org.springframework.context.annotation

Annotation Type Configuration

@Target(value=TYPE)
@Retention(value=RUNTIME)
@Documented
@Component
public @interface Configuration

Indicates that a class declares one or more @Bean methods and may be processed by the Spring container to generate bean definitions and service requests for those beans at runtime, for example:

 @Configuration
 public class AppConfig {
     @Bean
     public MyBean myBean() {
         // instantiate, configure and return bean ...
     }
 }

Bootstrapping @Configuration classes

Via AnnotationConfigApplicationContext

@Configuration classes are typically bootstrapped using either AnnotationConfigApplicationContext or its web-capable variant, AnnotationConfigWebApplicationContext. A simple example with the former follows:

 AnnotationConfigApplicationContext ctx =
     new AnnotationConfigApplicationContext();
 ctx.register(AppConfig.class);
 ctx.refresh();
 MyBean myBean = ctx.getBean(MyBean.class);
 // use myBean ...

See AnnotationConfigApplicationContext Javadoc for further details and see AnnotationConfigWebApplicationContext for web.xml configuration instructions.

Via Spring <beans> XML

As an alternative to registering @Configuration classes directly against an AnnotationConfigApplicationContext, @Configuration classes may be declared as normal <bean> definitions within Spring XML files:

 <beans>
    <context:annotation-config/>
    <bean class="com.acme.AppConfig"/>
 </beans>

In the example above, <context:annotation-config/> is required in order to enable ConfigurationClassPostProcessor and other annotation-related post processors that facilitate handling @Configuration classes.

Via component scanning

@Configuration is meta-annotated with @Component, therefore @Configuration classes are candidates for component scanning (typically using Spring XML's <context:component-scan/> element) and therefore may also take advantage of @Autowired/@Inject at the field and method level (but not at the constructor level).

@Configuration classes may not only be bootstrapped using component scanning, but may also themselves configure component scanning using the @ComponentScan annotation:

 @Configuration
 @ComponentScan("com.acme.app.services")
 public class AppConfig {
     // various @Bean definitions ...
 }

See @ComponentScan Javadoc for details.

Working with externalized values

Using the Environment API

Externalized values may be looked up by injecting the Spring Environment into a @Configuration class using the @Autowired or the @Inject annotation:

 @Configuration
 public class AppConfig {
     @Inject Environment env;

     @Bean
     public MyBean myBean() {
         MyBean myBean = new MyBean();
         myBean.setName(env.getProperty("bean.name"));
         return myBean;
     }
 }

Properties resolved through the Environment reside in one or more "property source" objects, and @Configuration classes may contribute property sources to the Environment object using the @PropertySources annotation:

 @Configuration
 @PropertySource("classpath:/com/acme/app.properties")
 public class AppConfig {
     @Inject Environment env;

     @Bean
     public MyBean myBean() {
         return new MyBean(env.getProperty("bean.name"));
     }
 }

See Environment and @PropertySource Javadoc for further details.

Using the @Value annotation

Externalized values may be 'wired into' @Configuration classes using the @Value annotation:

 @Configuration
 @PropertySource("classpath:/com/acme/app.properties")
 public class AppConfig {
     @Value("${bean.name}") String beanName;

     @Bean
     public MyBean myBean() {
         return new MyBean(beanName);
     }
 }

This approach is most useful when using Spring's PropertySourcesPlaceholderConfigurer, usually enabled via XML with <context:property-placeholder/>. See the section below on composing @Configuration classes with Spring XML using @ImportResource, see @Value Javadoc, and see @Bean Javadoc for details on working with BeanFactoryPostProcessor types such as PropertySourcesPlaceholderConfigurer.

Composing @Configuration classes

With the @Import annotation

@Configuration classes may be composed using the @Import annotation, not unlike the way that <import> works in Spring XML. Because @Configuration objects are managed as Spring beans within the container, imported configurations may be injected using @Autowired or @Inject:

 @Configuration
 public class DatabaseConfig {
     @Bean
     public DataSource dataSource() {
         // instantiate, configure and return DataSource
     }
 }

 @Configuration
 @Import(DatabaseConfig.class)
 public class AppConfig {
     @Inject DatabaseConfig dataConfig;

     @Bean
     public MyBean myBean() {
         // reference the dataSource() bean method
         return new MyBean(dataConfig.dataSource());
     }
 }

Now both AppConfig and the imported DatabaseConfig can be bootstrapped by registering only AppConfig against the Spring context:

 new AnnotationConfigApplicationContext(AppConfig.class);

With the @Profile annotation

@Configuration classes may be marked with the @Profile annotation to indicate they should be processed only if a given profile or profiles are active:

 @Profile("embedded")
 @Configuration
 public class EmbeddedDatabaseConfig {
     @Bean
     public DataSource dataSource() {
         // instantiate, configure and return embedded DataSource
     }
 }

 @Profile("production")
 @Configuration
 public class ProductionDatabaseConfig {
     @Bean
     public DataSource dataSource() {
         // instantiate, configure and return production DataSource
     }
 }

See @Profile and Environment Javadoc for further details.

With Spring XML using the @ImportResource annotation

As mentioned above, @Configuration classes may be declared as regular Spring <bean> definitions within Spring XML files. It is also possible to import Spring XML configuration files into @Configuration classes using the @ImportResource annotation. Bean definitions imported from XML can be injected using @Autowired or @Import:

 @Configuration
 @ImportResource("classpath:/com/acme/database-config.xml")
 public class AppConfig {
     @Inject DataSource dataSource; // from XML

     @Bean
     public MyBean myBean() {
         // inject the XML-defined dataSource bean
         return new MyBean(this.dataSource);
     }
 }

With nested @Configuration classes

@Configuration classes may be nested within one another as follows:

 @Configuration
 public class AppConfig {
     @Inject DataSource dataSource;

     @Bean
     public MyBean myBean() {
         return new MyBean(dataSource);
     }

     @Configuration
     static class DatabaseConfig {
         @Bean
         DataSource dataSource() {
             return new EmbeddedDatabaseBuilder().build();
         }
     }
 }

When bootstrapping such an arrangement, only AppConfig need be registered against the application context. By virtue of being a nested @Configuration class, DatabaseConfig will be registered automatically. This avoids the need to use an @Import annotation when the relationship between AppConfig DatabaseConfig is already implicitly clear.

Note also that nested @Configuration classes can be used to good effect with the @Profile annotation to provide two options of the same bean to the enclosing @Configuration class.

Configuring lazy initialization

By default, @Bean methods will be eagerly instantiated at container bootstrap time. To avoid this, @Configuration may be used in conjunction with the @Lazy annotation to indicate that all @Bean methods declared within the class are by default lazily initialized. Note that @Lazy may be used on individual @Bean methods as well.

Testing support for @Configuration classes

The Spring TestContext framework available in the spring-test module provides the @ContextConfiguration annotation, which as of Spring 3.1 can accept an array of @Configuration Class objects:

 @RunWith(SpringJUnit4ClassRunner.class)
 @ContextConfiguration(classes={AppConfig.class, DatabaseConfig.class})
 public class MyTests {

     @Autowired MyBean myBean;

     @Autowired DataSource dataSource;

     @Test
     public void test() {
         // assertions against myBean ...
     }
 }

See TestContext framework reference documentation for details.

Enabling built-in Spring features using @Enable annotations

Spring features such as asynchronous method execution, scheduled task execution, annotation driven transaction management, and even Spring MVC can be enabled and configured from @Configuration classes using their respective "@Enable" annotations. See @EnableAsync, @EnableScheduling, @EnableTransactionManagement, @EnableAspectJAutoProxy, and @EnableWebMvc for details.

Constraints when authoring @Configuration classes

• @Configuration classes must be non-final
• @Configuration classes must be non-local (may not be declared within a method)
• @Configuration classes must have a default/no-arg constructor and may not use @Autowired constructor parameters. Any nested configuration classes must be static

Since:

3.0

Author:

Rod Johnson, Chris Beams

See Also:

Bean, Profile, Import, ImportResource, ComponentScan, Lazy, PropertySource, AnnotationConfigApplicationContext, ConfigurationClassPostProcessor, Environment, ContextConfiguration

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element and Description
String	value Explicitly specify the name of the Spring bean definition associated with this Configuration class.

Element Detail

value

public abstract String value

Explicitly specify the name of the Spring bean definition associated with this Configuration class. If left unspecified (the common case), a bean name will be automatically generated.

The custom name applies only if the Configuration class is picked up via component scanning or supplied directly to a AnnotationConfigApplicationContext. If the Configuration class is registered as a traditional XML bean definition, the name/id of the bean element will take precedence.

Returns:

the specified bean name, if any

See Also:

DefaultBeanNameGenerator

Default:

""