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+ This page is a translated version of /site/dependencymanager-annotations-composition.html. In case of + doubt you might want to refer to the old page. +

+ + +

Dependency Manager Service Composition

This section shows how a component may be implemented using multiple object instances.

When implementing more complex services, you often find yourself using more than one instance for a given service. +However, several of these instances might want to have dependencies injected. In such cases you need to tell the +dependency manager which instances to consider. Within a Component (or an Aspect/Adapter), you can annotate a method +with the @Composition annotation. This method is meant to return such composition of service instances, and the objects +will be considered as part of the service implementation. So, all dependencies, as well as lifecycle annotations +(@Init, @Start, @Stop, @Destroy) will be applied on every objects returned by the method annotated with the @Composition annotation.

The following example illustrates a composition of two object instances, which are part of the implementation of a MyService service:

/**
+ * Main implementation for the MyService Service
+ */
+@Component
+public class MyServiceImpl implements MyService {
+  // This object instance is also used to implement the service.
+  private Helper helper = new Helper();
+
+  // MyServiceImpl, and Helper objects are part of the composition
+  @Composition
+  Object[] getComposition() {
+    return new Object[] { this, helper };
+  }
+
+  // This dependency is also applied to the Helper
+  @ServiceDependency
+  Dependency dep;
+
+  // Same thing for this dependency
+  @ServiceDependency
+  void bind(Dependency2 dep2) {}
+
+  // Lifecycle callbacks also applied on the Helper ...
+  @Init
+  void init() {}
+
+  @Start
+  void start() {}
+
+  @Stop
+  void stop() {}
+
+  @Destroy()
+  void destroy() {}
+}
+
+public class Helper {
+  // Also injected, since we are part of the composition
+  Dependency dep;
+
+  // But since we are not interested by the Dependency2, we don't have to declare the bind(Dependency2) method ...
+
+  // We only specify the start lifecycle callback because we need to return some extra service properties which will be published
+  // along with the provided service ...
+
+  Map start() {
+     Map extraServiceProperties = new HashMap();
+     extraServiceProperties.add("foo", "bar");
+     return extraServiceProperties;
+  }
+}
+

+ + +

Here, MyServiceImpl is the main component implementation, but is composed of the Helper object instance. So all Dependencies defined in MyServiceImpl +will be also injected to the Helper (if the Helper declares the fields or methods). The Helper may also declare lifecycle callbacks (optionally).

+ Rev. 1525175 by pderop on Fri, 20 Sep 2013 23:19:31 +0000 +

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