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== The Processor Callback

Processors reacts to method execution, from the implementation view, just much more likely the Java _Runnable_ interface, in other words, they provide an inner _start()_ method to be invoked by the implementation layer itself, while the user layer implements a _run()_ method containing the business logic. When you invoke the _start()_ method, the _run()_ method is called back from the runtime system to the user layer implementation.

Similar analogy could be applied to the _process()_ method, which is an insight behavior of the Processor model base implementation, in relation with the _doProcess()_ method left to higher user level implementation for the business logic.

This technical approach is so called _callback_ and is a useful way to hide the complexities of the execution task and leave to the user just those parts of the implementation focused on the business model of the application itself.

The chain subsystem uses this approach to make things simpler at the business layer logic and let developers focus on the logic associated with domain of competence.

.ProcessorCallBack.java
[source,java]
----
public interface ProcessorCallBack {
	public Message doProcess(Message message) throws ProcessorParsingException;
}
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[TIP]
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This way a Processor logic is simpler to implement in its business logic and the complexities of the execution view are left to the framework implementation layer on the DefaulProcessor class.
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.SimpleProcessor.java
[source,java]
----
public class SimpleProcessor extends DefaultProcessor implements ProcessorCallBack {

	private static transient Logger LOG = Logger.getLogger(SimpleProcessor.class);
	...

	public void doProcess(Message message) throws ProcessorParsingException  { 
		LOG.debug(">> doProcess() Callback from processor: "+getName());	
		
		if(message==null||message instanceof ImmutableMessage) {
			throw new ProcessorParsingException("Illegal Message data content");
		}
		MutableMessage m=(MutableMessage) message;;
		
		String content=(String) m.getWrappedObject();
		content="*"+content.replaceAll("little", "big");
		m.setWrappedObject(content);
		LOG.debug("<< doProcess() Callback from processor: "+getName());
		
		
	}


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