Can I add listeners/publisher subscriber to JAXB objects?

Question

I have a multithreaded application using JAXB. The JAXB object represent state of other system hardware. I have a thread monitoring the system hardware, and it updates the JAXB objects with the setters to update the state. I have another chunk of code that is supposed to "do something" when the state of an object changes. I'm wondering how to set up event listeners, or some sort of publisher subscriber scheme, so that when a JAXB object changes state, the appropriate response to the change is kicked off. I have not seen anything describing how something could register as a listener with a JAXB object.

Answer 1

JAXB objects themselves do not have built-in support for listeners or publisher/subscriber schemes. However, you can add this functionality to your application by implementing it yourself.

One approach is to use the Observer pattern, where you define a custom interface that your listener classes implement. This interface defines the callback method that will be called when the state of the JAXB object changes. Then, in your JAXB class, you can maintain a list of listeners and call their callback methods when the state changes. You can also define methods to add and remove listeners from the list.

Here's an example interface for a listener:

public interface JAXBListener {

   public void stateChanged(JAXBObject obj);

}

And here's an example implementation in the JAXB object:

public class JAXBObject {

   private List<JAXBListener> listeners = new ArrayList<>();

   // ... other JAXB properties and methods ...

   public void addListener(JAXBListener listener) {

       listeners.add(listener);

   }

   public void removeListener(JAXBListener listener) {

       listeners.remove(listener);

   }

   public void setState(String newState) {

       // update the state

       // ...

       // notify listeners

       for (JAXBListener listener : listeners) {

           listener.stateChanged(this);

       }

   }

}

In this example, the setState method updates the state of the object and then notifies all registered listeners by calling their stateChanged method.

You can then implement the JAXBListener interface in any classes that need to be notified of state changes and register them as listeners with the JAXB object using the addListener method.