Red Hat Training

A Red Hat training course is available for Red Hat Fuse

Chapter 43. Guava EventBus

Guava EventBus Component

Available since Camel 2.10.0
The Google Guava EventBus allows publish-subscribe-style communication between components without requiring the components to explicitly register with one another (and thus be aware of each other). The guava-eventbus: component provides integration bridge between Camel and Google Guava EventBus infrastructure. With the latter component, messages exchanged with the Guava EventBus can be transparently forwarded to the Camel routes. EventBus component allows also to route body of Camel exchanges to the Guava EventBus.
Maven users will need to add the following dependency to their pom.xml for this component: 
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-guava-eventbus</artifactId>
    <version>x.x.x</version>
    <!-- use the same version as your Camel core version -->
</dependency>

URI format

guava-eventbus:busName[?options]
Where busName represents the name of the com.google.common.eventbus.EventBus instance located in the Camel registry.

Options

Name Default Value Description
eventClass null Camel 2.10: If used on the consumer side of the route, will filter events received from the EventBus to the instances of the class and superclasses of eventClass. Null value of this option is equal to setting it to the java.lang.Object i.e. the consumer will capture all messages incoming to the event bus. This option cannot be used together with listenerInterface option.
listenerInterface null Camel 2.11: The interface with method(s) marked with the @Subscribe annotation. Dynamic proxy will be created over the interface so it could be registered as the EventBus listener. Particularly useful when creating multi-event listeners and for handling DeadEvent properly. This option cannot be used together with eventClass option.

Usage

Using guava-eventbus component on the consumer side of the route will capture messages sent to the Guava EventBus and forward them to the Camel route. Guava EventBus consumer processes incoming messages asynchronously. 
SimpleRegistry registry = new SimpleRegistry();
EventBus eventBus = new EventBus();
registry.put("busName", eventBus);
CamelContext camel = new DefaultCamelContext(registry);

from("guava-eventbus:busName").to("seda:queue");

eventBus.post("Send me to the SEDA queue.");
Using guava-eventbus component on the producer side of the route will forward body of the Camel exchanges to the Guava EventBus instance. 
SimpleRegistry registry = new SimpleRegistry();
EventBus eventBus = new EventBus();
registry.put("busName", eventBus);
CamelContext camel = new DefaultCamelContext(registry);

from("direct:start").to("guava-eventbus:busName");

ProducerTemplate producerTemplate = camel.createProducerTemplate();
producer.sendBody("direct:start", "Send me to the Guava EventBus.");

eventBus.register(new Object(){
  @Subscribe
  public void messageHander(String message) {
    System.out.println("Message received from the Camel: " + message);
  }
});

DeadEvent considerations

Keep in mind that due to the limitations caused by the design of the Guava EventBus, you cannot specify event class to be received by the listener without creating class annotated with @Subscribe method. This limitation implies that endpoint with eventClass option specified actually listens to all possible events (java.lang.Object) and filter appropriate messages programmatically at runtime. The snipped below demonstrates an appropriate excerpt from the Camel code base. 
@Subscribe
public void eventReceived(Object event) {
  if (eventClass == null || eventClass.isAssignableFrom(event.getClass())) {
    doEventReceived(event);
...
This drawback of this approach is that EventBus instance used by Camel will never generate com.google.common.eventbus.DeadEvent notifications. If you want Camel to listen only to the precisely specified event (and therefore enable DeadEvent support), use listenerInterface endpoint option. Camel will create dynamic proxy over the interface you specify with the latter option and listen only to messages specified by the interface handler methods. The example of the listener interface with single method handling only SpecificEvent instances is demonstrated below. 
package com.example;

public interface CustomListener {

  @Subscribe
  void eventReceived(SpecificEvent event);

}
The listener presented above could be used in the endpoint definition as follows. 
from("guava-eventbus:busName?listenerInterface=com.example.CustomListener").to("seda:queue");

Consuming multiple type of events

In order to define multiple type of events to be consumed by Guava EventBus consumer use listenerInterface endpoint option, as listener interface could provide multiple methods marked with the @Subscribe annotation. 
package com.example;

public interface MultipleEventsListener {

  @Subscribe
  void someEventReceived(SomeEvent event);

  @Subscribe
  void anotherEventReceived(AnotherEvent event);

}
The listener presented above could be used in the endpoint definition as follows. 
from("guava-eventbus:busName?listenerInterface=com.example.MultipleEventsListener").to("seda:queue");