Similar to Spring 2.0's support for XML namespaces, JavaConfig provides an extensibility mechanism with the @Plugin annotation. The general intent is to allow for a more expressive and declarative way to register commonly used bean definitions.

To get a sense of what @Plugin can do, consider each of the following annotations already discussed in this document: @AnnotationDrivenConfig, @AnnotationDrivenTx, @MBeanExport, @PropertiesValueSource. These annotations are all "@Plugin annotations". Take AnnotationDrivenConfig for example:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Plugin(handler=AnnotationDrivenConfigHandler.class)
public @interface AnnotationDrivenConfig { }
            

@AnnotationDrivenConfig is 'meta-annotated' as @Plugin. Notice the handler attribute to @Plugin accepts AnnotationDrivenConfigHandler.class:

class AnnotationDrivenConfigHandler implements ConfigurationPlugin<AnnotationDrivenConfig> {

    public void handle(AnnotationDrivenConfig annotation, BeanDefinitionRegistry registry) {
        AnnotationConfigUtils.registerAnnotationConfigProcessors(registry);
    }

}
            

The handle method of any ConfigurationPlugin implementation is provided with the annotation (AnnotationDrivenConfig in this case), as well as a BeanDefinitionRegistry - this registry is the same registry that JavaConfig is using to register all objects created from @Bean methods. Therefore, ConfigurationPlugin implementations have direct control over the container and can do with it as they please. Any number and type of bean definitions may be registered, offloading tedious or repetitive work from the @Configuration class author.

	Note
	This documentation is preliminary and the @Plugin/ ConfigurationPlugin API may change before JavaConfig's GA release. To find out more about programming @Plugin annotations, study the existing set of annotations mentioned above, and don't hesitate to interact with us via the JavaConfig forum.