part of the JavaBeans specification. Just as the BeanWrapper, it's best to explain the use of PropertyEditors in this chapter as well, since it's closely related to the BeanWrapper and the DataBinder.

6.2 Validation using Spring's Validator interface

Spring's features a Validator interface that you can use to validate objects. The Validator interface works using an Errors object so that while validating, validators can report validation failures to the Errors object.

Let's consider a small data object:

public class Person {

  private String name;
  private int age;

  // the usual getters and setters...
}

We're going to provide validation behavior for the Person class by implementing the following two methods of the org.springframework.validation.Validator interface:

  • supports(Class) - Can this Validator validate instances of the supplied Class?

  • validate(Object, org.springframework.validation.Errors) - validates the given object and in case of validation errors, registers those with the given Errors object

Implementing a Validator is fairly straightforward, especially when you know of the ValidationUtils helper class that the Spring Framework also provides.

public class PersonValidator implements Validator {
    
    /**
    * This Validator validates just Person instances
    */
    public boolean supports(Class clazz) {
        return Person.class.equals(clazz);
    }
    
    public void validate(Object obj, Errors e) {
        ValidationUtils.rejectIfEmpty(e, "name", "name.empty");
        Person p = (Person) obj;
        if (p.getAge() < 0) {
            e.rejectValue("age", "negativevalue");
        } else if (p.getAge() > 110) {
            e.rejectValue("age", "too.darn.old");
        }
    }
}

As you can see, the static rejectIfEmpty(..) method on the ValidationUtils class is used to reject the 'name' property if it is null or the empty string. Have a look at the Javadoc for the ValidationUtils class to see what functionality it provides besides the example shown previously.

While it is certainly possible to implement a single Validator class to validate each of the nested objects in a rich object, it may be better to encapsulate the validation logic for each nested class of object in its own Validator implementation. A simple example of a 'rich' object would be a Customer that is composed of two String properties (a first and second name) and a complex Address object. Address objects may be used independently of Customer objects, and so a distinct AddressValidator has been implemented. If you want your CustomerValidator to reuse the logic contained within the AddressValidator class without recourse to copy-n-paste you can dependency-inject or instantiate an AddressValidator within your CustomerValidator, and use it like so:

public class CustomerValidator implements Validator {

   private final Validator addressValidator;

   public CustomerValidator(Validator addressValidator) {
      if (addressValidator == null) {
          throw new IllegalArgumentException("The supplied [Validator] is required and must not be null.");
      }
      if (!addressValidator.supports(Address.class)) {
          throw new IllegalArgumentException(
            "The supplied [Validator] must support the validation of [Address] instances.");
      }
      this.addressValidator = addressValidator;
   }

    /**
    * This Validator validates Customer instances, and any subclasses of Customer too
    */
   public boolean supports(Class clazz) {
      return Customer.class.isAssignableFrom(clazz);
   }

   public void validate(Object target, Errors errors) {
      ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
      ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required");
      Customer customer = (Customer) target;
      try {
          errors.pushNestedPath("address");
          ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors);
      } finally {
          errors.popNestedPath();
      }
   }
}

Validation errors are reported to the Errors object passed to the validator. In case of Spring Web MVC you can use <spring:bind/> tag to inspect the error messages, but of course you can also inspect the errors object yourself. More information about the methods it offers can be found from the Javadoc.

6.3 Resolving codes to error messages

We've talked about databinding and validation. Outputting messages corresponding to validation errors is the last thing we need to discuss. In the example we've shown above, we rejected the name and the age field. If we're going to output the error messages by using a MessageSource, we will do so using the error code we've given when rejecting the field ('name' and 'age' in this case). When you call (either directly, or indirectly, using for example the ValidationUtils class) rejectValue or one of the other reject methods from the Errors interface, the underlying implementation will not only register the code you've passed in, but also a number of additional error codes. What error codes it registers is determined by the MessageCodesResolver that is used. By default, the DefaultMessageCodesResolver is used, which for example not only registers a message with the code you gave, but also messages that include the field name you passed to the reject method. So in case you reject a field using rejectValue("age", "too.darn.old"), apart from the too.darn.old code, Spring will also register too.darn.old.age and too.darn.old.age.int (so the first will include the field name and the second will include the type of the field); this is done as a convenience to aid developers in targeting error messages and suchlike.

More information on the MessageCodesResolver and the default strategy can be found online with the Javadocs for MessageCodesResolver and DefaultMessageCodesResolver respectively.

6.4 Bean manipulation and the BeanWrapper

The org.springframework.beans package adheres to the JavaBeans standard provided by Sun. A JavaBean is simply a class with a default no-argument constructor, which follows a naming convention where (by way of an example) a property named bingoMadness would have a setter method setBingoMadness(..) and a getter method getBingoMadness(). For more information about JavaBeans and the specification, please refer to Sun's website ( java.sun.com/products/javabeans).

One quite important class in the beans package is the BeanWrapper interface and its corresponding implementation (BeanWrapperImpl). As quoted from the Javadoc, the BeanWrapper offers functionality to set and get property values (individually or in bulk), get property descriptors, and to query properties to determine if they are readable or writable. Also, the BeanWrapper offers support for nested properties, enabling the setting of properties on sub-properties to an unlimited depth. Then, the BeanWrapper supports the ability to add standard JavaBeans PropertyChangeListeners and VetoableChangeListeners, without the need for supporting code in the target class. Last but not least, the BeanWrapper provides support for the setting of indexed properties. The BeanWrapper usually isn't used by application code directly, but by the DataBinder and the BeanFactory.

The way the BeanWrapper works is partly indicated by its name: it wraps a bean to perform actions on that bean, like setting and retrieving properties.

6.4.1 Setting and getting basic and nested properties

Setting and getting properties is done using the setPropertyValue(s) and getPropertyValue(s) methods that both come with a couple of overloaded variants. They're all described in more detail in the Javadoc Spring comes with. What's important to know is that there are a couple of conventions for indicating properties of an object. A couple of examples:

Table 6.1. Examples of properties

ExpressionExplanation
nameIndicates the property name corresponding to the methods getName() or isName() and setName(..)
account.nameIndicates the nested property name of the property account corresponding e.g. to the methods getAccount().setName() or getAccount().getName()
account[2]Indicates the third element of the indexed property account. Indexed properties can be of type array, list or other naturally ordered collection
account[COMPANYNAME]Indicates the value of the map entry indexed by the key COMPANYNAME of the Map property account

Below you'll find some examples of working with the BeanWrapper to get and set properties.

(This next section is not vitally important to you if you're not planning to work with the BeanWrapper directly. If you're just using the DataBinder and the BeanFactory and their out-of-the-box implementation, you should skip ahead to the section about PropertyEditors.)

Consider the following two classes:

public class Company {
    private String name;
    private Employee managingDirector;

    public String getName()	{ 
        return this.name; 
    }
    public void setName(String name) { 
        this.name = name; 
    } 
    public Employee getManagingDirector() { 
        return this.managingDirector; 
    }
    public void setManagingDirector(Employee managingDirector) {
        this.managingDirector = managingDirector;
    }
}
public class Employee {
    private String name;
    private float salary;

    public String getName()	{
        return this.name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getSalary() {
        return salary;
    }
    public void setSalary(float salary) {
        this.salary = salary;
    }
}

The following code snippets show some examples of how to retrieve and manipulate some of the properties of instantiated Companies and Employees:

BeanWrapper company = BeanWrapperImpl(new Company());
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.");
// ... can also be done like this:
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);

// ok, let's create the director and tie it to the company:
BeanWrapper jim = BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());

// retrieving the salary of the managingDirector through the company
Float salary = (Float) company.getPropertyValue("managingDirector.salary");

6.4.2 Built-in PropertyEditor implementations

Spring heavily uses the concept of PropertyEditors to effect the conversion between an Object and a String. If you think about it, it sometimes might be handy to be able to represent properties in a different way than the object itself. For example, a Date can be represented in a human readable way (as the String '2007-14-09'), while we're still able to convert the human readable form back to the original date (or even better: convert any date entered in a human readable form, back to Date objects). This behavior can be achieved by registering custom editors, of type java.beans.PropertyEditor. Registering custom editors on a BeanWrapper or alternately in a specific IoC container as mentioned in the previous chapter, gives it the knowledge of how to convert properties to the desired type. Read more about PropertyEditors in the Javadoc of the java.beans package provided by Sun.

A couple of examples where property editing is used in Spring:

  • setting properties on beans is done using PropertyEditors. When mentioning java.lang.String as the value of a property of some bean you're declaring in XML file, Spring will (if the setter of the corresponding property has a Class-parameter) use the ClassEditor to try to resolve the parameter to a Class object.

  • parsing HTTP request parameters in Spring's MVC framework is done using all kinds of PropertyEditors that you can manually bind in all subclasses of the CommandController.

Spring has a number of built-in PropertyEditors to make life easy. Each of those is listed below and they are all located in the org.springframework.beans.propertyeditors package. Most, but not all (as indicated below), are registered by default by BeanWrapperImpl. Where the property editor is configurable in some fashion, you can of course still register your own variant to override the default one:

Table 6.2. Built-in PropertyEditors

ClassExplanation
ByteArrayPropertyEditorEditor for byte arrays. Strings will simply be converted to their corresponding byte representations. Registered by default by BeanWrapperImpl.
ClassEditorParses Strings representing classes to actual classes and the other way around. When a class is not found, an IllegalArgumentException is thrown. Registered by default by BeanWrapperImpl.
CustomBooleanEditorCustomizable property editor for Boolean properties. Registered by default by BeanWrapperImpl, but, can be overridden by registering custom instance of it as custom editor.
CustomCollectionEditorProperty editor for Collections, converting any source Collection to a given target Collection type.
CustomDateEditorCustomizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user registered as needed with appropriate format.
CustomNumberEditorCustomizable property editor for any Number subclass like Integer, Long, Float, Double. Registered by default by BeanWrapperImpl, but can be overridden by registering custom instance of it as a custom editor.
FileEditorCapable of resolving Strings to java.io.File objects. Registered by default by BeanWrapperImpl.
InputStreamEditorOne-way property editor, capable of taking a text string and producing (via an intermediate ResourceEditor and Resource) an InputStream, so InputStream properties may be directly set as Strings. Note that the default usage will not close the InputStream for you! Registered by default by BeanWrapperImpl.
LocaleEditorCapable of resolving Strings to Locale objects and vice versa (the String format is [language]_[country]_[variant], which is the same thing the toString() method of Locale provides). Registered by default by BeanWrapperImpl.
PatternEditorCapable of resolving Strings to JDK 1.5 Pattern objects and vice versa.
PropertiesEditorCapable of converting Strings (formatted using the format as defined in the Javadoc for the java.lang.Properties class) to Properties objects. Registered by default by BeanWrapperImpl.
StringTrimmerEditorProperty editor that trims Strings. Optionally allows transforming an empty string into a null value. NOT registered by default; must be user registered as needed.
URLEditorCapable of resolving a String representation of a URL to an actual URL object. Registered by default by BeanWrapperImpl.

Spring uses the java.beans.PropertyEditorManager to set the search path for property editors that might be needed. The search path also includes sun.bean.editors, which includes PropertyEditor implementations for types such as Font, Color, and most of the primitive types. Note also that the standard JavaBeans infrastructure will automatically discover PropertyEditor classes (without you having to register them explicitly) if they are in the same package as the class they handle, and have the same name as that class, with 'Editor' appended; for example, one could have the following class and package structure, which would be sufficient for the FooEditor class to be recognized and used as the PropertyEditor for Foo-typed properties.

com
  chank
    pop
      Foo
      FooEditor   // the PropertyEditor for the Foo class

Note that you can also use the standard BeanInfo JavaBeans mechanism here as well (described in not-amazing-detail here). Find below an example of using the BeanInfo mechanism for explicitly registering one or more PropertyEditor instances with the properties of an associated class.

com
  chank
    pop
      Foo
      FooBeanInfo   // the BeanInfo for the Foo class

Here is the Java source code for the referenced FooBeanInfo class. This would associate a CustomNumberEditor with the age property of the Foo class.

public class FooBeanInfo extends SimpleBeanInfo {
      
    public PropertyDescriptor[] getPropertyDescriptors() {
        try {
            final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
            PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.class) {
                public PropertyEditor createPropertyEditor(Object bean) {
                    return numberPE;
                };
            };
            return new PropertyDescriptor[] { ageDescriptor };
        }
        catch (IntrospectionException ex) {
            throw new Error(ex.toString());
        }
    }
}

6.4.2.1 Registering additional custom PropertyEditors

When setting bean properties as a string value, a Spring IoC container ultimately uses standard JavaBeans PropertyEditors to convert these Strings to the complex type of the property. Spring pre-registers a number of custom PropertyEditors (for example, to convert a classname expressed as a string into a real Class object). Additionally, Java's standard JavaBeans PropertyEditor lookup mechanism allows a PropertyEditor for a class simply to be named appropriately and placed in the same package as the class it provides support for, to be found automatically.

If there is a need to register other custom PropertyEditors, there are several mechanisms available. The most manual approach, which is not normally convenient or recommended, is to simply use the registerCustomEditor() method of the ConfigurableBeanFactory interface, assuming you have a BeanFactory reference. Another, slightly more convenient, mechanism is to use a special bean factory post-processor called CustomEditorConfigurer. Although bean factory post-processors can be used with BeanFactory implementations, the CustomEditorConfigurer has a nested property setup, so it is strongly recommended that it is used with the ApplicationContext, where it may be deployed in similar fashion to any other bean, and automatically detected and applied.

Note that all bean factories and application contexts automatically use a number of built-in property editors, through their use of something called a BeanWrapper to handle property conversions. The standard property editors that the BeanWrapper registers are listed in the previous section. Additionally, ApplicationContexts also override or add an additional number of editors to handle resource lookups in a manner appropriate to the specific application context type.

Standard JavaBeans PropertyEditor instances are used to convert property values expressed as strings to the actual complex type of the property. CustomEditorConfigurer, a bean factory post-processor, may be used to conveniently add support for additional PropertyEditor instances to an ApplicationContext.

Consider a user class ExoticType, and another class DependsOnExoticType which needs ExoticType set as a property:

package example;
		
public class ExoticType {

    private String name;

    public ExoticType(String name) {
        this.name = name;
    }
}

public class DependsOnExoticType { 
   
    private ExoticType type;

    public void setType(ExoticType type) {
        this.type = type;
    }
}

When things are properly set up, we want to be able to assign the type property as a string, which a PropertyEditor will behind the scenes convert into an actual ExoticType instance:

<bean id="sample" class="example.DependsOnExoticType">
    <property name="type" value="aNameForExoticType"/>
</bean>

The PropertyEditor implementation could look similar to this:

// converts string representation to ExoticType object
package example;

public class ExoticTypeEditor extends PropertyEditorSupport {

    private String format;

    public void setFormat(String format) {
        this.format = format;
    }
    
    public void setAsText(String text) {
        if (format != null && format.equals("upperCase")) {
            text = text.toUpperCase();
        }
        ExoticType type = new ExoticType(text);
        setValue(type);
    }
}

Finally, we use CustomEditorConfigurer to register the new PropertyEditor with the ApplicationContext, which will then be able to use it as needed:

<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
  <property name="customEditors">
    <map>
      <entry key="example.ExoticType">
        <bean class="example.ExoticTypeEditor">
          <property name="format" value="upperCase"/>
        </bean>
      </entry>
    </map>
  </property>
</bean>
Using PropertyEditorRegistrars

Another mechanism for registering property editors with the Spring container is to create and use a PropertyEditorRegistrar. This interface is particularly useful when you need to use the same set of property editors in several different situations: write a corresponding registrar and reuse that in each case. PropertyEditorRegistrars work in conjunction with an interface called PropertyEditorRegistry, an interface that is implemented by the Spring BeanWrapper (and DataBinder). PropertyEditorRegistrars are particularly convenient when used in conjunction with the CustomEditorConfigurer (introduced here), which exposes a property called setPropertyEditorRegistrars(..): PropertyEditorRegistrars added to a CustomEditorConfigurer in this fashion can easily be shared with DataBinder and Spring MVC Controllers. Furthermore, it avoids the need for synchronization on custom editors: a PropertyEditorRegistrar is expected to create fresh PropertyEditor instances for each bean creation attempt.

Using a PropertyEditorRegistrar is perhaps best illustrated with an example. First off, you need to create your own PropertyEditorRegistrar implementation:

package com.foo.editors.spring;

public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {

    public void registerCustomEditors(PropertyEditorRegistry registry) {

        // it is expected that new PropertyEditor instances are created
        registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());

        // you could register as many custom property editors as are required here...
    }
}

See also the org.springframework.beans.support.ResourceEditorRegistrar for an example PropertyEditorRegistrar implementation. Notice how in its implementation of the registerCustomEditors(..) method it creates new instances of each property editor.

Next we configure a CustomEditorConfigurer and inject an instance of our CustomPropertyEditorRegistrar into it:

<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
    <property name="propertyEditorRegistrars">
        <list>
            <ref bean="customPropertyEditorRegistrar"/>
        </list>
    </property>
</bean>

<bean id="customPropertyEditorRegistrar" class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>

Finally, and in a bit of a departure from the focus of this chapter, for those of you using Spring's MVC web framework, using PropertyEditorRegistrars in conjunction with data-binding Controllers (such as SimpleFormController) can be very convenient. Find below an example of using a PropertyEditorRegistrar in the implementation of an initBinder(..) method:

public final class RegisterUserController extends SimpleFormController {

    private final PropertyEditorRegistrar customPropertyEditorRegistrar;

    public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) {
        this.customPropertyEditorRegistrar = propertyEditorRegistrar;
    }

    protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder) throws Exception {
        this.customPropertyEditorRegistrar.registerCustomEditors(binder);
    }

    // other methods to do with registering a User
}

This style of PropertyEditor registration can lead to concise code (the implementation of initBinder(..) is just one line long!), and allows common PropertyEditor registration code to be encapsulated in a class and then shared amongst as many Controllers as needed.

7. Spring Expression Language (SpEL)

7.1 Introduction

The Spring Expression Language (SpEL for short) is a powerful expression language that supports querying and manipulating an object graph at runtime. The language syntax is similar to Unified EL but offers additional features, most notably method invocation and basic string templating functionality.

While there are several other Java expression languages available, OGNL, MVEL, and JBoss EL, to name a few, the Spring Expression Language was created to provide the Spring community with a single well supported expression language that can used across all the products in the Spring portfolio. Its language features are driven by the requirements of the projects in the Spring portfolio, including tooling requirements for code completion support within the eclipse based SpringSource Tool Suite. That said, SpEL is based on an technology agnostic API allowing other expression language implementations to be integreated should the need arise.

While SpEL serves as the foundation for expression evaluation within the Spring portfolio, it is not directly tied to Spring and can be used independently. In order to be self contained, many of the examples in this chapter use SpEL as if it was an independent expression language. This requires creating a few boostrapping infrastructure classes such as the parser. Most Spring users will not need to deal with this infrastructure and will instead only author expression strings for evaluation. An example of this typical use is the integration of SpEL into creating XML or annotated based bean definitions as shown in the section Expression support for defining bean definitions.

This chapter covers the features of the expression language, its API, and its language sytnax. In several places an Inventor and Inventor's Society class are used as the target objects for expression evaluation. These class declarations and the data used to populate them are listed at the end of the chapter.

7.2 Feature Overview

The expression language support the following functionality

  • Literal expressions

  • Boolean and relational operators

  • Regular expressions

  • Class expressions

  • Accessing properties, arrays, lists, maps

  • Method invocation

  • Relational operators

  • Assignment

  • Calling constructors

  • Ternary operator

  • Variables

  • User defined functions

  • Collection projection

  • Collection selection

  • Templated expressions

7.3 Expression Evaluation using Spring's Expression Interface

This section introduces the simple use of SpEL interfaces and its expression language. The complete language reference can be found in the section Language Reference

The following code introduces the SpEL API to evaluate the literal string expression 'Hello World'

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("'Hello World'");
String message = (String) exp.getValue();

The value of the message variable is simply 'Hello World'.

The SpEL classes and interfaces you are most likely to use are located in the packages org.springframework.expression and its subpackages spel.antlr and spel.support.

The expression language is based on a grammar and uses ANTLR to construct the lexer and parser. The interface ExpressionParser is responsible for parsing an expression string. In this example the expression string is a string literal denoted by the surrounding single quotes. The interface Expression is responsible for evaluating the previously defined expression string. There are two exceptions that can be thrown, ParseException and EvaluationException when calling 'parser.parseExpression' and 'exp.getValue' respectedly.

SpEL supports a wide range of features, such a calling methods, accessing properties and calling constructors.

As an example of method invocation, we call the 'concat' method on the string literal

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.concat('!')");
String message = (String) exp.getValue();

The value of message is now 'Hello World!'.

As an example of calling a JavaBean property, the String property 'Bytes' can be called as shown below

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.bytes");  // invokes 'getBytes()'
byte[] bytes = (byte[]) exp.getValue();

SpEL also supports nested properties using standard 'dot' notation, i.e. prop1.prop2.prop3 and the setting of property values

Public fields may also be accessed

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("'Hello World'.bytes.length");  // invokes 'getBytes().length'
int length = (Integer) exp.getValue();

The String's constructor can be called instead of using a string literal

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("new String('hello world').toUpperCase()");
String message = exp.getValue(String.class);

Note the use of the generic method public <T> T getValue(Class<T> desiredResultType). Using this method removes the need to cast the value of the expression to the desired result type. An EvaluationException will be thrown if the value an not be cast to the type T or converted using the registered type converter.

The more common usage of SpEL is provide an expression string that is evaluated against a specific object instance. In the following example we retrieve the Name property from an instance of the Inventor class.

// Create and set a calendar 
GregorianCalendar c = new GregorianCalendar();
c.set(1856, 7, 9);

//  The constructor arguments are name, birthday, and nationaltiy.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");

ExpressionParser parser = new SpelAntlrExpressionParser();
Expression exp = parser.parseExpression("name");

EvaluationContext context = new StandardEvaluationContext();
context.setRootObject(tesla);

String name = (String) exp.getValue(context);

In the last line, the value of the string variable 'name' will be set to "Nikola Tesla". The class StandardEvaluationContext is where you can specify which object the "Name" property will be evaluated against. You can reuse the same expression over and over again and set a new root object on the evaluation context. Expressions are evaluated using reflection.

[Note]Note

In standalone usage of SpEL you will need to create the parser as well as provide an evaluation context. However, more common usage is to provide only the SpEL expression string as part of a configuration file, for example for Spring bean or Spring Web Flow definitions. In this case, the parser, evaluation context, root object and any predefined variables will be set up for you implicitly.

As a final introductory example, the use of a boolean operator is shown using the Inventor object in the previous example

Expression exp = parser.parseExpression("name == 'Nikola Tesla'");
boolean result = exp.getValue(context, Boolean.class);  // evaluates to true

7.3.1 The EvaluationContext interface

The interface EvaluationContext is used when evaluating an expression to resolve properties, methods, fields, and to help perform type conversion. The out-of-the-box implementation, StandardEvaluationContext, uses reflection to manipulate the object, caching java.lang.reflect's Method, Field, and Constructor instances for increased performance.

The StandardEvaluationContext is where you specify the root object to evaluate against via the method setRootObject . You can also specify variables and functions that will be used in the expression using the methods setVariable and registerFunction. The use of variables and functions are described in the language reference sections Variables and Functions. The StandardEvaluationContext is also where you can register custom ConstructorResolvers, MethodResolvers, and PropertyAccessors to extend how SpEL evaluates expressions. Please refer to the JavaDoc of these classes for more details.

7.3.1.1 Type Conversion

By default SpEL uses the conversion service available in Spring core (org.springframework.core.convert.ConversionService). This conversion service comes with many converters built in for common conversions but is also fully extensible so custom conversions between types can be added. Additionally it has the key capability that it is generics aware. This means that when working with generic types in expressions, SpEL will attempt conversions to maintain type correctness for any objects it encounters.

What does this mean in practice? Suppose assignment, using setValue(), is being used to set a List property. The type of the property is actually List<Boolean>. SpEL will recognize that the elements of the list need to be converted to Boolean before being placed in it. A simple example:

class Simple {
    public List<Boolean> booleanList = new ArrayList<Boolean>();
}
    	
Simple simple = new Simple();

simple.booleanList.add(true);

StandardEvaluationContext simpleContext = new StandardEvaluationContext(simple);

// false is passed in here as a string.  SpEL and the conversion service will 
// correctly recognize that it needs to be a Boolean and convert it
parser.parseExpression("booleanList[0]").setValue(simpleContext, "false");

// b will be false
Boolean b = simple.booleanList.get(0);
        

7.4 Expression support for defining bean definitions

SpEL expressions can be used with XML or annotation based configuration metadata for defining BeanDefinitions. In both cases the syntax to define the expression is of the form #{ <expression string> }.

7.4.1 XML based configuration

A property or constructor-arg value can be set using expressions as shown below

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>

The variable 'systemProperties' is predefined, so you can use it in your expressions as shown below. Note that you do not have to prefix the predefined variable with the '#' symbol in this context.

<bean id="taxCalculator" class="org.spring.samples.TaxCalculator">
    <property name="defaultLocale" value="#{ systemProperties['user.region'] }"/>

    <!-- other properties -->
</bean>

You can also refer to other bean properties by name, for example

<bean id="numberGuess" class="org.spring.samples.NumberGuess">
    <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/>

    <!-- other properties -->
</bean>


<bean id="shapeGuess" class="org.spring.samples.ShapeGuess">
    <property name="initialShapeSeed" value="#{ numberGuess.randomNumber }"/>

    <!-- other properties -->
</bean>

7.4.2 Annotation-based configuration

The @Value annotation can be placed on fields, methods and method/constructor parameters to specify a default value.

Here is an example to set the default value of a field variable

public static class FieldValueTestBean

  @Value("#{ systemProperties['user.region'] }")
  private String defaultLocale;

  public void setDefaultLocale(String defaultLocale)
  {
    this.defaultLocale = defaultLocale;
  }

  public String getDefaultLocale() 
  {
    return this.defaultLocale;
  }

}

The equivalent but on a property setter method is shown below

public static class PropertyValueTestBean

  private String defaultLocale;

  @Value("#{ systemProperties['user.region'] }")
  public void setDefaultLocale(String defaultLocale)
  {
    this.defaultLocale = defaultLocale;
  }

  public String getDefaultLocale() 
  {
    return this.defaultLocale;
  }

}

Autowired methods and constructors can also use the @Value annotation.

public class SimpleMovieLister {

    private MovieFinder movieFinder;
    private String defaultLocale;

    @Autowired
    public void configure(MovieFinder movieFinder, 
                          @Value("#{ systemProperties['user.region'] } String defaultLocale) {
        this.movieFinder = movieFinder;
        this.defaultLocale = defaultLocale;
    }

    // ...
}
public class MovieRecommender {

    private String defaultLocale;
    
    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao,
                            @Value("#{ systemProperties['user.country'] } String defaultLocale) {
        this.customerPreferenceDao = customerPreferenceDao;
        this.defaultLocale = defaultLocale;
    }

    // ...
}

7.5 Language Reference

7.5.1 Literal expressions

The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), boolean and null. String are delimited by single quotes. To put a single quote itself in a string use the backslash character. The following listing shows simple usage of literals. Typically they would not be used in isolation like this, but as part of a more complex expression, for example using a literal on one side of a logical comparison operator.

ExpressionParser parser = new SpelAntlrExpressionParser();

String helloWorld = (String) parser.parseExpression("'Hello World'").getValue(); // evals to "Hello World"

double avogadrosNumber  = (Double) parser.parseExpression("6.0221415E+23").getValue();  

int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();  // evals to 2147483647

boolean trueValue = (Boolean) parser.parseExpression("true").getValue();

Object nullValue = parser.parseExpression("null").getValue();

Numbers support the use of the negative sign, exponential notation, and decimal points. By default real numbers are parsed using Double.parseDouble().

7.5.2 Properties, Arrays, Lists, Maps, Indexers

Navigating with property references is easy, just use a period to indicate a nested property value. The instances of Inventor class, pupin and tesla, were populated with data listed in section Section Classes used in the examples. To navigate "down" and get Tesla's year of birth and Pupin's city of birth the following expressions are used

int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); // 1856


String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);

Case insensitivity is allowed for the first letter of property names. The contents of arrays and lists are obtained using square bracket notation.

ExpressionParser parser = new SpelAntlrExpressionParser();

// Inventions Array
StandardEvaluationContext teslaContext = new StandardEvaluationContext();
teslaContext.setRootObject(tesla);

// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(teslaContext, String.class); 


// Members List
StandardEvaluationContext societyContext = new StandardEvaluationContext();
societyContext.setRootObject(ieee);

// evaluates to "Nikola Tesla"
String name = parser.parseExpression("Members[0].Name").getValue(societyContext, String.class);

// List and Array navigation
// evaluates to "Wireless communication"
String invention = parser.parseExpression("Members[0].Inventions[6]").getValue(societyContext, String.class);

The contents of maps are obtained by specifying the literal key value within the brackets. In this case, because keys for the Officers map are strings, we can specify string literal.

// Officer's Dictionary

Inventor pupin = parser.parseExpression("Officers['president']").getValue(societyContext, Inventor.class);

// evaluates to "Idvor"
String city = parser.parseExpression("Officers['president'].PlaceOfBirth.City").getValue(societyContext, String.class);

// setting values
parser.parseExpression("Officers['advisors'][0].PlaceOfBirth.Country").setValue(societyContext, "Croatia");

7.5.3 Methods

Methods are invoked using typical Java programming syntax. You may also invoke methods on literals. Varargs are also supported.

// string literal, evaluates to "bc"
String c = parser.parseExpression("'abc'.substring(2, 3)").getValue(String.class);

// evaluates to true
boolean isMember = parser.parseExpression("isMember('Mihajlo Pupin')").getValue(societyContext, Boolean.class);

7.5.4 Operators

7.5.4.1 Relational operators

The relational operators; equal, not equal, less than, less than or equal, greater than, and greater than or equal are supported using standard operator notation.

// evaluates to true
boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class);

// evaluates to false
boolean falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean.class);

// evaluates to true
boolean trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean.class);

In addition to standard relational operators SpEL supports the 'instanceof' and regular expression based 'matches' operator.

// evaluates to false
boolean falseValue = parser.parseExpression("'xyz' instanceof T(int)").getValue(Boolean.class);

// evaluates to true
boolean trueValue = parser.parseExpression("'5.00' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);

//evaluates to false
boolean falseValue = parser.parseExpression("'5.0067' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);

7.5.4.2 Logical operators

The logical operators that are supported are and, or, and not. Their use is demonstrated below

// -- AND --

// evaluates to false
boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class);

// evaluates to true
String expression =  "isMember('Nikola Tesla') and isMember('Mihajlo Pupin')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- OR --

// evaluates to true
boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class);

// evaluates to true
String expression =  "isMember('Nikola Tesla') or isMember('Albert Einstien')";
boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

// -- NOT --

// evaluates to false
boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class);


// -- AND and NOT --
String expression =  "isMember('Nikola Tesla') and !isMember('Mihajlo Pupin')";
boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);

7.5.4.3 Mathematical operators

The addition operator can be used on numbers, strings and dates. Subtraction can be used on numbers and dates. Multiplication and division can be used only on numbers. Other mathematical operators supported are modulus (%) and exponential power (^). Standard operator precedence is enforced. These operators are demonstrated below

// Addition
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2

String testString = parser.parseExpression("'test' + ' ' + 'string'").getValue(String.class); // 'test string'

// Subtraction
int four =  parser.parseExpression("1 - -3").getValue(Integer.class); // 4

double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class); // -9000

// Multiplication
int six =  parser.parseExpression("-2 * -3").getValue(Integer.class); // 6

double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); // 24.0

// Division
int minusTwo =  parser.parseExpression("6 / -3").getValue(Integer.class); // -2

double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class); // 1.0

// Modulus
int three =  parser.parseExpression("7 % 4").getValue(Integer.class); // 3

int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class); // 1

// Operator precedence
int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class); // -21

7.5.5 Assignment

Setting of a property is done by using the assignment operator. This would typically be done within a call to setValue but can also be done inside a call to getValue

Inventor inventor = new Inventor();		
StandardEvaluationContext inventorContext = new StandardEvaluationContext();
inventorContext.setRootObject(inventor);

parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2");

// alternatively

String aleks = parser.parseExpression("Name = 'Alexandar Seovic'").getValue(inventorContext, String.class);

7.5.6 Types

The special 'T' operator can be used to specify an instance of java.lang.Class (the 'type'). Static methods are invoked using this operator as well. The StandardEvaluationContext uses a TypeLocator to find types and the StandardTypeLocator (which can be replaced) is built with an understanding of the java.lang package. This means T() references to types within java.lang do not need to be fully qualified, but all other type references must be.

Class dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class.class);

Class stringClass = parser.parseExpression("T(String)").getValue(Class.class);

boolean trueValue = parser.parseExpression("T(java.math.RoundingMode).CEILING < T(java.math.RoundingMode).FLOOR").getValue(Boolean.class);

7.5.7 Constructors

Constructors can be invoked using the new operator. The fully qualified class name should be used for all but the primitive type and String (where int, float, etc, can be used).

Inventor einstein = 
   parser.parseExpression("new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German')").getValue(Inventor.class);

//create new inventor instance within add method of List
parser.parseExpression("Members.add(new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 'German'))").getValue(societyContext);

7.5.8 Variables

Variables can referenced in the expression using the syntax #variableName. Variables are set using the method setVariable on the StandardEvaluationContext.

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("newName", "Mike Tesla");


context.setRootObject(tesla);

parser.parseExpression("Name = #newName").getValue(context);

System.out.println(tesla.getName()) // "Mike Tesla"

7.5.8.1 The #this variable

The variable #this is always defined and refers to the current evaluation object (the object against which unqualified references will be resolved).

// create an array of integers
List<Integer> primes = new ArrayList<Integer>();
primes.addAll(Arrays.asList(2,3,5,7,11,13,17));

// create parser and set variable 'primes' as the array of integers
ExpressionParser parser = new SpelAntlrExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("primes",primes);

// all prime numbers > 10 from the list (using selection ?{...})
List<Integer> primesGreaterThanTen = (List<Integer>) parser.parseExpression("#primes.?[#this>10]").getValue(context);

//evaluates to [11, 13, 17]

7.5.9 Functions

You can extend SpEL by registering user defined functions that can be called within the expression string. The function is registered with the StandardEvaluationContext using the method

public void registerFunction(String name, Method m)

A reference to a Java Method provides the implementation of the function. For example, a utility method to reverse a string is shown below.

public abstract class StringUtils {

  public static String reverseString(String input) {
    StringBuilder backwards = new StringBuilder();
    for (int i = 0; i < input.length(); i++) {
      backwards.append(input.charAt(input.length() - 1 - i));
    }
    return backwards.toString();
  }
}

This method is then registered with the evaluation context and can be used within an expression string

ExpressionParser parser = new SpelAntlrExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();

context.registerFunction("reverseString", 
                         StringUtils.class.getDeclaredMethod("reverseString", new Class[] { String.class }));

String helloWorldReversed = parser.parseExpression("#reverseString('hello')").getValue(context, String.class);

7.5.10 Ternary Operator (If-Then-Else)

You can use the ternary operator for performing if-then-else conditional logic inside the expression. A minimal example is;

String falseString = parser.parseExpression("false ? 'trueExp' : 'falseExp'").getValue(String.class);

In this case, the boolean false results in returning the string value 'falseExp'. A less artificial example is shown below.

parser.parseExpression("Name").setValue(societyContext, "IEEE");
societyContext.setVariable("queryName", "Nikola Tesla");

expression = "isMember(#queryName)? #queryName + ' is a member of the ' " + 
             "+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'";

String queryResultString = parser.parseExpression(expression).getValue(societyContext, String.class);
// queryResultString = "Nikola Tesla is a member of the IEEE Society"

7.5.11 Collection Selection

Selection is a powerful expression language feature that allow you to transform some source collection into another by selecting from its entries.

Selection uses the syntax ?[selectionExpression]. This will filter the collection and return a new collection containing a subset of the original elements. For example, selection would allow us to easily get a list of Serbian inventors:

List<Inventor> list = (List<Inventor>) parser.parseExpression("Members.?[Nationality == 'Serbian']").getValue(societyContext);

Selection is possible upon both lists and maps. In the former case the selection criteria is evaluated against each individual list element whilst against a map the selection criteria is evaluated against each map entry (objects of the Java type Map.Entry). Map entries have their key and value accessible as properties for use in the selection.

This expression will return a new map consisting of those elements of the original map where the entry value is less than 27.

Map newMap = parser.parseExpression("map.?[value<27]").getValue();

In addition to returning all the selected elements, it is possible to retrieve just the first or the last value. To obtain the first entry matching the selection the syntax is ^[...] whilst to obtain the last matching selection the syntax is $[...].

7.5.12 Collection Projection

Projection allows a collection to drive the evaluation of a sub-expression and the result is a new collection. The syntax for projection is ![projectionExpression]. Most easily understood by example, suppose we have a list of inventors but want the list of cities where they were born. Effectively we want to evaluate 'placeOfBirth.city' for every entry in the inventor list. Using projection:

// returns [ 'Smiljan', 'Idvor' ]
List placesOfBirth = (List)parser.parseExpression("Members.![placeOfBirth.city]");

A map can also be used to drive projection and in this case the projection expression is evaluated against each entry in the map (represented as a Java Map.Entry). The result of a projection across a map is a list consisting of the evaluation of the projection expression against each map entry.

7.5.13 Expression templating

Expression templates allow a mixing of literal text with one or more evaluation blocks. Each evaluation block is delimited with a prefix and suffix characters that you can define, a common choice is to use ${} as the delimiters. For example,

String randomPhrase = 
   parser.parseExpression("random number is ${T(java.lang.Math).random()}", new TemplatedParserContext()).getValue(String.class);

// evaluates to "random number is 0.7038186818312008"

The string is evaluated by concatenating the literal text 'random number is' with the result of evaluating the expression inside the ${} delimiter, in this case the result of calling that random() method. The second argument to the method parseExpression() of the type ParserContext. The ParserContext interface is used to influence how the expression is parsed in order to support the expression templating functionality. The definition of TemplatedParserContext is shown below

public class TemplatedParserContext implements ParserContext {

  public String getExpressionPrefix() {
    return "${";
  }

  public String getExpressionSuffix() {
    return "}";
  }
  
  public boolean isTemplate() {
    return true;
  }
}

7.6 Classes used in the examples

Inventor.java

package org.spring.samples.spel.inventor;

import java.util.Date;
import java.util.GregorianCalendar;

public class Inventor {

  private String name;
  private String nationality;
  private String[] inventions;
  private Date birthdate;
  private PlaceOfBirth placeOfBirth;
  
  
  public Inventor(String name, String nationality)
  {
    GregorianCalendar c= new GregorianCalendar();
    this.name = name;
    this.nationality = nationality;
    this.birthdate = c.getTime();
  }
  public Inventor(String name, Date birthdate, String nationality) {
    this.name = name;
    this.nationality = nationality;
    this.birthdate = birthdate;
  }
  
  public Inventor() {
  }

  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getNationality() {
    return nationality;
  }
  public void setNationality(String nationality) {
    this.nationality = nationality;
  }
  public Date getBirthdate() {
    return birthdate;
  }
  public void setBirthdate(Date birthdate) {
    this.birthdate = birthdate;
  }
  public PlaceOfBirth getPlaceOfBirth() {
    return placeOfBirth;
  }
  public void setPlaceOfBirth(PlaceOfBirth placeOfBirth) {
    this.placeOfBirth = placeOfBirth;
  }
  public void setInventions(String[] inventions) {
    this.inventions = inventions;
  }
  public String[] getInventions() {
    return inventions;
  }       
}

PlaceOfBirth.java

package org.spring.samples.spel.inventor;

public class PlaceOfBirth {

	private String city;
	private String country;
	
	public PlaceOfBirth(String city) {
		this.city=city;
	}
	public PlaceOfBirth(String city, String country)
	{
		this(city);
		this.country = country;
	}
	
	
	public String getCity() {
		return city;
	}
	public void setCity(String s) {
		this.city = s;
	}
	public String getCountry() {
		return country;
	}
	public void setCountry(String country) {
		this.country = country;
	}

	
	
}

Society.java

package org.spring.samples.spel.inventor;

import java.util.*;

public class Society {

	private String name;
	
	public static String Advisors = "advisors";
	public static String President = "president";
	
	private List<Inventor> members = new ArrayList<Inventor>();
	private Map officers = new HashMap();

	public List getMembers() {
		return members;
	}

	public Map getOfficers() {
		return officers;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public boolean isMember(String name)
	{
		boolean found = false;
		for (Inventor inventor : members) {
			if (inventor.getName().equals(name))
			{
				found = true;
				break;
			}
		}		
		return found;
	}

	
}

8. Aspect Oriented Programming with Spring

8.1 Introduction

Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects. (Such concerns are often termed crosscutting concerns in AOP literature.)

One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you do not need to use AOP if you don't want to, AOP complements Spring IoC to provide a very capable middleware solution.

AOP is used in the Spring Framework to...

  • ... provide declarative enterprise services, especially as a replacement for EJB declarative services. The most important such service is declarative transaction management.

  • ... allow users to implement custom aspects, complementing their use of OOP with AOP.

If you are interested only in generic declarative services or other pre-packaged declarative middleware services such as pooling, you do not need to work directly with Spring AOP, and can skip most of this chapter.

8.1.1 AOP concepts

Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific... unfortunately, AOP terminology is not particularly intuitive; however, it would be even more confusing if Spring used its own terminology.

  • Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in J2EE applications. In Spring AOP, aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (the @AspectJ style).

  • Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

  • Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. (Advice types are discussed below.) Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.

  • Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.

  • Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)

  • Target object: object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.

  • AOP proxy: an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.

  • Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

Types of advice:

  • Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).

  • After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.

  • After throwing advice: Advice to be executed if a method exits by throwing an exception.

  • After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).

  • Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.

Around advice is the most general kind of advice. Since Spring AOP, like AspectJ, provides a full range of advice types, we recommend that you use the least powerful advice type that can implement the required behavior. For example, if you need only to update a cache with the return value of a method, you are better off implementing an after returning advice than an around advice, although an around advice can accomplish the same thing. Using the most specific advice type provides a simpler programming model with less potential for errors. For example, you do not need to invoke the proceed() method on the JoinPoint used for around advice, and hence cannot fail to invoke it.

In Spring 2.0, all advice parameters are statically typed, so that you work with advice parameters of the appropriate type (the type of the return value from a method execution for example) rather than Object arrays.

The concept of join points, matched by pointcuts, is the key to AOP which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the Object-Oriented hierarchy. For example, an around advice providing declarative transaction management can be applied to a set of methods spanning multiple objects (such as all business operations in the service layer).

8.1.2 Spring AOP capabilities and goals

Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP does not need to control the class loader hierarchy, and is thus suitable for use in a J2EE web container or application server.

Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.

Spring AOP's approach to AOP differs from that of most other AOP frameworks. The aim is not to provide the most complete AOP implementation (although Spring AOP is quite capable); it is rather to provide a close integration between AOP implementation and Spring IoC to help solve common problems in enterprise applications.

Thus, for example, the Spring Framework's AOP functionality is normally used in conjunction with the Spring IoC container. Aspects are configured using normal bean definition syntax (although this allows powerful "autoproxying" capabilities): this is a crucial difference from other AOP implementations. There are some things you cannot do easily or efficiently with Spring AOP, such as advise very fine-grained objects (such as domain objects typically): AspectJ is the best choice in such cases. However, our experience is that Spring AOP provides an excellent solution to most problems in J2EE applications that are amenable to AOP.

Spring AOP will never strive to compete with AspectJ to provide a comprehensive AOP solution. We believe that both proxy-based frameworks like Spring AOP and full-blown frameworks such as AspectJ are valuable, and that they are complementary, rather than in competition. Spring 2.0 seamlessly integrates Spring AOP and IoC with AspectJ, to enable all uses of AOP to be catered for within a consistent Spring-based application architecture. This integration does not affect the Spring AOP API or the AOP Alliance API: Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring AOP APIs.

[Note]Note

One of the central tenets of the Spring Framework is that of non-invasiveness; this is the idea that you should not be forced to introduce framework-specific classes and interfaces into your business/domain model. However, in some places the Spring Framework does give you the option to introduce Spring Framework-specific dependencies into your codebase: the rationale in giving you such options is because in certain scenarios it might be just plain easier to read or code some specific piece of functionality in such a way. The Spring Framework (almost) always offers you the choice though: you have the freedom to make an informed decision as to which option best suits your particular use case or scenario.

One such choice that is relevant to this chapter is that of which AOP framework (and which AOP style) to choose. You have the choice of AspectJ and/or Spring AOP, and you also have the choice of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach. The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be taken as an indication that the Spring team favors the @AspectJ annotation-style approach over the Spring XML configuration-style.

See the section entitled Section 8.4, “Choosing which AOP declaration style to use” for a fuller discussion of the whys and wherefores of each style.

8.1.3 AOP Proxies

Spring AOP defaults to using standard J2SE dynamic proxies for AOP proxies. This enables any interface (or set of interfaces) to be proxied.

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes, rather than interfaces. CGLIB is used by default if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally will implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface, or where you need to pass a proxied object to a method as a concrete type.

It is important to grasp the fact that Spring AOP is proxy-based. See the section entitled Section 8.6.1, “Understanding AOP proxies” for a thorough examination of exactly what this implementation detail actually means.

8.2 @AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with Java 5 annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring 2.0 interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime is still pure Spring AOP though, and there is no dependency on the AspectJ compiler or weaver.

Using the AspectJ compiler and weaver enables use of the full AspectJ language, and is discussed in Section 8.8, “Using AspectJ with Spring applications”.

8.2.1 Enabling @AspectJ Support

To use @AspectJ aspects in a Spring configuration you need to enable Spring support for configuring Spring AOP based on @AspectJ aspects, and autoproxying beans based on whether or not they are advised by those aspects. By autoproxying we mean that if Spring determines that a bean is advised by one or more aspects, it will automatically generate a proxy for that bean to intercept method invocations and ensure that advice is executed as needed.

The @AspectJ support is enabled by including the following element inside your spring configuration:

<aop:aspectj-autoproxy/>

This assumes that you are using schema support as described in Appendix A, XML Schema-based configuration. See Section A.2.7, “The aop schema” for how to import the tags in the aop namespace.

If you are using the DTD, it is still possible to enable @AspectJ support by adding the following definition to your application context:

<bean class="org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator" />

You will also need two AspectJ libraries on the classpath of your application: aspectjweaver.jar and aspectjrt.jar. These libraries are available in the 'lib' directory of an AspectJ installation (version 1.5.1 or later required), or in the 'lib/aspectj' directory of the Spring-with-dependencies distribution.

8.2.2 Declaring an aspect

With the @AspectJ support enabled, any bean defined in your application context with a class that is an @AspectJ aspect (has the @Aspect annotation) will be automatically detected by Spring and used to configure Spring AOP. The following example shows the minimal definition required for a not-very-useful aspect:

A regular bean definition in the application context, pointing to a bean class that has the @Aspect annotation:

<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect">
   <!-- configure properties of aspect here as normal -->
</bean>

And the NotVeryUsefulAspect class definition, annotated with org.aspectj.lang.annotation.Aspect annotation;

package org.xyz;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class NotVeryUsefulAspect {

}

Aspects (classes annotated with @Aspect) may have methods and fields just like any other class. They may also contain pointcut, advice, and introduction (inter-type) declarations.

[Note]Advising aspects

In Spring AOP, it is not possible to have aspects themselves be the target of advice from other aspects. The @Aspect annotation on a class marks it as an aspect, and hence excludes it from auto-proxying.

8.2.3 Declaring a pointcut

Recall that pointcuts determine join points of interest, and thus enable us to control when advice executes. Spring AOP only supports method execution join points for Spring beans, so you can think of a pointcut as matching the execution of methods on Spring beans. A pointcut declaration has two parts: a signature comprising a name and any parameters, and a pointcut expression that determines exactly which method executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method definition, and the pointcut expression is indicated using the @Pointcut annotation (the method serving as the pointcut signature must have a void return type).

An example will help make this distinction between a pointcut signature and a pointcut expression clear. The following example defines a pointcut named 'anyOldTransfer' that will match the execution of any method named 'transfer':

@Pointcut("execution(* transfer(..))")// the pointcut expression
private void anyOldTransfer() {}// the pointcut signature

The pointcut expression that forms the value of the @Pointcut annotation is a regular AspectJ 5 pointcut expression. For a full discussion of AspectJ's pointcut language, see the AspectJ Programming Guide (and for Java 5 based extensions, the AspectJ 5 Developers Notebook) or one of the books on AspectJ such as “Eclipse AspectJ” by Colyer et. al. or “AspectJ in Action” by Ramnivas Laddad.

8.2.3.1 Supported Pointcut Designators

Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:

  • execution - for matching method execution join points, this is the primary pointcut designator you will use when working with Spring AOP

  • within - limits matching to join points within certain types (simply the execution of a method declared within a matching type when using Spring AOP)

  • this - limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type

  • target - limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type

  • args - limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types

  • @target - limits matching to join points (the execution of methods when using Spring AOP) where the class of the executing object has an annotation of the given type

  • @args - limits matching to join points (the execution of methods when using Spring AOP) where the runtime type of the actual arguments passed have annotations of the given type(s)

  • @within - limits matching to join points within types that have the given annotation (the execution of methods declared in types with the given annotation when using Spring AOP)

  • @annotation - limits matching to join points where the subject of the join point (method being executed in Spring AOP) has the given annotation

Because Spring AOP limits matching to only method execution join points, the discussion of the pointcut designators above gives a narrower definition than you will find in the AspectJ programming guide. In addition, AspectJ itself has type-based semantics and at an execution join point both 'this' and 'target' refer to the same object - the object executing the method. Spring AOP is a proxy-based system and differentiates between the proxy object itself (bound to 'this') and the target object behind the proxy (bound to 'target').

[Note]Note

Due to the proxy-based nature of Spring's AOP framework, protected methods are by definition not intercepted, neither for JDK proxies (where this isn't applicable) nor for CGLIB proxies (where this is technically possible but not recommendable for AOP purposes). As a consequence, any given pointcut will be matched against public methods only!

If your interception needs include protected/private methods or even constructors, consider the use of Spring-driven native AspectJ weaving instead of Spring's proxy-based AOP framework. This constitutes a different mode of AOP usage with different characteristics, so be sure to make yourself familiar with weaving first before making a decision.

Spring AOP also supports an additional PCD named 'bean'. This PCD allows you to limit the matching of join points to a particular named Spring bean, or to a set of named Spring beans (when using wildcards). The 'bean' PCD has the following form:

bean(idOrNameOfBean)

The 'idOrNameOfBean' token can be the name of any Spring bean: limited wildcard support using the '*' character is provided, so if you establish some naming conventions for your Spring beans you can quite easily write a 'bean' PCD expression to pick them out. As is the case with other pointcut designators, the 'bean' PCD can be &&'ed, ||'ed, and ! (negated) too.

[Note]Note

Please note that the 'bean' PCD is only supported in Spring AOP - and not in native AspectJ weaving. It is a Spring-specific extension to the standard PCDs that AspectJ defines.

The 'bean' PCD operates at the instance level (building on the Spring bean name concept) rather than at the type level only (which is what weaving-based AOP is limited to). Instance-based pointcut designators are a special capability of Spring's proxy-based AOP framework and its close integration with the Spring bean factory, where it is natural and straightforward to identify specific beans by name.

8.2.3.2 Combining pointcut expressions

Pointcut expressions can be combined using '&&', '||' and '!'. It is also possible to refer to pointcut expressions by name. The following example shows three pointcut expressions: anyPublicOperation (which matches if a method execution join point represents the execution of any public method); inTrading (which matches if a method execution is in the trading module), and tradingOperation (which matches if a method execution represents any public method in the trading module).

    @Pointcut("execution(public * *(..))")
    private void anyPublicOperation() {}
    
    @Pointcut("within(com.xyz.someapp.trading..*)")
    private void inTrading() {}
    
    @Pointcut("anyPublicOperation() && inTrading()")
    private void tradingOperation() {}

It is a best practice to build more complex pointcut expressions out of smaller named components as shown above. When referring to pointcuts by name, normal Java visibility rules apply (you can see private pointcuts in the same type, protected pointcuts in the hierarchy, public pointcuts anywhere and so on). Visibility does not affect pointcut matching.

8.2.3.3 Sharing common pointcut definitions

When working with enterprise applications, you often want to refer to modules of the application and particular sets of operations from within several aspects. We recommend defining a "SystemArchitecture" aspect that captures common pointcut expressions for this purpose. A typical such aspect would look as follows:

package com.xyz.someapp;

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;

@Aspect
public class SystemArchitecture {

  /**
   * A join point is in the web layer if the method is defined
   * in a type in the com.xyz.someapp.web package or any sub-package
   * under that.
   */
  @Pointcut("within(com.xyz.someapp.web..*)")
  public void inWebLayer() {}

  /**
   * A join point is in the service layer if the method is defined
   * in a type in the com.xyz.someapp.service package or any sub-package
   * under that.
   */
  @Pointcut("within(com.xyz.someapp.service..*)")
  public void inServiceLayer() {}

  /**
   * A join point is in the data access layer if the method is defined
   * in a type in the com.xyz.someapp.dao package or any sub-package
   * under that.
   */
  @Pointcut("within(com.xyz.someapp.dao..*)")
  public void inDataAccessLayer() {}

  /**
   * A business service is the execution of any method defined on a service
   * interface. This definition assumes that interfaces are placed in the
   * "service" package, and that implementation types are in sub-packages.
   * 
   * If you group service interfaces by functional area (for example, 
   * in packages com.xyz.someapp.abc.service and com.xyz.def.service) then
   * the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
   * could be used instead.
   *
   * Alternatively, you can write the expression using the 'bean'
   * PCD, like so "bean(*Service)". (This assumes that you have
   * named your Spring service beans in a consistent fashion.)
   */
  @Pointcut("execution(* com.xyz.someapp.service.*.*(..))")
  public void businessService() {}
  
  /**
   * A data access operation is the execution of any method defined on a 
   * dao interface. This definition assumes that interfaces are placed in the
   * "dao" package, and that implementation types are in sub-packages.
   */
  @Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
  public void dataAccessOperation() {}

}

The pointcuts defined in such an aspect can be referred to anywhere that you need a pointcut expression. For example, to make the service layer transactional, you could write:

<aop:config>
  <aop:advisor 
      pointcut="com.xyz.someapp.SystemArchitecture.businessService()"
      advice-ref="tx-advice"/>
</aop:config>

<tx:advice id="tx-advice">
  <tx:attributes>
    <tx:method name="*" propagation="REQUIRED"/>
  </tx:attributes>
</tx:advice>

The <aop:config> and <aop:advisor> elements are discussed in Section 8.3, “Schema-based AOP support”. The transaction elements are discussed in Chapter 11, Transaction management.

8.2.3.4 Examples

Spring AOP users are likely to use the execution pointcut designator the most often. The format of an execution expression is:

execution(modifiers-pattern? ret-type-pattern declaring-type-pattern? name-pattern(param-pattern)
          throws-pattern?)

All parts except the returning type pattern (ret-type-pattern in the snippet above), name pattern, and parameters pattern are optional. The returning type pattern determines what the return type of the method must be in order for a join point to be matched. Most frequently you will use * as the returning type pattern, which matches any return type. A fully-qualified type name will match only when the method returns the given type. The name pattern matches the method name. You can use the * wildcard as all or part of a name pattern. The parameters pattern is slightly more complex: () matches a method that takes no parameters, whereas (..) matches any number of parameters (zero or more). The pattern (*) matches a method taking one parameter of any type, (*,String) matches a method taking two parameters, the first can be of any type, the second must be a String. Consult the Language Semantics section of the AspectJ Programming Guide for more information.

Some examples of common pointcut expressions are given below.

  • the execution of any public method:

    execution(public * *(..))
  • the execution of any method with a name beginning with "set":

    execution(* set*(..))
  • the execution of any method defined by the AccountService interface:

    execution(* com.xyz.service.AccountService.*(..))
  • the execution of any method defined in the service package:

    execution(* com.xyz.service.*.*(..))
  • the execution of any method defined in the service package or a sub-package:

    execution(* com.xyz.service..*.*(..))
  • any join point (method execution only in Spring AOP) within the service package:

    within(com.xyz.service.*)
  • any join point (method execution only in Spring AOP) within the service package or a sub-package:

    within(com.xyz.service..*)
  • any join point (method execution only in Spring AOP) where the proxy implements the AccountService interface:

    this(com.xyz.service.AccountService)

    'this' is more commonly used in a binding form :- see the following section on advice for how to make the proxy object available in the advice body.

  • any join point (method execution only in Spring AOP) where the target object implements the AccountService interface:

    target(com.xyz.service.AccountService)

    'target' is more commonly used in a binding form :- see the following section on advice for how to make the target object available in the advice body.

  • any join point (method execution only in Spring AOP) which takes a single parameter, and where the argument passed at runtime is Serializable:

    args(java.io.Serializable)

    'args' is more commonly used in a binding form :- see the following section on advice for how to make the method arguments available in the advice body.

    Note that the pointcut given in this example is different to execution(* *(java.io.Serializable)): the args version matches if the argument passed at runtime is Serializable, the execution version matches if the method signature declares a single parameter of type Serializable.

  • any join point (method execution only in Spring AOP) where the target object has an @Transactional annotation:

    @target(org.springframework.transaction.annotation.Transactional)

    '@target' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.

  • any join point (method execution only in Spring AOP) where the declared type of the target object has an @Transactional annotation:

    @within(org.springframework.transaction.annotation.Transactional)

    '@within' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.

  • any join point (method execution only in Spring AOP) where the executing method has an @Transactional annotation:

    @annotation(org.springframework.transaction.annotation.Transactional)

    '@annotation' can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body.

  • any join point (method execution only in Spring AOP) which takes a single parameter, and where the runtime type of the argument passed has the @Classified annotation:

    @args(com.xyz.security.Classified)

    '@args' can also be used in a binding form :- see the following section on advice for how to make the annotation object(s) available in the advice body.

  • any join point (method execution only in Spring AOP) on a Spring bean named 'tradeService':

    bean(tradeService)
  • any join point (method execution only in Spring AOP) on Spring beans having names that match the wildcard expression '*Service':

    bean(*Service)

8.2.4 Declaring advice

Advice is associated with a pointcut expression, and runs before, after, or around method executions matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut, or a pointcut expression declared in place.

8.2.4.1 Before advice

Before advice is declared in an aspect using the @Before annotation:

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

  @Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
  public void doAccessCheck() {
    // ...
  }

}

If using an in-place pointcut expression we could rewrite the above example as:

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Before;

@Aspect
public class BeforeExample {

  @Before("execution(* com.xyz.myapp.dao.*.*(..))")
  public void doAccessCheck() {
    // ...
  }

}

8.2.4.2 After returning advice

After returning advice runs when a matched method execution returns normally. It is declared using the @AfterReturning annotation:

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

  @AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
  public void doAccessCheck() {
    // ...
  }

}

Note: it is of course possible to have multiple advice declarations, and other members as well, all inside the same aspect. We're just showing a single advice declaration in these examples to focus on the issue under discussion at the time.

Sometimes you need access in the advice body to the actual value that was returned. You can use the form of @AfterReturning that binds the return value for this:

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterReturning;

@Aspect
public class AfterReturningExample {

  @AfterReturning(
    pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
    returning="retVal")
  public void doAccessCheck(Object retVal) {
    // ...
  }
  
}

The name used in the returning attribute must correspond to the name of a parameter in the advice method. When a method execution returns, the return value will be passed to the advice method as the corresponding argument value. A returning clause also restricts matching to only those method executions that return a value of the specified type (Object in this case, which will match any return value).

Please note that it is not possible to return a totally different reference when using after-returning advice.

8.2.4.3 After throwing advice

After throwing advice runs when a matched method execution exits by throwing an exception. It is declared using the @AfterThrowing annotation:

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

  @AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
  public void doRecoveryActions() {
    // ...
  }

}

Often you want the advice to run only when exceptions of a given type are thrown, and you also often need access to the thrown exception in the advice body. Use the throwing attribute to both restrict matching (if desired, use Throwable as the exception type otherwise) and bind the thrown exception to an advice parameter.

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.AfterThrowing;

@Aspect
public class AfterThrowingExample {

  @AfterThrowing(
    pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()",
    throwing="ex")
  public void doRecoveryActions(DataAccessException ex) {
    // ...
  }

}

The name used in the throwing attribute must correspond to the name of a parameter in the advice method. When a method execution exits by throwing an exception, the exception will be passed to the advice method as the corresponding argument value. A throwing clause also restricts matching to only those method executions that throw an exception of the specified type (DataAccessException in this case).

8.2.4.4 After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared using the @After annotation. After advice must be prepared to handle both normal and exception return conditions. It is typically used for releasing resources, etc.

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.After;

@Aspect
public class AfterFinallyExample {

  @After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()")
  public void doReleaseLock() {
    // ...
  }

}

8.2.4.5 Around advice

The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements (i.e. don't use around advice if simple before advice would do).

Around advice is declared using the @Around annotation. The first parameter of the advice method must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the ProceedingJoinPoint causes the underlying method to execute. The proceed method may also be called passing in an Object[] - the values in the array will be used as the arguments to the method execution when it proceeds.

The behavior of proceed when called with an Object[] is a little different than the behavior of proceed for around advice compiled by the AspectJ compiler. For around advice written using the traditional AspectJ language, the number of arguments passed to proceed must match the number of arguments passed to the around advice (not the number of arguments taken by the underlying join point), and the value passed to proceed in a given argument position supplants the original value at the join point for the entity the value was bound to (Don't worry if this doesn't make sense right now!). The approach taken by Spring is simpler and a better match to its proxy-based, execution only semantics. You only need to be aware of this difference if you are compiling @AspectJ aspects written for Spring and using proceed with arguments with the AspectJ compiler and weaver. There is a way to write such aspects that is 100% compatible across both Spring AOP and AspectJ, and this is discussed in the following section on advice parameters.

import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.ProceedingJoinPoint;

@Aspect
public class AroundExample {

  @Around("com.xyz.myapp.SystemArchitecture.businessService()")
  public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
    // start stopwatch
    Object retVal = pjp.proceed();
    // stop stopwatch
    return retVal;
  }

}

The value returned by the around advice will be the return value seen by the caller of the method. A simple caching aspect for example could return a value from a cache if it has one, and invoke proceed() if it does not. Note that proceed may be invoked once, many times, or not at all within the body of the around advice, all of these are quite legal.

8.2.4.6 Advice parameters

Spring 2.0 offers fully typed advice - meaning that you declare the parameters you need in the advice signature (as we saw for the returning and throwing examples above) rather than work with Object[] arrays all the time. We'll see how to make argument and other contextual values available to the advice body in a moment. First let's take a look at how to write generic advice that can find out about the method the advice is currently advising.

Access to the current JoinPoint

Any advice method may declare as its first parameter, a parameter of type org.aspectj.lang.JoinPoint (please note that around advice is required to declare a first parameter of type ProceedingJoinPoint, which is a subclass of JoinPoint. The JoinPoint interface provides a number of useful methods such as getArgs() (returns the method arguments), getThis() (returns the proxy object), getTarget() (returns the target object), getSignature() (returns a description of the method that is being advised) and toString() (prints a useful description of the method being advised). Please do consult the Javadocs for full details.

Passing parameters to advice

We've already seen how to bind the returned value or exception value (using after returning and after throwing advice). To make argument values available to the advice body, you can use the binding form of args. If a parameter name is used in place of a type name in an args expression, then the value of the corresponding argument will be passed as the parameter value when the advice is invoked. An example should make this clearer. Suppose you want to advise the execution of dao operations that take an Account object as the first parameter, and you need access to the account in the advice body. You could write the following:

@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() &&" + 
        "args(account,..)")
public void validateAccount(Account account) {
  // ...
}

The args(account,..) part of the pointcut expression serves two purposes: firstly, it restricts matching to only those method executions where the method takes at least one parameter, and the argument passed to that parameter is an instance of Account; secondly, it makes the actual Account object available to the advice via the account parameter.

Another way of writing this is to declare a pointcut that "provides" the Account object value when it matches a join point, and then just refer to the named pointcut from the advice. This would look as follows:

@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() &&" + 
          "args(account,..)")
private void accountDataAccessOperation(Account account) {}

@Before("accountDataAccessOperation(account)")
public void validateAccount(Account account) {
  // ...
}

The interested reader is once more referred to the AspectJ programming guide for more details.

The proxy object (this), target object (target), and annotations (@within, @target, @annotation, @args) can all be bound in a similar fashion. The following example shows how you could match the execution of methods annotated with an @Auditable annotation, and extract the audit code.

First the definition of the @Auditable annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Auditable {
	AuditCode value();
}

And then the advice that matches the execution of @Auditable methods:

@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && " + 
        "@annotation(auditable)")
public void audit(Auditable auditable) {
  AuditCode code = auditable.value();
  // ...
}
Determining argument names

The parameter binding in advice invocations relies on matching names used in pointcut expressions to declared parameter names in (advice and pointcut) method signatures. Parameter names are not available through Java reflection, so Spring AOP uses the following strategies to determine parameter names:

  1. If the parameter names have been specified by the user explicitly, then the specified parameter names are used: both the advice and the pointcut annotations have an optional "argNames" attribute which can be used to specify the argument names of the annotated method - these argument names are available at runtime. For example:

    @Before(
       value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
       argNames="bean,auditable")
    public void audit(Object bean, Auditable auditable) {
      AuditCode code = auditable.value();
      // ... use code and bean
    }

    If the first parameter is of the JoinPoint, ProceedingJoinPoint, or JoinPoint.StaticPart type, you may leave out the name of the parameter from the value of the "argNames" attribute. For example, if you modify the preceding advice to receive the join point object, the "argNames" attribute need not include it:

    @Before(
       value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)",
       argNames="bean,auditable")
    public void audit(JoinPoint jp, Object bean, Auditable auditable) {
      AuditCode code = auditable.value();
      // ... use code, bean, and jp
    }

    The special treatment given to the first parameter of the JoinPoint, ProceedingJoinPoint, and JoinPoint.StaticPart types is particularly convenient for advice that do not collect any other join point context. In such situations, you may simply omit the "argNames" attribute. For example, the following advice need not declare the "argNames" attribute:

    @Before(
       "com.xyz.lib.Pointcuts.anyPublicMethod()")
    public void audit(JoinPoint jp) {
      // ... use jp
    }
  2. Using the 'argNames' attribute is a little clumsy, so if the 'argNames' attribute has not been specified, then Spring AOP will look at the debug information for the class and try to determine the parameter names from the local variable table. This information will be present as long as the classes have been compiled with debug information ('-g:vars' at a minimum). The consequences of compiling with this flag on are: (1) your code will be slightly easier to understand (reverse engineer), (2) the class file sizes will be very slightly bigger (typically inconsequential), (3) the optimization to remove unused local variables will not be applied by your compiler. In other words, you should encounter no difficulties building with this flag on.

    If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) even without the debug information then there is no need to add the argNames attribute as the compiler will retain the needed information.

  3. If the code has been compiled without the necessary debug information, then Spring AOP will attempt to deduce the pairing of binding variables to parameters (for example, if only one variable is bound in the pointcut expression, and the advice method only takes one parameter, the pairing is obvious!). If the binding of variables is ambiguous given the available information, then an AmbiguousBindingException will be thrown.

  4. If all of the above strategies fail then an IllegalArgumentException will be thrown.

Proceeding with arguments

We remarked earlier that we would describe how to write a proceed call with arguments that works consistently across Spring AOP and AspectJ. The solution is simply to ensure that the advice signature binds each of the method parameters in order. For example:

@Around("execution(List<Account> find*(..)) &&" +
        "com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " +
        "args(accountHolderNamePattern)")		
public Object preProcessQueryPattern(ProceedingJoinPoint pjp, String accountHolderNamePattern)
throws Throwable {
  String newPattern = preProcess(accountHolderNamePattern);
  return pjp.proceed(new Object[] {newPattern});
}        

In many cases you will be doing this binding anyway (as in the example above).

8.2.4.7 Advice ordering

What happens when multiple pieces of advice all want to run at the same join point? Spring AOP follows the same precedence rules as AspectJ to determine the order of advice execution. The highest precedence advice runs first "on the way in" (so given two pieces of before advice, the one with highest precedence runs first). "On the way out" from a join point, the highest precedence advice runs last (so given two pieces of after advice, the one with the highest precedence will run second).

When two pieces of advice defined in different aspects both need to run at the same join point, unless you specify otherwise the order of execution is undefined. You can control the order of execution by specifying precedence. This is done in the normal Spring way by either implementing the org.springframework.core.Ordered interface in the aspect class or annotating it with the Order annotation. Given two aspects, the aspect returning the lower value from Ordered.getValue() (or the annotation value) has the higher precedence.

When two pieces of advice defined in the same aspect both need to run at the same join point, the ordering is undefined (since there is no way to retrieve the declaration order via reflection for javac-compiled classes). Consider collapsing such advice methods into one advice method per join point in each aspect class, or refactor the pieces of advice into separate aspect classes - which can be ordered at the aspect level.

8.2.5 Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.

An introduction is made using the @DeclareParents annotation. This annotation is used to declare that matching types have a new parent (hence the name). For example, given an interface UsageTracked, and an implementation of that interface DefaultUsageTracked, the following aspect declares that all implementors of service interfaces also implement the UsageTracked interface. (In order to expose statistics via JMX for example.)

@Aspect
public class UsageTracking {

  @DeclareParents(value="com.xzy.myapp.service.*+",
                  defaultImpl=DefaultUsageTracked.class)
  public static UsageTracked mixin;
  
  @Before("com.xyz.myapp.SystemArchitecture.businessService() &&" +
          "this(usageTracked)")
  public void recordUsage(UsageTracked usageTracked) {
    usageTracked.incrementUseCount();
  }
  
}

The interface to be implemented is determined by the type of the annotated field. The value attribute of the @DeclareParents annotation is an AspectJ type pattern :- any bean of a matching type will implement the UsageTracked interface. Note that in the before advice of the above example, service beans can be directly used as implementations of the UsageTracked interface. If accessing a bean programmatically you would write the following:

UsageTracked usageTracked = (UsageTracked) context.getBean("myService");

8.2.6 Aspect instantiation models

(This is an advanced topic, so if you are just starting out with AOP you can safely skip it until later.)

By default there will be a single instance of each aspect within the application context. AspectJ calls this the singleton instantiation model. It is possible to define aspects with alternate lifecycles :- Spring supports AspectJ's perthis and pertarget instantiation models (percflow, percflowbelow, and pertypewithin are not currently supported).

A "perthis" aspect is declared by specifying a perthis clause in the @Aspect annotation. Let's look at an example, and then we'll explain how it works.

@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())")
public class MyAspect {

  private int someState;
	
  @Before(com.xyz.myapp.SystemArchitecture.businessService())
  public void recordServiceUsage() {
    // ...
  }
  	
}

The effect of the 'perthis' clause is that one aspect instance will be created for each unique service object executing a business service (each unique object bound to 'this' at join points matched by the pointcut expression). The aspect instance is created the first time that a method is invoked on the service object. The aspect goes out of scope when the service object goes out of scope. Before the aspect instance is created, none of the advice within it executes. As soon as the aspect instance has been created, the advice declared within it will execute at matched join points, but only when the service object is the one this aspect is associated with. See the AspectJ programming guide for more information on per-clauses.

The 'pertarget' instantiation model works in exactly the same way as perthis, but creates one aspect instance for each unique target object at matched join points.

8.2.7 Example

Now that you have seen how all the constituent parts work, let's put them together to do something useful!

The execution of business services can sometimes fail due to concurrency issues (for example, deadlock loser). If the operation is retried, it is quite likely to succeed next time round. For business services where it is appropriate to retry in such conditions (idempotent operations that don't need to go back to the user for conflict resolution), we'd like to transparently retry the operation to avoid the client seeing a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple services in the service layer, and hence is ideal for implementing via an aspect.

Because we want to retry the operation, we will need to use around advice so that we can call proceed multiple times. Here's how the basic aspect implementation looks:

@Aspect
public class ConcurrentOperationExecutor implements Ordered {
   
   private static final int DEFAULT_MAX_RETRIES = 2;

   private int maxRetries = DEFAULT_MAX_RETRIES;
   private int order = 1;

   public void setMaxRetries(int maxRetries) {
      this.maxRetries = maxRetries;
   }
   
   public int getOrder() {
      return this.order;
   }
   
   public void setOrder(int order) {
      this.order = order;
   }
   
   @Around("com.xyz.myapp.SystemArchitecture.businessService()")
   public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { 
      int numAttempts = 0;
      PessimisticLockingFailureException lockFailureException;
      do {
         numAttempts++;
         try { 
            return pjp.proceed();
         }
         catch(PessimisticLockingFailureException ex) {
            lockFailureException = ex;
         }
      }
      while(numAttempts <= this.maxRetries);
      throw lockFailureException;
   }

}

Note that the aspect implements the Ordered interface so we can set the precedence of the aspect higher than the transaction advice (we want a fresh transaction each time we retry). The maxRetries and order properties will both be configured by Spring. The main action happens in the doConcurrentOperation around advice. Notice that for the moment we're applying the retry logic to all businessService()s. We try to proceed, and if we fail with an PessimisticLockingFailureException we simply try again unless we have exhausted all of our retry attempts.

The corresponding Spring configuration is:

<aop:aspectj-autoproxy/>

<bean id="concurrentOperationExecutor"
  class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
     <property name="maxRetries" value="3"/>
     <property name="order" value="100"/>  
</bean>

To refine the aspect so that it only retries idempotent operations, we might define an Idempotent annotation:

@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
  // marker annotation
}

and use the annotation to annotate the implementation of service operations. The change to the aspect to only retry idempotent operations simply involves refining the pointcut expression so that only @Idempotent operations match:

@Around("com.xyz.myapp.SystemArchitecture.businessService() && " + 
        "@annotation(com.xyz.myapp.service.Idempotent)")
public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { 
  ...	
}

8.3 Schema-based AOP support

If you are unable to use Java 5, or simply prefer an XML-based format, then Spring 2.0 also offers support for defining aspects using the new "aop" namespace tags. The exact same pointcut expressions and advice kinds are supported as when using the @AspectJ style, hence in this section we will focus on the new syntax and refer the reader to the discussion in the previous section (Section 8.2, “@AspectJ support”) for an understanding of writing pointcut expressions and the binding of advice parameters.

To use the aop namespace tags described in this section, you need to import the spring-aop schema as described in Appendix A, XML Schema-based configuration. See Section A.2.7, “The aop schema” for how to import the tags in the aop namespace.

Within your Spring configurations, all aspect and advisor elements must be placed within an <aop:config> element (you can have more than one <aop:config> element in an application context configuration). An <aop:config> element can contain pointcut, advisor, and aspect elements (note these must be declared in that order).

[Warning]Warning

The <aop:config> style of configuration makes heavy use of Spring's auto-proxying mechanism. This can cause issues (such as advice not being woven) if you are already using explicit auto-proxying via the use of BeanNameAutoProxyCreator or suchlike. The recommended usage pattern is to use either just the <aop:config> style, or just the AutoProxyCreator style.

8.3.1 Declaring an aspect

Using the schema support, an aspect is simply a regular Java object defined as a bean in your Spring application context. The state and behavior is captured in the fields and methods of the object, and the pointcut and advice information is captured in the XML.

An aspect is declared using the <aop:aspect> element, and the backing bean is referenced using the ref attribute:

<aop:config>
  <aop:aspect id="myAspect" ref="aBean">
    ...
  </aop:aspect>
</aop:config>

<bean id="aBean" class="...">
  ...
</bean>

The bean backing the aspect ("aBean" in this case) can of course be configured and dependency injected just like any other Spring bean.

8.3.2 Declaring a pointcut

A named pointcut can be declared inside an <aop:config> element, enabling the pointcut definition to be shared across several aspects and advisors.

A pointcut representing the execution of any business service in the service layer could be defined as follows:

<aop:config>

  <aop:pointcut id="businessService" 
        expression="execution(* com.xyz.myapp.service.*.*(..))"/>

</aop:config>

Note that the pointcut expression itself is using the same AspectJ pointcut expression language as described in Section 8.2, “@AspectJ support”. If you are using the schema based declaration style with Java 5, you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression, but this feature is not available on JDK 1.4 and below (it relies on the Java 5 specific AspectJ reflection APIs). On JDK 1.5 therefore, another way of defining the above pointcut would be:

<aop:config>

  <aop:pointcut id="businessService" 
        expression="com.xyz.myapp.SystemArchitecture.businessService()"/>

</aop:config>

Assuming you have a SystemArchitecture aspect as described in Section 8.2.3.3, “Sharing common pointcut definitions”.

Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut:

<aop:config>

  <aop:aspect id="myAspect" ref="aBean">

    <aop:pointcut id="businessService" 
          expression="execution(* com.xyz.myapp.service.*.*(..))"/>
          
    ...
    
  </aop:aspect>

</aop:config>

Much the same way in an @AspectJ aspect, pointcuts declared using the schema based definition style may collect join point context. For example, the following pointcut collects the 'this' object as the join point context and passes it to advice:

<aop:config>

  <aop:aspect id="myAspect" ref="aBean">

    <aop:pointcut id="businessService" 
          expression="execution(* com.xyz.myapp.service.*.*(..)) &amp;&amp; this(service)"/>
    <aop:before pointcut-ref="businessService" method="monitor"/>
    ...
    
  </aop:aspect>

</aop:config>

The advice must be declared to receive the collected join point context by including parameters of the matching names:

public void monitor(Object service) {
    ...
}

When combining pointcut sub-expressions, '&&' is awkward within an XML document, and so the keywords 'and', 'or' and 'not' can be used in place of '&&', '||' and '!' respectively. For example, the previous pointcut may be better written as:

<aop:config>

  <aop:aspect id="myAspect" ref="aBean">

    <aop:pointcut id="businessService" 
          expression="execution(* com.xyz.myapp.service.*.*(..)) and this(service)"/>
    <aop:before pointcut-ref="businessService" method="monitor"/>
    ...
    
  </aop:aspect>

</aop:config>

Note that pointcuts defined in this way are referred to by their XML id and cannot be used as named pointcuts to form composite pointcuts. The named pointcut support in the schema based definition style is thus more limited than that offered by the @AspectJ style.

8.3.3 Declaring advice

The same five advice kinds are supported as for the @AspectJ style, and they have exactly the same semantics.

8.3.3.1 Before advice

Before advice runs before a matched method execution. It is declared inside an <aop:aspect> using the <aop:before> element.

<aop:aspect id="beforeExample" ref="aBean">

    <aop:before 
      pointcut-ref="dataAccessOperation" 
      method="doAccessCheck"/>
          
    ...
    
</aop:aspect>

Here dataAccessOperation is the id of a pointcut defined at the top (<aop:config>) level. To define the pointcut inline instead, replace the pointcut-ref attribute with a pointcut attribute:

<aop:aspect id="beforeExample" ref="aBean">

    <aop:before 
      pointcut="execution(* com.xyz.myapp.dao.*.*(..))" 
      method="doAccessCheck"/>
          
    ...
    
</aop:aspect>

As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve the readability of your code.

The method attribute identifies a method (doAccessCheck) that provides the body of the advice. This method must be defined for the bean referenced by the aspect element containing the advice. Before a data access operation is executed (a method execution join point matched by the pointcut expression), the "doAccessCheck" method on the aspect bean will be invoked.

8.3.3.2 After returning advice

After returning advice runs when a matched method execution completes normally. It is declared inside an <aop:aspect> in the same way as before advice. For example:

<aop:aspect id="afterReturningExample" ref="aBean">

    <aop:after-returning 
      pointcut-ref="dataAccessOperation" 
      method="doAccessCheck"/>
          
    ...
    
</aop:aspect>

Just as in the @AspectJ style, it is possible to get hold of the return value within the advice body. Use the returning attribute to specify the name of the parameter to which the return value should be passed:

<aop:aspect id="afterReturningExample" ref="aBean">

    <aop:after-returning 
      pointcut-ref="dataAccessOperation"
      returning="retVal" 
      method="doAccessCheck"/>
          
    ...
    
</aop:aspect>

The doAccessCheck method must declare a parameter named retVal. The type of this parameter constrains matching in the same way as described for @AfterReturning. For example, the method signature may be declared as:

public void doAccessCheck(Object retVal) {...

8.3.3.3 After throwing advice

After throwing advice executes when a matched method execution exits by throwing an exception. It is declared inside an <aop:aspect> using the after-throwing element:

<aop:aspect id="afterThrowingExample" ref="aBean">

    <aop:after-throwing
      pointcut-ref="dataAccessOperation" 
      method="doRecoveryActions"/>
          
    ...
    
</aop:aspect>

Just as in the @AspectJ style, it is possible to get hold of the thrown exception within the advice body. Use the throwing attribute to specify the name of the parameter to which the exception should be passed:

<aop:aspect id="afterThrowingExample" ref="aBean">

    <aop:after-throwing 
      pointcut-ref="dataAccessOperation"
      throwing="dataAccessEx" 
      method="doRecoveryActions"/>
          
    ...
    
</aop:aspect>

The doRecoveryActions method must declare a parameter named dataAccessEx. The type of this parameter constrains matching in the same way as described for @AfterThrowing. For example, the method signature may be declared as:

public void doRecoveryActions(DataAccessException dataAccessEx) {...

8.3.3.4 After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared using the after element:

<aop:aspect id="afterFinallyExample" ref="aBean">

    <aop:after
      pointcut-ref="dataAccessOperation" 
      method="doReleaseLock"/>
          
    ...
    
</aop:aspect>

8.3.3.5 Around advice

The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements; don't use around advice if simple before advice would do.

Around advice is declared using the aop:around element. The first parameter of the advice method must be of type ProceedingJoinPoint. Within the body of the advice, calling proceed() on the ProceedingJoinPoint causes the underlying method to execute. The proceed method may also be calling passing in an Object[] - the values in the array will be used as the arguments to the method execution when it proceeds. See Section 8.2.4.5, “Around advice” for notes on calling proceed with an Object[].

<aop:aspect id="aroundExample" ref="aBean">

    <aop:around
      pointcut-ref="businessService" 
      method="doBasicProfiling"/>
          
    ...
    
</aop:aspect>

The implementation of the doBasicProfiling advice would be exactly the same as in the @AspectJ example (minus the annotation of course):

public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable {
    // start stopwatch
    Object retVal = pjp.proceed();
    // stop stopwatch
    return retVal;
}

8.3.3.6 Advice parameters

The schema based declaration style supports fully typed advice in the same way as described for the @AspectJ support - by matching pointcut parameters by name against advice method parameters. See Section 8.2.4.6, “Advice parameters” for details. If you wish to explicitly specify argument names for the advice methods (not relying on the detection strategies previously described) then this is done using the arg-names attribute of the advice element, which is treated in the same manner to the "argNames" attribute in an advice annotation as described in the section called “Determining argument names”. For example:

<aop:before
  pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)"
  method="audit"
  arg-names="auditable"/>

The arg-names attribute accepts a comma-delimited list of parameter names.

Find below a slightly more involved example of the XSD-based approach that illustrates some around advice used in conjunction with a number of strongly typed parameters.

package x.y.service;

public interface FooService {

   Foo getFoo(String fooName, int age);
}

public class DefaultFooService implements FooService {

   public Foo getFoo(String name, int age) {
      return new Foo(name, age);
   }
}

Next up is the aspect. Notice the fact that the profile(..) method accepts a number of strongly-typed parameters, the first of which happens to be the join point used to proceed with the method call: the presence of this parameter is an indication that the profile(..) is to be used as around advice:

package x.y;

import org.aspectj.lang.ProceedingJoinPoint;
import org.springframework.util.StopWatch;

public class SimpleProfiler {

   public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable {
      StopWatch clock = new StopWatch(
            "Profiling for '" + name + "' and '" + age + "'");
      try {
         clock.start(call.toShortString());
         return call.proceed();
      } finally {
         clock.stop();
         System.out.println(clock.prettyPrint());
      }
   }
}

Finally, here is the XML configuration that is required to effect the execution of the above advice for a particular join point:

<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xmlns:aop="http://www.springframework.org/schema/aop"
      xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd">

   <!-- this is the object that will be proxied by Spring's AOP infrastructure -->
   <bean id="fooService" class="x.y.service.DefaultFooService"/>

   <!-- this is the actual advice itself -->
   <bean id="profiler" class="x.y.SimpleProfiler"/>

   <aop:config>
      <aop:aspect ref="profiler">

         <aop:pointcut id="theExecutionOfSomeFooServiceMethod"
                    expression="execution(* x.y.service.FooService.getFoo(String,int))
                    and args(name, age)"/>

         <aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod"
                  method="profile"/>

      </aop:aspect>
   </aop:config>

</beans>

If we had the following driver script, we would get output something like this on standard output:

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import x.y.service.FooService;

public final class Boot {

   public static void main(final String[] args) throws Exception {
      BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml");
      FooService foo = (FooService) ctx.getBean("fooService");
      foo.getFoo("Pengo", 12);
   }
}
StopWatch 'Profiling for 'Pengo' and '12'': running time (millis) = 0
-----------------------------------------
ms     %     Task name
-----------------------------------------
00000  ?  execution(getFoo)

8.3.3.7 Advice ordering

When multiple advice needs to execute at the same join point (executing method) the ordering rules are as described in Section 8.2.4.7, “Advice ordering”. The precedence between aspects is determined by either adding the Order annotation to the bean backing the aspect or by having the bean implement the Ordered interface.

8.3.4 Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.

An introduction is made using the aop:declare-parents element inside an aop:aspect This element is used to declare that matching types have a new parent (hence the name). For example, given an interface UsageTracked, and an implementation of that interface DefaultUsageTracked, the following aspect declares that all implementors of service interfaces also implement the UsageTracked interface. (In order to expose statistics via JMX for example.)

<aop:aspect id="usageTrackerAspect" ref="usageTracking">

  <aop:declare-parents
      types-matching="com.xzy.myapp.service.*+"
      implement-interface="com.xyz.myapp.service.tracking.UsageTracked"
      default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/>
  
  <aop:before
    pointcut="com.xyz.myapp.SystemArchitecture.businessService()
              and this(usageTracked)"
    method="recordUsage"/>
  
</aop:aspect>

The class backing the usageTracking bean would contain the method:

public void recordUsage(UsageTracked usageTracked) {
    usageTracked.incrementUseCount();
}

The interface to be implemented is determined by implement-interface attribute. The value of the types-matching attribute is an AspectJ type pattern :- any bean of a matching type will implement the UsageTracked interface. Note that in the before advice of the above example, service beans can be directly used as implementations of the UsageTracked interface. If accessing a bean programmatically you would write the following:

UsageTracked usageTracked = (UsageTracked) context.getBean("myService");

8.3.5 Aspect instantiation models

The only supported instantiation model for schema-defined aspects is the singleton model. Other instantiation models may be supported in future releases.

8.3.6 Advisors

The concept of "advisors" is brought forward from the AOP support defined in Spring 1.2 and does not have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single piece of advice. The advice itself is represented by a bean, and must implement one of the advice interfaces described in Section 9.3.2, “Advice types in Spring”. Advisors can take advantage of AspectJ pointcut expressions though.

Spring 2.0 supports the advisor concept with the <aop:advisor> element. You will most commonly see it used in conjunction with transactional advice, which also has its own namespace support in Spring 2.0. Here's how it looks:

<aop:config>

  <aop:pointcut id="businessService"
        expression="execution(* com.xyz.myapp.service.*.*(..))"/>

  <aop:advisor 
      pointcut-ref="businessService"
      advice-ref="tx-advice"/>
      
</aop:config>

<tx:advice id="tx-advice">
  <tx:attributes>
    <tx:method name="*" propagation="REQUIRED"/>
  </tx:attributes>
</tx:advice>

As well as the pointcut-ref attribute used in the above example, you can also use the pointcut attribute to define a pointcut expression inline.

To define the precedence of an advisor so that the advice can participate in ordering, use the order attribute to define the Ordered value of the advisor.

8.3.7 Example

Let's see how the concurrent locking failure retry example from Section 8.2.7, “Example” looks when rewritten using the schema support.

The execution of business services can sometimes fail due to concurrency issues (for example, deadlock loser). If the operation is retried, it is quite likely it will succeed next time round. For business services where it is appropriate to retry in such conditions (idempotent operations that don't need to go back to the user for conflict resolution), we'd like to transparently retry the operation to avoid the client seeing a PessimisticLockingFailureException. This is a requirement that clearly cuts across multiple services in the service layer, and hence is ideal for implementing via an aspect.

Because we want to retry the operation, we'll need to use around advice so that we can call proceed multiple times. Here's how the basic aspect implementation looks (it's just a regular Java class using the schema support):

public class ConcurrentOperationExecutor implements Ordered {
   
   private static final int DEFAULT_MAX_RETRIES = 2;

   private int maxRetries = DEFAULT_MAX_RETRIES;
   private int order = 1;

   public void setMaxRetries(int maxRetries) {
      this.maxRetries = maxRetries;
   }
   
   public int getOrder() {
      return this.order;
   }
   
   public void setOrder(int order) {
      this.order = order;
   }
   
   public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { 
      int numAttempts = 0;
      PessimisticLockingFailureException lockFailureException;
      do {
         numAttempts++;
         try { 
            return pjp.proceed();
         }
         catch(PessimisticLockingFailureException ex) {
            lockFailureException = ex;
         }
      }
      while(numAttempts <= this.maxRetries);
      throw lockFailureException;
   }

}

Note that the aspect implements the Ordered interface so we can set the precedence of the aspect higher than the transaction advice (we want a fresh transaction each time we retry). The maxRetries and order properties will both be configured by Spring. The main action happens in the doConcurrentOperation around advice method. We try to proceed, and if we fail with a PessimisticLockingFailureException we simply try again unless we have exhausted all of our retry attempts.

This class is identical to the one used in the @AspectJ example, but with the annotations removed.

The corresponding Spring configuration is:

<aop:config>

  <aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor">

    <aop:pointcut id="idempotentOperation"
        expression="execution(* com.xyz.myapp.service.*.*(..))"/>
       
    <aop:around
       pointcut-ref="idempotentOperation"
       method="doConcurrentOperation"/>
  
  </aop:aspect>

</aop:config>

<bean id="concurrentOperationExecutor"
  class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor">
     <property name="maxRetries" value="3"/>
     <property name="order" value="100"/>  
</bean>

Notice that for the time being we assume that all business services are idempotent. If this is not the case we can refine the aspect so that it only retries genuinely idempotent operations, by introducing an Idempotent annotation:

@Retention(RetentionPolicy.RUNTIME)
public @interface Idempotent {
  // marker annotation
}

and using the annotation to annotate the implementation of service operations. The change to the aspect to retry only idempotent operations simply involves refining the pointcut expression so that only @Idempotent operations match:

  <aop:pointcut id="idempotentOperation"
        expression="execution(* com.xyz.myapp.service.*.*(..)) and
                    @annotation(com.xyz.myapp.service.Idempotent)"/>

8.4 Choosing which AOP declaration style to use

Once you have decided that an aspect is the best approach for implementing a given requirement, how do you decide between using Spring AOP or AspectJ, and between the Aspect language (code) style, @AspectJ annotation style, or the Spring XML style? These decisions are influenced by a number of factors including application requirements, development tools, and team familiarity with AOP.

8.4.1 Spring AOP or full AspectJ?

Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ as there is no requirement to introduce the AspectJ compiler / weaver into your development and build processes. If you only need to advise the execution of operations on Spring beans, then Spring AOP is the right choice. If you need to advise objects not managed by the Spring container (such as domain objects typically), then you will need to use AspectJ. You will also need to use AspectJ if you wish to advise join points other than simple method executions (for example, field get or set join points, and so on).

When using AspectJ, you have the choice of the AspectJ language syntax (also known as the "code style") or the @AspectJ annotation style. Clearly, if you are not using Java 5+ then the choice has been made for you... use the code style. If aspects play a large role in your design, and you are able to use the AspectJ Development Tools (AJDT) plugin for Eclipse, then the AspectJ language syntax is the preferred option: it is cleaner and simpler because the language was purposefully designed for writing aspects. If you are not using Eclipse, or have only a few aspects that do not play a major role in your application, then you may want to consider using the @AspectJ style and sticking with a regular Java compilation in your IDE, and adding an aspect weaving phase to your build script.

8.4.2 @AspectJ or XML for Spring AOP?

If you have chosen to use Spring AOP, then you have a choice of @AspectJ or XML style. Clearly if you are not running on Java 5+, then the XML style is the appropriate choice; for Java 5 projects there are various tradeoffs to consider.

The XML style will be most familiar to existing Spring users. It can be used with any JDK level (referring to named pointcuts from within pointcut expressions does still require Java 5+ though) and is backed by genuine POJOs. When using AOP as a tool to configure enterprise services then XML can be a good choice (a good test is whether you consider the pointcut expression to be a part of your configuration you might want to change independently). With the XML style arguably it is clearer from your configuration what aspects are present in the system.

The XML style has two disadvantages. Firstly it does not fully encapsulate the implementation of the requirement it addresses in a single place. The DRY principle says that there should be a single, unambiguous, authoritative representation of any piece of knowledge within a system. When using the XML style, the knowledge of how a requirement is implemented is split across the declaration of the backing bean class, and the XML in the configuration file. When using the @AspectJ style there is a single module - the aspect - in which this information is encapsulated. Secondly, the XML style is slightly more limited in what it can express than the @AspectJ style: only the "singleton" aspect instantiation model is supported, and it is not possible to combine named pointcuts declared in XML. For example, in the @AspectJ style you can write something like:

  @Pointcut(execution(* get*()))
  public void propertyAccess() {}

  @Pointcut(execution(org.xyz.Account+ *(..))
  public void operationReturningAnAccount() {}

  @Pointcut(propertyAccess() && operationReturningAnAccount())
  public void accountPropertyAccess() {}

In the XML style I can declare the first two pointcuts:

  <aop:pointcut id="propertyAccess"
      expression="execution(* get*())"/>

  <aop:pointcut id="operationReturningAnAccount"
      expression="execution(org.xyz.Account+ *(..))"/>

The downside of the XML approach is that you cannot define the 'accountPropertyAccess' pointcut by combining these definitions.

The @AspectJ style supports additional instantiation models, and richer pointcut composition. It has the advantage of keeping the aspect as a modular unit. It also has the advantage the @AspectJ aspects can be understood (and thus consumed) both by Spring AOP and by AspectJ - so if you later decide you need the capabilities of AspectJ to implement additional requirements then it is very easy to migrate to an AspectJ-based approach. On balance the Spring team prefer the @AspectJ style whenever you have aspects that do more than simple "configuration" of enterprise services.

8.5 Mixing aspect types

It is perfectly possible to mix @AspectJ style aspects using the autoproxying support, schema-defined <aop:aspect> aspects, <aop:advisor> declared advisors and even proxies and interceptors defined using the Spring 1.2 style in the same configuration. All of these are implemented using the same underlying support mechanism and will co-exist without any difficulty.

8.6 Proxying mechanisms

Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given target object. (JDK dynamic proxies are preferred whenever you have a choice).

If the target object to be proxied implements at least one interface then a JDK dynamic proxy will be used. All of the interfaces implemented by the target type will be proxied. If the target object does not implement any interfaces then a CGLIB proxy will be created.

If you want to force the use of CGLIB proxying (for example, to proxy every method defined for the target object, not just those implemented by its interfaces) you can do so. However, there are some issues to consider:

  • final methods cannot be advised, as they cannot be overriden.

  • You will need the CGLIB 2 binaries on your classpath, whereas dynamic proxies are available with the JDK. Spring will automatically warn you when it needs CGLIB and the CGLIB library classes are not found on the classpath.

  • The constructor of your proxied object will be called twice. This is a natural consequence of the CGLIB proxy model whereby a subclass is generated for each proxied object. For each proxied instance, two objects are created: the actual proxied object and an instance of the subclass that implements the advice. This behavior is not exhibited when using JDK proxies. Usually, calling the constructor of the proxied type twice, is not an issue, as there are usually only assignments taking place and no real logic is implemented in the constructor.

To force the use of CGLIB proxies set the value of the proxy-target-class attribute of the <aop:config> element to true:

<aop:config proxy-target-class="true">
    <!-- other beans defined here... -->
</aop:config>

To force CGLIB proxying when using the @AspectJ autoproxy support, set the 'proxy-target-class' attribute of the <aop:aspectj-autoproxy> element to true:

<aop:aspectj-autoproxy proxy-target-class="true"/>
[Note]Note

Multiple <aop:config/> sections are collapsed into a single unified auto-proxy creator at runtime, which applies the strongest proxy settings that any of the <aop:config/> sections (typically from different XML bean definition files) specified. This also applies to the <tx:annotation-driven/> and <aop:aspectj-autoproxy/> elements.

To be clear: using 'proxy-target-class="true"' on <tx:annotation-driven/>, <aop:aspectj-autoproxy/> or <aop:config/> elements will force the use of CGLIB proxies for all three of them.

8.6.1 Understanding AOP proxies

Spring AOP is proxy-based. It is vitally important that you grasp the semantics of what that last statement actually means before you write your own aspects or use any of the Spring AOP-based aspects supplied with the Spring Framework.

Consider first the scenario where you have a plain-vanilla, un-proxied, nothing-special-about-it, straight object reference, as illustrated by the following code snippet.

public class SimplePojo implements Pojo {

   public void foo() {
      // this next method invocation is a direct call on the 'this' reference
      this.bar();
   }
   
   public void bar() {
      // some logic...
   }
}

If you invoke a method on an object reference, the method is invoked directly on that object reference, as can be seen below.

public class Main {

   public static void main(String[] args) {
   
      Pojo pojo = new SimplePojo();
      
      // this is a direct method call on the 'pojo' reference
      pojo.foo();
   }
}

Things change slightly when the reference that client code has is a proxy. Consider the following diagram and code snippet.

public class Main {

   public static void main(String[] args) {
   
      ProxyFactory factory = new ProxyFactory(new SimplePojo());
      factory.addInterface(Pojo.class);
      factory.addAdvice(new RetryAdvice());

      Pojo pojo = (Pojo) factory.getProxy();
      
      // this is a method call on the proxy!
      pojo.foo();
   }
}

The key thing to understand here is that the client code inside the main(..) of the Main class has a reference to the proxy. This means that method calls on that object reference will be calls on the proxy, and as such the proxy will be able to delegate to all of the interceptors (advice) that are relevant to that particular method call. However, once the call has finally reached the target object, the SimplePojo reference in this case, any method calls that it may make on itself, such as this.bar() or this.foo(), are going to be invoked against the this reference, and not the proxy. This has important implications. It means that self-invocation is not going to result in the advice associated with a method invocation getting a chance to execute.

Okay, so what is to be done about this? The best approach (the term best is used loosely here) is to refactor your code such that the self-invocation does not happen. For sure, this does entail some work on your part, but it is the best, least-invasive approach. The next approach is absolutely horrendous, and I am almost reticent to point it out precisely because it is so horrendous. You can (choke!) totally tie the logic within your class to Spring AOP by doing this:

public class SimplePojo implements Pojo {

   public void foo() {
      // this works, but... gah!
      ((Pojo) AopContext.currentProxy()).bar();
   }
   
   public void bar() {
      // some logic...
   }
}

This totally couples your code to Spring AOP, and it makes the class itself aware of the fact that it is being used in an AOP context, which flies in the face of AOP. It also requires some additional configuration when the proxy is being created:

public class Main {

   public static void main(String[] args) {
   
      ProxyFactory factory = new ProxyFactory(new SimplePojo());
      factory.adddInterface(Pojo.class);
      factory.addAdvice(new RetryAdvice());
      factory.setExposeProxy(true);

      Pojo pojo = (Pojo) factory.getProxy();

      // this is a method call on the proxy!
      pojo.foo();
   }
}

Finally, it must be noted that AspectJ does not have this self-invocation issue because it is not a proxy-based AOP framework.

8.7 Programmatic creation of @AspectJ Proxies

In addition to declaring aspects in your configuration using either <aop:config> or <aop:aspectj-autoproxy>, it is also possible programmatically to create proxies that advise target objects. For the full details of Spring's AOP API, see the next chapter. Here we want to focus on the ability to automatically create proxies using @AspectJ aspects.

The class org.springframework.aop.aspectj.annotation.AspectJProxyFactory can be used to create a proxy for a target object that is advised by one or more @AspectJ aspects. Basic usage for this class is very simple, as illustrated below. See the Javadocs for full information.

// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject); 

// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);

// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
factory.addAspect(usageTracker);	

// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();

8.8 Using AspectJ with Spring applications

Everything we've covered so far in this chapter is pure Spring AOP. In this section, we're going to look at how you can use the AspectJ compiler/weaver instead of, or in addition to, Spring AOP if your needs go beyond the facilities offered by Spring AOP alone.

Spring ships with a small AspectJ aspect library, which is available standalone in your distribution as spring-aspects.jar; you'll need to add this to your classpath in order to use the aspects in it. Section 8.8.1, “Using AspectJ to dependency inject domain objects with Spring” and Section 8.8.2, “Other Spring aspects for AspectJ” discuss the content of this library and how you can use it. Section 8.8.3, “Configuring AspectJ aspects using Spring IoC” discusses how to dependency inject AspectJ aspects that are woven using the AspectJ compiler. Finally, Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework” provides an introduction to load-time weaving for Spring applications using AspectJ.

8.8.1 Using AspectJ to dependency inject domain objects with Spring

The Spring container instantiates and configures beans defined in your application context. It is also possible to ask a bean factory to configure a pre-existing object given the name of a bean definition containing the configuration to be applied. The spring-aspects.jar contains an annotation-driven aspect that exploits this capability to allow dependency injection of any object. The support is intended to be used for objects created outside of the control of any container. Domain objects often fall into this category because they are often created programmatically using the new operator, or by an ORM tool as a result of a database query.

The @Configurable annotation marks a class as eligible for Spring-driven configuration. In the simplest case it can be used just as a marker annotation:

package com.xyz.myapp.domain;

import org.springframework.beans.factory.annotation.Configurable;

@Configurable
public class Account {
   // ...
}

When used as a marker interface in this way, Spring will configure new instances of the annotated type (Account in this case) using a prototype-scoped bean definition with the same name as the fully-qualified type name (com.xyz.myapp.domain.Account). Since the default name for a bean is the fully-qualified name of its type, a convenient way to declare the prototype definition is simply to omit the id attribute:

<bean class="com.xyz.myapp.domain.Account" scope="prototype">
  <property name="fundsTransferService" ref="fundsTransferService"/>
</bean>

If you want to explicitly specify the name of the prototype bean definition to use, you can do so directly in the annotation:

package com.xyz.myapp.domain;

import org.springframework.beans.factory.annotation.Configurable;

@Configurable("account")
public class Account {
   // ...
}

Spring will now look for a bean definition named "account" and use that as the definition to configure new Account instances.

You can also use autowiring to avoid having to specify a prototype-scoped bean definition at all. To have Spring apply autowiring use the 'autowire' property of the @Configurable annotation: specify either @Configurable(autowire=Autowire.BY_TYPE) or @Configurable(autowire=Autowire.BY_NAME for autowiring by type or by name respectively. As an alternative, as of Spring 2.5 it is preferable to specify explicit, annotation-driven dependency injection for your @Configurable beans by using @Autowired and @Resource at the field or method level (see Section 4.11, “Annotation-based configuration” for further details).

Finally you can enable Spring dependency checking for the object references in the newly created and configured object by using the dependencyCheck attribute (for example: @Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)). If this attribute is set to true, then Spring will validate after configuration that all properties (which are not primitives or collections) have been set.

Using the annotation on its own does nothing of course. It is the AnnotationBeanConfigurerAspect in spring-aspects.jar that acts on the presence of the annotation. In essence the aspect says "after returning from the initialization of a new object of a type annotated with @Configurable, configure the newly created object using Spring in accordance with the properties of the annotation". In this context, initialization refers to newly instantiated objects (e.g., objects instantiated with the 'new' operator) as well as to Serializable objects that are undergoing deserialization (e.g., via readResolve()).

[Note]Note

One of the key phrases in the above paragraph is 'in essence'. For most cases, the exact semantics of 'after returning from the initialization of a new object' will be fine... in this context, 'after initialization' means that the dependencies will be injected after the object has been constructed - this means that the dependencies will not be available for use in the constructor bodies of the class. If you want the dependencies to be injected before the constructor bodies execute, and thus be available for use in the body of the constructors, then you need to define this on the @Configurable declaration like so:

@Configurable(preConstruction=true)

You can find out more information about the language semantics of the various pointcut types in AspectJ in this appendix of the AspectJ Programming Guide.

For this to work the annotated types must be woven with the AspectJ weaver - you can either use a build-time Ant or Maven task to do this (see for example the AspectJ Development Environment Guide) or load-time weaving (see Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”). The AnnotationBeanConfigurerAspect itself needs configuring by Spring (in order to obtain a reference to the bean factory that is to be used to configure new objects). The Spring context namespace defines a convenient tag for doing this: just include the following in your application context configuration:

<context:spring-configured/>

If you are using the DTD instead of schema, the equivalent definition is:

<bean 
      class="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect"
      factory-method="aspectOf"/>

Instances of @Configurable objects created before the aspect has been configured will result in a warning being issued to the log and no configuration of the object taking place. An example might be a bean in the Spring configuration that creates domain objects when it is initialized by Spring. In this case you can use the "depends-on" bean attribute to manually specify that the bean depends on the configuration aspect.

<bean id="myService"
  class="com.xzy.myapp.service.MyService"
  depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect">

  <!-- ... -->

</bean>

8.8.1.1 Unit testing @Configurable objects

One of the goals of the @Configurable support is to enable independent unit testing of domain objects without the difficulties associated with hard-coded lookups. If @Configurable types have not been woven by AspectJ then the annotation has no affect during unit testing, and you can simply set mock or stub property references in the object under test and proceed as normal. If @Configurable types have been woven by AspectJ then you can still unit test outside of the container as normal, but you will see a warning message each time that you construct an @Configurable object indicating that it has not been configured by Spring.

8.8.1.2 Working with multiple application contexts

The AnnotationBeanConfigurerAspect used to implement the @Configurable support is an AspectJ singleton aspect. The scope of a singleton aspect is the same as the scope of static members, that is to say there is one aspect instance per classloader that defines the type. This means that if you define multiple application contexts within the same classloader hierarchy you need to consider where to define the <context:spring-configured/> bean and where to place spring-aspects.jar on the classpath.

Consider a typical Spring web-app configuration with a shared parent application context defining common business services and everything needed to support them, and one child application context per servlet containing definitions particular to that servlet. All of these contexts will co-exist within the same classloader hierarchy, and so the AnnotationBeanConfigurerAspect can only hold a reference to one of them. In this case we recommend defining the <context:spring-configured/> bean in the shared (parent) application context: this defines the services that you are likely to want to inject into domain objects. A consequence is that you cannot configure domain objects with references to beans defined in the child (servlet-specific) contexts using the @Configurable mechanism (probably not something you want to do anyway!).

When deploying multiple web-apps within the same container, ensure that each web-application loads the types in spring-aspects.jar using its own classloader (for example, by placing spring-aspects.jar in 'WEB-INF/lib'). If spring-aspects.jar is only added to the container wide classpath (and hence loaded by the shared parent classloader), all web applications will share the same aspect instance which is probably not what you want.

8.8.2 Other Spring aspects for AspectJ

In addition to the @Configurable aspect, spring-aspects.jar contains an AspectJ aspect that can be used to drive Spring's transaction management for types and methods annotated with the @Transactional annotation. This is primarily intended for users who want to use the Spring Framework's transaction support outside of the Spring container.

The aspect that interprets @Transactional annotations is the AnnotationTransactionAspect. When using this aspect, you must annotate the implementation class (and/or methods within that class), not the interface (if any) that the class implements. AspectJ follows Java's rule that annotations on interfaces are not inherited.

A @Transactional annotation on a class specifies the default transaction semantics for the execution of any public operation in the class.

A @Transactional annotation on a method within the class overrides the default transaction semantics given by the class annotation (if present). Methods with public, protected, and default visibility may all be annotated. Annotating protected and default visibility methods directly is the only way to get transaction demarcation for the execution of such methods.

For AspectJ programmers that want to use the Spring configuration and transaction management support but don't want to (or cannot) use annotations, spring-aspects.jar also contains abstract aspects you can extend to provide your own pointcut definitions. See the sources for the AbstractBeanConfigurerAspect and AbstractTransactionAspect aspects for more information. As an example, the following excerpt shows how you could write an aspect to configure all instances of objects defined in the domain model using prototype bean definitions that match the fully-qualified class names:

public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect {

  public DomainObjectConfiguration() {
    setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver());
  }

  // the creation of a new bean (any object in the domain model)
  protected pointcut beanCreation(Object beanInstance) :
    initialization(new(..)) &&
    SystemArchitecture.inDomainModel() && 
    this(beanInstance);
		   		   
}

8.8.3 Configuring AspectJ aspects using Spring IoC

When using AspectJ aspects with Spring applications, it is natural to both want and expect to be able to configure such aspects using Spring. The AspectJ runtime itself is responsible for aspect creation, and the means of configuring the AspectJ created aspects via Spring depends on the AspectJ instantiation model (the 'per-xxx' clause) used by the aspect.

The majority of AspectJ aspects are singleton aspects. Configuration of these aspects is very easy: simply create a bean definition referencing the aspect type as normal, and include the bean attribute 'factory-method="aspectOf"'. This ensures that Spring obtains the aspect instance by asking AspectJ for it rather than trying to create an instance itself. For example:

<bean id="profiler" class="com.xyz.profiler.Profiler"
      factory-method="aspectOf">
  <property name="profilingStrategy" ref="jamonProfilingStrategy"/>
</bean>

Non-singleton aspects are harder to configure: however it is possible to do so by creating prototype bean definitions and using the @Configurable support from spring-aspects.jar to configure the aspect instances once they have bean created by the AspectJ runtime.

If you have some @AspectJ aspects that you want to weave with AspectJ (for example, using load-time weaving for domain model types) and other @AspectJ aspects that you want to use with Spring AOP, and these aspects are all configured using Spring, then you will need to tell the Spring AOP @AspectJ autoproxying support which exact subset of the @AspectJ aspects defined in the configuration should be used for autoproxying. You can do this by using one or more <include/> elements inside the <aop:aspectj-autoproxy/> declaration. Each <include/> element specifies a name pattern, and only beans with names matched by at least one of the patterns will be used for Spring AOP autoproxy configuration:

<aop:aspectj-autoproxy>
  <aop:include name="thisBean"/>
  <aop:include name="thatBean"/>
</aop:aspectj-autoproxy>
[Note]Note

Do not be misled by the name of the <aop:aspectj-autoproxy/> element: using it will result in the creation of Spring AOP proxies. The @AspectJ style of aspect declaration is just being used here, but the AspectJ runtime is not involved.

8.8.4 Load-time weaving with AspectJ in the Spring Framework

Load-time weaving (LTW) refers to the process of weaving AspectJ aspects into an application's class files as they are being loaded into the Java virtual machine (JVM). The focus of this section is on configuring and using LTW in the specific context of the Spring Framework: this section is not an introduction to LTW though. For full details on the specifics of LTW and configuring LTW with just AspectJ (with Spring not being involved at all), see the LTW section of the AspectJ Development Environment Guide.

The value-add that the Spring Framework brings to AspectJ LTW is in enabling much finer-grained control over the weaving process. 'Vanilla' AspectJ LTW is effected using a Java (5+) agent, which is switched on by specifying a VM argument when starting up a JVM. It is thus a JVM-wide setting, which may be fine in some situations, but often is a little too coarse. Spring-enabled LTW enables you to switch on LTW on a per-ClassLoader basis, which obviously is more fine-grained and which can make more sense in a 'single-JVM-multiple-application' environment (such as is found in a typical application server environment).

Further, in certain environments, this support enables load-time weaving without making any modifications to the application server's launch script that will be needed to add -javaagent:path/to/aspectjweaver.jar or (as we describe later in this section) -javaagent:path/to/spring-agent.jar. Developers simply modify one or more files that form the application context to enable load-time weaving instead of relying on administrators who typically are in charge of the deployment configuration such as the launch script.

Now that the sales pitch is over, let us first walk through a quick example of AspectJ LTW using Spring, followed by detailed specifics about elements introduced in the following example. For a complete example, please see the Petclinic sample application.

8.8.4.1 A first example

Let us assume that you are an application developer who has been tasked with diagnosing the cause of some performance problems in a system. Rather than break out a profiling tool, what we are going to do is switch on a simple profiling aspect that will enable us to very quickly get some performance metrics, so that we can then apply a finer-grained profiling tool to that specific area immediately afterwards.

Here is the profiling aspect. Nothing too fancy, just a quick-and-dirty time-based profiler, using the @AspectJ-style of aspect declaration.

package foo;

import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Pointcut;
import org.springframework.util.StopWatch;
import org.springframework.core.annotation.Order;

@Aspect
public class ProfilingAspect {

    @Around("methodsToBeProfiled()")
    public Object profile(ProceedingJoinPoint pjp) throws Throwable {
        StopWatch sw = new StopWatch(getClass().getSimpleName());
        try {
            sw.start(pjp.getSignature().getName());
            return pjp.proceed();
        } finally {
            sw.stop();
            System.out.println(sw.prettyPrint());
        }
    }

    @Pointcut("execution(public * foo..*.*(..))")
    public void methodsToBeProfiled(){}
}

We will also need to create an 'META-INF/aop.xml' file, to inform the AspectJ weaver that we want to weave our ProfilingAspect into our classes. This file convention, namely the presence of a file (or files) on the Java classpath called ' META-INF/aop.xml' is standard AspectJ.

<!DOCTYPE aspectj PUBLIC
        "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd">
<aspectj>

    <weaver>

        <!-- only weave classes in our application-specific packages -->
        <include within="foo.*"/>

    </weaver>

    <aspects>

        <!-- weave in just this aspect -->        
        <aspect name="foo.ProfilingAspect"/>

    </aspects>

  </aspectj>

Now to the Spring-specific portion of the configuration. We need to configure a LoadTimeWeaver (all explained later, just take it on trust for now). This load-time weaver is the essential component responsible for weaving the aspect configuration in one or more 'META-INF/aop.xml' files into the classes in your application. The good thing is that it does not require a lot of configuration, as can be seen below (there are some more options that you can specify, but these are detailed later).

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <!-- a service object; we will be profiling its methods -->
    <bean id="entitlementCalculationService"
          class="foo.StubEntitlementCalculationService"/>

    <!-- this switches on the load-time weaving -->
    <context:load-time-weaver/>

</beans>

Now that all the required artifacts are in place - the aspect, the 'META-INF/aop.xml' file, and the Spring configuration -, let us create a simple driver class with a main(..) method to demonstrate the LTW in action.

package foo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Main {

    public static void main(String[] args) {

        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml", Main.class);

        EntitlementCalculationService entitlementCalculationService
            = (EntitlementCalculationService) ctx.getBean("entitlementCalculationService");

        // the profiling aspect is 'woven' around this method execution
        entitlementCalculationService.calculateEntitlement();
    }
}

There is one last thing to do. The introduction to this section did say that one could switch on LTW selectively on a per-ClassLoader basis with Spring, and this is true. However, just for this example, we are going to use a Java agent (supplied with Spring) to switch on the LTW. This is the command line we will use to run the above Main class:

java -javaagent:C:/projects/foo/lib/global/spring-agent.jar foo.Main

The '-javaagent' is a Java 5+ flag for specifying and enabling agents to instrument programs running on the JVM. The Spring Framework ships with such an agent, the InstrumentationSavingAgent, which is packaged in the spring-agent.jar that was supplied as the value of the -javaagent argument in the above example.

The output from the execution of the Main program will look something like that below. (I have introduced a Thread.sleep(..) statement into the calculateEntitlement() implementation so that the profiler actually captures something other than 0 milliseconds - the 01234 milliseconds is not an overhead introduced by the AOP :) )

Calculating entitlement

StopWatch 'ProfilingAspect': running time (millis) = 1234
------ ----- ----------------------------
ms     %     Task name
------ ----- ----------------------------
01234  100%  calculateEntitlement

Since this LTW is effected using full-blown AspectJ, we are not just limited to advising Spring beans; the following slight variation on the Main program will yield the same result.

package foo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Main {

    public static void main(String[] args) {

        new ClassPathXmlApplicationContext("beans.xml", Main.class);

        EntitlementCalculationService entitlementCalculationService =
            new StubEntitlementCalculationService();

        // the profiling aspect will be 'woven' around this method execution
        entitlementCalculationService.calculateEntitlement();
    }
}

Notice how in the above program we are simply bootstrapping the Spring container, and then creating a new instance of the StubEntitlementCalculationService totally outside the context of Spring... the profiling advice still gets woven in.

The example admittedly is simplistic... however the basics of the LTW support in Spring have all been introduced in the above example, and the rest of this section will explain the 'why' behind each bit of configuration and usage in detail.

[Note]Note

The ProfilingAspect used in this example may be basic, but it is quite useful. It is a nice example of a development-time aspect that developers can use during development (of course), and then quite easily exclude from builds of the application being deployed into UAT or production.

8.8.4.2 Aspects

The aspects that you use in LTW have to be AspectJ aspects. They can be written in either the AspectJ language itself or you can write your aspects in the @AspectJ-style. The latter option is of course only an option if you are using Java 5+, but it does mean that your aspects are then both valid AspectJ and Spring AOP aspects. Furthermore, the compiled aspect classes need to be available on the classpath.

8.8.4.3 'META-INF/aop.xml'

The AspectJ LTW infrastructure is configured using one or more 'META-INF/aop.xml' files, that are on the Java classpath (either directly, or more typically in jar files).

The structure and contents of this file is detailed in the main AspectJ reference documentation, and the interested reader is referred to that resource. (I appreciate that this section is brief, but the 'aop.xml' file is 100% AspectJ - there is no Spring-specific information or semantics that apply to it, and so there is no extra value that I can contribute either as a result), so rather than rehash the quite satisfactory section that the AspectJ developers wrote, I am just directing you there.)

8.8.4.4 Required libraries (JARS)

At a minimum you will need the following libraries to use the Spring Framework's support for AspectJ LTW:

  1. spring.jar (version 2.5 or later)

  2. aspectjrt.jar (version 1.5 or later)

  3. aspectjweaver.jar (version 1.5 or later)

If you are using the Spring-provided agent to enable instrumentation, you will also need:

  1. spring-agent.jar

8.8.4.5 Spring configuration

The key component in Spring's LTW support is the LoadTimeWeaver interface (in the org.springframework.instrument.classloading package), and the numerous implementations of it that ship with the Spring distribution. A LoadTimeWeaver is responsible for adding one or more java.lang.instrument.ClassFileTransformers to a ClassLoader at runtime, which opens the door to all manner of interesting applications, one of which happens to be the LTW of aspects.

[Tip]Tip

If you are unfamiliar with the idea of runtime class file transformation, you are encouraged to read the Javadoc API documentation for the java.lang.instrument package before continuing. This is not a huge chore because there is - rather annoyingly - precious little documentation there... the key interfaces and classes will at least be laid out in front of you for reference as you read through this section.

Configuring a LoadTimeWeaver using XML for a particular ApplicationContext can be as easy as adding one line. (Please note that you almost certainly will need to be using an ApplicationContext as your Spring container - typically a BeanFactory will not be enough because the LTW support makes use of BeanFactoryPostProcessors.)

To enable the Spring Framework's LTW support, you need to configure a LoadTimeWeaver, which typically is done using the <context:load-time-weaver/> element. Find below a valid <context:load-time-weaver/> definition that uses default settings.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:load-time-weaver/>

</beans>

The above <context:load-time-weaver/> bean definition will define and register a number of LTW-specific infrastructure beans for you automatically, such as a LoadTimeWeaver and an AspectJWeavingEnabler. Notice how the <context:load-time-weaver/> is defined in the 'context' namespace; note also that the referenced XML Schema file is only available in versions of Spring 2.5 and later.

What the above configuration does is define and register a default LoadTimeWeaver bean for you. The default LoadTimeWeaver is the DefaultContextLoadTimeWeaver class, which attempts to decorate an automatically detected LoadTimeWeaver: the exact type of LoadTimeWeaver that will be 'automatically detected' is dependent upon your runtime environment (summarised in the following table).

Table 8.1. DefaultContextLoadTimeWeaver LoadTimeWeavers

Runtime EnvironmentLoadTimeWeaver implementation

Running in BEA's Weblogic 10

WebLogicLoadTimeWeaver

Running in Oracle's OC4J

OC4JLoadTimeWeaver

Running in GlassFish

GlassFishLoadTimeWeaver

JVM started with Spring InstrumentationSavingAgent

(java -javaagent:path/to/spring-agent.jar)

InstrumentationLoadTimeWeaver

Fallback, expecting the underlying ClassLoader to follow common conventions (e.g. applicable to TomcatInstrumentableClassLoader and to Resin)

ReflectiveLoadTimeWeaver


Note that these are just the LoadTimeWeavers that are autodetected when using the DefaultContextLoadTimeWeaver: it is of course possible to specify exactly which LoadTimeWeaver implementation that you wish to use by specifying the fully-qualified classname as the value of the 'weaver-class' attribute of the <context:load-time-weaver/> element. Find below an example of doing just that:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <context:load-time-weaver
            weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/>

</beans>

The LoadTimeWeaver that is defined and registered by the <context:load-time-weaver/> element can be later retrieved from the Spring container using the well-known name 'loadTimeWeaver'. Remember that the LoadTimeWeaver exists just as a mechanism for Spring's LTW infrastructure to add one or more ClassFileTransformers. The actual ClassFileTransformer that does the LTW is the ClassPreProcessorAgentAdapter (from the org.aspectj.weaver.loadtime package) class. See the class-level Javadoc for the ClassPreProcessorAgentAdapter class for further details, because the specifics of how the weaving is actually effected is beyond the scope of this section.

There is one final attribute of the <context:load-time-weaver/> left to discuss: the 'aspectj-weaving' attribute. This is a simple attribute that controls whether LTW is enabled or not, it is as simple as that. It accepts one of three possible values, summarised below, with the default value if the attribute is not present being ' autodetect'

Table 8.2. 'aspectj-weaving' attribute values

Attribute ValueExplanation

on

AspectJ weaving is on, and aspects will be woven at load-time as appropriate.

off

LTW is off... no aspect will be woven at load-time.

autodetect

If the Spring LTW infrastructure can find at least one 'META-INF/aop.xml' file, then AspectJ weaving is on, else it is off. This is the default value.


8.8.4.6 Environment-specific configuration

This last section contains any additional settings and configuration that you will need when using Spring's LTW support in environments such as application servers and web containers.

Generic Java applications

You may enable Spring's support for LTW in any Java application (standalone as well as application server based) through the use of the Spring-provided instrumentation agent. To do so, start the VM by by specifying the -javaagent:path/to/spring-agent.jar option. Note that this requires modification of the VM launch script which may prevent you from using this in application server environments (depending on your operation policies).

Tomcat

For web applications deployed onto Apache Tomcat 5.0 and above, Spring provides a TomcatInstrumentableClassLoader to be registered as the web app class loader. The required Tomcat setup looks as follows, to be included either in Tomcat's central server.xml file or in an application-specific META-INF/context.xml file within the WAR root. Spring's spring-tomcat-weaver.jar needs to be included in Tomcat's common lib directory in order to make this setup work.

<Context path="/myWebApp" docBase="/my/webApp/location">
    <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"
            useSystemClassLoaderAsParent="false"/>
</Context>

Note: We generally recommend Tomcat 5.5.20 or above when enabling load-time weaving. Prior versions have known issues with custom ClassLoader setup.

Alternatively, consider the use of the Spring-provided generic VM agent, to be specified in Tomcat's launch script (see above). This will make instrumentation available to all deployed web applications, no matter which ClassLoader they happen to run on.

For a more detailed discussion of Tomcat-based weaving setup, check out the the section called “Tomcat load-time weaving setup (5.0+)” section which discusses specifics of various Tomcat versions. While the primary focus of that section is on JPA persistence provider setup, the Tomcat setup characteristics apply to general load-time weaving as well.

WebLogic, OC4J, Resin, GlassFish

Recent versions of BEA WebLogic (version 10 and above), Oracle Containers for Java EE (OC4J 10.1.3.1 and above) and Resin (3.1 and above) provide a ClassLoader that is capable of local instrumentation. Spring's native LTW leverages such ClassLoaders to enable AspectJ weaving. You can enable LTW by simply activating context:load-time-weaver as described earlier. Specifically, you do not need to modify the launch script to add -javaagent:path/to/spring-agent.jar.

GlassFish provides an instrumentation-capable ClassLoader as well, but only in its EAR environment. For GlassFish web applications, follow the Tomcat setup instructions as outlined above.

8.9 Further Resources

More information on AspectJ can be found on the AspectJ website.

The book Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a comprehensive introduction and reference for the AspectJ language.

The book AspectJ in Action by Ramnivas Laddad (Manning, 2003) comes highly recommended; the focus of the book is on AspectJ, but a lot of general AOP themes are explored (in some depth).

9. Spring AOP APIs

9.1 Introduction

The previous chapter described the Spring 2.0 support for AOP using @AspectJ and schema-based aspect definitions. In this chapter we discuss the lower-level Spring AOP APIs and the AOP support used in Spring 1.2 applications. For new applications, we recommend the use of the Spring 2.0 AOP support described in the previous chapter, but when working with existing applications, or when reading books and articles, you may come across Spring 1.2 style examples. Spring 2.0 is fully backwards compatible with Spring 1.2 and everything described in this chapter is fully supported in Spring 2.0.

9.2 Pointcut API in Spring

Let's look at how Spring handles the crucial pointcut concept.

9.2.1 Concepts

Spring's pointcut model enables pointcut reuse independent of advice types. It's possible to target different advice using the same pointcut.

The org.springframework.aop.Pointcut interface is the central interface, used to target advices to particular classes and methods. The complete interface is shown below:

public interface Pointcut {

    ClassFilter getClassFilter();

    MethodMatcher getMethodMatcher();

}

Splitting the Pointcut interface into two parts allows reuse of class and method matching parts, and fine-grained composition operations (such as performing a "union" with another method matcher).

The ClassFilter interface is used to restrict the pointcut to a given set of target classes. If the matches() method always returns true, all target classes will be matched:

public interface ClassFilter {

    boolean matches(Class clazz);
}

The MethodMatcher interface is normally more important. The complete interface is shown below:

public interface MethodMatcher {

    boolean matches(Method m, Class targetClass);

    boolean isRuntime();

    boolean matches(Method m, Class targetClass, Object[] args);
}

The matches(Method, Class) method is used to test whether this pointcut will ever match a given method on a target class. This evaluation can be performed when an AOP proxy is created, to avoid the need for a test on every method invocation. If the 2-argument matches method returns true for a given method, and the isRuntime() method for the MethodMatcher returns true, the 3-argument matches method will be invoked on every method invocation. This enables a pointcut to look at the arguments passed to the method invocation immediately before the target advice is to execute.

Most MethodMatchers are static, meaning that their isRuntime() method returns false. In this case, the 3-argument matches method will never be invoked.

[Tip]Tip

If possible, try to make pointcuts static, allowing the AOP framework to cache the results of pointcut evaluation when an AOP proxy is created.

9.2.2 Operations on pointcuts

Spring supports operations on pointcuts: notably, union and intersection.

  • Union means the methods that either pointcut matches.

  • Intersection means the methods that both pointcuts match.

  • Union is usually more useful.

  • Pointcuts can be composed using the static methods in the org.springframework.aop.support.Pointcuts class, or using the ComposablePointcut class in the same package. However, using AspectJ pointcut expressions is usually a simpler approach.

9.2.3 AspectJ expression pointcuts

Since 2.0, the most important type of pointcut used by Spring is org.springframework.aop.aspectj.AspectJExpressionPointcut. This is a pointcut that uses an AspectJ supplied library to parse an AspectJ pointcut expression string.

See the previous chapter for a discussion of supported AspectJ pointcut primitives.

9.2.4 Convenience pointcut implementations

Spring provides several convenient pointcut implementations. Some can be used out of the box; others are intended to be subclassed in application-specific pointcuts.

9.2.4.1 Static pointcuts

Static pointcuts are based on method and target class, and cannot take into account the method's arguments. Static pointcuts are sufficient - and best - for most usages. It's possible for Spring to evaluate a static pointcut only once, when a method is first invoked: after that, there is no need to evaluate the pointcut again with each method invocation.

Let's consider some static pointcut implementations included with Spring.

Regular expression pointcuts

One obvious way to specify static pointcuts is regular expressions. Several AOP frameworks besides Spring make this possible. org.springframework.aop.support.Perl5RegexpMethodPointcut is a generic regular expression pointcut, using Perl 5 regular expression syntax. The Perl5RegexpMethodPointcut class depends on Jakarta ORO for regular expression matching. Spring also provides the JdkRegexpMethodPointcut class that uses the regular expression support in JDK 1.4+.

Using the Perl5RegexpMethodPointcut class, you can provide a list of pattern Strings. If any of these is a match, the pointcut will evaluate to true. (So the result is effectively the union of these pointcuts.)

The usage is shown below:

<bean id="settersAndAbsquatulatePointcut" 
    class="org.springframework.aop.support.Perl5RegexpMethodPointcut">
    <property name="patterns">
        <list>
            <value>.*set.*</value>
            <value>.*absquatulate</value>
        </list>
    </property>
</bean>

Spring provides a convenience class, RegexpMethodPointcutAdvisor, that allows us to also reference an Advice (remember that an Advice can be an interceptor, before advice, throws advice etc.). Behind the scenes, Spring will use a JdkRegexpMethodPointcut. Using RegexpMethodPointcutAdvisor simplifies wiring, as the one bean encapsulates both pointcut and advice, as shown below:

<bean id="settersAndAbsquatulateAdvisor" 
    class="org.springframework.aop.support.RegexpMethodPointcutAdvisor">
    <property name="advice">
        <ref local="beanNameOfAopAllianceInterceptor"/>
    </property>
    <property name="patterns">
        <list>
            <value>.*set.*</value>
            <value>.*absquatulate</value>
        </list>
    </property>
</bean>

RegexpMethodPointcutAdvisor can be used with any Advice type.

Attribute-driven pointcuts

An important type of static pointcut is a metadata-driven pointcut. This uses the values of metadata attributes: typically, source-level metadata.

9.2.4.2 Dynamic pointcuts

Dynamic pointcuts are costlier to evaluate than static pointcuts. They take into account method arguments, as well as static information. This means that they must be evaluated with every method invocation; the result cannot be cached, as arguments will vary.

The main example is the control flow pointcut.

Control flow pointcuts

Spring control flow pointcuts are conceptually similar to AspectJ cflow pointcuts, although less powerful. (There is currently no way to specify that a pointcut executes below a join point matched by another pointcut.) A control flow pointcut matches the current call stack. For example, it might fire if the join point was invoked by a method in the com.mycompany.web package, or by the SomeCaller class. Control flow pointcuts are specified using the org.springframework.aop.support.ControlFlowPointcut class.

[Note]Note

Control flow pointcuts are significantly more expensive to evaluate at runtime than even other dynamic pointcuts. In Java 1.4, the cost is about 5 times that of other dynamic pointcuts.

9.2.5 Pointcut superclasses

Spring provides useful pointcut superclasses to help you to implement your own pointcuts.

Because static pointcuts are most useful, you'll probably subclass StaticMethodMatcherPointcut, as shown below. This requires implementing just one abstract method (although it's possible to override other methods to customize behavior):

class TestStaticPointcut extends StaticMethodMatcherPointcut {

    public boolean matches(Method m, Class targetClass) {
        // return true if custom criteria match
    }
}

There are also superclasses for dynamic pointcuts.

You can use custom pointcuts with any advice type in Spring 1.0 RC2 and above.

9.2.6 Custom pointcuts

Because pointcuts in Spring AOP are Java classes, rather than language features (as in AspectJ) it's possible to declare custom pointcuts, whether static or dynamic. Custom pointcuts in Spring can be arbitrarily complex. However, using the AspectJ pointcut expression language is recommended if possible.

[Note]Note

Later versions of Spring may offer support for "semantic pointcuts" as offered by JAC: for example, "all methods that change instance variables in the target object."

9.3 Advice API in Spring

Let's now look at how Spring AOP handles advice.

9.3.1 Advice lifecycles

Each advice is a Spring bean. An advice instance can be shared across all advised objects, or unique to each advised object. This corresponds to per-class or per-instance advice.

Per-class advice is used most often. It is appropriate for generic advice such as transaction advisors. These do not depend on the state of the proxied object or add new state; they merely act on the method and arguments.

Per-instance advice is appropriate for introductions, to support mixins. In this case, the advice adds state to the proxied object.

It's possible to use a mix of shared and per-instance advice in the same AOP proxy.

9.3.2 Advice types in Spring

Spring provides several advice types out of the box, and is extensible to support arbitrary advice types. Let us look at the basic concepts and standard advice types.

9.3.2.1 Interception around advice

The most fundamental advice type in Spring is interception around advice.

Spring is compliant with the AOP Alliance interface for around advice using method interception. MethodInterceptors implementing around advice should implement the following interface:

public interface MethodInterceptor extends Interceptor {
  
    Object invoke(MethodInvocation invocation) throws Throwable;
}

The MethodInvocation argument to the invoke() method exposes the method being invoked; the target join point; the AOP proxy; and the arguments to the method. The invoke() method should return the invocation's result: the return value of the join point.

A simple MethodInterceptor implementation looks as follows:

public class DebugInterceptor implements MethodInterceptor {

    public Object invoke(MethodInvocation invocation) throws Throwable {
        System.out.println("Before: invocation=[" + invocation + "]");
        Object rval = invocation.proceed();
        System.out.println("Invocation returned");
        return rval;
    }
}

Note the call to the MethodInvocation's proceed() method. This proceeds down the interceptor chain towards the join point. Most interceptors will invoke this method, and return its return value. However, a MethodInterceptor, like any around advice, can return a different value or throw an exception rather than invoke the proceed method. However, you don't want to do this without good reason!

[Note]Note

MethodInterceptors offer interoperability with other AOP Alliance-compliant AOP implementations. The other advice types discussed in the remainder of this section implement common AOP concepts, but in a Spring-specific way. While there is an advantage in using the most specific advice type, stick with MethodInterceptor around advice if you are likely to want to run the aspect in another AOP framework. Note that pointcuts are not currently interoperable between frameworks, and the AOP Alliance does not currently define pointcut interfaces.

9.3.2.2 Before advice

A simpler advice type is a before advice. This does not need a MethodInvocation object, since it will only be called before entering the method.

The main advantage of a before advice is that there is no need to invoke the proceed() method, and therefore no possibility of inadvertently failing to proceed down the interceptor chain.

The MethodBeforeAdvice interface is shown below. (Spring's API design would allow for field before advice, although the usual objects apply to field interception and it's unlikely that Spring will ever implement it).

public interface MethodBeforeAdvice extends BeforeAdvice {

    void before(Method m, Object[] args, Object target) throws Throwable;
}

Note the return type is void. Before advice can insert custom behavior before the join point executes, but cannot change the return value. If a before advice throws an exception, this will abort further execution of the interceptor chain. The exception will propagate back up the interceptor chain. If it is unchecked, or on the signature of the invoked method, it will be passed directly to the client; otherwise it will be wrapped in an unchecked exception by the AOP proxy.

An example of a before advice in Spring, which counts all method invocations:

public class CountingBeforeAdvice implements MethodBeforeAdvice {

    private int count;

    public void before(Method m, Object[] args, Object target) throws Throwable {
        ++count;
    }

    public int getCount() { 
        return count; 
    }
}
[Tip]Tip

Before advice can be used with any pointcut.

9.3.2.3 Throws advice

Throws advice is invoked after the return of the join point if the join point threw an exception. Spring offers typed throws advice. Note that this means that the org.springframework.aop.ThrowsAdvice interface does not contain any methods: It is a tag interface identifying that the given object implements one or more typed throws advice methods. These should be in the form of:

afterThrowing([Method, args, target], subclassOfThrowable) 

Only the last argument is required. The method signatures may have either one or four arguments, depending on whether the advice method is interested in the method and arguments. The following classes are examples of throws advice.

The advice below is invoked if a RemoteException is thrown (including subclasses):

public class RemoteThrowsAdvice implements ThrowsAdvice {

    public void afterThrowing(RemoteException ex) throws Throwable {
        // Do something with remote exception
    }
}

The following advice is invoked if a ServletException is thrown. Unlike the above advice, it declares 4 arguments, so that it has access to the invoked method, method arguments and target object:

public class ServletThrowsAdviceWithArguments implements ThrowsAdvice {

    public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
        // Do something with all arguments
    }
}

The final example illustrates how these two methods could be used in a single class, which handles both RemoteException and ServletException. Any number of throws advice methods can be combined in a single class.

public static class CombinedThrowsAdvice implements ThrowsAdvice {

    public void afterThrowing(RemoteException ex) throws Throwable {
        // Do something with remote exception
    }
 
    public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
        // Do something with all arguments
    }
}

Note: If a throws-advice method throws an exception itself, it will override the original exception (i.e. change the exception thrown to the user). The overriding exception will typically be a RuntimeException; this is compatible with any method signature. However, if a throws-advice method throws a checked exception, it will have to match the declared exceptions of the target method and is hence to some degree coupled to specific target method signatures. Do not throw an undeclared checked exception that is incompatible with the target method's signature!

[Tip]Tip

Throws advice can be used with any pointcut.

9.3.2.4 After Returning advice

An after returning advice in Spring must implement the org.springframework.aop.AfterReturningAdvice interface, shown below:

public interface AfterReturningAdvice extends Advice {

    void afterReturning(Object returnValue, Method m, Object[] args, Object target) 
            throws Throwable;
}

An after returning advice has access to the return value (which it cannot modify), invoked method, methods arguments and target.

The following after returning advice counts all successful method invocations that have not thrown exceptions:

public class CountingAfterReturningAdvice implements AfterReturningAdvice {

    private int count;

    public void afterReturning(Object returnValue, Method m, Object[] args, Object target)
            throws Throwable {
        ++count;
    }

    public int getCount() {
        return count;
    }
}

This advice doesn't change the execution path. If it throws an exception, this will be thrown up the interceptor chain instead of the return value.

[Tip]Tip

After returning advice can be used with any pointcut.

9.3.2.5 Introduction advice

Spring treats introduction advice as a special kind of interception advice.

Introduction requires an IntroductionAdvisor, and an IntroductionInterceptor, implementing the following interface:

public interface IntroductionInterceptor extends MethodInterceptor {

    boolean implementsInterface(Class intf);
}

The invoke() method inherited from the AOP Alliance MethodInterceptor interface must implement the introduction: that is, if the invoked method is on an introduced interface, the introduction interceptor is responsible for handling the method call - it cannot invoke proceed().

Introduction advice cannot be used with any pointcut, as it applies only at class, rather than method, level. You can only use introduction advice with the IntroductionAdvisor, which has the following methods:

public interface IntroductionAdvisor extends Advisor, IntroductionInfo {

	ClassFilter getClassFilter();

	void validateInterfaces() throws IllegalArgumentException;
}

public interface IntroductionInfo {

	Class[] getInterfaces();
}

There is no MethodMatcher, and hence no Pointcut, associated with introduction advice. Only class filtering is logical.

The getInterfaces() method returns the interfaces introduced by this advisor.

The validateInterfaces() method is used internally to see whether or not the introduced interfaces can be implemented by the configured IntroductionInterceptor .

Let's look at a simple example from the Spring test suite. Let's suppose we want to introduce the following interface to one or more objects:

public interface Lockable {
    void lock();
    void unlock();
    boolean locked();
}

This illustrates a mixin. We want to be able to cast advised objects to Lockable, whatever their type, and call lock and unlock methods. If we call the lock() method, we want all setter methods to throw a LockedException. Thus we can add an aspect that provides the ability to make objects immutable, without them having any knowledge of it: a good example of AOP.

Firstly, we'll need an IntroductionInterceptor that does the heavy lifting. In this case, we extend the org.springframework.aop.support.DelegatingIntroductionInterceptor convenience class. We could implement IntroductionInterceptor directly, but using DelegatingIntroductionInterceptor is best for most cases.

The DelegatingIntroductionInterceptor is designed to delegate an introduction to an actual implementation of the introduced interface(s), concealing the use of interception to do so. The delegate can be set to any object using a constructor argument; the default delegate (when the no-arg constructor is used) is this. Thus in the example below, the delegate is the LockMixin subclass of DelegatingIntroductionInterceptor. Given a delegate (by default itself), a DelegatingIntroductionInterceptor instance looks for all interfaces implemented by the delegate (other than IntroductionInterceptor), and will support introductions against any of them. It's possible for subclasses such as LockMixin to call the suppressInterface(Class intf) method to suppress interfaces that should not be exposed. However, no matter how many interfaces an IntroductionInterceptor is prepared to support, the IntroductionAdvisor used will control which interfaces are actually exposed. An introduced interface will conceal any implementation of the same interface by the target.

Thus LockMixin subclasses DelegatingIntroductionInterceptor and implements Lockable itself. The superclass automatically picks up that Lockable can be supported for introduction, so we don't need to specify that. We could introduce any number of interfaces in this way.

Note the use of the locked instance variable. This effectively adds additional state to that held in the target object.

public class LockMixin extends DelegatingIntroductionInterceptor 
    implements Lockable {

    private boolean locked;

    public void lock() {
        this.locked = true;
    }

    public void unlock() {
        this.locked = false;
    }

    public boolean locked() {
        return this.locked;
    }

    public Object invoke(MethodInvocation invocation) throws Throwable {
        if (locked() && invocation.getMethod().getName().indexOf("set") == 0)
            throw new LockedException();
        return super.invoke(invocation);
    }

}

Often it isn't necessary to override the invoke() method: the DelegatingIntroductionInterceptor implementation - which calls the delegate method if the method is introduced, otherwise proceeds towards the join point - is usually sufficient. In the present case, we need to add a check: no setter method can be invoked if in locked mode.

The introduction advisor required is simple. All it needs to do is hold a distinct LockMixin instance, and specify the introduced interfaces - in this case, just Lockable. A more complex example might take a reference to the introduction interceptor (which would be defined as a prototype): in this case, there's no configuration relevant for a LockMixin, so we simply create it using new.

public class LockMixinAdvisor extends DefaultIntroductionAdvisor {

    public LockMixinAdvisor() {
        super(new LockMixin(), Lockable.class);
    }
}

We can apply this advisor very simply: it requires no configuration. (However, it is necessary: It's impossible to use an IntroductionInterceptor without an IntroductionAdvisor.) As usual with introductions, the advisor must be per-instance, as it is stateful. We need a different instance of LockMixinAdvisor, and hence LockMixin, for each advised object. The advisor comprises part of the advised object's state.

We can apply this advisor programmatically, using the Advised.addAdvisor() method, or (the recommended way) in XML configuration, like any other advisor. All proxy creation choices discussed below, including "auto proxy creators," correctly handle introductions and stateful mixins.

9.4 Advisor API in Spring

In Spring, an Advisor is an aspect that contains just a single advice object associated with a pointcut expression.

Apart from the special case of introductions, any advisor can be used with any advice. org.springframework.aop.support.DefaultPointcutAdvisor is the most commonly used advisor class. For example, it can be used with a MethodInterceptor, BeforeAdvice or ThrowsAdvice.

It is possible to mix advisor and advice types in Spring in the same AOP proxy. For example, you could use a interception around advice, throws advice and before advice in one proxy configuration: Spring will automatically create the necessary interceptor chain.

9.5 Using the ProxyFactoryBean to create AOP proxies

If you're using the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects - and you should be! - you will want to use one of Spring's AOP FactoryBeans. (Remember that a factory bean introduces a layer of indirection, enabling it to create objects of a different type.)

[Note]Note

The Spring 2.0 AOP support also uses factory beans under the covers.

The basic way to create an AOP proxy in Spring is to use the org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts and advice that will apply, and their ordering. However, there are simpler options that are preferable if you don't need such control.

9.5.1 Basics

The ProxyFactoryBean, like other Spring FactoryBean implementations, introduces a level of indirection. If you define a ProxyFactoryBean with name foo, what objects referencing foo see is not the ProxyFactoryBean instance itself, but an object created by the ProxyFactoryBean's implementation of the getObject() method. This method will create an AOP proxy wrapping a target object.

One of the most important benefits of using a ProxyFactoryBean or another IoC-aware class to create AOP proxies, is that it means that advices and pointcuts can also be managed by IoC. This is a powerful feature, enabling certain approaches that are hard to achieve with other AOP frameworks. For example, an advice may itself reference application objects (besides the target, which should be available in any AOP framework), benefiting from all the pluggability provided by Dependency Injection.

9.5.2 JavaBean properties

In common with most FactoryBean implementations provided with Spring, the ProxyFactoryBean class is itself a JavaBean. Its properties are used to:

Some key properties are inherited from org.springframework.aop.framework.ProxyConfig (the superclass for all AOP proxy factories in Spring). These key properties include:

  • proxyTargetClass: true if the target class is to be proxied, rather than the target class' interfaces. If this property value is set to true, then CGLIB proxies will be created (but see also below the section entitled Section 9.5.3, “JDK- and CGLIB-based proxies”).

  • optimize: controls whether or not aggressive optimizations are applied to proxies created via CGLIB. One should not blithely use this setting unless one fully understands how the relevant AOP proxy handles optimization. This is currently used only for CGLIB proxies; it has no effect with JDK dynamic proxies.

  • frozen: if a proxy configuration is frozen, then changes to the configuration are no longer allowed. This is useful both as a slight optimization and for those cases when you don't want callers to be able to manipulate the proxy (via the Advised interface) after the proxy has been created. The default value of this property is false, so changes such as adding additional advice are allowed.

  • exposeProxy: determines whether or not the current proxy should be exposed in a ThreadLocal so that it can be accessed by the target. If a target needs to obtain the proxy and the exposeProxy property is set to true, the target can use the AopContext.currentProxy() method.

  • aopProxyFactory: the implementation of AopProxyFactory to use. Offers a way of customizing whether to use dynamic proxies, CGLIB or any other proxy strategy. The default implementation will choose dynamic proxies or CGLIB appropriately. There should be no need to use this property; it is intended to allow the addition of new proxy types in Spring 1.1.

Other properties specific to ProxyFactoryBean include:

  • proxyInterfaces: array of String interface names. If this isn't supplied, a CGLIB proxy for the target class will be used (but see also below the section entitled Section 9.5.3, “JDK- and CGLIB-based proxies”).

  • interceptorNames: String array of Advisor, interceptor or other advice names to apply. Ordering is significant, on a first come-first served basis. That is to say that the first interceptor in the list will be the first to be able to intercept the invocation.

    The names are bean names in the current factory, including bean names from ancestor factories. You can't mention bean references here since doing so would result in the ProxyFactoryBean ignoring the singleton setting of the advice.

    You can append an interceptor name with an asterisk (*). This will result in the application of all advisor beans with names starting with the part before the asterisk to be applied. An example of using this feature can be found in Section 9.5.6, “Using 'global' advisors”.

  • singleton: whether or not the factory should return a single object, no matter how often the getObject() method is called. Several FactoryBean implementations offer such a method. The default value is true. If you want to use stateful advice - for example, for stateful mixins - use prototype advices along with a singleton value of false.

9.5.3 JDK- and CGLIB-based proxies

This section serves as the definitive documentation on how the ProxyFactoryBean chooses to create one of either a JDK- and CGLIB-based proxy for a particular target object (that is to be proxied).

[Note]Note

The behavior of the ProxyFactoryBean with regard to creating JDK- or CGLIB-based proxies changed between versions 1.2.x and 2.0 of Spring. The ProxyFactoryBean now exhibits similar semantics with regard to auto-detecting interfaces as those of the TransactionProxyFactoryBean class.

If the class of a target object that is to be proxied (hereafter simply referred to as the target class) doesn't implement any interfaces, then a CGLIB-based proxy will be created. This is the easiest scenario, because JDK proxies are interface based, and no interfaces means JDK proxying isn't even possible. One simply plugs in the target bean, and specifies the list of interceptors via the interceptorNames property. Note that a CGLIB-based proxy will be created even if the proxyTargetClass property of the ProxyFactoryBean has been set to false. (Obviously this makes no sense, and is best removed from the bean definition because it is at best redundant, and at worst confusing.)

If the target class implements one (or more) interfaces, then the type of proxy that is created depends on the configuration of the ProxyFactoryBean.

If the proxyTargetClass property of the ProxyFactoryBean has been set to true, then a CGLIB-based proxy will be created. This makes sense, and is in keeping with the principle of least surprise. Even if the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully qualified interface names, the fact that the proxyTargetClass property is set to true will cause CGLIB-based proxying to be in effect.

If the proxyInterfaces property of the ProxyFactoryBean has been set to one or more fully qualified interface names, then a JDK-based proxy will be created. The created proxy will implement all of the interfaces that were specified in the proxyInterfaces property; if the target class happens to implement a whole lot more interfaces than those specified in the proxyInterfaces property, that is all well and good but those additional interfaces will not be implemented by the returned proxy.

If the proxyInterfaces property of the ProxyFactoryBean has not been set, but the target class does implement one (or more) interfaces, then the ProxyFactoryBean will auto-detect the fact that the target class does actually implement at least one interface, and a JDK-based proxy will be created. The interfaces that are actually proxied will be all of the interfaces that the target class implements; in effect, this is the same as simply supplying a list of each and every interface that the target class implements to the proxyInterfaces property. However, it is significantly less work, and less prone to typos.

9.5.4 Proxying interfaces

Let's look at a simple example of ProxyFactoryBean in action. This example involves:

  • A target bean that will be proxied. This is the "personTarget" bean definition in the example below.

  • An Advisor and an Interceptor used to provide advice.

  • An AOP proxy bean definition specifying the target object (the personTarget bean) and the interfaces to proxy, along with the advices to apply.

<bean id="personTarget" class="com.mycompany.PersonImpl">
    <property name="name"><value>Tony</value></property>
    <property name="age"><value>51</value></property>
</bean>

<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
    <property name="someProperty"><value>Custom string property value</value></property>
</bean>

<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor">
</bean>

<bean id="person" 
    class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="proxyInterfaces"><value>com.mycompany.Person</value></property>

    <property name="target"><ref local="personTarget"/></property>
    <property name="interceptorNames">
        <list>
            <value>myAdvisor</value>
            <value>debugInterceptor</value>
        </list>
    </property>
</bean>

Note that the interceptorNames property takes a list of String: the bean names of the interceptor or advisors in the current factory. Advisors, interceptors, before, after returning and throws advice objects can be used. The ordering of advisors is significant.

[Note]Note

You might be wondering why the list doesn't hold bean references. The reason for this is that if the ProxyFactoryBean's singleton property is set to false, it must be able to return independent proxy instances. If any of the advisors is itself a prototype, an independent instance would need to be returned, so it's necessary to be able to obtain an instance of the prototype from the factory; holding a reference isn't sufficient.

The "person" bean definition above can be used in place of a Person implementation, as follows:

Person person = (Person) factory.getBean("person");

Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary Java object:

<bean id="personUser" class="com.mycompany.PersonUser">
  <property name="person"><ref local="person" /></property>
</bean>

The PersonUser class in this example would expose a property of type Person. As far as it's concerned, the AOP proxy can be used transparently in place of a "real" person implementation. However, its class would be a dynamic proxy class. It would be possible to cast it to the Advised interface (discussed below).

It's possible to conceal the distinction between target and proxy using an anonymous inner bean, as follows. Only the ProxyFactoryBean definition is different; the advice is included only for completeness:

<bean id="myAdvisor" class="com.mycompany.MyAdvisor">
  <property name="someProperty"><value>Custom string property value</value></property>
</bean>

<bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/>

<bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean">
  <property name="proxyInterfaces"><value>com.mycompany.Person</value></property>
  <!-- Use inner bean, not local reference to target -->
  <property name="target">
    <bean class="com.mycompany.PersonImpl">
      <property name="name"><value>Tony</value></property>
      <property name="age"><value>51</value></property>
    </bean>
  </property>
  <property name="interceptorNames">
    <list>
      <value>myAdvisor</value>
      <value>debugInterceptor</value>
    </list>
  </property>
</bean>

This has the advantage that there's only one object of type Person: useful if we want to prevent users of the application context from obtaining a reference to the un-advised object, or need to avoid any ambiguity with Spring IoC autowiring. There's also arguably an advantage in that the ProxyFactoryBean definition is self-contained. However, there are times when being able to obtain the un-advised target from the factory might actually be an advantage: for example, in certain test scenarios.

9.5.5 Proxying classes

What if you need to proxy a class, rather than one or more interfaces?

Imagine that in our example above, there was no Person interface: we needed to advise a class called Person that didn't implement any business interface. In this case, you can configure Spring to use CGLIB proxying, rather than dynamic proxies. Simply set the proxyTargetClass property on the ProxyFactoryBean above to true. While it's best to program to interfaces, rather than classes, the ability to advise classes that don't implement interfaces can be useful when working with legacy code. (In general, Spring isn't prescriptive. While it makes it easy to apply good practices, it avoids forcing a particular approach.)

If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.

CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this generated subclass to delegate method calls to the original target: the subclass is used to implement the Decorator pattern, weaving in the advice.

CGLIB proxying should generally be transparent to users. However, there are some issues to consider:

  • Final methods can't be advised, as they can't be overridden.

  • You'll need the CGLIB 2 binaries on your classpath; dynamic proxies are available with the JDK.

There's little performance difference between CGLIB proxying and dynamic proxies. As of Spring 1.0, dynamic proxies are slightly faster. However, this may change in the future. Performance should not be a decisive consideration in this case.

9.5.6 Using 'global' advisors

By appending an asterisk to an interceptor name, all advisors with bean names matching the part before the asterisk, will be added to the advisor chain. This can come in handy if you need to add a standard set of 'global' advisors:

<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean">
  <property name="target" ref="service"/>
  <property name="interceptorNames">
    <list>
      <value>global*</value>
    </list>
  </property>
</bean>

<bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/>
<bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/>

9.6 Concise proxy definitions

Especially when defining transactional proxies, you may end up with many similar proxy definitions. The use of parent and child bean definitions, along with inner bean definitions, can result in much cleaner and more concise proxy definitions.

First a parent, template, bean definition is created for the proxy:

<bean id="txProxyTemplate" abstract="true"
        class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
  <property name="transactionManager" ref="transactionManager"/>
  <property name="transactionAttributes">
    <props>
      <prop key="*">PROPAGATION_REQUIRED</prop>
    </props>
  </property>
</bean>

This will never be instantiated itself, so may actually be incomplete. Then each proxy which needs to be created is just a child bean definition, which wraps the target of the proxy as an inner bean definition, since the target will never be used on its own anyway.

<bean id="myService" parent="txProxyTemplate">
  <property name="target">
    <bean class="org.springframework.samples.MyServiceImpl">
    </bean>
  </property>
</bean>

It is of course possible to override properties from the parent template, such as in this case, the transaction propagation settings:

<bean id="mySpecialService" parent="txProxyTemplate">
  <property name="target">
    <bean class="org.springframework.samples.MySpecialServiceImpl">
    </bean>
  </property>
  <property name="transactionAttributes">
    <props>
      <prop key="get*">PROPAGATION_REQUIRED,readOnly</prop>
      <prop key="find*">PROPAGATION_REQUIRED,readOnly</prop>
      <prop key="load*">PROPAGATION_REQUIRED,readOnly</prop>
      <prop key="store*">PROPAGATION_REQUIRED</prop>
    </props>
  </property>
</bean>

Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute, as described previously, so that it may not actually ever be instantiated. Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. It is therefore important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually try to pre-instantiate it.

9.7 Creating AOP proxies programmatically with the ProxyFactory

It's easy to create AOP proxies programmatically using Spring. This enables you to use Spring AOP without dependency on Spring IoC.

The following listing shows creation of a proxy for a target object, with one interceptor and one advisor. The interfaces implemented by the target object will automatically be proxied:

ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl);
factory.addInterceptor(myMethodInterceptor);
factory.addAdvisor(myAdvisor);
MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();

The first step is to construct an object of type org.springframework.aop.framework.ProxyFactory. You can create this with a target object, as in the above example, or specify the interfaces to be proxied in an alternate constructor.

You can add interceptors or advisors, and manipulate them for the life of the ProxyFactory. If you add an IntroductionInterceptionAroundAdvisor you can cause the proxy to implement additional interfaces.

There are also convenience methods on ProxyFactory (inherited from AdvisedSupport) which allow you to add other advice types such as before and throws advice. AdvisedSupport is the superclass of both ProxyFactory and ProxyFactoryBean.

[Tip]Tip

Integrating AOP proxy creation with the IoC framework is best practice in most applications. We recommend that you externalize configuration from Java code with AOP, as in general.

9.8 Manipulating advised objects

However you create AOP proxies, you can manipulate them using the org.springframework.aop.framework.Advised interface. Any AOP proxy can be cast to this interface, whichever other interfaces it implements. This interface includes the following methods:

Advisor[] getAdvisors();

void addAdvice(Advice advice) throws AopConfigException;

void addAdvice(int pos, Advice advice) 
        throws AopConfigException;

void addAdvisor(Advisor advisor) throws AopConfigException;

void addAdvisor(int pos, Advisor advisor) throws AopConfigException;

int indexOf(Advisor advisor);

boolean removeAdvisor(Advisor advisor) throws AopConfigException;

void removeAdvisor(int index) throws AopConfigException;

boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException;

boolean isFrozen();

The getAdvisors() method will return an Advisor for every advisor, interceptor or other advice type that has been added to the factory. If you added an Advisor, the returned advisor at this index will be the object that you added. If you added an interceptor or other advice type, Spring will have wrapped this in an advisor with a pointcut that always returns true. Thus if you added a MethodInterceptor, the advisor returned for this index will be an DefaultPointcutAdvisor returning your MethodInterceptor and a pointcut that matches all classes and methods.

The addAdvisor() methods can be used to add any Advisor. Usually the advisor holding pointcut and advice will be the generic DefaultPointcutAdvisor, which can be used with any advice or pointcut (but not for introductions).

By default, it's possible to add or remove advisors or interceptors even once a proxy has been created. The only restriction is that it's impossible to add or remove an introduction advisor, as existing proxies from the factory will not show the interface change. (You can obtain a new proxy from the factory to avoid this problem.)

A simple example of casting an AOP proxy to the Advised interface and examining and manipulating its advice:

Advised advised = (Advised) myObject;
Advisor[] advisors = advised.getAdvisors();
int oldAdvisorCount = advisors.length;
System.out.println(oldAdvisorCount + " advisors");

// Add an advice like an interceptor without a pointcut
// Will match all proxied methods
// Can use for interceptors, before, after returning or throws advice
advised.addAdvice(new DebugInterceptor());

// Add selective advice using a pointcut
advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice));

assertEquals("Added two advisors",
     oldAdvisorCount + 2, advised.getAdvisors().length);
[Note]Note

It's questionable whether it's advisable (no pun intended) to modify advice on a business object in production, although there are no doubt legitimate usage cases. However, it can be very useful in development: for example, in tests. I have sometimes found it very useful to be able to add test code in the form of an interceptor or other advice, getting inside a method invocation I want to test. (For example, the advice can get inside a transaction created for that method: for example, to run SQL to check that a database was correctly updated, before marking the transaction for roll back.)

Depending on how you created the proxy, you can usually set a frozen flag, in which case the Advised isFrozen() method will return true, and any attempts to modify advice through addition or removal will result in an AopConfigException. The ability to freeze the state of an advised object is useful in some cases, for example, to prevent calling code removing a security interceptor. It may also be used in Spring 1.1 to allow aggressive optimization if runtime advice modification is known not to be required.

9.9 Using the "autoproxy" facility

So far we've considered explicit creation of AOP proxies using a ProxyFactoryBean or similar factory bean.

Spring also allows us to use "autoproxy" bean definitions, which can automatically proxy selected bean definitions. This is built on Spring "bean post processor" infrastructure, which enables modification of any bean definition as the container loads.

In this model, you set up some special bean definitions in your XML bean definition file to configure the auto proxy infrastructure. This allows you just to declare the targets eligible for autoproxying: you don't need to use ProxyFactoryBean.

There are two ways to do this:

  • Using an autoproxy creator that refers to specific beans in the current context.

  • A special case of autoproxy creation that deserves to be considered separately; autoproxy creation driven by source-level metadata attributes.

9.9.1 Autoproxy bean definitions

The org.springframework.aop.framework.autoproxy package provides the following standard autoproxy creators.

9.9.1.1 BeanNameAutoProxyCreator

The BeanNameAutoProxyCreator class is a BeanPostProcessor that automatically creates AOP proxies for beans with names matching literal values or wildcards.

<bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator">
  <property name="beanNames"><value>jdk*,onlyJdk</value></property>
  <property name="interceptorNames">
    <list>
      <value>myInterceptor</value>
    </list>
  </property>
</bean>

As with ProxyFactoryBean, there is an interceptorNames property rather than a list of interceptors, to allow correct behavior for prototype advisors. Named "interceptors" can be advisors or any advice type.

As with auto proxying in general, the main point of using BeanNameAutoProxyCreator is to apply the same configuration consistently to multiple objects, with minimal volume of configuration. It is a popular choice for applying declarative transactions to multiple objects.

Bean definitions whose names match, such as "jdkMyBean" and "onlyJdk" in the above example, are plain old bean definitions with the target class. An AOP proxy will be created automatically by the BeanNameAutoProxyCreator. The same advice will be applied to all matching beans. Note that if advisors are used (rather than the interceptor in the above example), the pointcuts may apply differently to different beans.

9.9.1.2 DefaultAdvisorAutoProxyCreator

A more general and extremely powerful auto proxy creator is DefaultAdvisorAutoProxyCreator. This will automagically apply eligible advisors in the current context, without the need to include specific bean names in the autoproxy advisor's bean definition. It offers the same merit of consistent configuration and avoidance of duplication as BeanNameAutoProxyCreator.

Using this mechanism involves:

  • Specifying a DefaultAdvisorAutoProxyCreator bean definition.

  • Specifying any number of Advisors in the same or related contexts. Note that these must be Advisors, not just interceptors or other advices. This is necessary because there must be a pointcut to evaluate, to check the eligibility of each advice to candidate bean definitions.

The DefaultAdvisorAutoProxyCreator will automatically evaluate the pointcut contained in each advisor, to see what (if any) advice it should apply to each business object (such as "businessObject1" and "businessObject2" in the example).

This means that any number of advisors can be applied automatically to each business object. If no pointcut in any of the advisors matches any method in a business object, the object will not be proxied. As bean definitions are added for new business objects, they will automatically be proxied if necessary.

Autoproxying in general has the advantage of making it impossible for callers or dependencies to obtain an un-advised object. Calling getBean("businessObject1") on this ApplicationContext will return an AOP proxy, not the target business object. (The "inner bean" idiom shown earlier also offers this benefit.)

<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>

<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
  <property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>

<bean id="customAdvisor" class="com.mycompany.MyAdvisor"/>

<bean id="businessObject1" class="com.mycompany.BusinessObject1">
  <!-- Properties omitted -->
</bean>

<bean id="businessObject2" class="com.mycompany.BusinessObject2"/>

The DefaultAdvisorAutoProxyCreator is very useful if you want to apply the same advice consistently to many business objects. Once the infrastructure definitions are in place, you can simply add new business objects without including specific proxy configuration. You can also drop in additional aspects very easily - for example, tracing or performance monitoring aspects - with minimal change to configuration.

The DefaultAdvisorAutoProxyCreator offers support for filtering (using a naming convention so that only certain advisors are evaluated, allowing use of multiple, differently configured, AdvisorAutoProxyCreators in the same factory) and ordering. Advisors can implement the org.springframework.core.Ordered interface to ensure correct ordering if this is an issue. The TransactionAttributeSourceAdvisor used in the above example has a configurable order value; the default setting is unordered.

9.9.1.3 AbstractAdvisorAutoProxyCreator

This is the superclass of DefaultAdvisorAutoProxyCreator. You can create your own autoproxy creators by subclassing this class, in the unlikely event that advisor definitions offer insufficient customization to the behavior of the framework DefaultAdvisorAutoProxyCreator.

9.9.2 Using metadata-driven auto-proxying

A particularly important type of autoproxying is driven by metadata. This produces a similar programming model to .NET ServicedComponents. Instead of using XML deployment descriptors as in EJB, configuration for transaction management and other enterprise services is held in source-level attributes.

In this case, you use the DefaultAdvisorAutoProxyCreator, in combination with Advisors that understand metadata attributes. The metadata specifics are held in the pointcut part of the candidate advisors, rather than in the autoproxy creation class itself.

This is really a special case of the DefaultAdvisorAutoProxyCreator, but deserves consideration on its own. (The metadata-aware code is in the pointcuts contained in the advisors, not the AOP framework itself.)

The /attributes directory of the JPetStore sample application shows the use of attribute-driven autoproxying. In this case, there's no need to use the TransactionProxyFactoryBean. Simply defining transactional attributes on business objects is sufficient, because of the use of metadata-aware pointcuts. The bean definitions include the following code, in /WEB-INF/declarativeServices.xml. Note that this is generic, and can be used outside the JPetStore:

<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>

<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
  <property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>

<bean id="transactionInterceptor"
    class="org.springframework.transaction.interceptor.TransactionInterceptor">
  <property name="transactionManager" ref="transactionManager"/>
  <property name="transactionAttributeSource">
    <bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource">
      <property name="attributes" ref="attributes"/>
    </bean>
  </property>
</bean>

<bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/>

The DefaultAdvisorAutoProxyCreator bean definition (the name is not significant, hence it can even be omitted) will pick up all eligible pointcuts in the current application context. In this case, the "transactionAdvisor" bean definition, of type TransactionAttributeSourceAdvisor, will apply to classes or methods carrying a transaction attribute. The TransactionAttributeSourceAdvisor depends on a TransactionInterceptor, via constructor dependency. The example resolves this via autowiring. The AttributesTransactionAttributeSource depends on an implementation of the org.springframework.metadata.Attributes interface. In this fragment, the "attributes" bean satisfies this, using the Jakarta Commons Attributes API to obtain attribute information. (The application code must have been compiled using the Commons Attributes compilation task.)

The /annotation directory of the JPetStore sample application contains an analogous example for auto-proxying driven by JDK 1.5+ annotations. The following configuration enables automatic detection of Spring's Transactional annotation, leading to implicit proxies for beans containing that annotation:

<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/>

<bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor">
  <property name="transactionInterceptor" ref="transactionInterceptor"/>
</bean>

<bean id="transactionInterceptor"
    class="org.springframework.transaction.interceptor.TransactionInterceptor">
  <property name="transactionManager" ref="transactionManager"/>
  <property name="transactionAttributeSource">
    <bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/>
  </property>
</bean>

The TransactionInterceptor defined here depends on a PlatformTransactionManager definition, which is not included in this generic file (although it could be) because it will be specific to the application's transaction requirements (typically JTA, as in this example, or Hibernate, JDO or JDBC):

<bean id="transactionManager" 
    class="org.springframework.transaction.jta.JtaTransactionManager"/>
[Tip]Tip

If you require only declarative transaction management, using these generic XML definitions will result in Spring automatically proxying all classes or methods with transaction attributes. You won't need to work directly with AOP, and the programming model is similar to that of .NET ServicedComponents.

This mechanism is extensible. It's possible to do autoproxying based on custom attributes. You need to:

  • Define your custom attribute.

  • Specify an Advisor with the necessary advice, including a pointcut that is triggered by the presence of the custom attribute on a class or method. You may be able to use an existing advice, merely implementing a static pointcut that picks up the custom attribute.

It's possible for such advisors to be unique to each advised class (for example, mixins): they simply need to be defined as prototype, rather than singleton, bean definitions. For example, the LockMixin introduction interceptor from the Spring test suite, shown above, could be used in conjunction with an attribute-driven pointcut to target a mixin, as shown here. We use the generic DefaultPointcutAdvisor, configured using JavaBean properties:

<bean id="lockMixin" class="org.springframework.aop.LockMixin"
    scope="prototype"/>

<bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor"
    scope="prototype">
  <property name="pointcut" ref="myAttributeAwarePointcut"/>
  <property name="advice" ref="lockMixin"/>
</bean>

<bean id="anyBean" class="anyclass" ...

If the attribute aware pointcut matches any methods in the anyBean or other bean definitions, the mixin will be applied. Note that both lockMixin and lockableAdvisor definitions are prototypes. The myAttributeAwarePointcut pointcut can be a singleton definition, as it doesn't hold state for individual advised objects.

9.10 Using TargetSources

Spring offers the concept of a TargetSource, expressed in the org.springframework.aop.TargetSource interface. This interface is responsible for returning the "target object" implementing the join point. The TargetSource implementation is asked for a target instance each time the AOP proxy handles a method invocation.

Developers using Spring AOP don't normally need to work directly with TargetSources, but this provides a powerful means of supporting pooling, hot swappable and other sophisticated targets. For example, a pooling TargetSource can return a different target instance for each invocation, using a pool to manage instances.

If you do not specify a TargetSource, a default implementation is used that wraps a local object. The same target is returned for each invocation (as you would expect).

Let's look at the standard target sources provided with Spring, and how you can use them.

[Tip]Tip

When using a custom target source, your target will usually need to be a prototype rather than a singleton bean definition. This allows Spring to create a new target instance when required.

9.10.1 Hot swappable target sources

The org.springframework.aop.target.HotSwappableTargetSource exists to allow the target of an AOP proxy to be switched while allowing callers to keep their references to it.

Changing the target source's target takes effect immediately. The HotSwappableTargetSource is threadsafe.

You can change the target via the swap() method on HotSwappableTargetSource as follows:

HotSwappableTargetSource swapper = 
    (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);

The XML definitions required look as follows:

<bean id="initialTarget" class="mycompany.OldTarget"/>

<bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource">
  <constructor-arg ref="initialTarget"/>
</bean>

<bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean">
  <property name="targetSource" ref="swapper"/>
</bean>

The above swap() call changes the target of the swappable bean. Clients who hold a reference to that bean will be unaware of the change, but will immediately start hitting the new target.

Although this example doesn't add any advice - and it's not necessary to add advice to use a TargetSource - of course any TargetSource can be used in conjunction with arbitrary advice.

9.10.2 Pooling target sources

Using a pooling target source provides a similar programming model to stateless session EJBs, in which a pool of identical instances is maintained, with method invocations going to free objects in the pool.

A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to any POJO. As with Spring in general, this service can be applied in a non-invasive way.

Spring provides out-of-the-box support for Jakarta Commons Pool 1.3, which provides a fairly efficient pooling implementation. You'll need the commons-pool Jar on your application's classpath to use this feature. It's also possible to subclass org.springframework.aop.target.AbstractPoolingTargetSource to support any other pooling API.

Sample configuration is shown below:

<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject" 
    scope="prototype">
  ... properties omitted
</bean>

<bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource">
  <property name="targetBeanName" value="businessObjectTarget"/>
  <property name="maxSize" value="25"/>
</bean>

<bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean">
  <property name="targetSource" ref="poolTargetSource"/>
  <property name="interceptorNames" value="myInterceptor"/>
</bean>

Note that the target object - "businessObjectTarget" in the example - must be a prototype. This allows the PoolingTargetSource implementation to create new instances of the target to grow the pool as necessary. See the havadoc for AbstractPoolingTargetSource and the concrete subclass you wish to use for information about its properties: "maxSize" is the most basic, and always guaranteed to be present.

In this case, "myInterceptor" is the name of an interceptor that would need to be defined in the same IoC context. However, it isn't necessary to specify interceptors to use pooling. If you want only pooling, and no other advice, don't set the interceptorNames property at all.

It's possible to configure Spring so as to be able to cast any pooled object to the org.springframework.aop.target.PoolingConfig interface, which exposes information about the configuration and current size of the pool through an introduction. You'll need to define an advisor like this:

<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetObject" ref="poolTargetSource"/>
  <property name="targetMethod" value="getPoolingConfigMixin"/>
</bean>

This advisor is obtained by calling a convenience method on the AbstractPoolingTargetSource class, hence the use of MethodInvokingFactoryBean. This advisor's name ("poolConfigAdvisor" here) must be in the list of interceptors names in the ProxyFactoryBean exposing the pooled object.

The cast will look as follows:

PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject");
System.out.println("Max pool size is " + conf.getMaxSize());
[Note]Note

Pooling stateless service objects is not usually necessary. We don't believe it should be the default choice, as most stateless objects are naturally thread safe, and instance pooling is problematic if resources are cached.

Simpler pooling is available using autoproxying. It's possible to set the TargetSources used by any autoproxy creator.

9.10.3 Prototype target sources

Setting up a "prototype" target source is similar to a pooling TargetSource. In this case, a new instance of the target will be created on every method invocation. Although the cost of creating a new object isn't high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be more expensive. Thus you shouldn't use this approach without very good reason.

To do this, you could modify the poolTargetSource definition shown above as follows. (I've also changed the name, for clarity.)

<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource">
  <property name="targetBeanName" ref="businessObjectTarget"/>
</bean>

There's only one property: the name of the target bean. Inheritance is used in the TargetSource implementations to ensure consistent naming. As with the pooling target source, the target bean must be a prototype bean definition.

9.10.4 ThreadLocal target sources

ThreadLocal target sources are useful if you need an object to be created for each incoming request (per thread that is). The concept of a ThreadLocal provide a JDK-wide facility to transparently store resource alongside a thread. Setting up a ThreadLocalTargetSource is pretty much the same as was explained for the other types of target source:

<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource">
  <property name="targetBeanName" value="businessObjectTarget"/>
</bean>
[Note]Note

ThreadLocals come with serious issues (potentially resulting in memory leaks) when incorrectly using them in a multi-threaded and multi-classloader environments. One should always consider wrapping a threadlocal in some other class and never directly use the ThreadLocal itself (except of course in the wrapper class). Also, one should always remember to correctly set and unset (where the latter simply involved a call to ThreadLocal.set(null)) the resource local to the thread. Unsetting should be done in any case since not unsetting it might result in problematic behavior. Spring's ThreadLocal support does this for you and should always be considered in favor of using ThreadLocals without other proper handling code.

9.11 Defining new Advice types

Spring AOP is designed to be extensible. While the interception implementation strategy is presently used internally, it is possible to support arbitrary advice types in addition to the out-of-the-box interception around advice, before, throws advice and after returning advice.

The org.springframework.aop.framework.adapter package is an SPI package allowing support for new custom advice types to be added without changing the core framework. The only constraint on a custom Advice type is that it must implement the org.aopalliance.aop.Advice tag interface.

Please refer to the org.springframework.aop.framework.adapter package's Javadocs for further information.

9.12 Further resources

Please refer to the Spring sample applications for further examples of Spring AOP:

  • The JPetStore's default configuration illustrates the use of the TransactionProxyFactoryBean for declarative transaction management.

  • The /attributes directory of the JPetStore illustrates the use of attribute-driven declarative transaction management.

10. Testing

10.1 Introduction

The Spring team considers developer testing to be an absolutely integral part of enterprise software development. A thorough treatment of testing in the enterprise is beyond the scope of this chapter; rather, the focus here is on the value-add that the adoption of the IoC principle can bring to unit testing and on the benefits that the Spring Framework provides in integration testing.

10.2 Unit testing

One of the main benefits of Dependency Injection is that your code should really depend far less on the container than in traditional J2EE development. The POJOs that make up your application should be testable in JUnit or TestNG tests, with objects simply instantiated using the new operator, without Spring or any other container. You can use mock objects (in conjunction with many other valuable testing techniques) to test your code in isolation. If you follow the architecture recommendations around Spring you will find that the resulting clean layering and componentization of your codebase will naturally facilitate easier unit testing. For example, you will be able to test service layer objects by stubbing or mocking DAO or Repository interfaces, without any need to access persistent data while running unit tests.

True unit tests typically will run extremely quickly, as there is no runtime infrastructure to set up, whether application server, database, ORM tool, or whatever. Thus emphasizing true unit tests as part of your development methodology will boost your productivity. The upshot of this is that you often do not need this section of the testing chapter to help you write effective unit tests for your IoC-based applications. For certain unit testing scenarios, however, the Spring Framework provides the following mock objects and testing support classes.

10.2.1 Mock objects

10.2.1.1 JNDI

The org.springframework.mock.jndi package contains an implementation of the JNDI SPI, which is useful for setting up a simple JNDI environment for test suites or stand-alone applications. If, for example, JDBC DataSources get bound to the same JNDI names in test code as within a J2EE container, both application code and configuration can be reused in testing scenarios without modification.

10.2.1.2 Servlet API

The org.springframework.mock.web package contains a comprehensive set of Servlet API mock objects, targeted at usage with Spring's Web MVC framework, which are useful for testing web contexts and controllers. These mock objects are generally more convenient to use than dynamic mock objects (e.g., EasyMock) or existing Servlet API mock objects (e.g., MockObjects).

10.2.1.3 Portlet API

The org.springframework.mock.web.portlet package contains a set of Portlet API mock objects, targeted at usage with Spring's Portlet MVC framework.

10.2.2 Unit testing support classes

10.2.2.1 General utilities

The org.springframework.test.util package contains ReflectionTestUtils, which is a collection of reflection-based utility methods for use in unit and integration testing scenarios in which the developer would benefit from being able to set a non-public field or invoke a non-public setter method when testing application code involving, for example:

  • ORM frameworks such as JPA and Hibernate which condone the usage of private or protected field access as opposed to public setter methods for properties in a domain entity

  • Spring's support for annotations such as @Autowired and @Resource which provides dependency injection for private or protected fields, setter methods, and configuration methods

10.2.2.2 Spring MVC

The org.springframework.test.web package contains ModelAndViewAssert, which can be used in combination with any testing framework (e.g., JUnit 4+, TestNG, etc.) for unit tests dealing with Spring MVC ModelAndView objects.

[Tip]Unit testing Spring MVC Controllers

To test your Spring MVC Controllers, use ModelAndViewAssert combined with MockHttpServletRequest, MockHttpSession, etc. from the org.springframework.mock.web package.

10.3 Integration testing

10.3.1 Overview

It is important to be able to perform some integration testing without requiring deployment to your application server or connecting to other enterprise infrastructure. This will enable you to test things such as:

  • The correct wiring of your Spring IoC container contexts.

  • Data access using JDBC or an ORM tool. This would include such things as the correctness of SQL statements, Hibernate queries, JPA entity mappings, etc.

The Spring Framework provides first class support for integration testing in the org.springframework.test-VERSION.jar library (where VERSION is the release version). In this library, you will find the org.springframework.test package which contains valuable classes for integration testing using a Spring container, while at the same time not being reliant on an application server or other deployment environment. Such tests will be slower to run than unit tests but much faster to run than the equivalent Cactus tests or remote tests relying on deployment to an application server.

Since Spring 2.5, unit and integration testing support is provided in the form of the annotation-driven Spring TestContext Framework. The TestContext Framework is agnostic of the actual testing framework in use, thus allowing instrumentation of tests in various environments including JUnit 3.8, JUnit 4.5, TestNG, etc.

[Note]Legacy JUnit 3.8 class hierarchy is deprecated

As of Spring 3.0, the legacy JUnit 3.8 base class hierarchy (e.g., AbstractDependencyInjectionSpringContextTests, AbstractTransactionalDataSourceSpringContextTests, etc.) is officially deprecated and will be removed in a later release. Thus any code which depends on the legacy JUnit 3.8 support should be migrated to the Spring TestContext Framework.

10.3.2 Goals

The following bullet points highlight the fundamental goals of Spring's integration testing support:

In the next few sections each of the above goals is discussed in greater detail, and at the end of each section you will find a direct link to implementation and configuration details pertaining to that particular goal.

10.3.2.1 Context management and caching

The Spring TestContext Framework provides consistent loading of Spring ApplicationContexts and caching of those contexts. Support for the caching of loaded contexts is important, because if you are working on a large project, startup time may become an issue - not because of the overhead of Spring itself, but because the objects instantiated by the Spring container will themselves take time to instantiate. For example, a project with 50-100 Hibernate mapping files might take 10-20 seconds to load the mapping files, and incurring that cost before running every single test in every single test fixture will lead to slower overall test runs that could reduce productivity.

Test classes provide an array containing the resource locations of XML configuration metadata - typically on the classpath - used to configure the application. This will be the same, or nearly the same, as the list of configuration locations specified in web.xml or other deployment configuration.

By default, once loaded, the configured ApplicationContext will be reused for each test. Thus the setup cost will be incurred only once (per test fixture), and subsequent test execution will be much faster. In the unlikely case that a test may 'dirty' the application context, requiring reloading - for example, by changing a bean definition or the state of an application object - Spring's testing support provides a mechanism to cause the test fixture to reload the configurations and rebuild the application context before executing the next test.

See: context management and caching with the TestContext Framework.

10.3.2.2 Dependency Injection of test fixtures

When the TestContext framework loads your application context, it can optionally configure instances of your test classes via Dependency Injection. This provides a convenient mechanism for setting up test fixtures using pre-configured beans from your application context. A strong benefit here is that you can reuse application contexts across various testing scenarios (e.g., for configuring Spring-managed object graphs, transactional proxies, DataSources, etc.), thus avoiding the need to duplicate complex test fixture set up for individual test cases.

As an example, consider the scenario where we have a class, HibernateTitleDao, that performs data access logic for say, the Title domain object. We want to write integration tests that test all of the following areas:

  • The Spring configuration: basically, is everything related to the configuration of the HibernateTitleDao bean correct and present?

  • The Hibernate mapping file configuration: is everything mapped correctly and are the correct lazy-loading settings in place?

  • The logic of the HibernateTitleDao: does the configured instance of this class perform as anticipated?

See: dependency injection of test fixtures with the TestContext Framework.

10.3.2.3 Transaction management

One common issue in tests that access a real database is their affect on the state of the persistence store. Even when you're using a development database, changes to the state may affect future tests. Also, many operations - such as inserting or modifying persistent data - cannot be performed (or verified) outside a transaction.

The TestContext framework addresses this issue. By default, the framework will create and roll back a transaction for each test. You simply write code that can assume the existence of a transaction. If you call transactionally proxied objects in your tests, they will behave correctly, according to their transactional semantics. In addition, if test methods delete the contents of selected tables while running within a transaction, the transaction will roll back by default, and the database will return to its state prior to execution of the test. Transactional support is provided to your test class via a PlatformTransactionManager bean defined in the test's application context.

If you want a transaction to commit - unusual, but occasionally useful when you want a particular test to populate or modify the database - the TestContext framework can be instructed to cause the transaction to commit instead of roll back via the @TransactionConfiguration and @Rollback annotations.

See: transaction management with the TestContext Framework.

10.3.2.4 Integration testing support classes

The Spring TestContext Framework provides several abstract support classes that can simplify writing integration tests. These base test classes provide well defined hooks into the testing framework as well as convenient instance variables and methods, allowing access to such things as:

  • The ApplicationContext: useful for performing explicit bean lookups or testing the state of the context as a whole.

  • A SimpleJdbcTemplate: useful for querying to confirm state. For example, you might query before and after testing application code that creates an object and persists it using an ORM tool, to verify that the data appears in the database. (Spring will ensure that the query runs in the scope of the same transaction.) You will need to tell your ORM tool to 'flush' its changes for this to work correctly, for example using the flush() method on Hibernate's Session interface.

In addition, you may find it desirable to provide your own custom, application-wide superclass for integration tests that provides further useful instance variables and methods specific to your project.

See: support classes for the TestContext Framework.

10.3.3 JDBC testing support

The org.springframework.test.jdbc package contains SimpleJdbcTestUtils, which is a Java-5-based collection of JDBC related utility functions intended to simplify standard database testing scenarios. Note that AbstractTransactionalJUnit38SpringContextTests, AbstractTransactionalJUnit4SpringContextTests, and AbstractTransactionalTestNGSpringContextTests provide convenience methods which delegate to SimpleJdbcTestUtils internally.

10.3.4 Annotations

The Spring Framework provides the following set of Spring-specific annotations that you can use in your unit and integration tests in conjunction with the TestContext framework. Refer to the respective JavaDoc for further information, including default attribute values, etc.

  • @ContextConfiguration

    Defines class-level metadata which is used to determine how to load and configure an ApplicationContext. Specifically, @ContextConfiguration defines the application context resource locations to load as well as the ContextLoader strategy to use for loading the context.

    @ContextConfiguration(locations={"example/test-context.xml"}, loader=CustomContextLoader.class)
    public class CustomConfiguredApplicationContextTests {
        // class body...
    }

    Note: @ContextConfiguration provides support for inherited resource locations by default. See the Context management and caching section and JavaDoc for an example and further details.

  • @DirtiesContext

    The presence of this annotation on a test method indicates that the underlying Spring container is 'dirtied' during the execution of the test method, and thus must be rebuilt after the test method finishes execution (regardless of whether the test passed or not).

    @DirtiesContext
    @Test
    public void testProcessWhichDirtiesAppCtx() {
        // some logic that results in the Spring container being dirtied
    }
  • @TestExecutionListeners

    Defines class-level metadata for configuring which TestExecutionListeners should be registered with a TestContextManager. Typically, @TestExecutionListeners will be used in conjunction with @ContextConfiguration.

    @ContextConfiguration
    @TestExecutionListeners({CustomTestExecutionListener.class, AnotherTestExecutionListener.class})
    public class CustomTestExecutionListenerTests {
        // class body...
    }

    Note: @TestExecutionListeners provides support for inherited listeners by default. See the JavaDoc for an example and further details.

  • @TransactionConfiguration

    Defines class-level metadata for configuring transactional tests. Specifically, the bean name of the PlatformTransactionManager that is to be used to drive transactions can be explicitly configured if the bean name of the desired PlatformTransactionManager is not "transactionManager". In addition, the defaultRollback flag can optionally be changed to false. Typically, @TransactionConfiguration will be used in conjunction with @ContextConfiguration.

    @ContextConfiguration
    @TransactionConfiguration(transactionManager="txMgr", defaultRollback=false)
    public class CustomConfiguredTransactionalTests {
        // class body...
    }
  • @Rollback

    Indicates whether or not the transaction for the annotated test method should be rolled back after the test method has completed. If true, the transaction will be rolled back; otherwise, the transaction will be committed. Use @Rollback to override the default rollback flag configured at the class level.

    @Rollback(false)
    @Test
    public void testProcessWithoutRollback() {
        // ...
    }
  • @BeforeTransaction

    Indicates that the annotated public void method should be executed before a transaction is started for test methods configured to run within a transaction via the @Transactional annotation.

    @BeforeTransaction
    public void beforeTransaction() {
        // logic to be executed before a transaction is started
    }
  • @AfterTransaction

    Indicates that the annotated public void method should be executed after a transaction has been ended for test methods configured to run within a transaction via the @Transactional annotation.

    @AfterTransaction
    public void afterTransaction() {
        // logic to be executed after a transaction has ended
    }
  • @NotTransactional

    The presence of this annotation indicates that the annotated test method must not execute in a transactional context.

    @NotTransactional 
    @Test
    public void testProcessWithoutTransaction() {
        // ...
    }

The following annotations are only supported when used in conjunction with JUnit (i.e., with the SpringJUnit4ClassRunner or the JUnit 3.8 and JUnit 4.5 support classes.

  • @IfProfileValue

    Indicates that the annotated test is enabled for a specific testing environment. If the configured ProfileValueSource returns a matching value for the provided name, the test will be enabled. This annotation can be applied to an entire class or individual methods.

    @IfProfileValue(name="java.vendor", value="Sun Microsystems Inc.")
    @Test
    public void testProcessWhichRunsOnlyOnSunJvm() {
        // some logic that should run only on Java VMs from Sun Microsystems
    }

    Alternatively @IfProfileValue may be configured with a list of values (with OR semantics) to achieve TestNG-like support for test groups in a JUnit environment. Consider the following example:

    @IfProfileValue(name="test-groups", values={"unit-tests", "integration-tests"})
    @Test
    public void testProcessWhichRunsForUnitOrIntegrationTestGroups() {
        // some logic that should run only for unit and integration test groups
    }
  • @ProfileValueSourceConfiguration

    Class-level annotation which is used to specify what type of ProfileValueSource to use when retrieving profile values configured via the @IfProfileValue annotation. If @ProfileValueSourceConfiguration is not declared for a test, SystemProfileValueSource will be used by default.

    @ProfileValueSourceConfiguration(CustomProfileValueSource.class)
    public class CustomProfileValueSourceTests {
        // class body...
    }
  • @ExpectedException

    Indicates that the annotated test method is expected to throw an exception during execution. The type of the expected exception is provided in the annotation, and if an instance of the exception is thrown during the test method execution then the test passes. Likewise if an instance of the exception is not thrown during the test method execution then the test fails.

    @ExpectedException(SomeBusinessException.class)
    public void testProcessRainyDayScenario() {
        // some logic that should result in an Exception being thrown
    }

    Using Spring's @ExpectedException annotation in conjunction with JUnit 4's @Test(expected=...) configuration would lead to an unresolvable conflict. Developers must therefore choose one or the other when integrating with JUnit 4, in which case it is generally preferable to use the explicit JUnit 4 configuration.

  • @Timed

    Indicates that the annotated test method has to finish execution in a specified time period (in milliseconds). If the text execution time takes longer than the specified time period, the test fails.

    Note that the time period includes execution of the test method itself, any repetitions of the test (see @Repeat), as well as any set up or tear down of the test fixture.

    @Timed(millis=1000)
    public void testProcessWithOneSecondTimeout() {
        // some logic that should not take longer than 1 second to execute
    }

    Spring's @Timed annotation has different semantics than JUnit 4's @Test(timeout=...) support. Specifically, due to the manner in which JUnit 4 handles test execution timeouts (i.e., by executing the test method in a separate Thread), @Test(timeout=...) applies to each iteration in the case of repetitions and preemptively fails the test if the test takes too long. Spring's @Timed, on the other hand, times the total test execution time (including all repetitions) and does not preemptively fail the test but rather waits for the test to actually complete before failing.

  • @Repeat

    Indicates that the annotated test method must be executed repeatedly. The number of times that the test method is to be executed is specified in the annotation.

    Note that the scope of execution to be repeated includes execution of the test method itself as well as any set up or tear down of the test fixture.

    @Repeat(10)
    @Test
    public void testProcessRepeatedly() {
        // ...
    }

The following non-test-specific annotations are supported with standard semantics for all configurations of the Spring TestContext Framework.

  • @Autowired

  • @Qualifier

  • @Resource (javax.annotation) if JSR-250 is present

  • @PersistenceContext (javax.persistence) if JPA is present

  • @PersistenceUnit (javax.persistence) if JPA is present

  • @Required

  • @Transactional

10.3.5 Spring TestContext Framework

The Spring TestContext Framework (located in the org.springframework.test.context package) provides generic, annotation-driven unit and integration testing support that is agnostic of the testing framework in use, for example JUnit 3.8, JUnit 4.5, TestNG 5.8, etc. The TestContext framework also places a great deal of importance on convention over configuration with reasonable defaults that can be overridden via annotation-based configuration.

In addition to generic testing infrastructure, the TestContext framework provides explicit support for JUnit 3.8, JUnit 4.5, and TestNG 5.8 in the form of abstract support classes. For JUnit 4.5, the framework also provides a custom Runner which allows one to write test classes that are not required to extend a particular class hierarchy.

The following section provides an overview of the internals of the TestContext framework. If you are only interested in using the framework and not necessarily interested in extending it with your own custom listeners, feel free to go directly to the configuration (context management, dependency injection, transaction management), support classes, and annotation support sections.

10.3.5.1 Key abstractions

The core of the framework consists of the TestContext and TestContextManager classes and the TestExecutionListener interface. A TestContextManager is created on a per-test basis. The TestContextManager in turn manages a TestContext which is responsible for holding the context of the current test. The TestContextManager is also responsible for updating the state of the TestContext as the test progresses and delegating to TestExecutionListeners, which instrument the actual test execution (e.g., providing dependency injection, managing transactions, etc.). Consult the JavaDoc and the Spring test suite for further information and examples of various configurations.

  • TestContext: encapsulates the context in which a test is executed, agnostic of the actual testing framework in use.

  • TestContextManager: the main entry point into the Spring TestContext Framework, which is responsible for managing a single TestContext and signaling events to all registered TestExecutionListeners at well defined test execution points: test instance preparation, prior to any before methods of a particular testing framework, and after any after methods of a particular testing framework.

  • TestExecutionListener: defines a listener API for reacting to test execution events published by the TestContextManager with which the listener is registered.

    Spring provides three TestExecutionListener implementations which are configured by default: DependencyInjectionTestExecutionListener, DirtiesContextTestExecutionListener, and TransactionalTestExecutionListener, which provide support for dependency injection of the test instance, handling of the @DirtiesContext annotation, and transactional test execution support with default rollback semantics, respectively.

The following three sections explain how to configure the TestContext framework via annotations and provide working examples of how to actually write unit and integration tests with the framework.

10.3.5.2 Context management and caching

Each TestContext provides context management and caching support for the test instance for which it is responsible. Test instances do not automatically receive access to the configured ApplicationContext; however, if a test class implements the ApplicationContextAware interface, a reference to the ApplicationContext will be supplied to the test instance (provided the DependencyInjectionTestExecutionListener has been configured, which is the default). Note that AbstractJUnit38SpringContextTests, AbstractJUnit4SpringContextTests, and AbstractTestNGSpringContextTests already implement ApplicationContextAware and therefore provide this functionality out-of-the-box.

[Tip]@Autowired ApplicationContext

As an alternative to implementing the ApplicationContextAware interface, your test class can have its application context injected via the @Autowired annotation on either a field or setter method, for example:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class MyTest {
    @Autowired
    private ApplicationContext applicationContext;

    // class body...
}

In contrast to the now deprecated JUnit 3.8 legacy class hierarchy, test classes which use the TestContext framework do not need to override any protected instance methods to configure their application context. Rather, configuration is achieved merely by declaring the @ContextConfiguration annotation at the class level. If your test class does not explicitly declare any application context resource locations, the configured ContextLoader will determine how and whether or not to load a context from a default set of locations. For example, GenericXmlContextLoader - which is the default ContextLoader - will generate a default location based on the name of the test class. If your class is named com.example.MyTest, GenericXmlContextLoader will load your application context from "classpath:/com/example/MyTest-context.xml".

package com.example;

@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "classpath:/com/example/MyTest-context.xml"
@ContextConfiguration
public class MyTest {
    // class body...
}

If the default location does not suit your needs, you are free to explicitly configure the locations attribute of @ContextConfiguration (see code listing below) with an array containing the resource locations of XML configuration metadata (assuming an XML-capable ContextLoader has been configured) - typically on the classpath - used to configure the application. This will be the same, or nearly the same, as the list of configuration locations specified in web.xml or other deployment configuration. As an alternative you may choose to implement and configure your own custom ContextLoader.

@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "/applicationContext.xml" and "/applicationContext-test.xml"
// in the root of the classpath
@ContextConfiguration(locations={"/applicationContext.xml", "/applicationContext-test.xml"})
public class MyTest {
    // class body...
}

@ContextConfiguration supports an alias for the locations attribute via the standard value attribute. Thus, if you do not need to configure a custom ContextLoader, you can omit the declaration of the locations attribute name and declare the resource locations using the shorthand format demonstrated in the following example. @ContextConfiguration also supports a boolean inheritLocations attribute which denotes whether or not resource locations from superclasses should be inherited. The default value is true, which means that an annotated class will inherit the resource locations defined by an annotated superclass. Specifically, the resource locations for an annotated class will be appended to the list of resource locations defined by an annotated superclass. Thus, subclasses have the option of extending the list of resource locations. In the following example, the ApplicationContext for ExtendedTest will be loaded from "/base-context.xml" and "/extended-context.xml", in that order. Beans defined in "/extended-context.xml" may therefore override those defined in "/base-context.xml".

@RunWith(SpringJUnit4ClassRunner.class)
// ApplicationContext will be loaded from "/base-context.xml" in the root of the classpath
@ContextConfiguration("/base-context.xml")
public class BaseTest {
    // class body...
}

// ApplicationContext will be loaded from "/base-context.xml" and "/extended-context.xml"
// in the root of the classpath
@ContextConfiguration("/extended-context.xml")
public class ExtendedTest extends BaseTest {
    // class body...
}

If inheritLocations is set to false, the resource locations for the annotated class will shadow and effectively replace any resource locations defined by a superclass.

By default, once loaded, the configured ApplicationContext will be reused for each test. Thus the setup cost will be incurred only once (per test fixture), and subsequent test execution will be much faster. In the unlikely case that a test may dirty the application context, requiring reloading - for example, by changing a bean definition or the state of an application object - you may annotate your test method with @DirtiesContext (assuming DirtiesContextTestExecutionListener has been configured, which is the default) to cause the test fixture to reload the configurations and rebuild the application context before executing the next test.

10.3.5.3 Dependency Injection of test fixtures

When you configure the DependencyInjectionTestExecutionListener - which is configured by default - via the @TestExecutionListeners annotation, the dependencies of your test instances will be