Annotation Type @bean，@Import，@configuration使用--官方文档

@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 @Configurationclasses.

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 @PropertySourcesannotation:

 @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 @Configurationclasses 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 @Configurationclasses 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.