Instance Controller or Service Controller ?¶

When the component implementation needs to impact the instance container, it uses controllers on Boolean fields. iPOJO provides two controllers:

• Instance Lifecycle Controller (@Controller)
• Service Lifecycle Controller (@ServiceController)

This page explains the differences between the two kinds of controllers.

The Instance Lifecycle Controller¶

The instance lifecycle controller impacts the instance state. Such controller must only be used if the instance code has detected an irremediable situation, where the instance cannot behave correctly.

Such controller is generally used when the component code:

• check the configuration
• has caught an error

Instance Lifecycle Controller cannot be initialized, and generally are only set to false:

@Component
public class MyComponent {

    @Controller
    private boolean controller;

    @Property(name="my-property")
    public void setMyProperty(String value) {
        if (! ACCEPTABLE_VALUES.contains(value) {
                // Let's image that if the given 
                // value is not acceptable, we invalidate
                // the instance
                controller = false;
        }
    }
}

When set to false:

• the service exposed by the instance are withdrawn from the service registry
• the @Invalidate callback is called

The service lifecycle controller¶

The service lifecycle controller impacts only the services exposed by the instance. It is used when the component implementation decides whether or not to publish the services.

Unlike the instance lifecycle controller, such controller can:

• be set to false at startup (to not publish the services by default).
• fluctuate between true and false to reflect the decision of the component implementation to publish or not the services.

The following example publishes the component's provided service only after initialization.

@Component
@Provides
public class MyComponent implements MyService {

    // Service not published by default
    @ServiceController(value=false)
    private boolean controller;

    @Validate
    public void initialize() {
        //...
        // publish the service once the initialization
        // is completed.
        controller = true;
    }
}

When the controller is set to true, the @PostRegistration callback is called. When it is set to false the @PostUnregistration callback is called. In both cases, the service is already registered or unregistered when the callback is invoked.

Notice that a @ServiceController controls all exposed services by default, but can be restricted to specific service interface with the specification attribute.