Bean Binding

Bean binding

The binding of a Camel Message to a bean method call can occur in different ways

• if the bean can be converted to a Processor using the Type Converter mechanism then this is used to process the message. This mechanism is used by the ActiveMQ component to allow any MessageListener to be invoked by the Bean component
• if the body of the message can be converted to a BeanInvocation (the default payload used by the ProxyHelper) - then that its used to invoke the method and pass the arguments
• if the message contains the header org.apache.camel.MethodName then that method is invoked, converting the body to whatever the argument is to the method
• otherwise the type of the method body is used to try find a method which matches; an error is thrown if a single method cannot be chosen unambiguously.
• you can also use Exchange as the parameter itself, but then the return type must be void.

By default the return value is set on the outbound message body.

For example a POJO such as:

public class Bar {

    public String doSomething(String body) {
      // process the in body and return whatever you want
      return "Bye World";
   }

Or the Exchange example. Notice that the return type must be void:

public class Bar {

    public void doSomething(Exchange exchange) {
      // process the exchange
      exchange.getIn().setBody("Bye World");
   }

For example you could write a method like this (showing also a feature in Camel, the @MessageDrive annotation):

public class Foo {

    @MessageDriven(uri = "activemq:my.queue")
    public void doSomething(String body) {
		// process the inbound message here
    }

}

Here Camel with subscribe to an ActiveMQ queue, then convert the message payload to a String (so dealing with TextMessage, ObjectMessage and BytesMessage in JMS), then process this method.

Using Annotations to bind parameters to the Exchange

The annotations can be used to bind in situations where traditional methods would result in ambiguous methods. So by adding annotations you can decorate your bean to help Camel invoke the correct method.

You can also use the following annotations to bind parameters to different kinds of Expression

For example

public class Foo {
	
    @MessageDriven(uri = "activemq:my.queue")
    public void doSomething(@Header('JMSCorrelationID') String correlationID, @Body String body) {
		// process the inbound message here
    }

}

In the above you can now pass the Message.getJMSCorrelationID() as a parameter to the method (using the Type Converter to adapt the value to the type of the parameter).

Finally you don't need the @MessageDriven annotation; as the Camel route could describe which method to invoke.

e.g. a route could look like

from("activemq:someQueue").
  to("bean:myBean");

Here myBean would be looked up in the Registry (such as JNDI or the Spring ApplicationContext), then the body of the message would be used to try figure out what method to call.

If you want to be explicit you can use

from("activemq:someQueue").
  to("bean:myBean?methodName=doSomething");

And here we have a nifty example for you to show some great power in Camel. You can mix and match the annotations with the normal parameters, so we can have this example with annotations and the Exchange also:

    public void doSomething(@Header(name = "user") String user, @Body String body, Exchange exchange) {
        exchange.getIn().setBody(body + "MyBean");
    }

Using Expression Languages

You can also use any of the Languages supported in Camel to bind expressions to method parameters when using bean integration. For example you can use any of these annotations...

For example

public class Foo {
	
    @MessageDriven(uri = "activemq:my.queue")
    public void doSomething(@Path("/foo/bar/text()") String correlationID, @Body String body) {
		// process the inbound message here
    }
}