If you’re interested in messages sent to a destination, you need to “listen” for them. That is, you must create and register a message listener for the destination.
To create a message listener, implement the
MessageListener interface
and override its receive(Message) method to process messages your way.
public void receive(Message message) {
// Process messages your way
}
Here are the ways to register your listener with Message Bus:
-
Automatic Registration as a Component: Publish the listener to the OSGi registry as a Declarative Services Component that specifies a destination. Message Bus automatically wires the listener to the destination.
-
Registering via MessageBus: Obtain a reference to the Message Bus and use it directly to register the listener to a destination.
-
Registering directly to a Destination: Obtain a reference to a specific destination and use it directly to register the listener with that destination.
The Declarative Services component module provides the easiest way to register a message listener.
Automatic Registration as a Component
You can specify a message listener in the Declarative Services (DS) @Component
annotation:
@Component (
immediate = true,
property = {"destination.name=myCustom/Destination"},
service = MessageListener.class
)
public class MyMessageListener implements MessageListener {
...
public void receive(Message message) {
// Handle the message
}
}
The Message Bus listens for MessageListener service components like this one
to publish themselves to the OSGi service registry. The attribute immediate = true tells the OSGi framework to activate the component as soon as its
dependencies resolve. Message Bus wires each registered listener to the
destination its destination.name property specifies. If the destination is not
yet registered, Message Bus queues the listener until the destination registers.
Registration as a component is the preferred way to register message listeners to destinations.
Registering via MessageBus
You can use the
MessageBus instance
directly to register message listeners to destinations. You might want to do
this if, for example, you want to create some special proxy wrappers. Here’s a
registrator that demonstrates registering a listener this way:
@Component (
immediate = true,
service = MyMessageListenerRegistrator.class
)
public class MyMessageListenerRegistrator {
...
@Activate
protected void activate() {
_messageListener = new MessageListener() {
public void receive(Message message) {
// Handle the message
}
};
_messageBus.registerMessageListener("myDestinationName",
_messageListener);
}
@Deactivate
protected void deactivate() {
_messageBus.unregisterMessageListener("myDestinationName",
_messageListener);
}
@Reference
private MessageBus _messageBus;
private MessageListener _messageListener;
}
The _messageBus field’s @Reference annotation binds it to the MessageBus
instance. The activate method creates the listener and uses the Message Bus to
register the listener to a destination named "myDestination". When this
registrator component is destroyed, the deactivate method unregisters the
listener.
Registering directly to the Destination
You can use a Destination instance to register a listener to that destination.
You might want to do this if, for example, you want to create some special proxy
wrappers. Here’s a registrator that demonstrates registering a listener this
way:
@Component (
immediate = true,
service = MyMessageListenerRegistrator.class
)
public class MyMessageListenerRegistrator {
...
@Activate
protected void activate() {
_messageListener = new MessageListener() {
public void receive(Message message) {
// Handle the message
}
};
_destination.register(_messageListener);
}
@Deactivate
protected void deactivate() {
_destination.unregister(_messageListener);
}
@Reference(target = "(destination.name=someDestination)")
private Destination _destination;
private MessageListener _messageListener;
}
The _destination field’s @Reference annotation binds it to a destination
named "someDestination". The activate method creates the listener and
registers it to the destination. When this registrator component is destroyed,
the deactivate method unregisters the listener.
Now you know how to create and register message listeners for receiving messages sent to the destinations.