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>
guava-eventbus:busName[?options]
Where busName represents the name of the
com.google.common.eventbus.EventBus
instance located in the Camel
registry.
Name |
Default Value |
Description |
---|---|---|
|
|
Camel 2.10: If used on the consumer side of
the route, will filter events received from the |
|
|
Camel 2.11: The interface with method(s) marked
with the |
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");