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Following snippet will install an window listener and run your clean up code before the application is exited. However when user selects Quit instead of hitting the close button, clean up code will not run.

addWindowListener(new WindowAdapter(){
        public void windowClosing(WindowEvent we){

            //do something here
            System.exit(0);
        }});

In order to handle quit menu item. Apple's Application class class provides handlers, but these classes are Apple only, so you should not try loading them when you are not OS X. So wrap them in a class and load that class only when you are on OS X and use standard window listener on other operating systems.

import com.apple.eawt.*;
import com.apple.mrj.*;

public class MacApplication extends Application {
    public MacApplication() {
        addApplicationListener( new ApplicationAdapter(){
                public void handleReOpenApplication(ApplicationEvent event) {

                }
                public void handleQuit( ApplicationEvent event ) {
                    //do something here...
                    System.exit(0);
                }
                public void handleAbout(ApplicationEvent event){

                }
            });
    }
}

Now when Quit menu item is selected your application will run your clean up code.

When you close a window on a Mac OS X, application is kept running and only the window is hidden. In order for our application to act more like a native application we should also implement this. Fortunately Application class provides a handler for that too.

public void handleReOpenApplication(ApplicationEvent event) {
    mainFrame.setVisible(true);
}

Now when the dock icon is clicked your application will be visible and when closed, it will be hidden. Don't forget to set your main frame's default close operation to hide.

If you are not on OS X, you shouldn't load MacApplication class. On Mac OS X mrj.version system property is always set you can check it's value to see if you are on OS X. If it's set create your application object.

if(System.getProperty("mrj.version") == null){
    addWindowListener(new WindowAdapter(){
            public void windowClosing(WindowEvent we){
                //not on a mac cleanup
                System.exit(0);
            }});
}else{      
    MacApplication macApplication = new MacApplication();
}

• Application (Apple Java Extensions)