Annotations - Dependencies

This section describes the various dependencies supported with annotations.

@ServiceDependency

Annotates a method or a field for injecting a Service Dependency. When applied on a class field, optional unavailable dependencies are injected with a NullObject.

Annotation attributes:

• added: The callback method to be invoked when the service is available. This attribute is only meaningful when the annotation is applied on a class field.
• changed: The callback method to be invoked when the service properties have changed.
• removed: The callback method to invoke when the service is lost.
• timeout: The max time in millis to wait for when the dependency is temporarily unavailable. Specifying a positive number allow to block the caller thread between service updates. Only useful for required stateless dependencies that can be replaced transparently. A Dynamic Proxy is used to wrap the actual service dependency (which must be an interface). When the dependency goes away, an attempt is made to replace it with another one which satisfies the service dependency criteria. If no service replacement is available, then any method invocation (through the dynamic proxy) will block during a configurable timeout. On timeout, an unchecked IllegalStateException exception is raised (but the service is not deactivated).
  Notice that the changed/removed callbacks are not used when the timeout parameter is > -1.
  -1 means no timeout at all (default). 0 means that invocation on a missing service will fail immediately. A positive number represents the max timeout in millis to wait for the service availability.
• name: The name used when dynamically configuring this dependency from the init method. Specifying this attribute allows to dynamically configure the dependency filter and required flag from the Service's init method. All unnamed dependencies will be injected before the init() method; so from the init() method, you can then pick up whatever information needed from already injected (unnamed) dependencies, and configure dynamically your named dependencies, which will then be calculated once the init() method returns. Please refer to the "Dynamic Dependency Configuration" in the Lifecycle section.
• propagate: Returns true if the dependency service properties must be published along with the service. Any additional service properties specified directly are merged with these.

Usage Example: Here, the MyComponent component is injected with a dependency over a "MyDependency" service.

     @Component
     class MyComponent {
         @ServiceDependency(timeout=15000)
         MyDependency dependency;
         ...
     }

Usage example of a Service whose dependency filter is configured from ConfigAdmin:

      /**
        * A Service whose service dependency "otherService" filter is configured from ConfigAdmin
        */
      @Service
      class X {
          private Dictionary m_config;
          
          /**
           * Initialize our service from config ... and store the config for later usage (from our init method)
           */ 
          @ConfigurationDependency(pid="MyPid")
          void configure(Dictionary conf) {
               m_config = config;
          }
     
          /**
           * All unnamed dependencies are injected: we can now configure other named
           * dependencies, using the already injected configuration.
           * The returned Map will be used to configure our "otherService" Dependency.
           */
          @Init
          Map init() {
              return new HashMap() {{
                  put("otherService.filter", m_config.get("filter"));
                  put("otherService.required", m_config.get("required"));
              }};
          } 

          /**
           * This named dependency filter/required flag will be configured by our init method (see above).
           */
          @ServiceDependency(name="otherService") 
          void bindOtherService(OtherService other) {
          }
          
          /**
           * All dependencies are injected and our service is now ready to be published.
           * Notice that you can also use the publisher service attribute if you need 
           * to take control on service exposition.
           */
          @Start
          void start() {
          }
      }

@ConfigurationDependency

A configuration dependency is always required, and allows you to depend on the availability of a valid configuration for your component. This dependency requires the OSGi Configuration Admin Service.

Annotation attributes:

• pid: Returns the pid for a given service (by default, the pid is the service class name).
• propagate: Returns true if the configuration properties must be published along with the service. Any additional service properties specified directly are merged with these.
• heading: The label used to display the tab name (or section) where the properties are displayed. Example: "Printer Service".
• description: A human readable description of the PID this annotation is associated with. Example: "Configuration for the PrinterService bundle".
• metadata: an array of PropertyMetadaData[] annotation describing property types (see the FactoryConfigurationAdapterService section in the "Writing Components" section.

Usage Examples

In the following example, the "Printer" component depends on a configuration whose PID name is "org.apache.felix.sample.Printer". This service will initialize its ip/port number from the provided configuration:

     package org.apache.felix.sample;
     
     @Component
     public class Printer {
         @ConfigurationDependency
         void updated(Dictionary config) {
             // load printer ip/port from the provided dictionary.
         }
     }

This other example shows how to specify a configuration dependency, as well as meta data used to customize the WebConsole GUI. Using these meta data, you can specify for example the default value for your configurations data, some descriptions, the cardinality of configuration values, etc ...

     package org.apache.felix.sample;
     
     @Component
     public class Printer {
         @ConfigurationDependency(
             heading = "Printer Service",
             description = "Declare here parameters used to configure the Printer service", 
             metadata = { 
                 @PropertyMetaData(heading = "Ip Address", 
                                   description = "Enter the ip address for the Printer service",
                                   defaults = { "127.0.0.1" }, 
                                   type = String.class,
                                   id = "IPADDR", 
                                   cardinality = 0),
                 @PropertyMetaData(heading = "Port Number", 
                                   description = "Enter the port number for the Printer service",
                                   defaults = { "4444" }, 
                                   type = Integer.class,
                                   id = "PORTNUM", 
                                   cardinality = 0) 

             }
         )
         void updated(Dictionary config) {
             // load configuration from the provided dictionary.
         }

@BundleDependency

@ResourceDependency