Spring Framework

Reference Documentation

3.0

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Table of Contents

I. Overview of Spring Framework
1. Introduction to Spring Framework
1.1. Dependency Injection and Inversion of Control
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
1.3.1. Dependency Management and Naming Conventions
1.3.1.1. Spring Dependencies and Depending on Spring
1.3.1.2. Maven Dependency Management
1.3.1.3. Ivy Dependency Management
1.3.2. Logging
1.3.2.1. Not Using Commons Logging
1.3.2.2. Using SLF4J
1.3.2.3. Using Log4J
II. What's New in Spring 3.0
2. New Features and Enhancements in Spring 3.0
2.1. Java 5
2.2. Improved documentation
2.3. New articles and tutorials
2.4. New module organization and build system
2.5. Overview of new features
2.5.1. Core APIs updated for Java 5
2.5.2. Spring Expression Language
2.5.3. The Inversion of Control (IoC) container
2.5.3.1. Java based bean metadata
2.5.3.2. Defining bean metadata within components
2.5.4. General purpose type conversion system and field formatting system
2.5.5. The Data Tier
2.5.6. The Web Tier
2.5.6.1. Comprehensive REST support
2.5.6.2. @MVC additions
2.5.7. Declarative model validation
2.5.8. Early support for Java EE 6
2.5.9. Support for embedded databases
III. Core Technologies
3. The IoC container
3.1. Introduction to the Spring IoC container and beans
3.2. Container overview
3.2.1. Configuration metadata
3.2.2. Instantiating a container
3.2.2.1. Composing XML-based configuration metadata
3.2.3. Using the container
3.3. Bean overview
3.3.1. Naming beans
3.3.1.1. Aliasing a bean outside the bean definition
3.3.2. Instantiating beans
3.3.2.1. Instantiation with a constructor
3.3.2.2. Instantiation with a static factory method
3.3.2.3. Instantiation using an instance factory method
3.4. Dependencies
3.4.1. Dependency injection
3.4.1.1. Constructor-based dependency injection
3.4.1.2. Setter-based dependency injection
3.4.1.3. Dependency resolution process
3.4.1.4. Examples of dependency injection
3.4.2. Dependencies and configuration in detail
3.4.2.1. Straight values (primitives, Strings, and so on)
3.4.2.2. References to other beans (collaborators)
3.4.2.3. Inner beans
3.4.2.4. Collections
3.4.2.5. Null and empty string values
3.4.2.6. XML shortcut with the p-namespace
3.4.2.7. Compound property names
3.4.3. Using depends-on
3.4.4. Lazy-initialized beans
3.4.5. Autowiring collaborators
3.4.5.1. Limitations and disadvantages of autowiring
3.4.5.2. Excluding a bean from autowiring
3.4.6. Method injection
3.4.6.1. Lookup method injection
3.4.6.2. Arbitrary method replacement
3.5. Bean scopes
3.5.1. The singleton scope
3.5.2. The prototype scope
3.5.3. Singleton beans with prototype-bean dependencies
3.5.4. Request, session, and global session scopes
3.5.4.1. Initial web configuration
3.5.4.2. Request scope
3.5.4.3. Session scope
3.5.4.4. Global session scope
3.5.4.5. Scoped beans as dependencies
3.5.5. Custom scopes
3.5.5.1. Creating a custom scope
3.5.5.2. Using a custom scope
3.6. Customizing the nature of a bean
3.6.1. Lifecycle callbacks
3.6.1.1. Initialization callbacks
3.6.1.2. Destruction callbacks
3.6.1.3. Default initialization and destroy methods
3.6.1.4. Combining lifecycle mechanisms
3.6.1.5. Startup and shutdown callbacks
3.6.1.6. Shutting down the Spring IoC container gracefully in non-web applications
3.6.2. ApplicationContextAware and BeanNameAware
3.6.3. Other Aware interfaces
3.7. Bean definition inheritance
3.8. Container extension points
3.8.1. Customizing beans using the BeanPostProcessor Interface
3.8.1.1. Example: Hello World, BeanPostProcessor-style
3.8.1.2. Example: The RequiredAnnotationBeanPostProcessor
3.8.2. Customizing configuration metadata with BeanFactoryPostProcessor interface
3.8.2.1. Example: the PropertyPlaceholderConfigurer
3.8.2.2. Example: the PropertyOverrideConfigurer
3.8.3. Customizing instantiation logic with the FactoryBean Interface
3.9. Annotation-based container configuration
3.9.1. @Required
3.9.2. @Autowired and @Inject
3.9.3. Fine-tuning annotation-based autowiring with qualifiers
3.9.4. CustomAutowireConfigurer
3.9.5. @Resource
3.9.6. @PostConstruct and @PreDestroy
3.10. Classpath scanning and managed components
3.10.1. @Component and further stereotype annotations
3.10.2. Automatically detecting classes and registering bean definitions
3.10.3. Using filters to customize scanning
3.10.4. Defining bean metadata within components
3.10.5. Naming autodetected components
3.10.6. Providing a scope for autodetected components
3.10.7. Providing qualifier metadata with annotations
3.11. Java-based container configuration
3.11.1. Basic concepts: @Configuration and @Bean
3.11.2. Instantiating the Spring container using AnnotationConfigApplicationContext
3.11.2.1. Simple construction
3.11.2.2. Building the container programmatically using register(Class<?>...)
3.11.2.3. Enabling component scanning with scan(String...)
3.11.2.4. Support for web applications with AnnotationConfigWebApplicationContext
3.11.3. Composing Java-based configurations
3.11.3.1. Using the @Import annotation
3.11.3.2. Combining Java and XML configuration
3.11.4. Using the @Bean annotation
3.11.4.1. Declaring a bean
3.11.4.2. Injecting dependencies
3.11.4.3. Receiving lifecycle callbacks
3.11.4.4. Specifying bean scope
3.11.4.5. Customizing bean naming
3.11.4.6. Bean aliasing
3.11.5. Further information about how Java-based configuration works internally
3.12. Registering a LoadTimeWeaver
3.13. Additional Capabilities of the ApplicationContext
3.13.1. Internationalization using MessageSource
3.13.2. Standard and Custom Events
3.13.3. Convenient access to low-level resources
3.13.4. Convenient ApplicationContext instantiation for web applications
3.13.5. Deploying a Spring ApplicationContext as a J2EE RAR file
3.14. The BeanFactory
3.14.1. BeanFactory or ApplicationContext?
3.14.2. Glue code and the evil singleton
4. Resources
4.1. Introduction
4.2. The Resource interface
4.3. Built-in Resource implementations
4.3.1. UrlResource
4.3.2. ClassPathResource
4.3.3. FileSystemResource
4.3.4. ServletContextResource
4.3.5. InputStreamResource
4.3.6. ByteArrayResource
4.4. The ResourceLoader
4.5. The ResourceLoaderAware interface
4.6. Resources as dependencies
4.7. Application contexts and Resource paths
4.7.1. Constructing application contexts
4.7.1.1. Constructing ClassPathXmlApplicationContext instances - shortcuts
4.7.2. Wildcards in application context constructor resource paths
4.7.2.1. Ant-style Patterns
4.7.2.2. The classpath*: prefix
4.7.2.3. Other notes relating to wildcards
4.7.3. FileSystemResource caveats
5. Validation, Data Binding, and Type Conversion
5.1. Introduction
5.2. Validation using Spring's Validator interface
5.3. Resolving codes to error messages
5.4. Bean manipulation and the BeanWrapper
5.4.1. Setting and getting basic and nested properties
5.4.2. Built-in PropertyEditor implementations
5.4.2.1. Registering additional custom PropertyEditors
5.5. Spring 3 Type Conversion
5.5.1. Converter SPI
5.5.2. ConverterFactory
5.5.3. GenericConverter
5.5.3.1. ConditionalGenericConverter
5.5.4. ConversionService API
5.5.5. Configuring a ConversionService
5.5.6. Using a ConversionService programatically
5.6. Spring 3 Field Formatting
5.6.1. Formatter SPI
5.6.2. Annotation-driven Formatting
5.6.2.1. Format Annotation API
5.6.3. FormatterRegistry SPI
5.6.4. Configuring Formatting in Spring MVC
5.7. Spring 3 Validation
5.7.1. Overview of the JSR-303 Bean Validation API
5.7.2. Configuring a Bean Validation Implementation
5.7.2.1. Injecting a Validator
5.7.2.2. Configuring Custom Constraints
5.7.2.3. Additional Configuration Options
5.7.3. Configuring a DataBinder
5.7.4. Spring MVC 3 Validation
5.7.4.1. Triggering @Controller Input Validation
5.7.4.2. Configuring a Validator for use by Spring MVC
5.7.4.3. Configuring a JSR-303 Validator for use by Spring MVC
6. Spring Expression Language (SpEL)
6.1. Introduction
6.2. Feature Overview
6.3. Expression Evaluation using Spring's Expression Interface
6.3.1. The EvaluationContext interface
6.3.1.1. Type Conversion
6.4. Expression support for defining bean definitions
6.4.1. XML based configuration
6.4.2. Annotation-based configuration
6.5. Language Reference
6.5.1. Literal expressions
6.5.2. Properties, Arrays, Lists, Maps, Indexers
6.5.3. Inline lists
6.5.4. Array construction
6.5.5. Methods
6.5.6. Operators
6.5.6.1. Relational operators
6.5.6.2. Logical operators
6.5.6.3. Mathematical operators
6.5.7. Assignment
6.5.8. Types
6.5.9. Constructors
6.5.10. Variables
6.5.10.1. The #this and #root variables
6.5.11. Functions
6.5.12. Bean references
6.5.13. Ternary Operator (If-Then-Else)
6.5.14. The Elvis Operator
6.5.15. Safe Navigation operator
6.5.16. Collection Selection
6.5.17. Collection Projection
6.5.18. Expression templating
6.6. Classes used in the examples
7. Aspect Oriented Programming with Spring
7.1. Introduction
7.1.1. AOP concepts
7.1.2. Spring AOP capabilities and goals
7.1.3. AOP Proxies
7.2. @AspectJ support
7.2.1. Enabling @AspectJ Support
7.2.2. Declaring an aspect
7.2.3. Declaring a pointcut
7.2.3.1. Supported Pointcut Designators
7.2.3.2. Combining pointcut expressions
7.2.3.3. Sharing common pointcut definitions
7.2.3.4. Examples
7.2.3.5. Writing good pointcuts
7.2.4. Declaring advice
7.2.4.1. Before advice
7.2.4.2. After returning advice
7.2.4.3. After throwing advice
7.2.4.4. After (finally) advice
7.2.4.5. Around advice
7.2.4.6. Advice parameters
7.2.4.7. Advice ordering
7.2.5. Introductions
7.2.6. Aspect instantiation models
7.2.7. Example
7.3. Schema-based AOP support
7.3.1. Declaring an aspect
7.3.2. Declaring a pointcut
7.3.3. Declaring advice
7.3.3.1. Before advice
7.3.3.2. After returning advice
7.3.3.3. After throwing advice
7.3.3.4. After (finally) advice
7.3.3.5. Around advice
7.3.3.6. Advice parameters
7.3.3.7. Advice ordering
7.3.4. Introductions
7.3.5. Aspect instantiation models
7.3.6. Advisors
7.3.7. Example
7.4. Choosing which AOP declaration style to use
7.4.1. Spring AOP or full AspectJ?
7.4.2. @AspectJ or XML for Spring AOP?
7.5. Mixing aspect types
7.6. Proxying mechanisms
7.6.1. Understanding AOP proxies
7.7. Programmatic creation of @AspectJ Proxies
7.8. Using AspectJ with Spring applications
7.8.1. Using AspectJ to dependency inject domain objects with Spring
7.8.1.1. Unit testing @Configurable objects
7.8.1.2. Working with multiple application contexts
7.8.2. Other Spring aspects for AspectJ
7.8.3. Configuring AspectJ aspects using Spring IoC
7.8.4. Load-time weaving with AspectJ in the Spring Framework
7.8.4.1. A first example
7.8.4.2. Aspects
7.8.4.3. 'META-INF/aop.xml'
7.8.4.4. Required libraries (JARS)
7.8.4.5. Spring configuration
7.8.4.6. Environment-specific configuration
7.9. Further Resources
8. Spring AOP APIs
8.1. Introduction
8.2. Pointcut API in Spring
8.2.1. Concepts
8.2.2. Operations on pointcuts
8.2.3. AspectJ expression pointcuts
8.2.4. Convenience pointcut implementations
8.2.4.1. Static pointcuts
8.2.4.2. Dynamic pointcuts
8.2.5. Pointcut superclasses
8.2.6. Custom pointcuts
8.3. Advice API in Spring
8.3.1. Advice lifecycles
8.3.2. Advice types in Spring
8.3.2.1. Interception around advice
8.3.2.2. Before advice
8.3.2.3. Throws advice
8.3.2.4. After Returning advice
8.3.2.5. Introduction advice
8.4. Advisor API in Spring
8.5. Using the ProxyFactoryBean to create AOP proxies
8.5.1. Basics
8.5.2. JavaBean properties
8.5.3. JDK- and CGLIB-based proxies
8.5.4. Proxying interfaces
8.5.5. Proxying classes
8.5.6. Using 'global' advisors
8.6. Concise proxy definitions
8.7. Creating AOP proxies programmatically with the ProxyFactory
8.8. Manipulating advised objects
8.9. Using the "autoproxy" facility
8.9.1. Autoproxy bean definitions
8.9.1.1. BeanNameAutoProxyCreator
8.9.1.2. DefaultAdvisorAutoProxyCreator
8.9.1.3. AbstractAdvisorAutoProxyCreator
8.9.2. Using metadata-driven auto-proxying
8.10. Using TargetSources
8.10.1. Hot swappable target sources
8.10.2. Pooling target sources
8.10.3. Prototype target sources
8.10.4. ThreadLocal target sources
8.11. Defining new Advice types
8.12. Further resources
9. Testing
9.1. Introduction to testing
9.2. Unit testing
9.2.1. Mock objects
9.2.1.1. JNDI
9.2.1.2. Servlet API
9.2.1.3. Portlet API
9.2.2. Unit testing support classes
9.2.2.1. General utilities
9.2.2.2. Spring MVC
9.3. Integration testing
9.3.1. Overview
9.3.2. Goals of integration testing
9.3.2.1. Context management and caching
9.3.2.2. Dependency Injection of test fixtures
9.3.2.3. Transaction management
9.3.2.4. Support classes for integration testing
9.3.3. JDBC testing support
9.3.4. Annotations
9.3.5. Spring TestContext Framework
9.3.5.1. Key abstractions
9.3.5.2. Context management and caching
9.3.5.3. Dependency Injection of test fixtures
9.3.5.4. Transaction management
9.3.5.5. TestContext support classes
9.3.6. PetClinic example
9.4. Further Resources
IV. Data Access
10. Transaction Management
10.1. Introduction to Spring Framework transaction management
10.2. Advantages of the Spring Framework's transaction support model
10.2.1. Global transactions
10.2.2. Local transactions
10.2.3. Spring Framework's consistent programming model
10.3. Understanding the Spring Framework transaction abstraction
10.4. Synchronizing resources with transactions
10.4.1. High-level synchronization approach
10.4.2. Low-level synchronization approach
10.4.3. TransactionAwareDataSourceProxy
10.5. Declarative transaction management
10.5.1. Understanding the Spring Framework's declarative transaction implementation
10.5.2. Example of declarative transaction implementation
10.5.3. Rolling back a declarative transaction
10.5.4. Configuring different transactional semantics for different beans
10.5.5. <tx:advice/> settings
10.5.6. Using @Transactional
10.5.6.1. @Transactional settings
10.5.6.2. Multiple Transaction Managers with @Transactional
10.5.6.3. Custom shortcut annotations
10.5.7. Transaction propagation
10.5.7.1. Required
10.5.7.2. RequiresNew
10.5.7.3. Nested
10.5.8. Advising transactional operations
10.5.9. Using @Transactional with AspectJ
10.6. Programmatic transaction management
10.6.1. Using the TransactionTemplate
10.6.1.1. Specifying transaction settings
10.6.2. Using the PlatformTransactionManager
10.7. Choosing between programmatic and declarative transaction management
10.8. Application server-specific integration
10.8.1. IBM WebSphere
10.8.2. BEA WebLogic Server
10.8.3. Oracle OC4J
10.9. Solutions to common problems
10.9.1. Use of the wrong transaction manager for a specific DataSource
10.10. Further Resources
11. DAO support
11.1. Introduction
11.2. Consistent exception hierarchy
11.3. Annotations used for configuring DAO or Repository classes
12. Data access with JDBC
12.1. Introduction to Spring Framework JDBC
12.1.1. Choosing an approach for JDBC database access
12.1.2. Package hierarchy
12.2. Using the JDBC core classes to control basic JDBC processing and error handling
12.2.1. JdbcTemplate
12.2.1.1. Examples of JdbcTemplate class usage
12.2.1.2. JdbcTemplate best practices
12.2.2. NamedParameterJdbcTemplate
12.2.3. SimpleJdbcTemplate
12.2.4. SQLExceptionTranslator
12.2.5. Executing statements
12.2.6. Running queries
12.2.7. Updating the database
12.2.8. Retrieving auto-generated keys
12.3. Controlling database connections
12.3.1. DataSource
12.3.2. DataSourceUtils
12.3.3. SmartDataSource
12.3.4. AbstractDataSource
12.3.5. SingleConnectionDataSource
12.3.6. DriverManagerDataSource
12.3.7. TransactionAwareDataSourceProxy
12.3.8. DataSourceTransactionManager
12.3.9. NativeJdbcExtractor
12.4. JDBC batch operations
12.4.1. Batch operations with the JdbcTemplate
12.4.2. Batch operations with the SimpleJdbcTemplate
12.5. Simplifying JDBC operations with the SimpleJdbc classes
12.5.1. Inserting data using SimpleJdbcInsert
12.5.2. Retrieving auto-generated keys using SimpleJdbcInsert
12.5.3. Specifying columns for a SimpleJdbcInsert
12.5.4. Using SqlParameterSource to provide parameter values
12.5.5. Calling a stored procedure with SimpleJdbcCall
12.5.6. Explicitly declaring parameters to use for a SimpleJdbcCall
12.5.7. How to define SqlParameters
12.5.8. Calling a stored function using SimpleJdbcCall
12.5.9. Returning ResultSet/REF Cursor from a SimpleJdbcCall
12.6. Modeling JDBC operations as Java objects
12.6.1. SqlQuery
12.6.2. MappingSqlQuery
12.6.3. SqlUpdate
12.6.4. StoredProcedure
12.7. Common problems with parameter and data value handling
12.7.1. Providing SQL type information for parameters
12.7.2. Handling BLOB and CLOB objects
12.7.3. Passing in lists of values for IN clause
12.7.4. Handling complex types for stored procedure calls
12.8. Embedded database support
12.8.1. Why use an embedded database?
12.8.2. Creating an embedded database instance using Spring XML
12.8.3. Creating an embedded database instance programmatically
12.8.4. Extending the embedded database support
12.8.5. Using HSQL
12.8.6. Using H2
12.8.7. Using Derby
12.8.8. Testing data access logic with an embedded database
12.9. Initializing a DataSource
12.9.1. Initializing a database instance using Spring XML
12.9.1.1. Initialization of Other Components that Depend on the Database
13. Object Relational Mapping (ORM) Data Access
13.1. Introduction to ORM with Spring
13.2. General ORM integration considerations
13.2.1. Resource and transaction management
13.2.2. Exception translation
13.3. Hibernate
13.3.1. SessionFactory setup in a Spring container
13.3.2. Implementing DAOs based on plain Hibernate 3 API
13.3.3. Declarative transaction demarcation
13.3.4. Programmatic transaction demarcation
13.3.5. Transaction management strategies
13.3.6. Comparing container-managed and locally defined resources
13.3.7. Spurious application server warnings with Hibernate
13.4. JDO
13.4.1. PersistenceManagerFactory setup
13.4.2. Implementing DAOs based on the plain JDO API
13.4.3. Transaction management
13.4.4. JdoDialect
13.5. JPA
13.5.1. Three options for JPA setup in a Spring environment
13.5.1.1. LocalEntityManagerFactoryBean
13.5.1.2. Obtaining an EntityManagerFactory from JNDI
13.5.1.3. LocalContainerEntityManagerFactoryBean
13.5.1.4. Dealing with multiple persistence units
13.5.2. Implementing DAOs based on plain JPA
13.5.3. Transaction Management
13.5.4. JpaDialect
13.6. iBATIS SQL Maps
13.6.1. Setting up the SqlMapClient
13.6.2. Using SqlMapClientTemplate and SqlMapClientDaoSupport
13.6.3. Implementing DAOs based on plain iBATIS API
14. Marshalling XML using O/X Mappers
14.1. Introduction
14.2. Marshaller and Unmarshaller
14.2.1. Marshaller
14.2.2. Unmarshaller
14.2.3. XmlMappingException
14.3. Using Marshaller and Unmarshaller
14.4. XML Schema-based Configuration
14.5. JAXB
14.5.1. Jaxb2Marshaller
14.5.1.1. XML Schema-based Configuration
14.6. Castor
14.6.1. CastorMarshaller
14.6.2. Mapping
14.7. XMLBeans
14.7.1. XmlBeansMarshaller
14.7.1.1. XML Schema-based Configuration
14.8. JiBX
14.8.1. JibxMarshaller
14.8.1.1. XML Schema-based Configuration
14.9. XStream
14.9.1. XStreamMarshaller
V. The Web
15. Web MVC framework
15.1. Introduction to Spring Web MVC framework
15.1.1. Features of Spring Web MVC
15.1.2. Pluggability of other MVC implementations
15.2. The DispatcherServlet
15.3. Implementing Controllers
15.3.1. Defining a controller with @Controller
15.3.2. Mapping requests with @RequestMapping
15.3.2.1. URI Templates
15.3.2.2. Advanced @RequestMapping options
15.3.2.3. Supported handler method arguments and return types
15.3.2.4. Binding request parameters to method parameters with @RequestParam
15.3.2.5. Mapping the request body with the @RequestBody annotation
15.3.2.6. Mapping the response body with the @ResponseBody annotation
15.3.2.7. Using HttpEntity<?>
15.3.2.8. Providing a link to data from the model with @ModelAttribute
15.3.2.9. Specifying attributes to store in a session with @SessionAttributes
15.3.2.10. Mapping cookie values with the @CookieValue annotation
15.3.2.11. Mapping request header attributes with the @RequestHeader annotation
15.3.2.12. Customizing WebDataBinder initialization
15.4. Handler mappings
15.4.1. Intercepting requests - the HandlerInterceptor interface
15.5. Resolving views
15.5.1. Resolving views with the ViewResolver interface
15.5.2. Chaining ViewResolvers
15.5.3. Redirecting to views
15.5.3.1. RedirectView
15.5.3.2. The redirect: prefix
15.5.3.3. The forward: prefix
15.5.4. ContentNegotiatingViewResolver
15.6. Using locales
15.6.1. AcceptHeaderLocaleResolver
15.6.2. CookieLocaleResolver
15.6.3. SessionLocaleResolver
15.6.4. LocaleChangeInterceptor
15.7. Using themes
15.7.1. Overview of themes
15.7.2. Defining themes
15.7.3. Theme resolvers
15.8. Spring's multipart (fileupload) support
15.8.1. Introduction
15.8.2. Using the MultipartResolver
15.8.3. Handling a file upload in a form
15.9. Handling exceptions
15.9.1. HandlerExceptionResolver
15.9.2. @ExceptionHandler
15.10. Convention over configuration support
15.10.1. The Controller ControllerClassNameHandlerMapping
15.10.2. The Model ModelMap (ModelAndView)
15.10.3. The View - RequestToViewNameTranslator
15.11. ETag support
15.12. Configuring Spring MVC
15.12.1. mvc:annotation-driven
15.12.2. mvc:interceptors
15.12.3. mvc:view-controller
15.12.4. mvc:resources
15.12.5. mvc:default-servlet-handler
15.13. More Spring Web MVC Resources
16. View technologies
16.1. Introduction
16.2. JSP & JSTL
16.2.1. View resolvers
16.2.2. 'Plain-old' JSPs versus JSTL
16.2.3. Additional tags facilitating development
16.2.4. Using Spring's form tag library
16.2.4.1. Configuration
16.2.4.2. The form tag
16.2.4.3. The input tag
16.2.4.4. The checkbox tag
16.2.4.5. The checkboxes tag
16.2.4.6. The radiobutton tag
16.2.4.7. The radiobuttons tag
16.2.4.8. The password tag
16.2.4.9. The select tag
16.2.4.10. The option tag
16.2.4.11. The options tag
16.2.4.12. The textarea tag
16.2.4.13. The hidden tag
16.2.4.14. The errors tag
16.2.4.15. HTTP Method Conversion
16.3. Tiles
16.3.1. Dependencies
16.3.2. How to integrate Tiles
16.3.2.1. UrlBasedViewResolver
16.3.2.2. ResourceBundleViewResolver
16.3.2.3. SimpleSpringPreparerFactory and SpringBeanPreparerFactory
16.4. Velocity & FreeMarker
16.4.1. Dependencies
16.4.2. Context configuration
16.4.3. Creating templates
16.4.4. Advanced configuration
16.4.4.1. velocity.properties
16.4.4.2. FreeMarker
16.4.5. Bind support and form handling
16.4.5.1. The bind macros
16.4.5.2. Simple binding
16.4.5.3. Form input generation macros
16.4.5.4. HTML escaping and XHTML compliance
16.5. XSLT
16.5.1. My First Words
16.5.1.1. Bean definitions
16.5.1.2. Standard MVC controller code
16.5.1.3. Convert the model data to XML
16.5.1.4. Defining the view properties
16.5.1.5. Document transformation
16.5.2. Summary
16.6. Document views (PDF/Excel)
16.6.1. Introduction
16.6.2. Configuration and setup
16.6.2.1. Document view definitions
16.6.2.2. Controller code
16.6.2.3. Subclassing for Excel views
16.6.2.4. Subclassing for PDF views
16.7. JasperReports
16.7.1. Dependencies
16.7.2. Configuration
16.7.2.1. Configuring the ViewResolver
16.7.2.2. Configuring the Views
16.7.2.3. About Report Files
16.7.2.4. Using JasperReportsMultiFormatView
16.7.3. Populating the ModelAndView
16.7.4. Working with Sub-Reports
16.7.4.1. Configuring Sub-Report Files
16.7.4.2. Configuring Sub-Report Data Sources
16.7.5. Configuring Exporter Parameters
16.8. Feed Views
16.9. XML Marshalling View
16.10. JSON Mapping View
17. Integrating with other web frameworks
17.1. Introduction
17.2. Common configuration
17.3. JavaServer Faces 1.1 and 1.2
17.3.1. DelegatingVariableResolver (JSF 1.1/1.2)
17.3.2. SpringBeanVariableResolver (JSF 1.1/1.2)
17.3.3. SpringBeanFacesELResolver (JSF 1.2+)
17.3.4. FacesContextUtils
17.4. Apache Struts 1.x and 2.x
17.4.1. ContextLoaderPlugin
17.4.1.1. DelegatingRequestProcessor
17.4.1.2. DelegatingActionProxy
17.4.2. ActionSupport Classes
17.5. WebWork 2.x
17.6. Tapestry 3.x and 4.x
17.6.1. Injecting Spring-managed beans
17.6.1.1. Dependency Injecting Spring Beans into Tapestry pages
17.6.1.2. Component definition files
17.6.1.3. Adding abstract accessors
17.6.1.4. Dependency Injecting Spring Beans into Tapestry pages - Tapestry 4.x style
17.7. Further Resources
18. Portlet MVC Framework
18.1. Introduction
18.1.1. Controllers - The C in MVC
18.1.2. Views - The V in MVC
18.1.3. Web-scoped beans
18.2. The DispatcherPortlet
18.3. The ViewRendererServlet
18.4. Controllers
18.4.1. AbstractController and PortletContentGenerator
18.4.2. Other simple controllers
18.4.3. Command Controllers
18.4.4. PortletWrappingController
18.5. Handler mappings
18.5.1. PortletModeHandlerMapping
18.5.2. ParameterHandlerMapping
18.5.3. PortletModeParameterHandlerMapping
18.5.4. Adding HandlerInterceptors
18.5.5. HandlerInterceptorAdapter
18.5.6. ParameterMappingInterceptor
18.6. Views and resolving them
18.7. Multipart (file upload) support
18.7.1. Using the PortletMultipartResolver
18.7.2. Handling a file upload in a form
18.8. Handling exceptions
18.9. Annotation-based controller configuration
18.9.1. Setting up the dispatcher for annotation support
18.9.2. Defining a controller with @Controller
18.9.3. Mapping requests with @RequestMapping
18.9.4. Supported handler method arguments
18.9.5. Binding request parameters to method parameters with @RequestParam
18.9.6. Providing a link to data from the model with @ModelAttribute
18.9.7. Specifying attributes to store in a Session with @SessionAttributes
18.9.8. Customizing WebDataBinder initialization
18.9.8.1. Customizing data binding with @InitBinder
18.9.8.2. Configuring a custom WebBindingInitializer
18.10. Portlet application deployment
VI. Integration
19. Remoting and web services using Spring
19.1. Introduction
19.2. Exposing services using RMI
19.2.1. Exporting the service using the RmiServiceExporter
19.2.2. Linking in the service at the client
19.3. Using Hessian or Burlap to remotely call services via HTTP
19.3.1. Wiring up the DispatcherServlet for Hessian and co.
19.3.2. Exposing your beans by using the HessianServiceExporter
19.3.3. Linking in the service on the client
19.3.4. Using Burlap
19.3.5. Applying HTTP basic authentication to a service exposed through Hessian or Burlap
19.4. Exposing services using HTTP invokers
19.4.1. Exposing the service object
19.4.2. Linking in the service at the client
19.5. Web services
19.5.1. Exposing servlet-based web services using JAX-RPC
19.5.2. Accessing web services using JAX-RPC
19.5.3. Registering JAX-RPC Bean Mappings
19.5.4. Registering your own JAX-RPC Handler
19.5.5. Exposing servlet-based web services using JAX-WS
19.5.6. Exporting standalone web services using JAX-WS
19.5.7. Exporting web services using the JAX-WS RI's Spring support
19.5.8. Accessing web services using JAX-WS
19.6. JMS
19.6.1. Server-side configuration
19.6.2. Client-side configuration
19.7. Auto-detection is not implemented for remote interfaces
19.8. Considerations when choosing a technology
19.9. Accessing RESTful services on the Client
19.9.1. RestTemplate
19.9.1.1. Dealing with request and response headers
19.9.2. HTTP Message Conversion
19.9.2.1. StringHttpMessageConverter
19.9.2.2. FormHttpMessageConverter
19.9.2.3. ByteArrayMessageConverter
19.9.2.4. MarshallingHttpMessageConverter
19.9.2.5. MappingJacksonHttpMessageConverter
19.9.2.6. SourceHttpMessageConverter
19.9.2.7. BufferedImageHttpMessageConverter
20. Enterprise JavaBeans (EJB) integration
20.1. Introduction
20.2. Accessing EJBs
20.2.1. Concepts
20.2.2. Accessing local SLSBs
20.2.3. Accessing remote SLSBs
20.2.4. Accessing EJB 2.x SLSBs versus EJB 3 SLSBs
20.3. Using Spring's EJB implementation support classes
20.3.1. EJB 2.x base classes
20.3.2. EJB 3 injection interceptor
21. JMS (Java Message Service)
21.1. Introduction
21.2. Using Spring JMS
21.2.1. JmsTemplate
21.2.2. Connections
21.2.2.1. Caching Messaging Resources
21.2.2.2. SingleConnectionFactory
21.2.2.3. CachingConnectionFactory
21.2.3. Destination Management
21.2.4. Message Listener Containers
21.2.4.1. SimpleMessageListenerContainer
21.2.4.2. DefaultMessageListenerContainer
21.2.5. Transaction management
21.3. Sending a Message
21.3.1. Using Message Converters
21.3.2. SessionCallback and ProducerCallback
21.4. Receiving a message
21.4.1. Synchronous Reception
21.4.2. Asynchronous Reception - Message-Driven POJOs
21.4.3. The SessionAwareMessageListener interface
21.4.4. The MessageListenerAdapter
21.4.5. Processing messages within transactions
21.5. Support for JCA Message Endpoints
21.6. JMS Namespace Support
22. JMX
22.1. Introduction
22.2. Exporting your beans to JMX
22.2.1. Creating an MBeanServer
22.2.2. Reusing an existing MBeanServer
22.2.3. Lazy-initialized MBeans
22.2.4. Automatic registration of MBeans
22.2.5. Controlling the registration behavior
22.3. Controlling the management interface of your beans
22.3.1. The MBeanInfoAssembler Interface
22.3.2. Using Source-Level Metadata (JDK 5.0 annotations)
22.3.3. Source-Level Metadata Types
22.3.4. The AutodetectCapableMBeanInfoAssembler interface
22.3.5. Defining management interfaces using Java interfaces
22.3.6. Using MethodNameBasedMBeanInfoAssembler
22.4. Controlling the ObjectNames for your beans
22.4.1. Reading ObjectNames from Properties
22.4.2. Using the MetadataNamingStrategy
22.4.3. The <context:mbean-export/> element
22.5. JSR-160 Connectors
22.5.1. Server-side Connectors
22.5.2. Client-side Connectors
22.5.3. JMX over Burlap/Hessian/SOAP
22.6. Accessing MBeans via Proxies
22.7. Notifications
22.7.1. Registering Listeners for Notifications
22.7.2. Publishing Notifications
22.8. Further Resources
23. JCA CCI
23.1. Introduction
23.2. Configuring CCI
23.2.1. Connector configuration
23.2.2. ConnectionFactory configuration in Spring
23.2.3. Configuring CCI connections
23.2.4. Using a single CCI connection
23.3. Using Spring's CCI access support
23.3.1. Record conversion
23.3.2. The CciTemplate
23.3.3. DAO support
23.3.4. Automatic output record generation
23.3.5. Summary
23.3.6. Using a CCI Connection and Interaction directly
23.3.7. Example for CciTemplate usage
23.4. Modeling CCI access as operation objects
23.4.1. MappingRecordOperation
23.4.2. MappingCommAreaOperation
23.4.3. Automatic output record generation
23.4.4. Summary
23.4.5. Example for MappingRecordOperation usage
23.4.6. Example for MappingCommAreaOperation usage
23.5. Transactions
24. Email
24.1. Introduction
24.2. Usage
24.2.1. Basic MailSender and SimpleMailMessage usage
24.2.2. Using the JavaMailSender and the MimeMessagePreparator
24.3. Using the JavaMail MimeMessageHelper
24.3.1. Sending attachments and inline resources
24.3.1.1. Attachments
24.3.1.2. Inline resources
24.3.2. Creating email content using a templating library
24.3.2.1. A Velocity-based example
25. Task Execution and Scheduling
25.1. Introduction
25.2. The Spring TaskExecutor abstraction
25.2.1. TaskExecutor types
25.2.2. Using a TaskExecutor
25.3. The Spring TaskScheduler abstraction
25.3.1. The Trigger interface
25.3.2. Trigger implementations
25.3.3. TaskScheduler implementations
25.4. The Task Namespace
25.4.1. The 'scheduler' element
25.4.2. The 'executor' element
25.4.3. The 'scheduled-tasks' element
25.5. Annotation Support for Scheduling and Asynchronous Execution
25.5.1. The @Scheduled Annotation
25.5.2. The @Async Annotation
25.5.3. The <annotation-driven> Element
25.6. Using the OpenSymphony Quartz Scheduler
25.6.1. Using the JobDetailBean
25.6.2. Using the MethodInvokingJobDetailFactoryBean
25.6.3. Wiring up jobs using triggers and the SchedulerFactoryBean
25.7. Using JDK Timer support
25.7.1. Creating custom timers
25.7.2. Using the MethodInvokingTimerTaskFactoryBean
25.7.3. Wrapping up: setting up the tasks using the TimerFactoryBean
26. Dynamic language support
26.1. Introduction
26.2. A first example
26.3. Defining beans that are backed by dynamic languages
26.3.1. Common concepts
26.3.1.1. The <lang:language/> element
26.3.1.2. Refreshable beans
26.3.1.3. Inline dynamic language source files
26.3.1.4. Understanding Constructor Injection in the context of dynamic-language-backed beans
26.3.2. JRuby beans
26.3.3. Groovy beans
26.3.3.1. Customising Groovy objects via a callback
26.3.4. BeanShell beans
26.4. Scenarios
26.4.1. Scripted Spring MVC Controllers
26.4.2. Scripted Validators
26.5. Bits and bobs
26.5.1. AOP - advising scripted beans
26.5.2. Scoping
26.6. Further Resources
VII. Appendices
A. Classic Spring Usage
A.1. Classic ORM usage
A.1.1. Hibernate
A.1.1.1. The HibernateTemplate
A.1.1.2. Implementing Spring-based DAOs without callbacks
A.1.2. JDO
A.1.2.1. JdoTemplate and JdoDaoSupport
A.1.3. JPA
A.1.3.1. JpaTemplate and JpaDaoSupport
A.2. Classic Spring MVC
A.3. JMS Usage
A.3.1. JmsTemplate
A.3.2. Asynchronous Message Reception
A.3.3. Connections
A.3.4. Transaction Management
B. Classic Spring AOP Usage
B.1. Pointcut API in Spring
B.1.1. Concepts
B.1.2. Operations on pointcuts
B.1.3. AspectJ expression pointcuts
B.1.4. Convenience pointcut implementations
B.1.4.1. Static pointcuts
B.1.4.2. Dynamic pointcuts
B.1.5. Pointcut superclasses
B.1.6. Custom pointcuts
B.2. Advice API in Spring
B.2.1. Advice lifecycles
B.2.2. Advice types in Spring
B.2.2.1. Interception around advice
B.2.2.2. Before advice
B.2.2.3. Throws advice
B.2.2.4. After Returning advice
B.2.2.5. Introduction advice
B.3. Advisor API in Spring
B.4. Using the ProxyFactoryBean to create AOP proxies
B.4.1. Basics
B.4.2. JavaBean properties
B.4.3. JDK- and CGLIB-based proxies
B.4.4. Proxying interfaces
B.4.5. Proxying classes
B.4.6. Using 'global' advisors
B.5. Concise proxy definitions
B.6. Creating AOP proxies programmatically with the ProxyFactory
B.7. Manipulating advised objects
B.8. Using the "autoproxy" facility
B.8.1. Autoproxy bean definitions
B.8.1.1. BeanNameAutoProxyCreator
B.8.1.2. DefaultAdvisorAutoProxyCreator
B.8.1.3. AbstractAdvisorAutoProxyCreator
B.8.2. Using metadata-driven auto-proxying
B.9. Using TargetSources
B.9.1. Hot swappable target sources
B.9.2. Pooling target sources
B.9.3. Prototype target sources
B.9.4. ThreadLocal target sources
B.10. Defining new Advice types
B.11. Further resources
C. XML Schema-based configuration
C.1. Introduction
C.2. XML Schema-based configuration
C.2.1. Referencing the schemas
C.2.2. The util schema
C.2.2.1. <util:constant/>
C.2.2.2. <util:property-path/>
C.2.2.3. <util:properties/>
C.2.2.4. <util:list/>
C.2.2.5. <util:map/>
C.2.2.6. <util:set/>
C.2.3. The jee schema
C.2.3.1. <jee:jndi-lookup/> (simple)
C.2.3.2. <jee:jndi-lookup/> (with single JNDI environment setting)
C.2.3.3. <jee:jndi-lookup/> (with multiple JNDI environment settings)
C.2.3.4. <jee:jndi-lookup/> (complex)
C.2.3.5. <jee:local-slsb/> (simple)
C.2.3.6. <jee:local-slsb/> (complex)
C.2.3.7. <jee:remote-slsb/>
C.2.4. The lang schema
C.2.5. The jms schema
C.2.6. The tx (transaction) schema
C.2.7. The aop schema
C.2.8. The context schema
C.2.8.1. <property-placeholder/>
C.2.8.2. <annotation-config/>
C.2.8.3. <component-scan/>
C.2.8.4. <load-time-weaver/>
C.2.8.5. <spring-configured/>
C.2.8.6. <mbean-export/>
C.2.9. The tool schema
C.2.10. The beans schema
D. Extensible XML authoring
D.1. Introduction
D.2. Authoring the schema
D.3. Coding a NamespaceHandler
D.4. Coding a BeanDefinitionParser
D.5. Registering the handler and the schema
D.5.1. 'META-INF/spring.handlers'
D.5.2. 'META-INF/spring.schemas'
D.6. Using a custom extension in your Spring XML configuration
D.7. Meatier examples
D.7.1. Nesting custom tags within custom tags
D.7.2. Custom attributes on 'normal' elements
D.8. Further Resources
E. spring-beans-2.0.dtd
F. spring.tld
F.1. Introduction
F.2. The bind tag
F.3. The escapeBody tag
F.4. The hasBindErrors tag
F.5. The htmlEscape tag
F.6. The message tag
F.7. The nestedPath tag
F.8. The theme tag
F.9. The transform tag
F.10. The url tag
F.11. The eval tag
G. spring-form.tld
G.1. Introduction
G.2. The checkbox tag
G.3. The checkboxes tag
G.4. The errors tag
G.5. The form tag
G.6. The hidden tag
G.7. The input tag
G.8. The label tag
G.9. The option tag
G.10. The options tag
G.11. The password tag
G.12. The radiobutton tag
G.13. The radiobuttons tag
G.14. The select tag
G.15. The textarea tag

Part I. Overview of Spring Framework

The Spring Framework is a lightweight solution and a potential one-stop-shop for building your enterprise-ready applications. However, Spring is modular, allowing you to use only those parts that you need, without having to bring in the rest. You can use the IoC container, with Struts on top, but you can also use only the Hibernate integration code or the JDBC abstraction layer. The Spring Framework supports declarative transaction management, remote access to your logic through RMI or web services, and various options for persisting your data. It offers a full-featured MVC framework, and enables you to integrate AOP transparently into your software.

Spring is designed to be non-intrusive, meaning that your domain logic code generally has no dependencies on the framework itself. In your integration layer (such as the data access layer), some dependencies on the data access technology and the Spring libraries will exist. However, it should be easy to isolate these dependencies from the rest of your code base.

This document is a reference guide to Spring Framework features. If you have any requests, comments, or questions on this document, please post them on the user mailing list or on the support forums at http://forum.springsource.org/.

1. Introduction to Spring Framework

Spring Framework is a Java platform that provides comprehensive infrastructure support for developing Java applications. Spring handles the infrastructure so you can focus on your application.

Spring enables you to build applications from “plain old Java objects” (POJOs) and to apply enterprise services non-invasively to POJOs. This capability applies to the Java SE programming model and to full and partial Java EE.

Examples of how you, as an application developer, can use the Spring platform advantage:

  • Make a Java method execute in a database transaction without having to deal with transaction APIs.

  • Make a local Java method a remote procedure without having to deal with remote APIs.

  • Make a local Java method a management operation without having to deal with JMX APIs.

  • Make a local Java method a message handler without having to deal with JMS APIs.

1.1 Dependency Injection and Inversion of Control

Java applications -- a loose term that runs the gamut from constrained applets to n-tier server-side enterprise applications -- typically consist of objects that collaborate to form the application proper. Thus the objects in an application have dependencies on each other.

Although the Java platform provides a wealth of application development functionality, it lacks the means to organize the basic building blocks into a coherent whole, leaving that task to architects and developers. True, you can use design patterns such as Factory, Abstract Factory, Builder, Decorator, and Service Locator to compose the various classes and object instances that make up an application. However, these patterns are simply that: best practices given a name, with a description of what the pattern does, where to apply it, the problems it addresses, and so forth. Patterns are formalized best practices that you must implement yourself in your application.

The Spring Framework Inversion of Control (IoC) component addresses this concern by providing a formalized means of composing disparate components into a fully working application ready for use. The Spring Framework codifies formalized design patterns as first-class objects that you can integrate into your own application(s). Numerous organizations and institutions use the Spring Framework in this manner to engineer robust, maintainable applications.

1.2 Modules

The Spring Framework consists of features organized into about 20 modules. These modules are grouped into Core Container, Data Access/Integration, Web, AOP (Aspect Oriented Programming), Instrumentation, and Test, as shown in the following diagram.

Overview of the Spring Framework

1.2.1 Core Container

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

The Core and Beans modules provide the fundamental parts of the framework, including the IoC and Dependency Injection features. The BeanFactory is a sophisticated implementation of the factory pattern. It 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 builds on the solid base provided by the Core and Beans modules: it is a means to access objects in a framework-style manner that is similar to a JNDI registry. The Context module inherits its features from the Beans module and adds support for internationalization (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 supports Java EE features such as EJB, JMX ,and basic remoting. The ApplicationContext interface is the focal point of the Context module.

The Expression Language module provides a powerful expression language for querying and manipulating an object graph at runtime. It is an extension of the unified expression language (unified EL) as specified in the JSP 2.1 specification. The language supports setting and getting 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 aggregations.

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 of these O/R-mapping frameworks in combination with all of the other features Spring offers, such as the simple declarative transaction management feature mentioned previously.

The OXM module provides an abstraction layer that supports Object/XML mapping implementations for JAXB, Castor, XMLBeans, JiBX and XStream.

The Java Messaging Service (JMS) module contains features for producing and consuming messages.

The Transaction module supports programmatic and declarative transaction management for classes that implement special interfaces and for all your POJOs (plain old Java objects).

1.2.3 Web

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

Spring's Web module provides basic web-oriented integration features such as multipart file-upload functionality and 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 contains Spring's model-view-controller (MVC) implementation for web applications. Spring's MVC framework provides a clean separation between domain model code and web forms, and integrates with all the other features of the Spring Framework.

The Web-Struts module contains the support classes for integrating a classic Struts web tier within a Spring application. Note that this support is now deprecated as of Spring 3.0. Consider migrating your application to Struts 2.0 and its Spring integration or to a Spring MVC solution.

The Web-Portlet module provides the MVC implementation to be used in a portlet environment and mirrors the functionality of 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 that implements functionality that should be separated. Using source-level metadata functionality, you can also incorporate behavioral information into your code, in a manner similar to that of .NET attributes.

The separate Aspects module 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 supports the testing of Spring components with JUnit or TestNG. It provides consistent loading of Spring ApplicationContexts and caching of those contexts. It also provides mock objects that you can use to test your code in isolation.

1.3 Usage scenarios

The building blocks described previously make Spring a logical choice in many scenarios, from applets to full-fledged enterprise applications that use Spring's transaction management functionality and web framework integration.

Typical full-fledged Spring web application

Spring's declarative transaction management features make the web application fully transactional, just as it would be if you used EJB container-managed transactions. All your custom business logic can be implemented with simple POJOs and managed by Spring's IoC container. Additional services include support for sending email and validation that is independent of the web layer, which lets you 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 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 with WebWork, Struts, Tapestry, or other UI frameworks can be integrated with a Spring-based middle-tier, which allows you to use Spring transaction features. You simply need to wire up your business logic using an ApplicationContext and use a WebApplicationContext to integrate your web layer.

Remoting usage scenario

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

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, fail-safe web applications that might need declarative security.

1.3.1 Dependency Management and Naming Conventions

Dependency management and dependency injection are different things. To get those nice features of Spring into your application (like dependency injection) you need to assemble all the libraries needed (jar files) and get them onto your classpath at runtime, and possibly at compile time. These dependencies are not virtual components that are injected, but physical resources in a file system (typically). The process of dependency management involves locating those resources, storing them and adding them to classpaths. Dependencies can be direct (e.g. my application depends on Spring at runtime), or indirect (e.g. my application depends on commons-dbcp which depends on commons-pool). The indirect dependencies are also known as "transitive" and it is those dependencies that are hardest to identify and manage.

If you are going to use Spring you need to get a copy of the jar libraries that comprise the pieces of Spring that you need. To make this easier Spring is packaged as a set of modules that separate the dependencies as much as possible, so for example if you don't want to write a web application you don't need the spring-web modules. To refer to Spring library modules in this guide we use a shorthand naming convention spring-* or spring-*.jar, where "*" represents the short name for the module (e.g. spring-core, spring-webmvc, spring-jms, etc.). The actual jar file name that you use may be in this form (see below) or it may not, and normally it also has a version number in the file name (e.g. spring-core-3.0.0.RELEASE.jar).

In general, Spring publishes its artifacts to four different places:

  • On the community download site http://www.springsource.org/downloads/community. Here you find all the Spring jars bundled together into a zip file for easy download. The names of the jars here since version 3.0 are in the form org.springframework.*-<version>.jar.

  • Maven Central, which is the default repository that Maven queries, and does not require any special configuration to use. Many of the common libraries that Spring depends on also are available from Maven Central and a large section of the Spring community uses Maven for dependency management, so this is convenient for them. The names of the jars here are in the form spring-*-<version>.jar and the Maven groupId is org.springframework.

  • The Enterprise Bundle Repository (EBR), which is run by SpringSource and also hosts all the libraries that integrate with Spring. Both Maven and Ivy repositories are available here for all Spring jars and their dependencies, plus a large number of other common libraries that people use in applications with Spring. Both full releases and also milestones and development snapshots are deployed here. The names of the jar files are in the same form as the community download (org.springframework.*-<version>.jar), and the dependencies are also in this "long" form, with external libraries (not from SpringSource) having the prefix com.springsource. See the FAQ for more information.

  • In a public Maven repository hosted on Amazon S3 for development snapshots and milestone releases (a copy of the final releases is also held here). The jar file names are in the same form as Maven Central, so this is a useful place to get development versions of Spring to use with other libraries depoyed in Maven Central.

So the first thing you need to decide is how to manage your dependencies: most people use an automated system like Maven or Ivy, but you can also do it manually by downloading all the jars yourself. When obtaining Spring with Maven or Ivy you have then to decide which place you'll get it from. In general, if you care about OSGi, use the EBR, since it houses OSGi compatible artifacts for all of Spring's dependencies, such as Hibernate and Freemarker. If OSGi does not matter to you, either place works, though there are some pros and cons between them. In general, pick one place or the other for your project; do not mix them. This is particularly important since EBR artifacts necessarily use a different naming convention than Maven Central artifacts.

Table 1.1. Comparison of Maven Central and SpringSource EBR Repositories

FeatureMaven CentralEBR
OSGi CompatibleNot explicitYes
Number of ArtifactsTens of thousands; all kindsHundreds; those that Spring integrates with
Consistent Naming ConventionsNoYes
Naming Convention: GroupIdVaries. Newer artifacts often use domain name, e.g. org.slf4j. Older ones often just use the artifact name, e.g. log4j.Domain name of origin or main package root, e.g. org.springframework
Naming Convention: ArtifactIdVaries. Generally the project or module name, using a hyphen "-" separator, e.g. spring-core, logj4.Bundle Symbolic Name, derived from the main package root, e.g. org.springframework.beans. If the jar had to be patched to ensure OSGi compliance then com.springsource is appended, e.g. com.springsource.org.apache.log4j
Naming Convention: VersionVaries. Many new artifacts use m.m.m or m.m.m.X (with m=digit, X=text). Older ones use m.m. Some neither. Ordering is defined but not often relied on, so not strictly reliable.OSGi version number m.m.m.X, e.g. 3.0.0.RC3. The text qualifier imposes alphabetic ordering on versions with the same numeric values.
PublishingUsually automatic via rsync or source control updates. Project authors can upload individual jars to JIRA.Manual (JIRA processed by SpringSource)
Quality AssuranceBy policy. Accuracy is responsibility of authors.Extensive for OSGi manifest, Maven POM and Ivy metadata. QA performed by Spring team.
HostingContegix. Funded by Sonatype with several mirrors.S3 funded by SpringSource.
Search UtilitiesVarioushttp://www.springsource.com/repository
Integration with SpringSource ToolsIntegration through STS with Maven dependency managementExtensive integration through STS with Maven, Roo, CloudFoundry


1.3.1.1 Spring Dependencies and Depending on Spring

Although Spring provides integration and support for a huge range of enterprise and other external tools, it intentionally keeps its mandatory dependencies to an absolute minimum: you shouldn't have to locate and download (even automatically) a large number of jar libraries in order to use Spring for simple use cases. For basic dependency injection there is only one mandatory external dependency, and that is for logging (see below for a more detailed description of logging options).

Next we outline the basic steps needed to configure an application that depends on Spring, first with Maven and then with Ivy. In all cases, if anything is unclear, refer to the documentation of your dependency management system, or look at some sample code - Spring itself uses Ivy to manage dependencies when it is building, and our samples mostly use Maven.

1.3.1.2 Maven Dependency Management

If you are using Maven for dependency management you don't even need to supply the logging dependency explicitly. For example, to create an application context and use dependency injection to configure an application, your Maven dependencies will look like this:

<dependencies>
   <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>3.0.0.RELEASE</version>
      <scope>runtime</scope>
   </dependency>
</dependencies> 

That's it. Note the scope can be declared as runtime if you don't need to compile against Spring APIs, which is typically the case for basic dependency injection use cases.

We used the Maven Central naming conventions in the example above, so that works with Maven Central or the SpringSource S3 Maven repository. To use the S3 Maven repository (e.g. for milestones or developer snaphots), you need to specify the repository location in your Maven configuration. For full releases:

<repositories>
   <repository>
      <id>com.springsource.repository.maven.release</id>
      <url>http://maven.springframework.org/release/</url>
      <snapshots><enabled>false</enabled></snapshots>
   </repository>
</repositories>

For milestones:

<repositories>
   <repository>
      <id>com.springsource.repository.maven.milestone</id>
      <url>http://maven.springframework.org/milestone/</url>
      <snapshots><enabled>false</enabled></snapshots>
   </repository>
</repositories>

And for snapshots:

<repositories>
   <repository>
      <id>com.springsource.repository.maven.snapshot</id>
      <url>http://maven.springframework.org/snapshot/</url>
      <snapshots><enabled>true</enabled></snapshots>
   </repository>
</repositories>

To use the SpringSource EBR you would need to use a different naming convention for the dependencies. The names are usually easy to guess, e.g. in this case it is:

<dependencies>
   <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>org.springframework.context</artifactId>
      <version>3.0.0.RELEASE</version>
      <scope>runtime</scope>
   </dependency>
</dependencies>

You also need to declare the location of the repository explicitly (only the URL is important):

<repositories>
   <repository>
      <id>com.springsource.repository.bundles.release</id>
      <url>http://repository.springsource.com/maven/bundles/release/</url>
   </repository>
</repositories>

If you are managing your dependencies by hand, the URL in the repository declaration above is not browseable, but there is a user interface at http://www.springsource.com/repository that can be used to search for and download dependencies. It also has handy snippets of Maven and Ivy configuration that you can copy and paste if you are using those tools.

1.3.1.3 Ivy Dependency Management

If you prefer to use Ivy to manage dependencies then there are similar names and configuration options.

To configure Ivy to point to the SpringSource EBR add the following resolvers to your ivysettings.xml:

<resolvers>
  
  <url name="com.springsource.repository.bundles.release">

    <ivy pattern="http://repository.springsource.com/ivy/bundles/release/
      [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" />
    <artifact pattern="http://repository.springsource.com/ivy/bundles/release/
      [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" />

  </url>

  <url name="com.springsource.repository.bundles.external">

    <ivy pattern="http://repository.springsource.com/ivy/bundles/external/
       [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" />
    <artifact pattern="http://repository.springsource.com/ivy/bundles/external/
       [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> 

  </url>

</resolvers>

The XML above is not valid because the lines are too long - if you copy-paste then remove the extra line endings in the middle of the url patterns.

Once Ivy is configured to look in the EBR adding a dependency is easy. Simply pull up the details page for the bundle in question in the repository browser and you'll find an Ivy snippet ready for you to include in your dependencies section. For example (in ivy.xml):

<dependency org="org.springframework" 
      name="org.springframework.core" rev="3.0.0.RELEASE" conf="compile->runtime"/>

1.3.2 Logging

Logging is a very important dependency for Spring because a) it is the only mandatory external dependency, b) everyone likes to see some output from the tools they are using, and c) Spring integrates with lots of other tools all of which have also made a choice of logging dependency. One of the goals of an application developer is often to have unified logging configured in a central place for the whole application, including all external components. This is more difficult than it might have been since there are so many choices of logging framework.

The mandatory logging dependency in Spring is the Jakarta Commons Logging API (JCL). We compile against JCL and we also make JCL Log objects visible for classes that extend the Spring Framework. It's important to users that all versions of Spring use the same logging library: migration is easy because backwards compatibility is preserved even with applications that extend Spring. The way we do this is to make one of the modules in Spring depend explicitly on commons-logging (the canonical implementation of JCL), and then make all the other modules depend on that at compile time. If you are using Maven for example, and wondering where you picked up the dependency on commons-logging, then it is from Spring and specifically from the central module called spring-core.

The nice thing about commons-logging is that you don't need anything else to make your application work. It has a runtime discovery algorithm that looks for other logging frameworks in well known places on the classpath and uses one that it thinks is appropriate (or you can tell it which one if you need to). If nothing else is available you get pretty nice looking logs just from the JDK (java.util.logging or JUL for short). You should find that your Spring application works and logs happily to the console out of the box in most situations, and that's important.

1.3.2.1 Not Using Commons Logging

Unfortunately, the runtime discovery algorithm in commons-logging, while convenient for the end-user, is problematic. If we could turn back the clock and start Spring now as a new project it would use a different logging dependency. The first choice would probably be the Simple Logging Facade for Java (SLF4J), which is also used by a lot of other tools that people use with Spring inside their applications.

Switching off commons-logging is easy: just make sure it isn't on the classpath at runtime. In Maven terms you exclude the dependency, and because of the way that the Spring dependencies are declared, you only have to do that once.

<dependencies>
   <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>3.0.0.RELEASE</version>
      <scope>runtime</scope>
      <exclusions>
         <exclusion>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
         </exclusion>
      </exclusions>
   </dependency>
</dependencies> 

Now this application is probably broken because there is no implementation of the JCL API on the classpath, so to fix it a new one has to be provided. In the next section we show you how to provide an alternative implementation of JCL using SLF4J as an example.

1.3.2.2 Using SLF4J

SLF4J is a cleaner dependency and more efficient at runtime than commons-logging because it uses compile-time bindings instead of runtime discovery of the other logging frameworks it integrates. This also means that you have to be more explicit about what you want to happen at runtime, and declare it or configure it accordingly. SLF4J provides bindings to many common logging frameworks, so you can usually choose one that you already use, and bind to that for configuration and management.

SLF4J provides bindings to many common logging frameworks, including JCL, and it also does the reverse: bridges between other logging frameworks and itself. So to use SLF4J with Spring you need to replace the commons-logging dependency with the SLF4J-JCL bridge. Once you have done that then logging calls from within Spring will be translated into logging calls to the SLF4J API, so if other libraries in your application use that API, then you have a single place to configure and manage logging.

A common choice might be to bridge Spring to SLF4J, and then provide explicit binding from SLF4J to Log4J. You need to supply 4 dependencies (and exclude the existing commons-logging): the bridge, the SLF4J API, the binding to Log4J, and the Log4J implementation itself. In Maven you would do that like this

<dependencies>
   <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>3.0.0.RELEASE</version>
      <scope>runtime</scope>
      <exclusions>
         <exclusion>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
         </exclusion>
      </exclusions>
   </dependency>
   <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>jcl-over-slf4j</artifactId>
      <version>1.5.8</version>
      <scope>runtime</scope>
   </dependency>
   <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.5.8</version>
      <scope>runtime</scope>
   </dependency>
   <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.5.8</version>
      <scope>runtime</scope>
   </dependency>
   <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.14</version>
      <scope>runtime</scope>
   </dependency>
</dependencies> 

That might seem like a lot of dependencies just to get some logging. Well it is, but it is optional, and it should behave better than the vanilla commons-logging with respect to classloader issues, notably if you are in a strict container like an OSGi platform. Allegedly there is also a performance benefit because the bindings are at compile-time not runtime.

A more common choice amongst SLF4J users, which uses fewer steps and generates fewer dependencies, is to bind directly to Logback. This removes the extra binding step because Logback implements SLF4J directly, so you only need to depend on two libaries not four (jcl-over-slf4j and logback). If you do that you might also need to exlude the slf4j-api dependency from other external dependencies (not Spring), because you only want one version of that API on the classpath.

1.3.2.3 Using Log4J

Many people use Log4j as a logging framework for configuration and management purposes. It's efficient and well-established, and in fact it's what we use at runtime when we build and test Spring. Spring also provides some utilities for configuring and initializing Log4j, so it has an optional compile-time dependency on Log4j in some modules.

To make Log4j work with the default JCL dependency (commons-logging) all you need to do is put Log4j on the classpath, and provide it with a configuration file (log4j.properties or log4j.xml in the root of the classpath). So for Maven users this is your dependency declaration:

<dependencies>
   <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>3.0.0.RELEASE</version>
      <scope>runtime</scope>
   </dependency>
   <dependency>
      <groupId>log4j</groupId>
      <artifactId>log4j</artifactId>
      <version>1.2.14</version>
      <scope>runtime</scope>
   </dependency>
</dependencies> 

And here's a sample log4j.properties for logging to the console:

log4j.rootCategory=INFO, stdout

log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %t %c{2}:%L - %m%n

log4j.category.org.springframework.beans.factory=DEBUG
Runtime Containers with Native JCL

Many people run their Spring applications in a container that itself provides an implementation of JCL. IBM Websphere Application Server (WAS) is the archetype. This often causes problems, and unfortunately there is no silver bullet solution; simply excluding commons-logging from your application is not enough in most situations.

To be clear about this: the problems reported are usually not with JCL per se, or even with commons-logging: rather they are to do with binding commons-logging to another framework (often Log4J). This can fail because commons-logging changed the way they do the runtime discovery in between the older versions (1.0) found in some containers and the modern versions that most people use now (1.1). Spring does not use any unusual parts of the JCL API, so nothing breaks there, but as soon as Spring or your application tries to do any logging you can find that the bindings to Log4J are not working.

In such cases with WAS the easiest thing to do is to invert the class loader hierarchy (IBM calls it "parent last") so that the application controls the JCL dependency, not the container. That option isn't always open, but there are plenty of other suggestions in the public domain for alternative approaches, and your mileage may vary depending on the exact version and feature set of the container.

Part II. What's New in Spring 3.0

2. New Features and Enhancements 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 generic FactoryBeans, and also consistent resolution of bridge methods in the Spring AOP API. Generic 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 articles and tutorials

There are many excellent articles and tutorials that show how to get started with Spring 3 features. Read them at the Spring Documentation page.

The samples have been improved and updated to take advantage of the new features in Spring 3. Additionally, the samples have been moved out of the source tree into a dedicated SVN repository available at:

https://anonsvn.springframework.org/svn/spring-samples/

As such, the samples are no longer distributed alongside Spring 3 and need to be downloaded separately from the repository mentioned above. However, this documentation will continue to refer to some samples (in particular Petclinic) to illustrate various features.

[Note]Note
For more information on Subversion (or in short SVN), see the project homepage at: http://subversion.apache.org/

2.4 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

  • org.springframework.web.struts

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

2.5 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

  • General-purpose type conversion system and field formatting system

  • 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

  • Embedded database support

2.5.1 Core APIs updated for Java 5

BeanFactory interface returns typed bean instances as far as possible:

  • T getBean(Class<T> requiredType)

  • T getBean(String name, 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>

2.5.2 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 void setKeyGenerator(KeyGenerator kg) { … }
}

2.5.3 The Inversion of Control (IoC) container

2.5.3.1 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

  • @DependsOn

  • @Primary

  • @Lazy

  • @Import

  • @ImportResource

  • @Value

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

package org.example.config;

@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
        AnnotationSessionFactoryBean asFactoryBean = 
            new AnnotationSessionFactoryBean();
        asFactoryBean.setDataSource(dataSource());
        // additional config
        return 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="org.example.config"/>
<util:properties id="jdbcProperties" location="classpath:org/example/config/jdbc.properties"/>
        

Or you can bootstrap a @Configuration class directly using AnnotationConfigApplicationContext:

public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
    FooService fooService = ctx.getBean(FooService.class);
    fooService.doStuff();
}

See Section 3.11.2, “Instantiating the Spring container using AnnotationConfigApplicationContext” for full information on AnnotationConfigApplicationContext.

2.5.3.2 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

2.5.4 General purpose type conversion system and field formatting system

A general purpose type conversion system has been introduced. The system is currently used by SpEL for type conversion, and may also be used by a Spring Container and DataBinder when binding bean property values.

In addition, a formatter SPI has been introduced for formatting field values. This SPI provides a simpler and more robust alternative to JavaBean PropertyEditors for use in client environments such as Spring MVC.

2.5.5 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.

2.5.6 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.

2.5.6.1 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 requests and responses.

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

Refer to the sections on MVC and the RestTemplate for more information.

2.5.6.2 @MVC additions

A mvc namespace has been introduced that greatly simplifies Spring MVC configuration.

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.

2.5.7 Declarative model validation

Several validation enhancements, including JSR 303 support that uses Hibernate Validator as the default provider.

2.5.8 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).

JSR 303, JSF 2.0, JPA 2.0, etc

2.5.9 Support for embedded databases

Convenient support for embedded Java database engines, including HSQL, H2, and Derby, is now provided.

Part III. Core Technologies

This 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 has 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 in 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.

3. The IoC container

3.1 Introduction to the Spring IoC container and beans

This chapter covers the Spring Framework implementation of the Inversion of Control (IoC) [1]principle. IoC is also known as dependency injection (DI). It is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies by using direct construction of classes, or a mechanism such as the Service Locator pattern.

The org.springframework.beans and org.springframework.context packages are the basis for Spring Framework's IoC container. The BeanFactory interface provides an advanced configuration mechanism capable of managing any type of object. ApplicationContext is a sub-interface of BeanFactory. It adds easier integration with Spring's AOP features; message resource handling (for use in internationalization), event publication; 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, and the ApplicationContext adds more enterprise-specific functionality. The ApplicationContext is a complete superset of the BeanFactory, and is used exclusively in this chapter in descriptions of Spring's IoC container. For more information on using the BeanFactory instead of the ApplicationContext, refer to Section 3.14, “The BeanFactory”.

In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a bean is simply one of many objects in your application. Beans, and the dependencies among them, are reflected in the configuration metadata used by a container.

3.2 Container overview

The interface org.springframework.context.ApplicationContext represents the Spring IoC container and is responsible for instantiating, configuring, and assembling the aforementioned beans. The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java code. It allows you to express the objects that compose your application and the rich interdependencies between such objects.

Several implementations of the ApplicationContext interface are supplied out-of-the-box with Spring. In standalone applications it is common to create an instance of ClassPathXmlApplicationContext or FileSystemXmlApplicationContext. While XML has been the traditional format for defining configuration metadata you can instruct the container to use Java annotations or code as the metadata format by providng a small amount of XML configuration to declaratively enable support for these additional metadata formats.

In most 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 3.13.4, “Convenient ApplicationContext instantiation for web applications”). If you are using the SpringSource Tool Suite Eclipse-powered development environment or Spring Roo this boilerplate configuration can be easily created with few mouse clicks or keystrokes.

The following diagram is a high-level view of how Spring works. Your application classes are combined with configuration metadata so that after the ApplicationContext is created and initialized, you have a fully configured and executable system or application.

The Spring IoC container

3.2.1 Configuration metadata

As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata; this configuration metadata represents how you as an application developer tell the Spring container to instantiate, configure, and assemble the objects in your application.

Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most of this chapter uses to convey key concepts and features of the Spring IoC container.

[Note]Note

XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written.

For information about using other forms of metadata with the Spring container, see:

  • Annotation-based configuration: Spring 2.5 introduced support for annotation-based configuration metadata.

  • Java-based configuration: Starting with Spring 3.0, many features provided by the Spring JavaConfig project became part of the core Spring Framework. Thus you can define beans external to your application classes by using Java rather than XML files. To use these new features, see the @Configuration, @Bean, @Import and @DependsOn annotations.

Spring configuration consists of at least one and typically more than one bean definition that the container must manage. XML-based configuration metadata shows these beans 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 define service layer objects, 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 and load domain objects. However, you can use Spring's integration with AspectJ to configure objects that have been created outside the control of an IoC container. See Using AspectJ to dependency-inject domain objects with Spring.

The following example shows 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-3.0.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>

The id attribute is a string that you use to identify the individual bean definition. The class attribute defines the type of the bean and uses the fully qualified classname. The value of the id attribute refers to collaborating objects. The XML for referring to collaborating objects is not shown in this example; see Dependencies for more information.

3.2.2 Instantiating a container

Instantiating a Spring IoC container is straightforward. The location path or paths supplied to an ApplicationContext constructor are actually resource strings that allow the container to load configuration metadata from a variety of external resources such as the local file system, from the Java CLASSPATH, and so on.

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

After you learn about Spring's IoC container, you may want to know more about Spring's Resource abstraction, as described in Chapter 4, Resources, which provides a convenient mechanism for reading an InputSream from locations defined in a URI syntax. In particular, Resource paths are used to construct applications contexts as described in Section 4.7, “Application contexts and Resource paths”.

The following example shows the service layer objects (services.xml) configuration file:

<?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-3.0.xsd">

  <!-- services -->

  <bean id="petStore"
        class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl">
    <property name="accountDao" ref="accountDao"/>
    <property name="itemDao" ref="itemDao"/>
    <!-- additional collaborators and configuration for this bean go here -->
  </bean>

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

</beans>

The following example shows the data access objects daos.xml file:

<?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-3.0.xsd">

  <bean id="accountDao"
      class="org.springframework.samples.jpetstore.dao.ibatis.SqlMapAccountDao">
    <!-- additional collaborators and configuration for this bean go here -->
  </bean>

  <bean id="itemDao" class="org.springframework.samples.jpetstore.dao.ibatis.SqlMapItemDao">
    <!-- additional collaborators and configuration for this bean go here -->
  </bean>

  <!-- more bean definitions for data access objects go here -->

</beans>

In the preceding example, the service layer consists of the class PetStoreServiceImpl, and two data access objects of the type SqlMapAccountDao and SqlMapItemDao are based on the iBatis Object/Relational mapping framework. The property name element refers to the name of the JavaBean property, and the ref element refers to the name of another bean definition. This linkage between id and ref elements expresses the dependency between collaborating objects. For details of configuring an object's dependencies, see Dependencies.

3.2.2.1 Composing XML-based configuration metadata

It can be useful to have bean definitions span multiple XML files. Often each individual XML configuration file represents a logical layer or module in your architecture.

You can use the application context constructor to load bean definitions from all these XML fragments. This constructor takes multiple Resource locations, as was shown in the previous section. Alternatively, use one or more occurrences of the <import/> element to load bean definitions from another file or files. For example:

<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 the preceding example, external bean definitions are loaded from three files, services.xml, messageSource.xml, and themeSource.xml. All location paths are relative to the definition file doing the importing, so services.xml 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 ignored, but given that these paths are relative, it is better form not to use the slash at all. The contents of the files being imported, including the top level <beans/> element, must be valid XML bean definitions according to the Spring Schema or DTD.

[Note]Note

It is possible, but not recommended, to reference files in parent directories using a relative "../" path. Doing so creates a dependency on a file that is outside the current application. In particular, this reference is not recommended for "classpath:" URLs (for example, "classpath:../services.xml"), where the runtime resolution process chooses the "nearest" classpath root and then looks into its parent directory. Classpath configuration changes may lead to the choice of a different, incorrect directory.

You can always use fully qualified resource locations instead of relative paths: for example, "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. It is generally preferable to keep an indirection for such absolute locations, for example, through "${...}" placeholders that are resolved against JVM system properties at runtime.

3.2.3 Using the container

The ApplicationContext is the interface for an advanced factory capable of maintaining a registry of different beans and their dependencies. Using the method T getBean(Stringname, Class<T> requiredType) you can retrieve instances of your beans.

The ApplicationContext enables you to read bean definitions and access them as follows:

// create and configure beans
ApplicationContext context =
    new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});

// retrieve configured instance
PetStoreServiceImpl service = context.getBean("petStore", PetStoreServiceImpl.class);

// use configured instance
List userList service.getUsernameList();

You use getBean() to retrieve instances of your beans. The ApplicationContext interface has a few other methods for retrieving beans, but ideally your application code should never use them. Indeed, your application code should have no calls to the getBean() method at all, and thus no dependency on Spring APIs at all. For example, Spring's integration with web frameworks provides for dependency injection for various web framework classes such as controllers and JSF-managed beans.

3.3 Bean overview

A Spring IoC container manages one or more beans. These beans are created with the configuration metadata that you supply to the container, for example, 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 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 that 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, for example, the number of connections to use in a bean that manages a connection pool, or the size limit of the pool.

This metadata translates to a set of properties that make up each bean definition.


In addition to bean definitions that contain information on how to create a specific bean, the ApplicationContext implementations also permit the registration of existing objects that are created outside the container, by users. This is done by accessing the ApplicationContext's BeanFactory via the method getBeanFactory() which returns the BeanFactory implementation DefaultListableBeanFactory. DefaultListableBeanFactory supports this registration through the methods registerSingleton(..) and registerBeanDefinition(..). However, typical applications work solely with beans defined through metadata bean definitions.

3.3.1 Naming beans

Every bean has one or more identifiers. These identifiers must be unique within the container that hosts the bean. A bean usually has only one identifier, but if it requires more than one, the extra ones can be considered aliases.

In XML-based configuration metadata, you use the id and/or name attributes to specify the bean identifier(s). The id attribute allows you to specify exactly one id, and because it is a real XML element ID attribute, the XML parser can do some extra validation when other elements reference the id. As such, it is the preferred way to specify a bean identifier. However, the XML specification does limit the characters that are legal in XML ids. This is usually not a constraint, but if you need to use one of these special XML characters, or want to introduce other aliases to the bean, you can also specify them in the name attribute, separated by a comma (,), semicolon (;), or white space.

You are not required to supply a name or id for a bean. If no name or id is supplied explicitly, the container generates a unique name for that bean. However, if you want to refer to that bean by name, through the use of the ref element or Service Location style lookup, you must provide a name. Motivations for not supplying a name are related to using inner beans and autowiring collaborators.

3.3.1.1 Aliasing a bean outside the bean definition

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

Specifying all aliases where the bean is actually defined is not always adequate, however. It is sometimes desirable to introduce an alias for a bean that is defined elsewhere. This is commonly the case in large systems where configuration is split amongst each subsystem, each subsystem having its own set of object definitions. In XML-based configuration metadata, you can use the <alias/> element to accomplish this.

<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.

For example, the configuration metadata for subsystem A may refer to a DataSource via the name 'subsystemA-dataSource. The configuration metadata for subsystem B may refer to a DataSource via the name 'subsystemB-dataSource'. When composing the main application that uses both these subsystems the main application refers to the DataSource via the name 'myApp-dataSource'. To have all three names refer to the same object you add to the MyApp configuration metadata the following aliases definitions:

<alias name="subsystemA-dataSource" alias="subsystemB-dataSource"/>
<alias name="subsystemA-dataSource" alias="myApp-dataSource" />

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

3.3.2 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 use XML-based configuration metadata, you specify the type (or class) of object that is to be instantiated in the class attribute of the <bean/> element. This class attribute, which internally is a Class property on a BeanDefinition instance, is usually mandatory. (For exceptions, see Section 3.3.2.3, “Instantiation using an instance factory method” and Section 3.7, “Bean definition inheritance”.) You use the Class property in one of two ways:

  • Typically, to specify the bean class to be constructed in the case where the container itself directly creates the bean by calling its constructor reflectively, somewhat equivalent to Java code using the new operator.

  • To specify the actual class containing the static factory method that will be invoked to create the object, in the less common case where the container invokes a static, factory method on a class to create the bean. The object type returned from the invocation of the static factory method may be the same class or another class entirely.

3.3.2.1 Instantiation with a constructor

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

The Spring IoC container can manage virtually any class you want it to manage; it is not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument) constructor and appropriate setters and getters modeled after the properties in the container. You can also 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.

With XML-based configuration metadata you can specify your bean class as follows:

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

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

For details about the mechanism for supplying arguments to the constructor (if required) and setting object instance properties after the object is constructed, see Injecting Dependencies.

3.3.2.2 Instantiation with a static factory method

When defining a bean that you create with a static factory method, you use the class attribute to specify the class containing the static factory method and an attribute named factory-method to specify the name of the factory method itself. You should be able to call this method (with optional arguments as described later) and return a live object, which subsequently is treated as if it had been created through a constructor. One use for such a bean definition is to call static factories in legacy code.

The following bean definition specifies that the bean will be created by calling a factory-method. 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="clientService"
      class="examples.ClientService"
      factory-method="createInstance"/>
public class ClientService {
  private static ClientService clientService = new ClientService();
  private ClientService() {}

  public static ClientService createInstance() {
    return clientService;
  }
}

For details about the mechanism for supplying (optional) arguments to the factory method and setting object instance properties after the object is returned from the factory, see Dependencies and configuration in detail.

3.3.2.3 Instantiation using an instance factory method

Similar to instantiation through a static factory method, instantiation with an instance factory method invokes a non-static method of an existing bean from the container to create a new bean. To use this mechanism, leave the class attribute empty, and in the factory-bean attribute, 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. Set the name of the factory method itself with the factory-method attribute.

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

<!-- the bean to be created via the factory bean -->
<bean id="clientService"
      factory-bean="serviceLocator"
      factory-method="createClientServiceInstance"/>
public class DefaultServiceLocator {
  private static ClientService clientService = new ClientServiceImpl();
  private DefaultServiceLocator() {}

  public ClientService createClientServiceInstance() {
    return clientService;
  }
}

One factory class can also hold more than one factory method as shown here:

<bean id="serviceLocator" class="examples.DefaultServiceLocator">
  <!-- inject any dependencies required by this locator bean -->
</bean>
<bean id="clientService"
      factory-bean="serviceLocator"
      factory-method="createClientServiceInstance"/>

<bean id="accountService"
      factory-bean="serviceLocator"
      factory-method="createAccountServiceInstance"/>
public class DefaultServiceLocator {
  private static ClientService clientService = new ClientServiceImpl();
  private static AccountService accountService = new AccountServiceImpl();

  private DefaultServiceLocator() {}

  public ClientService createClientServiceInstance() {
    return clientService;
  }

  public AccountService createAccountServiceInstance() {
    return accountService;
  }
}

This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and configuration in detail.

[Note]Note

In Spring documentation, factory bean refers to a bean that is configured in the Spring container that will create objects through an instance or static factory method. By contrast, FactoryBean (notice the capitalization) refers to a Spring-specific FactoryBean .

3.4 Dependencies

A typical enterprise application does not consist of a single object (or bean in the Spring parlance). Even the simplest application has a few 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 to a fully realized application where objects collaborate to achieve a goal.

3.4.1 Dependency injection

Dependency injection (DI) is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies on its own by using direct construction of classes, or the Service Locator pattern.

Code is cleaner with the DI principle and decoupling is more effective when objects are provided with their dependencies. The object does not look up its dependencies, and does not know the location or class of the dependencies. As such, your classes become easier to test, in particular when the dependencies are on interfaces or abstract base classes, which allow for stub or mock implementations to be used in unit tests.

DI exists in two major variants, Constructor-based dependency injection and Setter-based dependency injection.

3.4.1.1 Constructor-based dependency injection

Constructor-based DI is accomplished by the container invoking a constructor with a number of arguments, each representing a dependency. Calling a static factory method with specific arguments to construct the bean is nearly equivalent, and this discussion treats arguments to a constructor and to a static factory method similarly. The following example shows a class that can only be dependency-injected with constructor injection. Notice that there is nothing special about this class, it is a POJO that has no dependencies on container specific interfaces, base classes or annotations.

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 no potential ambiguity exists 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 are supplied to the appropriate constructor when the bean is being instantiated. Consider the following class:

package x.y;

public class Foo {

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

No potential ambiguity exists, assuming that Bar and Baz classes are not related by inheritance. Thus the following configuration works fine, and you do not need to specify the constructor argument indexes and/or types explicitly in the <constructor-arg/> element.

<beans>
  <bean id="foo" class="x.y.Foo">
      <constructor-arg ref="bar"/>
      <constructor-arg ref="baz"/>
  </bean>

  <bean id="bar" class="x.y.Bar"/>
  <bean id="baz" class="x.y.Baz"/>

</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

In the preceding scenario, the container can use type matching with simple types if you explicitly specify 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

Use the index attribute to specify explicitly the index of constructor arguments. For example:

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

In addition to resolving the ambiguity of multiple simple values, specifying an index resolves ambiguity where a constructor has two arguments of the same type. Note that the index is 0 based.

Constructor argument name

As of Spring 3.0 you can also use the constructor parameter name for value disambiguation:

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg name="years" value="7500000"/>
<constructor-arg name="ultimateanswer" value="42"/>
</bean>

Keep in mind that to make this work out of the box your code must be compiled with the debug flag enabled so that Spring can look up the parameter name from the constructor. If you can't compile your code with debug flag (or don't want to) you can use @ConstructorProperties JDK annotation to explicitly name your constructor arguments. The sample class would then have to look as follows:

package examples;

public class ExampleBean {

  // Fields omitted

  @ConstructorProperties({"years", "ultimateAnswer"})
  public ExampleBean(int years, String ultimateAnswer) {
      this.years = years;
      this.ultimateAnswer = ultimateAnswer;
  }
}

3.4.1.2 Setter-based dependency injection

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

The following example shows a class that can only be dependency-injected using pure setter injection. This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces, base classes or annotations.

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 ApplicationContext supports constructor- and setter-based DI for the beans it manages. It also supports setter-based DI after some dependencies are already injected through the constructor approach. You configure the dependencies in the form of a BeanDefinition, which you use with PropertyEditor instances to convert properties from one format to another. However, most Spring users do not work with these classes directly (programmatically), but rather with an XML definition file that is then converted internally into instances of these classes, and used to load an entire Spring IoC container instance.

3.4.1.3 Dependency resolution process

The container performs bean dependency resolution as follows:

  1. The ApplicationContext is created and initialized with configuration metadata that describes all the beans. Configuration metadata can be specified via XML, Java code or annotations.

  2. For each bean, its dependencies are expressed in the form of properties, constructor arguments, or arguments to the static-factory method if you are using that instead of a normal constructor. These dependencies are provided to the bean, when the bean is actually created.

  3. Each property or constructor argument is 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 is converted from its specified format 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 of whether bean reference properties refer to valid beans. However, the bean properties themselves are not set until the bean is actually created. Beans that are singleton-scoped and set to be pre-instantiated (the default) are created when the container is created. Scopes are defined in Section 3.5, “Bean scopes” Otherwise, the bean is created only when it is requested. Creation of a bean potentially causes a graph of beans to be created, as the bean's dependencies and its dependencies' dependencies (and so on) are created and assigned.

You can generally trust Spring to do the right thing. It detects configuration problems, such as references to non-existent beans and circular dependencies, at container load-time. Spring sets properties and resolves dependencies as late as possible, when the bean is actually created. This means that a Spring container which has loaded correctly can later generate an exception when you request an object if there is a problem creating that object or one of its dependencies. For example, the bean throws an exception as a result of a missing or invalid property. 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 discover configuration issues when the ApplicationContext is created, not later. You can still override this default behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.

If no circular dependencies exist, when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being injected into the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container completely configures bean B prior to invoking the setter method on bean A. In other words, the bean is instantiated (if not a pre-instantiated singleton), its dependencies are set, and the relevant lifecycle methods (such as a configured init method or the IntializingBean callback method) are invoked.

3.4.1.4 Examples of dependency injection

The following example uses XML-based configuration metadata for setter-based DI. A small part of a Spring XML configuration file specifies 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;
  }
}

In the preceding example, setters are declared to match against the properties specified in the XML file. The following example uses 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;
  }
}

The constructor arguments specified in the bean definition will be used as arguments to the constructor of the ExampleBean.

Now consider a variant of this example, 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;
  }
}

Arguments to the static factory method are supplied via <constructor-arg/> elements, exactly the same as if a constructor had actually been used. The type of the class being returned by the factory method does not have to be of the same type as the class that 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.

3.4.2 Dependencies and configuration in detail

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

3.4.2.1 Straight values (primitives, Strings, and so on)

The value attribute of the <property/> 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"/>
<property name="url" value="jdbc:mysql://localhost:3306/mydb"/>
<property name="username" value="root"/>
<property name="password" value="masterkaoli"/>
</bean>

The following example uses the p-namespace for even more succinct XML configuration.

<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 id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource"
      destroy-method="close"
      p:driverClassName="com.mysql.jdbc.Driver"
      p:url="jdbc:mysql://localhost:3306/mydb"
      p:username="root"
      p:password="masterkaoli"/>

</beans>

The preceding XML is more succinct; however, typos are discovered at runtime rather than design time, unless you use an IDE such as IntelliJ IDEA or the SpringSource Tool Suite (STS) that support automatic property completion when you create bean definitions. Such IDE assistance is highly recommended.

You can also configure a java.util.Properties instance as:

<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>

The Spring container converts the text inside the <value/> element into a java.util.Properties instance by 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 (string value - not a reference) 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 first form is preferable to the second, because 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. Typos are only 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 deployed.

Additionally, if the referenced bean is in the same XML unit, and the bean name is the bean id, you can use the local attribute, which allows the XML parser itself to validate the bean id earlier, at XML document parse time.

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

A common place (at least in versions earlier than Spring 2.0) where the <idref/> element brings value is in the configuration of AOP interceptors in a ProxyFactoryBean bean definition. Using <idref/> elements when you specify the interceptor names prevents you from misspelling an interceptor id.

3.4.2.2 References to other beans (collaborators)

The ref element is the final element inside a <constructor-arg/> or <property/> definition element. Here you set the value of the specified property of a bean to be a reference to another bean (a collaborator) managed by the container. The referenced bean is a dependency of the bean whose property will be set, and it is initialized on demand as needed before the property is set. (If the collaborator is a singleton bean, it may be initialized already by the container.) All references are ultimately a reference to another object. Scoping and validation depend on whether you specify the id/name of the other object through the bean,local, or parent attributes.

Specifying the target bean through the bean attribute of the <ref/> tag is the most general form, and allows creation of a reference to any bean in the same container or parent container, regardless of whether it is in the same XML file. The value of the bean attribute may be the same as the id attribute of the target bean, or as one of the values in the name attribute of the target bean.

<ref bean="someBean"/>

Specifying the target bean through 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 issues 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 through the parent attribute creates a reference to a bean that 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 of the current one. You use this bean reference variant mainly when you have a hierarchy of containers and you want to wrap an existing bean in a parent container with a proxy that 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"  <-- bean name is the same as 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 here -->
</bean>

3.4.2.3 Inner beans

A <bean/> element inside the <property/> or <constructor-arg/> elements defines a so-called inner bean.

<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>

An inner bean definition does not require a defined id or name; the container ignores these values. It also ignores the scope flag. Inner beans are always anonymous and they are always scoped as prototypes. It is not possible to inject inner beans into collaborating beans other than into the enclosing bean.

3.4.2.4 Collections

In the <list/>, <set/>, <map/>, and <props/> elements, you set the properties and arguments of the Java Collection types List, Set, Map, and Properties, respectively.

<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="an entry" value="just some string"/>
      <entry key ="a ref" value-ref="myDataSource"/>
  </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>

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 supports the merging of collections. An application developer can 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, the child collection's values are the result of merging the elements of the parent and child collections, with the child's collection elements overriding values specified in the parent collection.

This section on merging discusses the parent-child bean mechanism. Readers unfamiliar with parent and child bean definitions may wish to read the relevant section before continuing.

The following example demonstrates collection merging:

<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 resolved and instantiated by the container, the resulting instance has an adminEmails Properties collection that contains the result of the merging of the child's adminEmails collection with the parent's adminEmails collection.

administrator=[email protected]
sales=[email protected]
support=[email protected]

The child Properties collection's value set inherits all property elements from the parent <props/>, and 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 precede all of the child list's values. In the case of the Map, Set, and Properties collection types, no ordering exists. Hence no ordering semantics are in effect for the collection types that underlie the associated Map, Set, and Properties implementation types that the container uses internally.

Limitations of collection merging

You cannot merge different collection types (such as a Map and a List), and if you do attempt to do so an appropriate Exception is thrown. The merge attribute must be specified on the lower, inherited, child definition; specifying the merge attribute on a parent collection definition is redundant and will not result in the desired merging. The merging feature is available only in Spring 2.0 and later.

Strongly-typed collection (Java 5+ only)

In Java 5 and later, you can use 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 are 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 prepared for injection, the generics information about the element type of the strongly-typed Map<String, Float> is available by reflection. Thus Spring's type conversion infrastructure recognizes the various value elements as being of type Float, and the string values 9.99, 2.75, and 3.99 are converted into an actual Float type.

3.4.2.5 Null and empty string values

Spring treats empty arguments for properties and the like as empty Strings. The following XML-based configuration metadata snippet sets the email property to the empty String value ("")

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

The preceding example is equivalent to the following Java code: exampleBean.setEmail(""). The <null/> element handles null values. For example:

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

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

3.4.2.6 XML shortcut with the p-namespace

The p-namespace enables you to use the bean element's attributes, instead of nested <property/> elements, to describe your property values and/or collaborating beans.

Spring 2.0 and later supports extensible configuration formats with namespaces, which are based on an XML Schema definition. The beans configuration format discussed in this chapter is defined in an XML Schema document. However, the p-namespace is not defined in an XSD file and exists only in the core of Spring.

The following example shows two XML snippets that resolve to the same result: The first uses standard XML format and the second uses 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>

The example shows an attribute in the p-namespace called email in the bean definition. This tells Spring to include a property declaration. As previously mentioned, the p-namespace does not have a schema definition, so you can set the name of the attribute to the property name.

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 includes not only 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

The p-namespace is not as flexible as the standard XML format. For example, the format for declaring property references clashes with properties that end in Ref, whereas the standard XML format does not. We recommend that you choose your approach carefully and communicate this to your team members, to avoid producing XML documents that use all three approaches at the same time.

3.4.2.7 Compound property names

You can use compound or nested property names when you set 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 after the bean is constructed, or a NullPointerException is thrown.

3.4.3 Using depends-on

If a bean is a dependency of another that usually means that one bean is set as a property of another. Typically you accomplish this with the <ref/> element in XML-based configuration metadata. However, sometimes dependencies between beans are less direct; for example, a static initializer in a class needs to be triggered, such as database driver registration. The depends-on attribute can explicitly force one or more beans to be initialized before the bean using this element is initialized. The following example uses 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" />

To express a dependency on multiple beans, supply a list of bean names as the value of the depends-on attribute, with commas, whitespace and semicolons, used as valid delimiters:

<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 in the bean definition can specify both an initialization time dependency and, in the case of singleton beans only, a corresponding destroy time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus depends-on can also control shutdown order.

3.4.4 Lazy-initialized beans

By default, ApplicationContext implementations eagerly create and configure all singleton beans as part of the initialization process. Generally, this pre-instantiation is desirable, because errors in the configuration or surrounding environment are discovered immediately, as opposed to hours or even days later. When this behavior is not desirable, you can prevent pre-instantiation of a singleton bean by marking the bean definition as lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean instance when it is first requested, rather than at startup.

In XML, this behavior 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 preceding configuration is consumed by an ApplicationContext, the bean named lazy is not eagerly pre-instantiated when the ApplicationContext is starting up, whereas the not.lazy bean is eagerly pre-instantiated.

However, when a lazy-initialized bean is a dependency of a singleton bean that is not lazy-initialized, the ApplicationContext creates the lazy-initialized bean at startup, because it must satisfy the singleton's dependencies. The lazy-initialized bean is injected into a singleton bean elsewhere that is not lazy-initialized.

You can also 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>

3.4.5 Autowiring collaborators

The Spring container can autowire relationships between collaborating beans. You can allow Spring to resolve collaborators (other beans) automatically for your bean by inspecting the contents of the ApplicationContext. Autowiring has the following advantages:

  • Autowiring can significantly reduce the need to specify properties or constructor arguments. (Other mechanisms such as a bean template discussed elsewhere in this chapter are also valuable in this regard.)

  • Autowiring can update a configuration as your objects evolve. For example, if you need to add a dependency to a class, that dependency can be satisfied automatically without you needing to modify the configuration. Thus autowiring can be especially useful during development, without negating the option of switching to explicit wiring when the code base becomes more stable.

When using XML-based configuration metadata[2], you specify autowire mode for a bean definition with the autowire attribute of the <bean/> element. The autowiring functionality has five modes. You specify autowiring per bean and thus can choose which ones to autowire.

Table 3.2. Autowiring modes

ModeExplanation
no

(Default) No autowiring. Bean references must be defined via a ref element. Changing the default setting is not recommended for larger deployments, because specifying collaborators explicitly gives greater control and clarity. To some extent, it documents the structure of a system.

byName

Autowiring by property name. Spring looks for a bean with the same name as the property that needs to be autowired. For example, if a bean definition is set to autowire by name, and it contains a master property (that is, it has a setMaster(..) method), Spring looks for a bean definition named master, and uses it to set the property.

byType

Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set.

constructor

Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised.


With byType or constructor autowiring mode, you can wire arrays and typed-collections. In such cases all autowire candidates within the container that match the expected type are provided to satisfy the dependency. You can autowire strongly-typed Maps if the expected key type is String. An autowired Maps values will consist of all bean instances that match the expected type, and the Maps keys will contain the corresponding bean names.

You can combine autowire behavior with dependency checking, which is performed after autowiring completes.

3.4.5.1 Limitations and disadvantages of autowiring

Autowiring works best when it is used consistently across a project. If autowiring is not used in general, it might be confusing to developers to use it to wire only one or two bean definitions.

Consider the limitations and disadvantages of autowiring:

  • Explicit dependencies in property and constructor-arg settings always override autowiring. You cannot autowire so-called simple properties such as primitives, Strings, and Classes (and arrays of such simple properties). This limitation is by-design.

  • Autowiring is less exact than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity that 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.

  • 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 is not arbitrarily resolved. If no unique bean definition is available, an exception is thrown.

In the latter scenario, you have several options:

  • Abandon autowiring in favor of explicit wiring.

  • Avoid autowiring for a bean definition by setting its autowire-candidate attributes to false as described in the next section.

  • Designate a single bean definition as the primary candidate by setting the primary attribute of its <bean/> element to true.

  • If you are using Java 5 or later, implement the more fine-grained control available with annotation-based configuration, as described in Section 3.9, “Annotation-based container configuration”.

3.4.5.2 Excluding a bean from autowiring

On a per-bean basis, you can exclude a bean from autowiring. In Spring's XML format, set the autowire-candidate attribute of the <bean/> element to false; the container makes that specific bean definition unavailable to the autowiring infrastructure (including annotation style configurations such as @Autowired).

You can also 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. An explicit value of true or false for a bean definitions autowire-candidate attribute always takes precedence, and for such beans, the pattern matching rules do not apply.

These techniques are useful for beans that you never want to be injected into other beans by autowiring. It does not mean that an excluded bean cannot itself be configured using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.

3.4.6 Method injection

In most application scenarios, most beans in the container are 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, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.

A solution is to forego some inversion of control. You can make bean A aware of the container by implementing the ApplicationContextAware interface, and by making a getBean("B") call to the container ask for (a typically new) bean B instance every time bean A needs it. The following is an example of this approach:

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

// Spring-API imports
import org.springframework.beans.BeansException;
import org.springframework.context.Applicationcontext;
import org.springframework.context.ApplicationContextAware;

public class CommandManager implements ApplicationContextAware {

 private ApplicationContext applicationContext;

 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();
 }

 protected Command createCommand() {
    // notice the Spring API dependency!
    return this.applicationContext.getBean("command", Command.class);
 }

 public void setApplicationContext(ApplicationContext applicationContext)
                                                                  throws BeansException {
    this.applicationContext = applicationContext;
 }
}

The preceding is not desirable, because the business code is 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.

3.4.6.1 Lookup method injection

Lookup method injection is the ability of the container to override methods on container managed beans, to return the lookup result for another named bean in the container. The lookup typically involves a prototype bean as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to generate dynamically a subclass that overrides the method.

[Note]Note

For this dynamic subclassing to work, you must have the CGLIB jar(s) in your classpath. The class that the Spring container will subclass cannot be final, and the method to be overridden cannot be final either. Also, testing a class that has an abstract method requires you to subclass the class yourself and to supply a stub implementation of the abstract method. Finally, objects that have been the target of method injection cannot be serialized.

Looking at the CommandManager class in the previous code snippet, you see that the Spring container will dynamically override the implementation of the createCommand() method. Your CommandManager class will not have any Spring dependencies, as can be seen in the reworked example:

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 to be injected requires a signature of the following form:

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

If the method is abstract, the dynamically-generated subclass implements the method. Otherwise, the dynamically-generated subclass overrides the concrete method defined in the original class. For 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 calls its own method createCommand() whenever it needs a new instance of the command bean. You 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 is returned each time.

[Tip]Tip

The interested reader may also find the ServiceLocatorFactoryBean (in the org.springframework.beans.factory.config package) to be of use. The approach used in ServiceLocatorFactoryBean is similar to that of another utility class, ObjectFactoryCreatingFactoryBean, but it allows you to specify your own lookup interface as opposed to a Spring-specific lookup interface. Consult the JavaDocs for these classes as well as this blog entry for additional information ServiceLocatorFactoryBean.

3.4.6.2 Arbitrary method replacement

A less 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 until the functionality is actually needed.

With XML-based configuration metadata, you can use the replaced-method element 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"/>

You can use one or more contained <arg-type/> elements within the <replaced-method/> element to indicate the method signature of the method being overridden. The signature for the arguments is necessary only if the method is overloaded and multiple variants exist within the class. For convenience, the type string for an argument may be a substring of the fully qualified type name. For example, the following all match java.lang.String:

    java.lang.String
  String
  Str

Because 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 only the shortest string that will match an argument type.

3.5 Bean scopes

When you create a bean definition, you create 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, as with a class, you can create many object instances 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 powerful and flexible in that you can 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 five scopes, three of which are available only if you use a web-aware ApplicationContext.

The following scopes are supported out of the box. You can also create a custom scope.

Table 3.3. Bean scopes

ScopeDescription

singleton

(Default) 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 HTTP request has 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 an 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.


[Note]Thread-scoped beans

As of Spring 3.0, a thread scope is available, but is not registered by default. For more information, see the documentation for SimpleThreadScope. For instructions on how to register this or any other custom scope, see Section 3.5.5.2, “Using a custom scope”.

3.5.1 The singleton scope

Only one shared instance of a singleton bean is managed, and all requests for beans with an id or ids matching that bean definition 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, the Spring IoC container creates exactly one instance of the object defined by that bean definition. This single instance is stored in a cache of such singleton beans, and all subsequent requests and references for that named bean return the cached object.

Spring's concept of a singleton bean differs from the Singleton pattern as defined in the 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 is 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 creates 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, for example:

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

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

3.5.2 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, the bean is injected into another bean or you request it through a getBean() method call on the container. As a rule, use the prototype scope for all stateful beans and the singleton scope for stateless beans.

The following diagram illustrates the Spring prototype scope. A data access object (DAO) is not typically configured as a prototype, because a typical DAO does not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.

The following example defines a bean as a prototype in XML:

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

In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean: the container instantiates, configures, and otherwise assembles a prototype object, and hands it to the client, with no further record of that prototype instance. Thus, although initialization lifecycle callback methods are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle callbacks are not called. The client code must clean up prototype-scoped objects and release expensive resources that the prototype bean(s) are holding. To get the Spring container to release resources held by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans that need to be cleaned up.

In some respects, the Spring container's role in regard to a prototype-scoped bean is a replacement for the Java new operator. All lifecycle management past that point must be handled by the client. (For details on the lifecycle of a bean in the Spring container, see Section 3.6.1, “Lifecycle callbacks”.)

3.5.3 Singleton beans with prototype-bean dependencies

When you use singleton-scoped beans with dependencies on prototype beans, be aware that dependencies are resolved at instantiation time. Thus if you dependency-inject a prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated and then dependency-injected into the singleton bean. The prototype instance is the sole instance that is ever supplied to the singleton-scoped bean.

However, suppose you want the singleton-scoped bean to acquire a new instance of the prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a prototype-scoped bean into your singleton bean, because that injection occurs only once, when the Spring container is instantiating the singleton bean and resolving and injecting its dependencies. If you need a new instance of a prototype bean at runtime more than once, see Section 3.4.6, “Method injection”

3.5.4 Request, session, and global session scopes

The request, session, and global session scopes are only available if you use a web-aware Spring ApplicationContext implementation (such as XmlWebApplicationContext). If you use these scopes with regular Spring IoC containers such as the ClassPathXmlApplicationContext, you get an IllegalStateException complaining about an unknown bean scope.

3.5.4.1 Initial web configuration

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 define your beans. (This initial setup is not required for the standard scopes, singleton and prototype.)

How you accomplish this initial setup depends on your particular Servlet environment..

If you access scoped beans within Spring Web MVC, in effect, 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.

If you use a Servlet 2.4+ web container, with requests processed outside of Spring's DispatcherServlet (for example, when using JSF or Struts), you need to add the following javax.servlet.ServletRequestListener to the declarations in your web applications web.xml file:

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

If you use an older web container (Servlet 2.3), use the provided javax.servlet.Filter implementation. The following snippet of XML configuration must be included in the web.xml file of your web application if you want to access 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, so you must 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>

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.

3.5.4.2 Request scope

Consider the following bean definition:

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

The Spring container creates a new instance of the LoginAction bean by using the loginAction bean definition for each and every HTTP request. That is, the loginAction bean is scoped at the HTTP request level. You can change the internal state of the instance that is created as much as you want, because other instances created from the same loginAction bean definition will not see these changes in state; they are particular to an individual request. When the request completes processing, the bean that is scoped to the request is discarded.

3.5.4.3 Session scope

Consider the following bean definition:

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

The Spring container creates a new instance of the UserPreferences bean by using the userPreferences bean definition for the lifetime of a single HTTP Session. In other words, the userPreferences bean is effectively scoped at the HTTP Session level. As with request-scoped beans, you can change the internal state of the instance that is created as much as you want, knowing that other HTTP Session instances that are also using instances created from the same userPreferences bean definition do not see these changes in state, because 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 is also discarded.

3.5.4.4 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 above), and applies only in the context of portlet-based web applications. The portlet specification defines the notion of a global Session that is shared among all 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.

If you write a standard Servlet-based web application and you define one or more beans as having global session scope, the standard HTTP Session scope is used, and no error is raised.

3.5.4.5 Scoped beans as dependencies

The Spring IoC container manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject (for example) an HTTP request scoped bean into another bean, you must 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 can also retrieve the real, target object from the relevant scope (for example, an 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. If you try to create a scoped proxy for a singleton bean, the BeanCreationException is raised.

The configuration in the following example is only 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">

  <!-- an 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 insert a child <aop:scoped-proxy/> element into a scoped bean definition. (If you choose class-based proxying, you also need the CGLIB library in your classpath. See the section called “Choosing the type of proxy to create” and Appendix C, XML Schema-based configuration.) Why do definitions of beans scoped at the request, session, globalSession and custom-scope levels require the <aop:scoped-proxy/> element ? Let's examine the following singleton bean definition and contrast it with what you need to define for the aforementioned scopes. (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>

In the preceding example, the singleton bean userManager is 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) are also injected only once. This means that the userManager bean will only operate on the exact same userPreferences object, that is, the one that it was originally injected with.

This is not the behavior you want when injecting a shorter-lived scoped bean into a longer-lived scoped bean, for example injecting an HTTP Session-scoped collaborating bean as a dependency into singleton bean. Rather, you need a single userManager object, and for the lifetime of an HTTP Session, you need a userPreferences object that is specific to said HTTP Session. Thus the container creates an object that exposes the exact same public interface as the UserPreferences class (ideally an object that is a UserPreferences instance) which can fetch the real UserPreferences object from the scoping mechanism (HTTP request, Session, etc.). The container injects this proxy object into the userManager bean, which is unaware that this UserPreferences reference is a proxy. In this example, when a UserManager instance invokes a method on the dependency-injected UserPreferences object, it actually is invoking a method on the proxy. The proxy then fetches the real UserPreferences object from (in this case) the HTTP Session, and delegates the method invocation onto the retrieved real UserPreferences object.

Thus 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 to create

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

Note: CGLIB proxies 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.

Alternatively, you can configure the Spring container to 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 means that you do not need additional libraries in your application classpath to effect such proxying. However, it also means that the class of the scoped bean must implement at least one interface, and that all collaborators into which the scoped bean is injected must reference the bean through 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>

For more detailed information about choosing class-based or interface-based proxying, see Section 7.6, “Proxying mechanisms”.

3.5.5 Custom scopes

As of Spring 2.0, the bean scoping mechanism is extensible. You can define your own scopes, or even redefine existing scopes, although the latter is considered bad practice and you cannot override the built-in singleton and prototype scopes.

3.5.5.1 Creating a custom scope

To integrate your custom scope(s) into the Spring container, you need to implement the org.springframework.beans.factory.config.Scope interface, which is described in this section. For an idea of how to implement your own scopes, see the Scope implementations that are supplied with the Spring Framework itself and the Scope Javadoc, which explains the methods you need to implement in more detail.

The Scope interface has four methods to get objects from the scope, remove them from the scope, and allow them to be destroyed.

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

Object get(String name, ObjectFactory objectFactory)

The following method removes 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, but you can return null if the object with the specified name is not found.

Object remove(String name)

The following method registers the callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed. Refer to the Javadoc or a Spring scope implementation for more information on destruction callbacks.

void registerDestructionCallback(String name, Runnable destructionCallback)

The following method obtains the conversation identifier for the underlying scope. This identifier is different for each scope. For a session scoped implementation, this identifier can be the session identifier.

String getConversationId()

3.5.5.2 Using a custom scope

After you write and test one or more custom Scope implementations, you need to make the Spring container aware of your new scope(s). The following method is the central method to register a new Scope with the Spring container:

void registerScope(String scopeName, Scope scope);

This method is declared on the ConfigurableBeanFactory interface, which is available on most of the concrete ApplicationContext implementations that ship with Spring via the BeanFactory property.

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.

Suppose that you write your custom Scope implementation, and then register it as below.

[Note]Note

The example below uses SimpleThreadScope which is included with Spring, but not registered by default. The instructions would be the same for your own custom Scope implementations.

Scope threadScope = new SimpleThreadScope();
beanFactory.registerScope("thread", threadScope);

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

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

With a custom Scope implementation, you are not limited to programmatic registration of the scope. You can also do the Scope registration declaratively, using the CustomScopeConfigurer 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: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="org.springframework.context.support.SimpleThreadScope"/>
              </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

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

3.6 Customizing the nature of a bean

3.6.1 Lifecycle callbacks

To interact with the container's management of the bean lifecycle, you can implement the Spring InitializingBean and DisposableBean interfaces. The container calls afterPropertiesSet() for the former and destroy() for the latter to allow the bean to perform certain actions upon initialization and destruction of your beans. You can also achieve the same integration with the container without coupling your classes to Spring interfaces through the use of init-method and destroy method object definition metadata.

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 does not offer out-of-the-box, you can implement a BeanPostProcessor yourself. For more information, see Section 3.8, “Container extension points”.

In addition to the initialization and destruction callbacks, Spring-managed objects may also implement the Lifecycle interface so that those objects can participate in the startup and shutdown process as driven by the container's own lifecycle.

The lifecycle callback interfaces are described in this section.

3.6.1.1 Initialization callbacks

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 a single method:

void afterPropertiesSet() throws Exception;

It is recommended that you do not use the InitializingBean interface because it unnecessarily couples the code to Spring. Alternatively, specify a POJO initialization method. In the case of XML-based configuration metadata, you use the init-method attribute to specify the name of the method that has a void no-argument signature. 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.

3.6.1.2 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;

It is recommended that you do not use the DisposableBean callback interface because it unnecessarily couples the code to Spring. Alternatively, specify a generic method that is supported by bean definitions. With XML-based configuration metadata, you use 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.

3.6.1.3 Default initialization and destroy methods

When you write initialization and destroy method callbacks that do not use the Spring-specific InitializingBean and DisposableBean callback interfaces, you typically write methods with names such as init(), initialize(), dispose(), and so on. Ideally, the names of such lifecycle callback methods are standardized across a project so that all developers use the same method names and ensure consistency.

You can configure the Spring container to look for named initialization and destroy callback method names on every bean. This means that you, as an application developer, can write your application classes and use an initialization callback called init(), without having to configure an init-method="init" attribute with each bean definition. The Spring IoC container calls that method when the bean is created (and in accordance with the standard lifecycle callback contract described previously). This feature also enforces a consistent naming convention for initialization and destroy method callbacks.

Suppose that your initialization callback methods are named init() and destroy callback methods are named destroy(). Your class will resemble the class in the following example.

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>

The presence of the default-init-method attribute on the top-level <beans/> element attribute causes the Spring IoC container to recognize a method called init on beans as the initialization method callback. When a bean is created and assembled, if the bean class has such a method, it is invoked at the appropriate time.

You configure destroy method callbacks similarly (in XML, that is) by using the default-destroy-method attribute on the top-level <beans/> element.

Where existing bean classes already have callback methods that are named at variance with the convention, you can override the default by specifying (in XML, that is) the method name using the init-method and destroy-method attributes of the <bean/> itself.

The Spring container guarantees that a configured initialization callback is called immediately after a bean is supplied with all dependencies. Thus the initialization callback is called on the raw bean reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. 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 inconsistent to apply the interceptors to the init method, because doing so would couple the lifecycle of the target bean with its proxy/interceptors and leave strange semantics when your code interacts directly to the raw target bean.

3.6.1.4 Combining lifecycle mechanisms

As of Spring 2.5, you have three options for controlling bean lifecycle behavior: the InitializingBean and DisposableBean callback interfaces; custom init() and destroy() methods; and the @PostConstruct and @PreDestroy annotations. You can combine these mechanisms to control a given bean.

[Note]Note

If multiple lifecycle mechanisms are configured for a bean, and each mechanism is configured with a different method name, then each configured method is executed in the order listed below. However, if the same method name is configured - for example, init() for an initialization method - for more than one of these lifecycle mechanisms, that method is executed once, as explained in the preceding section.

Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are called as follows:

  • 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

3.6.1.5 Startup and shutdown callbacks

The Lifecycle interface defines the essential methods for any object that has its own lifecycle requirements (e.g. starts and stops some background process):

public interface Lifecycle {

  void start();

  void stop();

  boolean isRunning();

}

Any Spring-managed object may implement that interface. Then, when the ApplicationContext itself starts and stops, it will cascade those calls to all Lifecycle implementations defined within that context. It does this by delegating to a LifecycleProcessor:

public interface LifecycleProcessor extends Lifecycle {

  void onRefresh();

  void onClose();

}

Notice that the LifecycleProcessor is itself an extension of the Lifecycle interface. It also adds two other methods for reacting to the context being refreshed and closed.

The order of startup and shutdown invocations can be important. If a "depends-on" relationship exists between any two objects, the dependent side will start after its dependency, and it will stop before its dependency. However, at times the direct dependencies are unknown. You may only know that objects of a certain type should start prior to objects of another type. In those cases, the SmartLifecycle interface defines another option, namely the getPhase() method as defined on its super-interface, Phased.

public interface Phased {

  int getPhase();

}


public interface SmartLifecycle extends Lifecycle, Phased {

  boolean isAutoStartup();

  void stop(Runnable callback);

}

When starting, the objects with the lowest phase start first, and when stopping, the reverse order is followed. Therefore, an object that implements SmartLifecycle and whose getPhase() method returns Integer.MIN_VALUE would be among the first to start and the last to stop. At the other end of the spectrum, a phase value of Integer.MAX_VALUE would indicate that the object should be started last and stopped first (likely because it depends on other processes to be running). When considering the phase value, it's also important to know that the default phase for any "normal" Lifecycle object that does not implement SmartLifecycle would be 0. Therefore, any negative phase value would indicate that an object should start before those standard components (and stop after them), and vice versa for any positive phase value.

As you can see the stop method defined by SmartLifecycle accepts a callback. Any implementation must invoke that callback's run() method after that implementation's shutdown process is complete. That enables asynchronous shutdown where necessary since the default implementation of the LifecycleProcessor interface, DefaultLifecycleProcessor, will wait up to its timeout value for the group of objects within each phase to invoke that callback. The default per-phase timeout is 30 seconds. You can override the default lifecycle processor instance by defining a bean named "lifecycleProcessor" within the context. If you only want to modify the timeout, then defining the following would be sufficient:

<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor">
  <!-- timeout value in milliseconds -->
  <property name="timeoutPerShutdownPhase" value="10000"/>
</bean>

As mentioned, the LifecycleProcessor interface defines callback methods for the refreshing and closing of the context as well. The latter will simply drive the shutdown process as if stop() had been called explicitly, but it will happen when the context is closing. The 'refresh' callback on the other hand enables another feature of SmartLifecycle beans. When the context is refreshed (after all objects have been instantiated and initialized), that callback will be invoked, and at that point the default lifecycle processor will check the boolean value returned by each SmartLifecycle object's isAutoStartup() method. If "true", then that object will be started at that point rather than waiting for an explicit invocation of the context's or its own start() method (unlike the context refresh, the context start does not happen automatically for a standard context implementation). The "phase" value as well as any "depends-on" relationships will determine the startup order in the same way as described above.

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

[Note]Note

This section applies only to non-web applications. Spring's web-based ApplicationContext implementations already have code in place to shut down the Spring IoC container gracefully when the relevant web application is shut down.

If you are using Spring's IoC container in a non-web application environment; for example, in a rich client desktop environment; you register a shutdown hook with the JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your singleton beans so that all resources are released. Of course, you must still configure and implement these destroy callbacks correctly.

To register a shutdown hook, you call the registerShutdownHook() method that is declared on the AbstractApplicationContext class:

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...
  }
}

3.6.2 ApplicationContextAware and BeanNameAware

When an ApplicationContext creates a class that implements the org.springframework.context.ApplicationContextAware interface, the class is provided with a reference to that ApplicationContext.

public interface ApplicationContextAware {

  void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
}

Thus beans can manipulate programmatically the ApplicationContext that created them, through the ApplicationContext interface, or by casting the reference to a known subclass of this interface, such as ConfigurableApplicationContext, which exposes additional functionality. One use would be the programmatic retrieval of other beans. Sometimes this capability is useful; however, in general you should avoid it, because it couples the code to Spring and does not follow the Inversion of Control style, where collaborators are provided to beans as properties. Other methods of the ApplicationContext provide access to file resources, publishing application events, and accessing a MessageSource. These additional features are described in Section 3.13, “Additional Capabilities of the ApplicationContext”

As of Spring 2.5, autowiring is another alternative to obtain reference to the ApplicationContext. The "traditional" constructor and byType autowiring modes (as described in Section 3.4.5, “Autowiring collaborators”) can provide a dependency of type ApplicationContext for a constructor argument or setter method parameter, respectively. For more flexibility, including the ability to autowire fields and multiple parameter methods, use the new annotation-based autowiring features. If you do, the ApplicationFactory is autowired into a field, constructor argument, or method parameter that is expecting the BeanFactory type if the field, constructor, or method in question carries the @Autowired annotation. For more information, see Section 3.9.2, “@Autowired and @Inject”.

When an ApplicationContext creates a class that implements the org.springframework.beans.factory.BeanNameAware interface, the class is provided with a reference to the name defined in its associated object definition.

public interface BeanNameAware {

  void setBeanName(string name) throws BeansException;
}

The callback is invoked after population of normal bean properties but before an initialization callback such as InitializingBeans afterPropertiesSet or a custom init-method.

3.6.3 Other Aware interfaces

Besides ApplicationContextAware and BeanNameAware discussed above, Spring offers a range of Aware interfaces that allow beans to indicate to the container that they require a certain infrastructure dependency. The most important Aware interfaces are summarized below - as a general rule, the name is a good indication of the dependency type:

Table 3.4. Aware interfaces

NameInjected DependencyExplained in...

ApplicationContextAware

Declaring ApplicationContext

Section 3.6.2, “ApplicationContextAware and BeanNameAware”

ApplicationEventPublisherAware

Event publisher of the enclosing ApplicationContext

Section 3.13, “Additional Capabilities of the ApplicationContext”

BeanClassLoaderAware

Class loader used to load the bean classes.

Section 3.3.2, “Instantiating beans”

BeanFactoryAware

Declaring BeanFactory

Section 3.6.2, “ApplicationContextAware and BeanNameAware”

BeanNameAware

Name of the declaring bean

Section 3.6.2, “ApplicationContextAware and BeanNameAware”

BootstrapContextAware

Resource adapter BootstrapContext the container runs in. Typically available only in JCA aware ApplicationContexts

Chapter 23, JCA CCI

LoadTimeWeaverAware

Defined weaver for processing class definition at load time

Section 7.8.4, “Load-time weaving with AspectJ in the Spring Framework”

MessageSourceAware

Configured strategy for resolving messages (with support for parametrization and internationalization)

Section 3.13, “Additional Capabilities of the ApplicationContext”

NotificationPublisherAware

Spring JMX notification publisher

Section 22.7, “Notifications”

PortletConfigAware

Current PortletConfig the container runs in. Valid only in a web-aware Spring ApplicationContext

Chapter 18, Portlet MVC Framework

PortletContextAware

Current PortletContext the container runs in. Valid only in a web-aware Spring ApplicationContext

Chapter 18, Portlet MVC Framework

ResourceLoaderAware

Configured loader for low-level access to resources

Chapter 4, Resources

ServletConfigAware

Current ServletConfig the container runs in. Valid only in a web-aware Spring ApplicationContext

Chapter 15, Web MVC framework

ServletContextAware

Current ServletContext the container runs in. Valid only in a web-aware Spring ApplicationContext

Chapter 15, Web MVC framework


Note again that usage of these interfaces ties your code to the Spring API and does not follow the Inversion of Control style. As such, they are recommended for infrastructure beans that require programmatic access to the container.

3.7 Bean definition inheritance

A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information such as initialization method, static factory method name, and so on. A child bean definition inherits configuration data from a parent definition. The child definition can override some values, or add others, as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.

If you work with an ApplicationContext interface programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users do not work with them on this level, instead configuring bean definitions declaratively in something like the ClassPathXmlApplicationContext. When you use XML-based configuration metadata, you indicate a child bean definition 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 uses 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 inherits constructor argument values, property values, and method overrides from the parent, with the option to add new values. Any initialization method, destroy method, and/or static factory method settings that you specify will override the corresponding parent settings.

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

The preceding example explicitly marks the parent bean definition as abstract by using the abstract attribute. If the parent definition does not specify a class, explicitly marking the parent bean definition as abstract is required, as follows:

<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 be instantiated on its own because it is incomplete, and it is also explicitly marked as abstract. When a definition is abstract like this, it is usable only as a pure template bean definition that serves 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, returns an error. Similarly, the container's internal preInstantiateSingletons() method ignores bean definitions that are defined as abstract.

[Note]Note

ApplicationContext pre-instantiates all singletons by default. 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.

3.8 Container extension points

Typically, an application developer does not need to subclass any ApplicationContext implementation classes. You can extend The Spring IoC container infinitely by plugging in implementations of special integration interfaces. The next few sections describe these integration interfaces.

3.8.1 Customizing beans using the BeanPostProcessor Interface

The BeanPostProcessor interface defines callback methods that you can implement to provide your own (or override the container's default) instantiation logic, dependency-resolution logic, and so forth. If you want to implement some custom logic after the Spring container finishes instantiating, configuring, and otherwise initializing a bean, you can plug in one or more BeanPostProcessor implementations.

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

[Note]Note

BeanPostProcessors operate on bean (or object) instances; that is to say, the Spring IoC container instantiates a bean instance and then BeanPostProcessor interfaces do their work.

BeanPostProcessor interfaces 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 work on the beans in that container. Beans that are defined in one container are not post-processed by a BeanPostProcessor in another container, even if both containers are part of the same hierarchy.

To change the actual bean definition (that is, the recipe that defines the bean), you instead need to use a BeanFactoryPostProcessor, described below in Section 3.8.2, “Customizing configuration metadata with BeanFactoryPostProcessor interface”.

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, for each bean instance that is created by the container, the post-processor gets a callback from the container both before container initialization methods (such as afterPropertiesSet and any declared init method) are called, and also afterwards. The post-processor can take any action with the bean instance, including ignoring the callback completely. A bean post-processor typically checks for callback interfaces, or may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as bean post-processors and they do this proxy-wrapping logic.

An ApplicationContext automatically detects any beans that are defined in the configuration metadata it receives that implement the BeanPostProcessor interface. The ApplicationContext registers these beans as post-processors, to be then called appropriately by the container upon bean creation. You can then deploy the post-processors as you would any bean.

[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 are instantiated on startup, as part of the special startup phase of the ApplicationContext. Next, all those BeanPostProcessors are registered in a sorted fashion - and applied to all further beans. Because AOP auto-proxying is implemented as a BeanPostProcessor itself, no BeanPostProcessors or directly referenced beans are eligible for auto-proxying, and thus do 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 BeanPostProcessor interfaces (for example: not eligible for auto-proxying)”.

The following examples show how to write, register, and use BeanPostProcessors in the context of an ApplicationContext.

3.8.1.1 Example: Hello World, BeanPostProcessor-style

This first example illustrates basic usage. The example shows a custom BeanPostProcessor implementation that invokes the toString() method of each bean as it is created by the container and prints the resulting string to the system console.

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 does not even have a name, and because it is a bean it can be dependency-injected just like any other bean. (The preceding configuration also defines a bean that is backed by a Groovy script. The Spring 2.0 dynamic language support is detailed in the chapter entitled Chapter 26, Dynamic language support.)

The following small driver script executes the preceding 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 the preceding execution resembles the following:

Bean 'messenger' created : org.springframework.scripting.groovy.GroovyMessenger@272961
org.springframework.scripting.groovy.GroovyMessenger@272961

3.8.1.2 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. An example is Spring's RequiredAnnotationBeanPostProcessor -- a 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.

3.8.2 Customizing configuration metadata with BeanFactoryPostProcessor interface

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 allows BeanFactoryPostProcessors to read the configuration metadata and potentially change it before the container instantiates any beans other than BeanFactoryPostProcessors.

You can configure multiple BeanFactoryPostProcessors. You can control the order in which these BeanFactoryPostProcessors execute by setting the order property. However, 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 instead need to use a BeanPostProcessor (described above in Section 3.8.1, “Customizing beans using the BeanPostProcessor Interface ”. While it is technically possible to work with bean instances within a BeanFactoryPostProcessor (e.g. using BeanFactory.getBean()), doing so causes premature bean instantiation, violating the usual containter lifecycle. This may cause negative side effects such as bypassing bean post processing.

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 automatically when it is declared inside of an ApplicationContext, in order to apply changes to the configuration metadata that defines a container. Spring includes a number of pre-existing bean factory post-processors, such as PropertyOverrideConfigurer and PropertyPlaceholderConfigurer. A custom BeanFactoryPostProcessor can also be used, for example, to register custom property editors.

An ApplicationContext detects any beans that are deployed into it and that implement the BeanFactoryPostProcessor interface. It automatically uses these beans as bean factory post-processors, at the appropriate time. You can then deploy these post-processor beans as you would any other bean.

[Note]Note

As with BeanPostProcessors, you typically do not want BeanFactoryPostProcessors marked as lazy-initialized. If they are marked as such, the Spring container never instantiates them, and thus they cannot apply their custom logic. If you use 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".

3.8.2.1 Example: the PropertyPlaceholderConfigurer

You use the PropertyPlaceholderConfigurer to externalize property values from a bean definition into another separate file in the standard Java Properties format. Doing so enables the person deploying an application to customize environment-specific properties such as database URLs 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. The example shows properties configured from an external Properties file. At runtime, a PropertyPlaceholderConfigurer is applied to the metadata that will replace some properties of the DataSource. The values to replace are specified as 'placeholders' of the form ${property-name} which follows the Ant / Log4J / JSP EL style.

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

Therefore, the string ${jdbc.username} is replaced at runtime with the value 'sa' and similarly for other placeholder values that match to keys in the property file. The PropertyPlaceholderConfigurer checks for placeholders in most locations of a bean definition and the placeholder prefix and suffix can be customized.

With the context namespace introduced in Spring 2.5, it is possible to configure property placeholders with a dedicated configuration element. You can provide multiple locations as a comma-separated list in the location attribute.

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

The PropertyPlaceholderConfigurer does not look for properties only 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. You can customize this behavior by setting the systemPropertiesMode property of the configurer. It has three values that specify configurer behavior: always override, never override, and override only if the property is not found in the properties file specified. Consult the Javadoc for the PropertyPlaceholderConfigurer for more information.

[Tip]Class name substitution

You can use the PropertyPlaceholderConfigurer 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 cannot be resolved at runtime to a valid class, resolution of the bean fails when it is about to be created, which is during the preInstantiateSingletons() phase of an ApplicationContext for a non-lazy-init bean.

3.8.2.2 Example: the PropertyOverrideConfigurer

The PropertyOverrideConfigurer, another bean factory post-processor, resembles the PropertyPlaceholderConfigurer, but unlike 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 definition is not aware of being overridden, so it is not immediately obvious from the XML definition file that the override configurer is used. In case of multiple PropertyOverrideConfigurer instances that define different values for the same bean property, the last one wins, due to the overriding mechanism.

Properties file configuration lines take this format:

beanName.property=value

For example:

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

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

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 set to the scalar value 123.

[Note]Note

Specified override values are always literal values; they are not translated into bean references. This convention 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"/>

3.8.3 Customizing instantiation logic with the FactoryBean Interface

You implement the org.springframework.beans.factory.FactoryBean interface for objects that are themselves factories.

The FactoryBean interface is a point of pluggability into the Spring IoC container's instantiation logic. If you have 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(): returns 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(): returns true if this FactoryBean returns singletons, false otherwise.

  • Class getObjectType(): returns the object type returned by the getObject() method or null if the type is not known in advance

The FactoryBean concept and interface is used in a number of places within the Spring Framework; more than 50 implementations of the FactoryBean interface ship with Spring itself.

When you need to ask a container for an actual FactoryBean instance itself, not the bean it produces, you preface the bean id with the ampersand symbol & (without quotes) when calling the getBean() method of the ApplicationContext. So for a given FactoryBean with an id of myBean, invoking getBean("myBean") on the container returns the product of the FactoryBean, and invoking getBean("&myBean") returns the FactoryBean instance itself.

3.9 Annotation-based container configuration

An alternative to XML setups is provided by annotation-based configuration which rely on the bytecode metadata for wiring up components instead of angle-bracket declarations. Instead of using XML to describe a bean wiring, the developer moves the configuration into the component class itself by using annotations on the relevant class, method, or field declaration. As mentioned in Section 3.8.1.2, “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 3.4.5, “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. Spring 3.0 adds support for JSR-330 (Dependency Injection for Java) annotations contained in the javax.inject package such as @Inject, @Qualifier, @Named, and @Provider if the JSR330 jar is present on the classpath. Use of these annotations also requires that certain BeanPostProcessors be registered within the Spring container.

[Note]Note
Annotation injection is performed before XML injection, thus the latter configuration will override the former for properties wired through both approaches.

As always, you can register them 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

<context:annotation-config/> only looks for annotations on beans in the same application context in which it is defined. 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 15.2, “The DispatcherServlet” for more information.

3.9.1 @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, through an explicit property value in a bean definition or through autowiring. The container throws 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. It is still recommended that you put assertions into the bean class itself, for example, into an init method. Doing so enforces those required references and values even when you use the class outside of a container.

3.9.2 @Autowired and @Inject

As expected, you can apply the @Autowired annotation to "traditional" setter methods:

[Note]Note

JSR 330's @Inject annotation can be used in place of Spring's @Autowired in the examples below. @Inject does not have a required property unlike Spring's @Autowired annotation which has a required property to indicate if the value being injected is optional. This behavior is enabled automatically if you have the JSR 330 JAR on the classpath.

public class SimpleMovieLister {

  private MovieFinder movieFinder;

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

  // ...
}

You can also apply the annotation 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;
  }

  // ...
}

You can apply @Autowired to 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 can 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 fails 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 can be marked as required, but multiple non-required constructors can be annotated. In that case, each is considered among the candidates and Spring uses the greediest constructor whose dependencies can be satisfied, that is the constructor that has the largest number of arguments.

@Autowired's required attribute is recommended over the @Required annotation. The required attribute indicates that the property is not required for autowiring purposes, the property is ignored if it cannot be autowired. @Required, on the other hand, is stronger in that it enforces the property that was set by any means supported by the container. If no value is injected, a corresponding exception is raised.

You can also use @Autowired for interfaces that are well-known resolvable dependencies: BeanFactory, ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource. These interfaces and their extended interfaces, such as ConfigurableApplicationContext or ResourcePatternResolver, are automatically resolved, with no special setup necessary.

public class MovieRecommender {

  @Autowired
  private ApplicationContext context;

  public MovieRecommender() {
  }

  // ...
}

3.9.3 Fine-tuning annotation-based autowiring with qualifiers

Because 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. You can associate 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:

[Note]Note

JSR 330's @Qualifier annotation can only be applied as a meta-annotation unlike Spring's @Qualifier which takes a string property to discriminate among multiple injection candidates and can be placed on annotations as well as types, fields, methods, constructors, and parameters.

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 appear as follows. The bean with qualifier value "main" is wired with the constructor argument that is 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 a default qualifier value. Thus you can define the bean with an id "main" instead of the nested qualifier element, leading to the same matching result. However, although you can use this convention to refer to specific beans by name, @Autowired is fundamentally about type-driven injection with optional semantic qualifiers. This means that qualifier values, even with 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 are "main" or "EMEA" or "persistent", expressing characteristics of a specific component that are independent from the bean id, which may be auto-generated in case of an anonymous bean definition like the one in the preceding example.

Qualifiers also apply to typed collections, as discussed above, for example, to Set<MovieCatalog>. In this case, all matching beans according to the declared qualifiers are injected as a collection. This implies that qualifiers do not have to be unique; they rather simply constitute filtering criteria. For example, you can define multiple MovieCatalog beans 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, use 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 that are themselves defined as a collection or map type cannot be injected through @Autowired, because type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection or map bean by unique name.

@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only 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 can create your own custom qualifier annotations. Simply define an annotation and provide the @Qualifier annotation within your definition:

[Note]Note

You can use JSR 330's @Qualifier annotation in the manner described below in place of Spring's @Qualifier annotation. This behavior is enabled automatically if you have the JSR 330 jar on the classpath.

@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;
  }

  // ...
}

Next, provide the information for 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 is matched against the fully-qualified class name of the annotation. Or, as a convenience if no risk of conflicting names exists, you can use the short class name. Both approaches 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 Section 3.10, “Classpath scanning and managed components”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Section 3.10.7, “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 can 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>

You can also 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 take precedence, but the autowiring mechanism falls back on the values provided within the <meta/> tags if no such qualifier is present, as in the last two bean definitions 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="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>

3.9.4 CustomAutowireConfigurer

The CustomAutowireConfigurer is a BeanFactoryPostProcessor that enables you to register your own custom qualifier annotation types even if they are not 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>

The particular implementation of AutowireCandidateResolver that is activated for the application context depends on the Java version. In versions earlier 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 by any default-autowire-candidates pattern(s) available on the <beans/> element. In Java 5 or later, the presence of @Qualifier annotations and any custom annotations registered with the CustomAutowireConfigurer will also play a role.

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

3.9.5 @Resource

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

@Resource takes a name attribute, and by default Spring interprets 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, the default name is derived from the field name or setter method. In case of a field, it takes the field name; in case of a setter method, it takes 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 is resolved as a bean name by the ApplicationContext of which the CommonAnnotationBeanPostProcessor is aware. The names can be resolved through JNDI if you configure Spring's SimpleJndiBeanFactory explicitly. However, it is recommended that you rely on the default behavior and simply use Spring's JNDI lookup capabilities to preserve the level of indirection.

In the exclusive case of @Resource usage with no explicit name specified, and similar to @Autowired, @Resource finds a primary type match instead of a specific named bean and resolves well-known resolvable dependencies: the BeanFactory, ApplicationContext, ResourceLoader, ApplicationEventPublisher, and MessageSource interfaces.

Thus in the following example, the customerPreferenceDao field first looks for a bean named customerPreferenceDao, then falls back to a primary type match for the type CustomerPreferenceDao. The "context" field is injected based on the known resolvable dependency type ApplicationContext.

public class MovieRecommender {

  @Resource
  private CustomerPreferenceDao customerPreferenceDao;

  @Resource
  private ApplicationContext context;

  public MovieRecommender() {
  }

  // ...
}

3.9.6 @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 initialization callbacks and destruction callbacks. Provided that the CommonAnnotationBeanPostProcessor is registered within the Spring ApplicationContext, a method carrying one of these annotations is invoked at the same point in the lifecycle as the corresponding Spring lifecycle interface 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 about the effects of combining various lifecycle mechanisms, see Section 3.6.1.4, “Combining lifecycle mechanisms”.

3.10 Classpath scanning and managed components

Most examples foo bar in this chapter use XML to specify the configuration metadata that produces each BeanDefinition within the Spring container. The previous section (Section 3.9, “Annotation-based container configuration”) demonstrates how to provide a lot of the configuration metadata through source-level annotations. Even in those examples, however, the "base" bean definitions are explicitly defined in the XML file, while the annotations only drive the dependency injection. This section describes an option for implicitly detecting the candidate components by scanning the classpath. Candidate components are classes that match against a filter criteria and have a corresponding bean definition registered with the container. This removes the need to use XML to perform bean registration, instead you can use annotations (for example @Component), AspectJ type expressions, or your own custom filter criteria to select which classes will have bean definitions registered with the container.

[Note]Note

Starting with Spring 3.0, many features provided by the Spring JavaConfig project are part of 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 examples of how to use these new features.

3.10.1 @Component and further stereotype annotations

In Spring 2.0 and later, the @Repository annotation is a marker for any class that fulfills the role or stereotype (also known as Data Access Object or DAO) of a repository. Among the uses of this marker is the automatic translation of exceptions as described in Section 13.2.2, “Exception translation”.

Spring 2.5 introduces further stereotype annotations: @Component, @Service, and @Controller. @Component is a generic stereotype for any Spring-managed component. @Repository, @Service, and @Controller are specializations of @Component for more specific use cases, for example, in the persistence, service, and presentation layers, respectively. Therefore, 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. It is also possible that @Repository, @Service, and @Controller may carry additional semantics in future releases of the Spring Framework. Thus, if you are choosing 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.

3.10.2 Automatically detecting classes and registering bean definitions

Spring can automatically detect stereotyped classes and register 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, you need to include the following element in XML, where the base-package element is a common parent package for the two classes. (Alternatively, you can specify a comma-separated list that includes 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

The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, make sure that you do not activate the files-only switch of the JAR task.

Furthermore, the AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor are both included implicitly when you use 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

You can disable the registration of AutowiredAnnotationBeanPostProcessor and CommonAnnotationBeanPostProcessor by including the annotation-config attribute with a value of false.

3.10.3 Using filters to customize scanning

By default, classes annotated with @Component, @Repository, @Service, @Controller, or a custom annotation that itself is annotated with @Component are the only detected candidate components. However, you can modify and extend this behavior simply by applying custom filters. Add them as include-filter or exclude-filter sub-elements of the component-scan element. Each filter element requires the type and expression attributes. The following table describes the filtering options.

Table 3.5. 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.MyTypeFilterA custom implementation of the org.springframework.core.type .TypeFilter interface.

The following example shows the XML configuration 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

You can also 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.

3.10.4 Defining bean metadata within components

Spring components can also contribute bean definition metadata to the container. You do this 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 that 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 other bean definition properties, such as a qualifier value through 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 previously discussed, with additional support for autowiring of @Bean methods:

@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(BeanDefinition.SCOPE_SINGLETON)
  private TestBean privateInstance() {
      return new TestBean("privateInstance", i++);
  }

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

The example autowires the String method parameter country to the value of the Age property on another bean named privateInstance. A Spring Expression Language element defines the value of the property through 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. Methods are not invoked with normal Java semantics. In contrast, calling a method or field within a @Component classes @Bean method has standard Java semantics.

3.10.5 Naming autodetected components

When a component is autodetected as part of the scanning process, its bean name is 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.

[Note]Note

JSR 330's @Named annotation can be used as a means to both detect components and to provide them with a name. This behavior is enabled automatically if you have the JSR 330 JAR on the classpath.

If such an annotation contains no name value or for any other detected component (such as those discovered by custom filters), the default bean name generator returns 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 do not want to rely on the default bean-naming strategy, you can 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.

3.10.6 Providing a scope for autodetected components

As with Spring-managed components in general, the default and most common scope for autodetected components is singleton. However, sometimes you need other scopes, which Spring 2.5 provides with a new @Scope annotation. Simply provide the name of the scope within the annotation:

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

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 Section 3.5.4.5, “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>

3.10.7 Providing qualifier metadata with annotations

The @Qualifier annotation is discussed in Section 3.9.3, “Fine-tuning annotation-based autowiring with qualifiers”. The examples in that section demonstrate the use of the @Qualifier annotation and custom qualifier annotations to provide fine-grained control when you resolve autowire candidates. Because 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, you provide the qualifier metadata 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 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, because that metadata is provided per-instance rather than per-class.

3.11 Java-based container configuration

3.11.1 Basic concepts: @Configuration and @Bean

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 to be managed by the Spring IoC container.

Annotating a class with the @Configuration indicates that the class can 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 {
  @Bean
  public MyService myService() {
      return new MyServiceImpl();
  }
}

For those more familiar with Spring <beans/> XML, the AppConfig class above would be equivalent to:

<beans>
  <bean id="myService" class="com.acme.services.MyServiceImpl"/>
</beans>

As you can see, the @Bean annotation plays the same role as the <bean/> element. The @Bean annotation will be discussed in depth in the sections below. First, however, we'll cover the various ways of creating a spring container using Java-based configuration.

3.11.2 Instantiating the Spring container using AnnotationConfigApplicationContext

The sections below document Spring's AnnotationConfigApplicationContext, new in Spring 3.0. This versatile ApplicationContext implementation is capable of accepting not only @Configuration classes as input, but also plain @Component classes and classes annotated with JSR-330 metadata.

When @Configuration classes are provided as input, the @Configuration class itself is registered as a bean definition, and all declared @Bean methods within the class are also registered as bean definitions.

When @Component and JSR-330 classes are provided, they are registered as bean definitions, and it is assumed that DI metadata such as @Autowired or @Inject are used within those classes where necessary.

3.11.2.1 Simple construction

In much the same way that Spring XML files are used as input when instantiating a ClassPathXmlApplicationContext, @Configuration classes may be used as input when instantiating an AnnotationConfigApplicationContext. This allows for completely XML-free usage of the Spring container:

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
  MyService myService = ctx.getBean(MyService.class);
  myService.doStuff();
}

As mentioned above, AnnotationConfigApplicationContext is not limited to working only with @Configuration classes. Any @Component or JSR-330 annotated class may be supplied as input to the constructor. For example:

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class, Dependency1.class, Dependency2.class);
  MyService myService = ctx.getBean(MyService.class);
  myService.doStuff();
}

The above assumes that MyServiceImpl, Dependency1 and Dependency2 use Spring dependency injection annotations such as @Autowired.

3.11.2.2 Building the container programmatically using register(Class<?>...)

An AnnotationConfigApplicationContext may be instantiated using a no-arg constructor and then configured using the register() method. This approach is particularly useful when programmatically building an AnnotationConfigApplicationContext.

public static void main(String[] args) {
  AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
  ctx.register(AppConfig.class, OtherConfig.class);
  ctx.register(AdditionalConfig.class);
  ctx.refresh();
  MyService myService = ctx.getBean(MyService.class);
  myService.doStuff();
}

3.11.2.3 Enabling component scanning with scan(String...)

Experienced Spring users will be familiar with the following commonly-used XML declaration from Spring's context: namespace

<beans>
  <context:component-scan base-package="com.acme"/>
</beans>

In the example above, the com.acme package will be scanned, looking for any @Component-annotated classes, and those classes will be registered as Spring bean definitions within the container. AnnotationConfigApplicationContext exposes the scan(String...) method to allow for the same component-scanning functionality:

public static void main(String[] args) {
  AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
  ctx.scan("com.acme");
  ctx.refresh();
  MyService myService = ctx.getBean(MyService.class);
}
[Note]Note

Remember that @Configuration classes are meta-annotated with @Component, so they are candidates for component-scanning! In the example above, assuming that AppConfig is declared within the com.acme package (or any package underneath), it will be picked up during the call to scan(), and upon refresh() all its @Bean methods will be processed and registered as bean definitions within the container.

3.11.2.4 Support for web applications with AnnotationConfigWebApplicationContext

A WebApplicationContext variant of AnnotationConfigApplicationContext is available with AnnotationConfigWebApplicationContext. This implementation may be used when configuring the Spring ContextLoaderListener servlet listener, Spring MVC DispatcherServlet, etc. What follows is a web.xml snippet that configures a typical Spring MVC web application. Note the use of the contextClass context-param and init-param:

<web-app>
  <!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext
       instead of the default XmlWebApplicationContext -->
  <context-param>
      <param-name>contextClass</param-name>
      <param-value>
          org.springframework.web.context.support.AnnotationConfigWebApplicationContext
      </param-value>
  </context-param>

  <!-- Configuration locations must consist of one or more comma- or space-delimited
       fully-qualified @Configuration classes. Fully-qualified packages may also be
       specified for component-scanning -->
  <context-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>com.acme.AppConfig</param-value>
  </context-param>

  <!-- Bootstrap the root application context as usual using ContextLoaderListener -->
  <listener>
      <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>

  <!-- Declare a Spring MVC DispatcherServlet as usual -->
  <servlet>
      <servlet-name>dispatcher</servlet-name>
      <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
      <!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext
           instead of the default XmlWebApplicationContext -->
      <init-param>
          <param-name>contextClass</param-name>
          <param-value>
              org.springframework.web.context.support.AnnotationConfigWebApplicationContext
          </param-value>
      </init-param>
      <!-- Again, config locations must consist of one or more comma- or space-delimited
           and fully-qualified @Configuration classes -->
      <init-param>
          <param-name>contextConfigLocation</param-name>
          <param-value>com.acme.web.MvcConfig</param-value>
      </init-param>
  </servlet>

  <!-- map all requests for /main/* to the dispatcher servlet -->
  <servlet-mapping>
      <servlet-name>dispatcher</servlet-name>
      <url-pattern>/main/*</url-pattern>
  </servlet-mapping>
</web-app>

3.11.3 Composing Java-based configurations

3.11.3.1 Using the @Import annotation

Much as the <import/> element is used within Spring XML files to aid in modularizing configurations, the @Import annotation allows for loading @Bean definitions from another configuration class:

@Configuration
public class ConfigA {
  public @Bean A a() { return new A(); }
}

@Configuration
@Import(ConfigA.class)
public class ConfigB {
  public @Bean B b() { return new B(); }
}

Now, rather than needing to specify both ConfigA.class and ConfigB.class when instantiating the context, only ConfigB needs to be supplied explicitly:

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class);

  // now both beans A and B will be available...
  A a = ctx.getBean(A.class);
  B b = ctx.getBean(B.class);
}

This approach simplifies container instantiation, as only one class needs to be dealt with, rather than requiring the developer to remember a potentially large number of @Configuration classes during construction.

Injecting dependencies on imported @Bean definitions

The example above works, but is simplistic. In most practical scenarios, beans will have dependencies on one another across configuration classes. When using XML, this is not an issue, per se, because there is no compiler involved, and one can simply declare ref="someBean" and trust that Spring will work it out during container initialization. Of course, when using @Configuration classes, the Java compiler places constraints on the configuration model, in that references to other beans must be valid Java syntax.

Fortunately, solving this problem is simple. Remember that @Configuration classes are ultimately just another bean in the container - this means that they can take advantage of @Autowired injection metadata just like any other bean!

Let's consider a more real-world scenario with several @Configuration classes, each depending on beans declared in the others:

@Configuration
public class ServiceConfig {
  private @Autowired AccountRepository accountRepository;

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

@Configuration
public class RepositoryConfig {
  private @Autowired DataSource dataSource;

  public @Bean AccountRepository accountRepository() {
      return new JdbcAccountRepository(dataSource);
  }
}

@Configuration
@Import({ServiceConfig.class, RepositoryConfig.class})
public class SystemTestConfig {
  public @Bean DataSource dataSource() { /* return new DataSource */ }
}

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
  // everything wires up across configuration classes...
  TransferService transferService = ctx.getBean(TransferService.class);
  transferService.transfer(100.00, "A123", "C456");
}
Fully-qualifying imported beans for ease of navigation

In the scenario above, using @Autowired works well and provides the desired modularity, but determining exactly where the autowired bean definitions are declared is still somewhat ambiguous. For example, as a developer looking at ServiceConfig, how do you know exactly where the @Autowired AccountRepository bean is declared? It's not explicit in the code, and this may be just fine. Remember that the SpringSource Tool Suite provides tooling that can render graphs showing how everything is wired up - that may be all you need. Also, your Java IDE can easily find all declarations and uses of the AccountRepository type, and will quickly show you the location of @Bean methods that return that type.

In cases where this ambiguity is not acceptable and you wish to have direct navigation from within your IDE from one @Configuration class to another, consider autowiring the configuration classes themselves:

@Configuration
public class ServiceConfig {
  private @Autowired RepositoryConfig repositoryConfig;

  public @Bean TransferService transferService() {
      // navigate 'through' the config class to the @Bean method!
      return new TransferServiceImpl(repositoryConfig.accountRepository());
  }
}

In the situation above, it is completely explicit where AccountRepository is defined. However, ServiceConfig is now tightly coupled to RepositoryConfig; that's the tradeoff. This tight coupling can be somewhat mitigated by using interface-based or abstract class-based @Configuration classes. Consider the following:

@Configuration
public class ServiceConfig {
  private @Autowired RepositoryConfig repositoryConfig;

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

@Configuration
public interface RepositoryConfig {
  @Bean AccountRepository accountRepository();
}

@Configuration
public class DefaultRepositoryConfig implements RepositoryConfig {
  public @Bean AccountRepository accountRepository() {
      return new JdbcAccountRepository(...);
  }
}

@Configuration
@Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config!
public class SystemTestConfig {
  public @Bean DataSource dataSource() { /* return DataSource */ }
}

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class);
  TransferService transferService = ctx.getBean(TransferService.class);
  transferService.transfer(100.00, "A123", "C456");
}

Now ServiceConfig is loosely coupled with respect to the concrete DefaultRepositoryConfig, and built-in IDE tooling is still useful: it will be easy for the developer to get a type hierarchy of RepositoryConfig implementations. In this way, navigating @Configuration classes and their dependencies becomes no different than the usual process of navigating interface-based code.

3.11.3.2 Combining Java and XML configuration

Spring's @Configuration class support does not aim to be a 100% complete replacement for Spring XML. Some facilities such as Spring XML namespaces remain an ideal way to configure the container. In cases where XML is convenient or necessary, you have a choice: either instantiate the container in an "XML-centric" way using, for example, ClassPathXmlApplicationContext, or in a "Java-centric" fashion using AnnotationConfigApplicationContext and the @ImportResource annotation to import XML as needed.

XML-centric use of @Configuration classes

It may be preferable to bootstrap the Spring container from XML and include @Configuration classes in an ad-hoc fashion. For example, in a large existing codebase that uses Spring XML, it will be easier to create @Configuration classes on an as-needed basis and include them from the existing XML files. Below you'll find the options for using @Configuration classes in this kind of "XML-centric" situation.

Declaring @Configuration classes as plain Spring <bean/> elements

Remember that @Configuration classes are ultimately just bean definitions in the container. In this example, we create a @Configuration class named AppConfig and include it within system-test-config.xml as a <bean/>definition. Because <context:annotation-config/> is switched on, the container will recognize the @Configuration annotation, and process the @Bean methods declared in AppConfig properly.

@Configuration
public class AppConfig {
  private @Autowired DataSource dataSource;

  public @Bean AccountRepository accountRepository() {
      return new JdbcAccountRepository(dataSource);
  }

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

system-test-config.xml
<beans>
  <!-- enable processing of annotations such as @Autowired and @Configuration -->
  <context:annotation-config/>
  <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

  <bean class="com.acme.AppConfig"/>

  <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
      <property name="url" value="${jdbc.url}"/>
      <property name="username" value="${jdbc.username}"/>
      <property name="password" value="${jdbc.password}"/>
  </bean>
</beans>

jdbc.properties
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=

public static void main(String[] args) {
  ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml");
  TransferService transferService = ctx.getBean(TransferService.class);
  // ...
}
[Note]Note

In system-test-config.xml above, the AppConfig<bean/> does not declare an id element. While it would be acceptable to do so, it is unnecessary given that no other bean will ever refer to it, and it is unlikely that it will be explicitly fetched from the container by name. Likewise with the DataSource bean - it is only ever autowired by type, so an explicit bean id is not strictly required.

Using <context:component-scan/> to pick up @Configuration classes

Because @Configuration is meta-annotated with @Component, @Configuration-annotated classes are automatically candidates for component scanning. Using the same scenario as above, we can redefine system-test-config.xml to take advantage of component-scanning. Note that in this case, we don't need to explicitly declare <context:annotation-config/>, because <context:component-scan/> enables all the same functionality.

system-test-config.xml
<beans>
  <!-- picks up and registers AppConfig as a bean definition -->
  <context:component-scan base-package="com.acme"/>
  <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>

  <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource">
      <property name="url" value="${jdbc.url}"/>
      <property name="username" value="${jdbc.username}"/>
      <property name="password" value="${jdbc.password}"/>
  </bean>
</beans>
@Configuration class-centric use of XML with @ImportResource

In applications where @Configuration classes are the primary mechanism for configuring the container, it will still likely be necessary to use at least some XML. In these scenarios, simply use @ImportResource and define only as much XML as is needed. Doing so achieves a "Java-centric" approach to configuring the container and keeps XML to a bare minimum.

@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
public class AppConfig {
  private @Value("${jdbc.url}") String url;
  private @Value("${jdbc.username}") String username;
  private @Value("${jdbc.password}") String password;

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

properties-config.xml
<beans>
  <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/>
</beans>

jdbc.properties
jdbc.url=jdbc:hsqldb:hsql://localhost/xdb
jdbc.username=sa
jdbc.password=

public static void main(String[] args) {
  ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class);
  TransferService transferService = ctx.getBean(TransferService.class);
  // ...
}

3.11.4 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 @Configuration-annotated or in a @Component-annotated class.

3.11.4.1 Declaring a bean

To declare a bean, simply annotate a method with the @Bean annotation. You use this method to register a bean definition within an ApplicationContext of the type specified as the method's return value. By default, the bean name will be the same as the method name. The following is a simple example of a @Bean method declaration:

@Configuration
public class AppConfig {

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

}

The preceding configuration is exactly equivalent to the following Spring XML:

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

Both declarations make a bean named transferService available in the ApplicationContext, bound to an object instance of type TransferServiceImpl:

transferService -> com.acme.TransferServiceImpl
              

3.11.4.2 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 receives a reference to bar via constructor injection.

3.11.4.3 Receiving lifecycle callbacks

Beans declared in a @Configuration-annotated class support the regular lifecycle callbacks. Any classes defined with the @Bean annotation can use the @PostConstruct and @PreDestroy annotations from JSR-250, see 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 are called by the container.

The standard set of *Aware interfaces such as BeanFactoryAware, BeanNameAware, MessageSourceAware, ApplicationContextAware, and so on 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 on the bean element:

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

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

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

When you work directly in Java, you can do anything you like with your objects and do not always need to rely on the container lifecycle!

3.11.4.4 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 default scope is singleton, but you can override this with the @Scope annotation:

@Configuration
public class MyConfiguration {
  @Bean
  @Scope("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 you port the scoped proxy example from the XML reference documentation (see preceding link) to our @Bean using Java, it would look like the following:

// an HTTP Session-scoped bean exposed as a proxy
@Bean
@Scope(value = "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.setUserPreferences(userPreferences());
 return service;
}                
Lookup method injection

As noted earlier, lookup method injection is an advanced feature that you should use rarely. 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-configuration support , you can create a subclass of CommandManager where the abstract createCommand() method is overridden in such a way that it looks up a new (prototype) command object:

@Bean
@Scope("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 createCommand() {
          return asyncCommand();
      }
  }
}                    

3.11.4.5 Customizing bean naming

By default, configuration classes use a @Bean method's name as the name of the resulting bean. This functionality can be overridden, however, with the name attribute.

@Configuration
public class AppConfig {

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

}        

3.11.4.6 Bean aliasing

As discussed in Section 3.3.1, “Naming beans”, it is sometimes desirable to give a single bean multiple names, otherwise known as bean aliasing. The name attribute of the @Bean annotation accepts a String array for this purpose.

@Configuration
public class AppConfig {

  @Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" })
  public DataSource dataSource() {
      // instantiate, configure and return DataSource bean...
  }

}        

3.11.5 Further information about how Java-based configuration works internally

The following example shows a @Bean annotated method being called twice:

@Configuration
public class AppConfig {

  @Bean
  public ClientService clientService1() {
    ClientServiceImpl clientService = new ClientServiceImpl();
    clientService.setClientDao(clientDao());
    return clientService;
  }
  @Bean
  public ClientService clientService2() {
    ClientServiceImpl clientService = new ClientServiceImpl();
    clientService.setClientDao(clientDao());
    return clientService;
  }

  @Bean
  public ClientDao clientDao() {
    return new ClientDaoImpl();
  }
}
    

clientDao() has been called once in clientService1() and once in clientService2(). Since this method creates a new instance of ClientDaoImpl and returns it, you would normally expect having 2 instances (one for each service). That definitely would be problematic: in Spring, instantiated beans have a singleton scope by default. This is where the magic comes in: All @Configuration classes are subclassed at startup-time with CGLIB. In the subclass, the child method checks the container first for any cached (scoped) beans before it calls the parent method and creates a new instance.

[Note]Note

The behavior could be different according to the scope of your bean. We are talking about singletons here.

[Note]Note

Beware that, in order for JavaConfig to work, you must include the CGLIB jar in your list of dependencies.

[Note]Note

There are a few restrictions due to the fact that CGLIB dynamically adds features at startup-time:

  • Configuration classes should not be final

  • They should have a constructor with no arguments

3.12 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 7.8.4, “Load-time weaving with AspectJ in the Spring Framework”.

3.13 Additional Capabilities of the ApplicationContext

As was discussed in the chapter introduction, the org.springframework.beans.factory package provides basic functionality for managing and manipulating beans, including in a programmatic way. The org.springframework.context package adds the ApplicationContext interface, which extends the BeanFactory interface, in addition to extending other interfaces to provide additional functionality in a more application framework-oriented style. Many people use the ApplicationContext in a completely declarative fashion, not even creating it programmatically, 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 application.

To enhance BeanFactory functionality in a more framework-oriented style the context package also provides the following functionality:

  • Access to messages in i18n-style, through the MessageSource interface.

  • Access to resources, such as URLs and files, through the ResourceLoader interface.

  • Event publication to beans implementing the ApplicationListener interface, through the use of the ApplicationEventPublisher interface.

  • Loading of multiple (hierarchical) contexts, allowing each to be focused on one particular layer, such as the web layer of an application, through the HierarchicalBeanFactory interface.

3.13.1 Internationalization using MessageSource

The ApplicationContext interface extends an interface called MessageSource, and therefore provides internationalization (i18n) functionality. Spring also provides the interface HierarchicalMessageSource, which can resolve messages hierarchically. Together these interfaces provide the foundation upon which Spring effects message resolution. The methods defined on these interfaces include:

  • 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 become 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 preceding methods are also wrapped in a class named MessageSourceResolvable, which you can use with this method.

When an ApplicationContext is loaded, it automatically searches for a MessageSource bean defined in the context. The bean must have the name messageSource. If such a bean is found, all calls to the preceding methods are delegated to the message source. If no message source is found, the ApplicationContext attempts to find a parent containing a bean with the same name. If it does, it uses that bean as the MessageSource. If the ApplicationContext cannot find any source for messages, an empty DelegatingMessageSource is instantiated in order to be able to accept calls to the methods defined above.

Spring provides two MessageSource implementations, ResourceBundleMessageSource and StaticMessageSource. Both implement HierarchicalMessageSource in order to do nested messaging. The StaticMessageSource is rarely used but provides programmatic ways to add messages to the source. The ResourceBundleMessageSource is shown in the following example:

<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>

In the example it is assumed you have three resource bundles defined in your classpath called format, exceptions and windows. Any request to resolve a message will be handled in the JDK standard way of resolving messages through ResourceBundles. For the purposes of the example, 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.

A program to execute the MessageSource functionality is shown in the next example. 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, which exists at the root of your classpath. The messageSource bean definition refers to a number of resource bundles through 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.

The next example shows arguments passed to the message lookup; these arguments will be converted into Strings and inserted into placeholders in the lookup message.

<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>

  <!-- lets 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.

Typically, locale resolution is managed by the surrounding environment of the application. In this example, the locale against which (British) messages will be resolved is specified manually.

# 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.

You can also use the MessageSourceAware interface to acquire a reference to any MessageSource that has been defined. Any bean that is defined in an ApplicationContext that implements the MessageSourceAware interface is injected with the application context's MessageSource when the bean is created and configured.

[Note]Note

As an alternative to ResourceBundleMessageSource, Spring 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.

3.13.2 Standard and Custom Events

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

Table 3.6. Built-in Events

EventExplanation
ContextRefreshedEventPublished when the ApplicationContext is initialized or refreshed, for example, 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. As long as the context has not been closed, a refresh can be triggered multiple times, provided that the chosen ApplicationContext actually supports such "hot" refreshes. For example, XmlWebApplicationContext supports hot refreshes, but GenericApplicationContext does not.
ContextStartedEventPublished when the ApplicationContext is started, using the start() method on the ConfigurableApplicationContext interface. "Started" here means that all Lifecycle beans receive an explicit start signal. Typically this signal is used to restart beans after an explicit stop, but it may also be used to start components that have not been configured for autostart , for example, components that have not already started on initialization.
ContextStoppedEventPublished when the ApplicationContext is stopped, using the stop() method on the ConfigurableApplicationContext interface. "Stopped" here means that all Lifecycle beans 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 reaches 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 event is published after the request is complete. This event is only applicable to web applications using Spring's DispatcherServlet.

You can also create and publish your own custom events. This example demonstrates a simple class that extends Spring's ApplicationEvent base class:

public class BlackListEvent extends ApplicationEvent {
  private final String address;
  private final String test;

  public BlackListEvent(Object source, String address, String test) {
      super(source);
      this.address = address;
      this.test = test;
  }

  // accessor and other methods...
}

To publish a custom ApplicationEvent, call the publishEvent() method on an ApplicationEventPublisher. Typically this is done by creating a class that implements ApplicationEventPublisherAware and registering it as a Spring bean. The following example demonstrates such a class:

public class EmailService implements ApplicationEventPublisherAware {

  private List<String> blackList;
  private ApplicationEventPublisher publisher;

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

  public void setApplicationEventPublisher(ApplicationEventPublisher publisher) {
      this.publisher = publisher;
  }

  public void sendEmail(String address, String text) {
      if (blackList.contains(address)) {
          BlackListEvent event = new BlackListEvent(this, address, text);
          publisher.publishEvent(event);
          return;
      }
      // send email...
  }
}

At configuration time, the Spring container will detect that EmailService implements ApplicationEventPublisherAware and will automatically call setApplicationEventPublisher(). In reality, the parameter passed in will be the Spring container itself; you're simply interacting with the application context via its ApplicationEventPublisher interface.

To receive the custom ApplicationEvent, create a class that implements ApplicationListener and register it as a Spring bean. The following example demonstrates such a class:

public class BlackListNotifier implements ApplicationListener<BlackListEvent> {

  private String notificationAddress;

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

  public void onApplicationEvent(BlackListEvent event) {
        // notify appropriate parties via notificationAddress...
  }
}

Notice that ApplicationListener is generically parameterized with the type of your custom event, BlackListEvent. This means that the onApplicationEvent() method can remain type-safe, avoiding any need for downcasting. You may register as many event listeners as you wish, but note that by default event listeners receive events synchronously. This means the publishEvent() method blocks until all listeners have finished processing the event. One advantage of this synchronous and single-threaded approach is that when a listener receives an event, it operates inside the transaction context of the publisher if a transaction context is available. If another strategy for event publication becomes necessary, refer to the JavaDoc for Spring's ApplicationEventMulticaster interface.

The following example demonstrates the bean definitions used to register and configure each of the classes above:

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

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

Putting it all together, when the sendEmail() method of the emailService bean is called, if there are any emails that should be blacklisted, a custom event of type BlackListEvent is published. The blackListNotifier bean is registered as an ApplicationListener and thus receives the BlackListEvent, at which point it can notify appropriate parties.

[Note]Note

Spring's eventing mechanism is designed for simple communication between Spring beans within the same application context. However, for more sophisticated enterprise integration needs, the separately-maintained Spring Integration project provides complete support for building lightweight, pattern-oriented, event-driven architectures that build upon the well-known Spring programming model.

3.13.3 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 Chapter 4, Resources.

An application context is a ResourceLoader, which can be used to load Resources. A Resource is essentially a more feature rich version of the JDK class java.net.URL, in fact, the implementations of the Resource wrap an instance of java.net.URL where appropriate. A Resource can 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.

You can configure a bean deployed into the application context to implement the special callback interface, ResourceLoaderAware, to be automatically called back at initialization time with the application context itself passed in as the ResourceLoader. You can also expose properties of type Resource, to be used to access static resources; they will be injected into it like any other properties. You can 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 when the bean is deployed.

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. You can also use location paths (resource strings) with special prefixes to force loading of definitions from the classpath or a URL, regardless of the actual context type.

3.13.4 Convenient ApplicationContext instantiation for web applications

You can create ApplicationContext instances declaratively by using, for example, a ContextLoader. Of course you can also create ApplicationContext instances programmatically by using one of the ApplicationContext implementations.

The ContextLoader mechanism comes in two flavors: the ContextLoaderListener and the ContextLoaderServlet. They have the same functionality but differ in that the listener version is not reliable in Servlet 2.3 containers. In the Servlet 2.4 specification, Servlet context listeners must execute immediately after the Servlet context for the web application is 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. 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 uses /WEB-INF/applicationContext.xml as a default. When the parameter does exist, the listener separates the String by using predefined delimiters (comma, semicolon and whitespace) and uses the values as locations where application contexts will be searched. Ant-style path patterns are supported as well. Examples are /WEB-INF/*Context.xml for all files with names ending with "Context.xml", residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml, for all such files in any subdirectory of "WEB-INF".

You can use ContextLoaderServlet instead of ContextLoaderListener. The Servlet uses the contextConfigLocation parameter just as the listener does.

3.13.5 Deploying a Spring ApplicationContext as a J2EE RAR file

In Spring 2.5 and later, 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 servers facilities. RAR deployment is a more natural alternative to scenario of deploying a headless WAR file, in effect, a WAR file without any HTTP entry points that is used only for bootstrapping a Spring ApplicationContext in a J2EE environment.

RAR deployment is ideal for application contexts that do not need HTTP entry points but rather consist only of message endpoints and scheduled jobs. Beans in such a context can 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 can also interact with the application server's JCA WorkManager through Spring's TaskExecutor abstraction.

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

For a simple deployment of a Spring ApplicationContext as a J2EE RAR file: package all application classes into a RAR file, which is 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 SpringContextResourceAdapters JavaDoc) and 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]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 occurs 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 needs to allow 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.

3.14 The BeanFactory

The BeanFactory provides the underlying basis for Spring's IoC functionality but it is only used directly in integration with other third-party frameworks and is now largely historical in nature for most users of Spring. The BeanFactory and related interfaces, such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in Spring for the purposes of backward compatibility with the large number of third-party frameworks that integrate with Spring. Often third-party components that can not use more modern equivalents such as @PostConstruct or @PreDestroy in order to remain compatible with JDK 1.4 or to avoid a dependency on JSR-250.

This section provides additional background into the differences between the BeanFactory and ApplicationContext and how one might access the IoC container directly through a classic singleton lookup.

3.14.1 BeanFactory or ApplicationContext?

Use an ApplicationContext unless you have a good reason for not doing so.

Because the ApplicationContext includes all functionality of the BeanFactory, it is generally recommended over the BeanFactory, except for a few 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. Spring 2.0 and later makes heavy use of the BeanPostProcessor extension point (to effect proxying and so on). If you use only a plain BeanFactory, a fair amount of support such as transactions and AOP will not take effect, at least not without some extra steps on your part. This situation could be confusing because nothing is actually wrong with the configuration.

The following table lists features provided by the BeanFactory and ApplicationContext interfaces and implementations.

Table 3.7. 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


To explicitly register a bean post-processor with a BeanFactory implementation, you must write 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

To explicitly register a BeanFactoryPostProcessor when using a BeanFactory implementation, you must write code like 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);

In both cases, the explicit registration step is inconvenient, which is one reason why the various ApplicationContext implementations are preferred above plain BeanFactory implementations in the vast majority of Spring-backed applications, especially when using BeanFactoryPostProcessors and BeanPostProcessors. These mechanisms implement important functionality such as property placeholder replacement and AOP.

3.14.2 Glue code and the evil singleton

It is best to write most application code in a dependency-injection (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, you sometimes need a 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 get these objects out of a Spring IoC container. If the object constructed by the third-party code is 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, retrieved from 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.

Looking up the application context in a service locator style is sometimes the only option for accessing shared Spring-managed components, such as in an EJB 2.1 environment, or when you want to share a single ApplicationContext as a parent to WebApplicationContexts across WAR files. In this case you should look into using the utility class ContextSingletonBeanFactoryLocator locator that is described in this SpringSource team blog entry.

4. Resources

4.1 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.

4.2 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.

4.3 Built-in Resource implementations

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

4.3.1 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.

4.3.2 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.

4.3.3 FileSystemResource

This is a Resource implementation for java.io.File handles. It obviously supports resolution as a File, and as a URL.

4.3.4 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.

4.3.5 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.

4.3.6 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.

4.4 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 4.1. 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.


4.5 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 Section 3.4.5, “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 Section 3.9.2, “@Autowired and @Inject”.

4.6 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"/>

4.7 Application contexts and Resource paths

4.7.1 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.

4.7.1.1 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.

4.7.2 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.

4.7.2.1 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.

4.7.2.2 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.

4.7.2.3 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.

4.7.3 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");

5. Validation, Data Binding, and Type Conversion

5.1 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 ands 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 have the need to use the BeanWrapper directly. Because this is reference documentation however, we felt that some explanation might be in order. We will explain the BeanWrapper in this chapter since, if you were going to use it at all, you would most likely do so when trying to bind data to objects.

Spring's DataBinder and the lower-level BeanWrapper both use PropertyEditors to parse and format property values. The PropertyEditor concept is part of the JavaBeans specification, and is also explained in this chapter. Spring 3 introduces a "core.convert" package that provides a general type conversion facility, as well as a higher-level "format" package for formatting UI field values. These new packages may be used as simpler alternatives to PropertyEditors, and will also be discussed in this chapter.

5.2 Validation using Spring's Validator interface

Spring 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 resorting to copy-and-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.

5.3 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.

5.4 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.

5.4.1 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 5.1. 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");

5.4.2 Built-in PropertyEditor implementations

Spring 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 5.2. 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());
        }
    }
}

5.4.2.1 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 {

    public void setAsText(String text) {
        setValue(new ExoticType(text.toUpperCase()));
    }
}

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" value="example.ExoticTypeEditor"/>
    </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.

5.5 Spring 3 Type Conversion

Spring 3 introduces a core.convert package that provides a general type conversion system. The system defines an SPI to implement type conversion logic, as well as an API to execute type conversions at runtime. Within a Spring container, this system can be used as an alternative to PropertyEditors to convert externalized bean property value strings to required property types. The public API may also be used anywhere in your application where type conversion is needed.

5.5.1 Converter SPI

The SPI to implement type conversion logic is simple and strongly typed:

package org.springframework.core.convert.converter;

public interface Converter<S, T> {

    T convert(S source);

}

To create your own Converter, simply implement the interface above. Parameterize S as the type you are converting from, and T as the type you are converting to. For each call to convert(S), the source argument is guaranteed to be NOT null. Your Converter may throw any Exception if conversion fails. An IllegalArgumentException should be thrown to report an invalid source value. Take care to ensure your Converter implementation is thread-safe.

Several converter implementations are provided in the core.convert.support package as a convenience. These include converters from Strings to Numbers and other common types. Consider StringToInteger as an example Converter implementation:

package org.springframework.core.convert.support;

final class StringToInteger implements Converter<String, Integer> {

    public Integer convert(String source) {
        return Integer.valueOf(source);
    }

}

5.5.2 ConverterFactory

When you need to centralize the conversion logic for an entire class hierarchy, for example, when converting from String to java.lang.Enum objects, implement ConverterFactory:

package org.springframework.core.convert.converter;

public interface ConverterFactory<S, R> {

    <T extends R> Converter<S, T> getConverter(Class<T> targetType);

}

Parameterize S to be the type you are converting from and R to be the base type defining the range of classes you can convert to. Then implement getConverter(Class<T>), where T is a subclass of R.

Consider the StringToEnum ConverterFactory as an example:

package org.springframework.core.convert.support;

final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {

    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new StringToEnumConverter(targetType);
    }

    private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {

        private Class<T> enumType;

        public StringToEnumConverter(Class<T> enumType) {
            this.enumType = enumType;
        }

        public T convert(String source) {
            return (T) Enum.valueOf(this.enumType, source.trim());
        }
    }
}

5.5.3 GenericConverter

When you require a sophisticated Converter implementation, consider the GenericConverter interface. With a more flexible but less strongly typed signature, a GenericConverter supports converting between multiple source and target types. In addition, a GenericConverter makes available source and target field context you can use when implementing your conversion logic. Such context allows a type conversion to be driven by a field annotation, or generic information declared on a field signature.

package org.springframework.core.convert.converter;

public interface GenericConverter {

    public Set<ConvertiblePair> getConvertibleTypes();

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

}

To implement a GenericConverter, have getConvertibleTypes() return the supported source->target type pairs. Then implement convert(Object, TypeDescriptor, TypeDescriptor) to implement your conversion logic. The source TypeDescriptor provides access to the source field holding the value being converted. The target TypeDescriptor provides access to the target field where the converted value will be set.

A good example of a GenericConverter is a converter that converts between a Java Array and a Collection. Such an ArrayToCollectionConverter introspects the field that declares the target Collection type to resolve the Collection's element type. This allows each element in the source array to be converted to the Collection element type before the Collection is set on the target field.

[Note]Note

Because GenericConverter is a more complex SPI interface, only use it when you need it. Favor Converter or ConverterFactory for basic type conversion needs.

5.5.3.1 ConditionalGenericConverter

Sometimes you only want a Converter to execute if a specific condition holds true. For example, you might only want to execute a Converter if a specific annotation is present on the target field. Or you might only want to execute a Converter if a specific method, such as static valueOf method, is defined on the target class. ConditionalGenericConverter is an subinterface of GenericConverter that allows you to define such custom matching criteria:

public interface ConditionalGenericConverter extends GenericConverter {

    boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);

}

A good example of a ConditionalGenericConverter is an EntityConverter that converts between an persistent entity identifier and an entity reference. Such a EntityConverter might only match if the target entity type declares a static finder method e.g. findAccount(Long). You would perform such a finder method check in the implementation of matches(TypeDescriptor, TypeDescriptor).

5.5.4 ConversionService API

The ConversionService defines a unified API for executing type conversion logic at runtime. Converters are often executed behind this facade interface:

package org.springframework.core.convert;

public interface ConversionService {

    boolean canConvert(Class<?> sourceType, Class<?> targetType);

    <T> T convert(Object source, Class<T> targetType);

    boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);

    Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

}

Most ConversionService implementations also implement ConverterRegistry, which provides an SPI for registering converters. Internally, a ConversionService implementation delegates to its registered converters to carry out type conversion logic.

A robust ConversionService implementation is provided in the core.convert.support package. GenericConversionService is the general-purpose implementation suitable for use in most environments. ConversionServiceFactory provides a convenient factory for creating common ConversionService configurations.

5.5.5 Configuring a ConversionService

A ConversionService is a stateless object designed to be instantiated at application startup, then shared between multiple threads. In a Spring application, you typically configure a ConversionService instance per Spring container (or ApplicationContext). That ConversionService will be picked up by Spring and then used whenever a type conversion needs to be performed by the framework. You may also inject this ConversionService into any of your beans and invoke it directly.

[Note]Note

If no ConversionService is registered with Spring, the original PropertyEditor-based system is used.

To register a default ConversionService with Spring, add the following bean definition with id conversionService:

<bean id="conversionService"
      class="org.springframework.context.support.ConversionServiceFactoryBean"/>
      

A default ConversionService can convert between strings, numbers, enums, collections, maps, and other common types. To suppliment or override the default converters with your own custom converter(s), set the converters property. Property values may implement either of the Converter, ConverterFactory, or GenericConverter interfaces.

<bean id="conversionService"
      class="org.springframework.context.support.ConversionServiceFactoryBean">
    <property name="converters">
        <list>
            <bean class="example.MyCustomConverter"/>
        </list>
    </property>
</bean>

It is also common to use a ConversionService within a Spring MVC application. See Section 5.6.4, “Configuring Formatting in Spring MVC” for details on use with <mvc:annotation-driven/>.

In certain situations you may wish to apply formatting during conversion. See Section 5.6.3, “FormatterRegistry SPI” for details on using FormattingConversionServiceFactoryBean.

5.5.6 Using a ConversionService programatically

To work with a ConversionService instance programatically, simply inject a reference to it like you would for any other bean:

@Service
public class MyService {

    @Autowired
    public MyService(ConversionService conversionService) {
        this.conversionService = conversionService;
    }

    public void doIt() {
        this.conversionService.convert(...)
    }
}

5.6 Spring 3 Field Formatting

As discussed in the previous section, core.convert is a general-purpose type conversion system. It provides a unified ConversionService API as well as a strongly-typed Converter SPI for implementing conversion logic from one type to another. A Spring Container uses this system to bind bean property values. In addition, both the Spring Expression Language (SpEL) and DataBinder use this system to bind field values. For example, when SpEL needs to coerce a Short to a Long to complete an expression.setValue(Object bean, Object value) attempt, the core.convert system performs the coercion.

Now consider the type conversion requirements of a typical client environment such as a web or desktop application. In such environments, you typically convert from String to support the client postback process, as well as back to String to support the view rendering process. In addition, you often need to localize String values. The more general core.convert Converter SPI does not address such formatting requirements directly. To directly address them, Spring 3 introduces a convenient Formatter SPI that provides a simple and robust alternative to PropertyEditors for client environments.

In general, use the Converter SPI when you need to implement general-purpose type conversion logic; for example, for converting between a java.util.Date and and java.lang.Long. Use the Formatter SPI when you're working in a client environment, such as a web application, and need to parse and print localized field values. The ConversionService provides a unified type conversion API for both SPIs.

5.6.1 Formatter SPI

The Formatter SPI to implement field formatting logic is simple and strongly typed:

package org.springframework.format;

public interface Formatter<T> extends Printer<T>, Parser<T> {
}

Where Formatter extends from the Printer and Parser building-block interfaces:

public interface Printer<T> {
    String print(T fieldValue, Locale locale);
}
import java.text.ParseException;

public interface Parser<T> {
    T parse(String clientValue, Locale locale) throws ParseException;
}

To create your own Formatter, simply implement the Formatter interface above. Parameterize T to be the type of object you wish to format, for example, java.util.Date. Implement the print() operation to print an instance of T for display in the client locale. Implement the parse() operation to parse an instance of T from the formatted representation returned from the client locale. Your Formatter should throw a ParseException or IllegalArgumentException if a parse attempt fails. Take care to ensure your Formatter implementation is thread-safe.

Several Formatter implementations are provided in format subpackages as a convenience. The number package provides a NumberFormatter, CurrencyFormatter, and PercentFormatter to format java.lang.Number objects using a java.text.NumberFormat. The datetime package provides a DateFormatter to format java.util.Date objects with a java.text.DateFormat. The datetime.joda package provides comprehensive datetime formatting support based on the Joda Time library.

Consider DateFormatter as an example Formatter implementation:

package org.springframework.format.datetime;

public final class DateFormatter implements Formatter<Date> {

    private String pattern;

    public DateFormatter(String pattern) {
        this.pattern = pattern;
    }

    public String print(Date date, Locale locale) {
        if (date == null) {
            return "";
        }
        return getDateFormat(locale).format(date);
    }

    public Date parse(String formatted, Locale locale) throws ParseException {
        if (formatted.length() == 0) {
            return null;
        }
        return getDateFormat(locale).parse(formatted);
    }

    protected DateFormat getDateFormat(Locale locale) {
        DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale);
        dateFormat.setLenient(false);
        return dateFormat;
    }

}

The Spring team welcomes community-driven Formatter contributions; see http://jira.springframework.org to contribute.

5.6.2 Annotation-driven Formatting

As you will see, field formatting can be configured by field type or annotation. To bind an Annotation to a formatter, implement AnnotationFormatterFactory:

package org.springframework.format;

public interface AnnotationFormatterFactory<A extends Annotation> {

    Set<Class<?>> getFieldTypes();

    Printer<?> getPrinter(A annotation, Class<?> fieldType);

    Parser<?> getParser(A annotation, Class<?> fieldType);

}

Parameterize A to be the field annotationType you wish to associate formatting logic with, for example org.springframework.format.annotation.DateTimeFormat. Have getFieldTypes() return the types of fields the annotation may be used on. Have getPrinter() return a Printer to print the value of an annotated field. Have getParser() return a Parser to parse a clientValue for an annotated field.

The example AnnotationFormatterFactory implementation below binds the @NumberFormat Annotation to a formatter. This annotation allows either a number style or pattern to be specified:

public final class NumberFormatAnnotationFormatterFactory
        implements AnnotationFormatterFactory<NumberFormat> {

    public Set<Class<?>> getFieldTypes() {
        return new HashSet<Class<?>>(asList(new Class<?>[] {
            Short.class, Integer.class, Long.class, Float.class,
            Double.class, BigDecimal.class, BigInteger.class }));
    }

    public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) {
        return configureFormatterFrom(annotation, fieldType);
    }

    private Formatter<Number> configureFormatterFrom(NumberFormat annotation,
                                                     Class<?> fieldType) {
        if (!annotation.pattern().isEmpty()) {
            return new NumberFormatter(annotation.pattern());
        } else {
            Style style = annotation.style();
            if (style == Style.PERCENT) {
                return new PercentFormatter();
            } else if (style == Style.CURRENCY) {
                return new CurrencyFormatter();
            } else {
                return new NumberFormatter();
            }
        }
    }
}

To trigger formatting, simply annotate fields with @NumberFormat:

public class MyModel {

    @NumberFormat(style=Style.CURRENCY)
    private BigDecimal decimal;

}

5.6.2.1 Format Annotation API

A portable format annotation API exists in the org.springframework.format.annotation package. Use @NumberFormat to format java.lang.Number fields. Use @DateTimeFormat to format java.util.Date, java.util.Calendar, java.util.Long, or Joda Time fields.

The example below uses @DateTimeFormat to format a java.util.Date as a ISO Date (yyyy-MM-dd):

public class MyModel {

    @DateTimeFormat(iso=ISO.DATE)
    private Date date;

}

5.6.3 FormatterRegistry SPI

At runtime, Formatters are registered in a FormatterRegistry. The FormatterRegistry SPI allows you to configure Formatting rules centrally, instead of duplicating such configuration across your Controllers. For example, you might want to enforce that all Date fields are formatted a certain way, or fields with a specific annotation are formatted in a certain way. With a shared FormatterRegistry, you define these rules once and they are applied whenever formatting is needed.

Review the FormatterRegistry SPI below:

package org.springframework.format;

public interface FormatterRegistry {

    void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser);

    void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter);

    void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory);

}

As shown above, Formatters can be registered by fieldType or annotation. FormattingConversionService is the implementation of FormatterRegistry suitable for most environments. This implementation may be configured programatically, or declaratively as a Spring bean using FormattingConversionServiceFactoryBean. Because this implemementation also implements ConversionService, it can be directly configured for use with Spring's DataBinder and the Spring Expression Language (SpEL).

5.6.4 Configuring Formatting in Spring MVC

In a Spring MVC application, you may configure a custom ConversionService instance explicity as an attribute of the annotation-driven element of the MVC namespace. This ConversionService will then be used anytime a type conversion is required during Controller model binding. If not configured explicitly, Spring MVC will automatically register default formatters and converters for common types such as numbers and dates.

To rely on default formatting rules, no custom configuration is required in your Spring MVC config XML:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    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-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven/>

</beans>

With this one-line of configuation, default formatters for Numbers and Date types will be installed, including support for the @NumberFormat and @DateTimeFormat annotations. Full support for the Joda Time formatting library is also installed if Joda Time is present on the classpath.

To inject a ConversionService instance with custom formatters and converters registered, set the conversion-service attribute:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    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-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven conversion-service="conversionService"/>

    <bean id="conversionService"
          class="org.springframework.format.support.FormattingConversionServiceFactoryBean"/>

</beans>

A custom ConversionService instance is often constructed by a FactoryBean that internally registers custom Formatters and Converters programatically before the ConversionService is returned. See FormattingConversionServiceFactoryBean for an example.

5.7 Spring 3 Validation

Spring 3 introduces several enhancements to its validation support. First, the JSR-303 Bean Validation API is now fully supported. Second, when used programatically, Spring's DataBinder can now validate objects as well as bind to them. Third, Spring MVC now has support for declaratively validating @Controller inputs.

5.7.1 Overview of the JSR-303 Bean Validation API

JSR-303 standardizes validation constraint declaration and metadata for the Java platform. Using this API, you annotate domain model properties with declarative validation constraints and the runtime enforces them. There are a number of built-in constraints you can take advantage of. You may also define your own custom constraints.

To illustrate, consider a simple PersonForm model with two properties:

public class PersonForm {
    private String name;
    private int age;
}

JSR-303 allows you to define declarative validation constraints against such properties:

public class PersonForm {

    @NotNull
    @Size(max=64)
    private String name;

    @Min(0)
    private int age;

}

When an instance of this class is validated by a JSR-303 Validator, these constraints will be enforced.

For general information on JSR-303, see the Bean Validation Specification. For information on the specific capabilities of the default reference implementation, see the Hibernate Validator documentation. To learn how to setup a JSR-303 implementation as a Spring bean, keep reading.

5.7.2 Configuring a Bean Validation Implementation

Spring provides full support for the JSR-303 Bean Validation API. This includes convenient support for bootstrapping a JSR-303 implementation as a Spring bean. This allows for a javax.validation.ValidatorFactory or javax.validation.Validator to be injected wherever validation is needed in your application.

Use the LocalValidatorFactoryBean to configure a default JSR-303 Validator as a Spring bean:

<bean id="validator"
      class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>

The basic configuration above will trigger JSR-303 to initialize using its default bootstrap mechanism. A JSR-303 provider, such as Hibernate Validator, is expected to be present in the classpath and will be detected automatically.

5.7.2.1 Injecting a Validator

LocalValidatorFactoryBean implements both javax.validation.ValidatorFactory and javax.validation.Validator, as well as Spring's org.springframework.validation.Validator. You may inject a reference to either of these interfaces into beans that need to invoke validation logic.

Inject a reference to javax.validation.Validator if you prefer to work with the JSR-303 API directly:

import javax.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;

Inject a reference to org.springframework.validation.Validator if your bean requires the Spring Validation API:

import org.springframework.validation.Validator;

@Service
public class MyService {

    @Autowired
    private Validator validator;

}

5.7.2.2 Configuring Custom Constraints

Each JSR-303 validation constraint consists of two parts. First, a @Constraint annotation that declares the constraint and its configurable properties. Second, an implementation of the javax.validation.ConstraintValidator interface that implements the constraint's behavior. To associate a declaration with an implementation, each @Constraint annotation references a corresponding ValidationConstraint implementation class. At runtime, a ConstraintValidatorFactory instantiates the referenced implementation when the constraint annotation is encountered in your domain model.

By default, the LocalValidatorFactoryBean configures a SpringConstraintValidatorFactory that uses Spring to create ConstraintValidator instances. This allows your custom ConstraintValidators to benefit from dependency injection like any other Spring bean.

Shown below is an example of a custom @Constraint declaration, followed by an associated ConstraintValidator implementation that uses Spring for dependency injection:

@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy=MyConstraintValidator.class)
public @interface MyConstraint {
}
import javax.validation.ConstraintValidator;

public class MyConstraintValidator implements ConstraintValidator {

    @Autowired;
    private Foo aDependency;

    ...
}

As you can see, a ConstraintValidator implementation may have its dependencies @Autowired like any other Spring bean.

5.7.2.3 Additional Configuration Options

The default LocalValidatorFactoryBean configuration should prove sufficient for most cases. There are a number of other configuration options for various JSR-303 constructs, from message interpolation to traversal resolution. See the JavaDocs of LocalValidatorFactoryBean for more information on these options.

5.7.3 Configuring a DataBinder

Since Spring 3, a DataBinder instance can be configured with a Validator. Once configured, the Validator may be invoked by calling binder.validate(). Any validation Errors are automatically added to the binder's BindingResult.

When working with the DataBinder programatically, this can be used to invoke validation logic after binding to a target object:

Foo target = new Foo();
DataBinder binder = new DataBinder(target);
binder.setValidator(new FooValidator());

// bind to the target object
binder.bind(propertyValues);

// validate the target object
binder.validate();

// get BindingResult that includes any validation errors
BindingResult results = binder.getBindingResult();

5.7.4 Spring MVC 3 Validation

Beginning with Spring 3, Spring MVC has the ability to automatically validate @Controller inputs. In previous versions it was up to the developer to manually invoke validation logic.

5.7.4.1 Triggering @Controller Input Validation

To trigger validation of a @Controller input, simply annotate the input argument as @Valid:

@Controller
public class MyController {

    @RequestMapping("/foo", method=RequestMethod.POST)
    public void processFoo(@Valid Foo foo) { /* ... */ }

Spring MVC will validate a @Valid object after binding so-long as an appropriate Validator has been configured.

[Note]Note

The @Valid annotation is part of the standard JSR-303 Bean Validation API, and is not a Spring-specific construct.

5.7.4.2 Configuring a Validator for use by Spring MVC

The Validator instance invoked when a @Valid method argument is encountered may be configured in two ways. First, you may call binder.setValidator(Validator) within a @Controller's @InitBinder callback. This allows you to configure a Validator instance per @Controller class:

@Controller
public class MyController {

    @InitBinder
    protected void initBinder(WebDataBinder binder) {
        binder.setValidator(new FooValidator());
    }

    @RequestMapping("/foo", method=RequestMethod.POST)
    public void processFoo(@Valid Foo foo) { ... }

}

Second, you may call setValidator(Validator) on the global WebBindingInitializer. This allows you to configure a Validator instance across all @Controllers. This can be achieved easily by using the Spring MVC namespace:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    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-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <mvc:annotation-driven validator="globalValidator"/>

</beans>

5.7.4.3 Configuring a JSR-303 Validator for use by Spring MVC

With JSR-303, a single javax.validation.Validator instance typically validates all model objects that declare validation constraints. To configure a JSR-303-backed Validator with Spring MVC, simply add a JSR-303 Provider, such as Hibernate Validator, to your classpath. Spring MVC will detect it and automatically enable JSR-303 support across all Controllers.

The Spring MVC configuration required to enable JSR-303 support is shown below:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    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-3.0.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

    <!-- JSR-303 support will be detected on classpath and enabled automatically -->
    <mvc:annotation-driven/>

</beans>

With this minimal configuration, anytime a @Valid @Controller input is encountered, it will be validated by the JSR-303 provider. JSR-303, in turn, will enforce any constraints declared against the input. Any ConstraintViolations will automatically be exposed as errors in the BindingResult renderable by standard Spring MVC form tags.

6. Spring Expression Language (SpEL)

6.1 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 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. That said, SpEL is based on a technology agnostic API allowing other expression language implementations to be integrated 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 were an independent expression language. This requires creating a few bootstrapping 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 syntax. 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.

6.2 Feature Overview

The expression language supports 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

  • Bean references

  • Array construction

  • Inline lists

  • Ternary operator

  • Variables

  • User defined functions

  • Collection projection

  • Collection selection

  • Templated expressions

6.3 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 SpelExpressionParser();
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 sub packages and spel.support.

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' respectively.

SpEL supports a wide range of features, such as 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 SpelExpressionParser();
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 SpelExpressionParser();

// invokes 'getBytes()'
Expression exp = parser.parseExpression("'Hello World'.bytes");  

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 SpelExpressionParser();

// invokes 'getBytes().length'
Expression exp = parser.parseExpression("'Hello World'.bytes.length");  

int length = (Integer) exp.getValue();

The String's constructor can be called instead of using a string literal.

ExpressionParser parser = new SpelExpressionParser();
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 cannot be cast to the type T or converted using the registered type converter.

The more common usage of SpEL is to provide an expression string that is evaluated against a specific object instance (called the root object). There are two options here and which to choose depends on whether the object against which the expression is being evaluated will be changing with each call to evaluate the expression. 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 nationality.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");

ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("name");
EvaluationContext context = new StandardEvaluationContext(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. This is the mechanism to use if the root object is unlikely to change, it can simply be set once in the evaluation context. If the root object is likely to change repeatedly, it can be supplied on each call to getValue, as this next example shows:

/ Create and set a calendar 
GregorianCalendar c = new GregorianCalendar();
c.set(1856, 7, 9);

//  The constructor arguments are name, birthday, and nationality.
Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian");

ExpressionParser parser = new SpelExpressionParser();
Expression exp = parser.parseExpression("name");

String name = (String) exp.getValue(tesla);
    

In this case the inventor tesla has been supplied directly to getValue and the expression evaluation infrastructure creates and manages a default evaluation context internally - it did not require one to be supplied.

The StandardEvaluationContext is relatively expensive to construct and during repeated usage it builds up cached state that enables subsequent expression evaluations to be performed more quickly. For this reason it is better to cache and reuse them where possible, rather than construct a new one for each expression evaluation.

In some cases it can be desirable to use a configured evaluation context and yet still supply a different root object on each call to getValue. getValue allows both to be specified on the same call. In these situations the root object passed on the call is considered to override any (which maybe null) specified on the evaluation context.

[Note]Note

In standalone usage of SpEL there is a need to create the parser, parse expressions and perhaps provide evaluation contexts and a root context object. 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 are all set up implicitly, requiring the user to specify nothing other than the expressions.

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

6.3.1 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 may specify the root object to evaluate against via the method setRootObject() or passing the root object into the constructor. 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.

6.3.1.1 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);
        

6.4 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> }.

6.4.1 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>

6.4.2 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;
  }

  // ...
}

6.5 Language Reference

6.5.1 Literal expressions

The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), boolean and null. Strings are delimited by single quotes. To put a single quote itself in a string use two single quote characters. 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 SpelExpressionParser();

// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue(); 

double avogadrosNumber  = (Double) parser.parseExpression("6.0221415E+23").getValue();  

// evals to 2147483647
int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue();  

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().

6.5.2 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 the 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.

// evals to 1856
int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); 


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 SpelExpressionParser();

// Inventions Array
StandardEvaluationContext teslaContext = new StandardEvaluationContext(tesla);

// evaluates to "Induction motor"
String invention = parser.parseExpression("inventions[3]").getValue(teslaContext, 
                                                                    String.class); 


// Members List
StandardEvaluationContext societyContext = new StandardEvaluationContext(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 literals.

// 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");

6.5.3 Inline lists

Lists can be expressed directly in an expression using {} notation.

// evaluates to a Java list containing the four numbers
List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context); 

List listOfLists = (List) parser.parseExpression("{{'a','b'},{'x','y'}}").getValue(context); 

{} by itself means an empty list. For performance reasons, if the list is itself entirely composed of fixed literals then a constant list is created to represent the expression, rather than building a new list on each evaluation.

6.5.4 Array construction

Arrays can be built using the familiar Java syntax, optionally supplying an initializer to have the array populated at construction time.

int[] numbers1 = (int[]) parser.parseExpression("new int[4]").getValue(context); 

// Array with initializer
int[] numbers2 = (int[]) parser.parseExpression("new int[]{1,2,3}").getValue(context); 

// Multi dimensional array
int[][] numbers3 = (int[][]) parser.parseExpression("new int[4][5]").getValue(context); 

It is not currently allowed to supply an initializer when constructing a multi-dimensional array.

6.5.5 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);

6.5.6 Operators

6.5.6.1 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);

Each symbolic operator can also be specified as a purely alphabetic equivalent. This avoids problems where the symbols used have special meaning for the document type in which the expression is embedded (eg. an XML document). The textual equivalents are shown here: lt ('<'), gt ('>'), le ('<='), ge ('>='), eq ('=='), ne ('!='), div ('/'), mod ('%'), not ('!'). These are case insensitive.

6.5.6.2 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);

6.5.6.3 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

6.5.7 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(inventor);

parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2");

// alternatively

String aleks = parser.parseExpression("Name = 'Alexandar Seovic'").getValue(inventorContext, 
                                                                            String.class);

6.5.8 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);

6.5.9 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 = 
  p.parseExpression("new org.spring.samples.spel.inventor.Inventor('Albert Einstein', 
                                                                   'German')")
                                                                   .getValue(Inventor.class);

//create new inventor instance within add method of List
p.parseExpression("Members.add(new org.spring.samples.spel.inventor.Inventor('Albert Einstein',
                                                                   'German'))")
                                                                   .getValue(societyContext);

6.5.10 Variables

Variables can be 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(tesla);
context.setVariable("newName", "Mike Tesla");

parser.parseExpression("Name = #newName").getValue(context);

System.out.println(tesla.getName()) // "Mike Tesla"

6.5.10.1 The #this and #root variables

The variable #this is always defined and refers to the current evaluation object (against which unqualified references are resolved). The variable #root is always defined and refers to the root context object. Although #this may vary as components of an expression are evaluated, #root always refers to the root.

// 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 SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setVariable("primes",primes);

// all prime numbers > 10 from the list (using selection ?{...})
// evaluates to [11, 13, 17]
List<Integer> primesGreaterThanTen = 
             (List<Integer>) parser.parseExpression("#primes.?[#this>10]").getValue(context);

6.5.11 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 SpelExpressionParser();
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);

6.5.12 Bean references

If the evaluation context has been configured with a bean resolver it is possible to lookup beans from an expression using the (@) symbol.

ExpressionParser parser = new SpelExpressionParser();
StandardEvaluationContext context = new StandardEvaluationContext();
context.setBeanResolver(new MyBeanResolver());

// This will end up calling resolve(context,"foo") on MyBeanResolver during evaluation
Object bean = parser.parseExpression("@foo").getValue(context);

6.5.13 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 more realistic 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"

Also see the next section on the Elvis operator for an even shorter syntax for the ternary operator.

6.5.14 The Elvis Operator

The Elvis operator is a shortening of the ternary operator syntax and is used in the Groovy language. With the ternary operator syntax you usually have to repeat a variable twice, for example:

String name = "Elvis Presley";
String displayName = name != null ? name : "Unknown";

Instead you can use the Elvis operator, named for the resemblance to Elvis' hair style.

ExpressionParser parser = new SpelExpressionParser();

String name = parser.parseExpression("null?:'Unknown'").getValue(String.class);

System.out.println(name);  // 'Unknown'

Here is a more complex example.

ExpressionParser parser = new SpelExpressionParser();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
StandardEvaluationContext context = new StandardEvaluationContext(tesla);

String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);

System.out.println(name); // Mike Tesla

tesla.setName(null);

name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);

System.out.println(name); // Elvis Presley

6.5.15 Safe Navigation operator

The Safe Navigation operator is used to avoid a NullPointerException and comes from the Groovy language. Typically when you have a reference to an object you might need to verify that it is not null before accessing methods or properties of the object. To avoid this, the safe navigation operator will simply return null instead of throwing an exception.

ExpressionParser parser = new SpelExpressionParser();

Inventor tesla = new Inventor("Nikola Tesla", "Serbian");
tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan"));

StandardEvaluationContext context = new StandardEvaluationContext(tesla);

String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);
System.out.println(city); // Smiljan

tesla.setPlaceOfBirth(null);

city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class);

System.out.println(city); // null - does not throw NullPointerException!!!

6.5.16 Collection Selection

Selection is a powerful expression language feature that allows 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 $[...].

6.5.17 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.

6.5.18 Expression templating

Expression templates allow a mixing of literal text with one or more evaluation blocks. Each evaluation block is delimited with 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 TemplateParserContext()).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() is 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 TemplateParserContext is shown below.

public class TemplateParserContext implements ParserContext {

  public String getExpressionPrefix() {
    return "#{";
  }

  public String getExpressionSuffix() {
    return "}";
  }
  
  public boolean isTemplate() {
    return true;
  }
}

6.6 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;
	}

	
}

7. Aspect Oriented Programming with Spring

7.1 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.

7.1.1 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 enterprise Java 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).

7.1.2 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 Servlet 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 enterprise Java 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 Section 7.4, “Choosing which AOP declaration style to use” for a more complete discussion of the whys and wherefores of each style.

7.1.3 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 Section 7.6.1, “Understanding AOP proxies” for a thorough examination of exactly what this implementation detail actually means.

7.2 @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 7.8, “Using AspectJ with Spring applications”.

7.2.1 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 C, XML Schema-based configuration. See Section C.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.

7.2.2 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]Autodetecting aspects through component scanning

You may register aspect classes as regular beans in your Spring XML configuration, or autodetect them throuch classpath scanning - just like any other Spring-managed bean. However, note that the @Aspect annotation is not sufficient for autodetection in the classpath: For that purpose, you need to add a separate @Component annotation (or alternatively a custom stereotype annotation that qualifies, as per the rules of Spring's component scanner).

[Note]Advising aspects with other 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.

7.2.3 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.

7.2.3.1 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.

7.2.3.2 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.

7.2.3.3 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 7.3, “Schema-based AOP support”. The transaction elements are discussed in Chapter 10, Transaction Management.

7.2.3.4 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)

7.2.3.5 Writing good pointcuts

During compilation, AspectJ processes pointcuts in order to try and optimize matching performance. Examining code and determining if each join point matches (statically or dynamically) a given pointcut is a costly process. (A dynamic match means the match cannot be fully determined from static analysis and a test will be placed in the code to determine if there is an actual match when the code is running). On first encountering a pointcut declaration, AspectJ will rewrite it into an optimal form for the matching process. What does this mean? Basically pointcuts are rewritten in DNF (Disjunctive Normal Form) and the components of the pointcut are sorted such that those components that are cheaper to evaluate are checked first. This means you do not have to worry about understanding the performance of various pointcut designators and may supply them in any order in a pointcut declaration.

However, AspectJ can only work with what it is told, and for optimal performance of matching you should think about what they are trying to achieve and narrow the search space for matches as much as possible in the definition. The existing designators naturally fall into one of three groups: kinded, scoping and context:

  • Kinded designators are those which select a particular kind of join point. For example: execution, get, set, call, handler

  • Scoping designators are those which select a group of join points of interest (of probably many kinds). For example: within, withincode

  • Contextual designators are those that match (and optionally bind) based on context. For example: this, target, @annotation

A well written pointcut should try and include at least the first two types (kinded and scoping), whilst the contextual designators may be included if wishing to match based on join point context, or bind that context for use in the advice. Supplying either just a kinded designator or just a contextual designator will work but could affect weaving performance (time and memory used) due to all the extra processing and analysis. Scoping designators are very fast to match and their usage means AspectJ can very quickly dismiss groups of join points that should not be further processed - that is why a good pointcut should always include one if possible.

7.2.4 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.

7.2.4.1 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() {
    // ...
  }

}

7.2.4.2 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.

7.2.4.3 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).

7.2.4.4 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() {
    // ...
  }

}

7.2.4.5 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.

7.2.4.6 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();
  // ...
}
Advice parameters and generics

Spring AOP can handle generics used in class declarations and method parameters. Suppose you have a generic type like this:

public interface Sample<T> {
  void sampleGenericMethod(T param);
  void sampleGenericCollectionMethod(Collection>T> param);
}

You can restrict interception of method types to certain parameter types by simply typing the advice parameter to the parameter type you want to intercept the method for:

@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)")
public void beforeSampleMethod(MyType param) {
  // Advice implementation
}

That this works is pretty obvious as we already discussed above. However, it's worth pointing out that this won't work for generic collections. So you cannot define a pointcut like this:

@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)")
public void beforeSampleMethod(Collection<MyType> param) {
  // Advice implementation
}

To make this work we would have to inspect every element of the collection, which is not reasonable as we also cannot decide how to treat null values in general. To achieve something similar to this you have to type the parameter to Collection<?> and manually check the type of the elements.

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).

7.2.4.7 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.

7.2.5 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");

7.2.6 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.

7.2.7 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 { 
  ...	
}

7.3 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 7.2, “@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 C, XML Schema-based configuration. See Section C.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.

7.3.1 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.

7.3.2 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 7.2, “@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 7.2.3.3, “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.

7.3.3 Declaring advice

The same five advice kinds are supported as for the @AspectJ style, and they have exactly the same semantics.

7.3.3.1 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.

7.3.3.2 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) {...

7.3.3.3 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) {...

7.3.3.4 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>

7.3.3.5 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 7.2.4.5, “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;
}

7.3.3.6 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 7.2.4.6, “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-3.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.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)

7.3.3.7 Advice ordering

When multiple advice needs to execute at the same join point (executing method) the ordering rules are as described in Section 7.2.4.7, “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.

7.3.4 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");

7.3.5 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.

7.3.6 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 8.3.2, “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.

7.3.7 Example

Let's see how the concurrent locking failure retry example from Section 7.2.7, “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)"/>

7.4 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.

7.4.1 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.

7.4.2 @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.

7.5 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.

7.6 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.

7.6.1 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.

7.7 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();

7.8 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 7.8.1, “Using AspectJ to dependency inject domain objects with Spring” and Section 7.8.2, “Other Spring aspects for AspectJ” discuss the content of this library and how you can use it. Section 7.8.3, “Configuring AspectJ aspects using Spring IoC” discusses how to dependency inject AspectJ aspects that are woven using the AspectJ compiler. Finally, Section 7.8.4, “Load-time weaving with AspectJ in the Spring Framework” provides an introduction to load-time weaving for Spring applications using AspectJ.

7.8.1 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 3.9, “Annotation-based container 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 7.8.4, “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>
[Note]Note

Do not activate @Configurable processing through the bean configurer aspect unless you really mean to rely on its semantics at runtime. In particular, make sure that you do not use @Configurable on bean classes which are registered as regular Spring beans with the container: You would get double initialization otherwise, once through the container and once through the aspect.

7.8.1.1 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.

7.8.1.2 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.

7.8.2 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);
		   		   
}

7.8.3 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.

7.8.4 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/org.springframework.instrument-{version}.jar (previously named 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.

7.8.4.1 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). T