org.apache.felix.dm.annotation.api

Annotation Type Composition

@Retention(value=CLASS)
 @Target(value=METHOD)
public @interface Composition

Annotates a method returning the list of objects which are part of a Component implementation. When implementing complex Components, you often need to use more than one object instances. Moreover, several of these instances might want to have dependencies injected, as well as lifecycle callbacks invoked, like the methods annotated with Init, Start, Stop, Destroy annotations. In such cases you can tell the dependency manager which instances to consider, by annotating a method in your Component, returning a list of objects which are part of the implementation.

This annotation may be applied on a method which is part of class annotated with either a Component, AspectService, or AdapterService.

Usage Examples

Here, the "MyComponent" component is composed of the Helper class, which is also injected with service dependencies. The lifecycle callbacks are also invoked in the Helper (if the Helper defines them):

 class Helper {
     volatile LogService logService; // Injected
     
     void start() {} // lifecycle callback
     void bind(OtherService otherService) {} // injected
 }
 
 @Component
 class MyComponent {
     // Helper which will also be injected with our service dependencies
     private Helper helper = new Helper();
      
     @Composition
     Object[] getComposition() {
         return new Object[] { this, helper }; 
     }

     @ServiceDependency
     private LogService logService; // Helper.logService will be also be injected, if defined.
     
     @Start
     void start() {} // the Helper.start() method will also be called, if defined
     
     @ServiceDependency
     void bind(OtherService otherService) {} // the Helper.bind() method will also be called, if defined     
 }