Chapter 121. Guava EventBus Component

guava-eventbus:busName[?options]

Where busName represents the name of the com.google.common.eventbus.EventBus instance located in the Camel registry.

The Guava EventBus component supports 3 options which are listed below.

The Guava EventBus endpoint is configured using URI syntax:

guava-eventbus:eventBusRef

with the following path and query parameters:

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);
  }
});

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");

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");

Available as of Camel 2.3

HawtDB is a very lightweight and embedable key value database. It allows together with Camel to provide persistent support for various Camel features such as Aggregator.

Deprecated

The HawtDB project is being deprecated and replaced by leveldb as the lightweight and embedable key value database. To make using leveldb easy there is a leveldbjni project for that. The Apache ActiveMQ project is planning on using leveldb as their primary file based message store in the future, to replace kahadb.

There is a camel-leveldb component we recommend to use instead of this.

Issue with HawtDB 1.4 or older

There is a bug in HawtDB 1.4 or older which means the filestore will not free unused space. That means the file keeps growing. This has been fixed in HawtDB 1.5 which is shipped with Camel 2.5 onwards.

Current features it provides:

<dependency>
  <groupId>org.apache.camel</groupId>
  <artifactId>camel-hawtdb</artifactId>
  <version>2.3.0</version>
</dependency>