Question

I have created a panel on which a set of objects is drawn. Each of the object is added as a mouse event listener to the panel. As I know, after an event occurs the listeners are notified and the code might be (or is?) executed in several threads. Is it possible to attach a custom code that will be executed after all listeners finish executing their code?

Answer 1

As for Noel comment:

A concern that this technique might trip on is if a particular listener implementation returns a notification early, and executes work “out-of-band” on a separate thread. In that case, the end of the notification call does not actually mark the end of the listener’s execution. If this is an actual concern (i.e, you’re aware of and concerned with other user-defined, elaborate listeners that do this sort of thing, you might want to refine and revisit the problem.

Using the SwingUtilities.invokeLater() you are sure the code is executed after all the listener were notified. But one of these listeners may perform it’s job in a separated thread.

If you need to execute your code after all the listeners not only were notified but they finished their job you could do something like this:

_pseudocode:

Listener implements MouseListener 
    +mouseClicked( event: MouseEvent ) 
        SwingUtilities.invokeLater( // returns immediately 
              someTask() 
        )

    -someTask()
        // perform some long task.

If you have your listeners as

 addListener( new Listener() )
 addListener( new Listener() )
 addListener( new Listener() )
 addListener( new ExecuteAtTheEnd() ) 

If all ( or some ) of your listeners perform their job in a different thread. Your ExecuteAtTheEnd may be executing it’s code at the end of the notification, but not at the end of the execution of the listeners code.

So, what you do?

You have to synchronize the usage of some lock flag and execute your code until that flag is not used.

It may be a counter that is incremented when the listener is executing and decremented when is not:

 Listener implements MouseListener 
    +mouseClicked( event: MouseEvent )
        lockFlagCount++ 
        SwingUtilities.invokeLater( 
              someTask() 
        )

    -someTask()
        // perform some long task.
        lockFlag--

And then you just proceed until this flag is false:

  ExecuteAtTheEndListener implements MouseListener 

       + mouseClicked( event: MouseEvent ) 
        SwingUtilities.invokeLater( 
              executeAtTheEnd() 
        )
       - executeAtTheEnd() 
            while( logFlagCount > 0 ) 
                wait()

             if( logFlagCount == 0 ) 
                 // continue with the task.                  

Of course it is much more complicated as I put it in the pseudo-code, but it should work, if you are in this kind of circumstance.