AWT Threading Issues

• All the threads that are not daemon threads terminate.
• Some thread invokes the exit method of class Runtime or class System, and the exit operation is permitted by the security manager.

This implies that if an application doesn't start any threads itself,the JVM will exit as soon as main terminates.This is not the case, however, for a simple applicationthat creates and displays a java.awt.Frame:

        public static void main(String[] args) {
            Frame frame = new Frame();
            frame.setVisible(true);
         }

• EventQueue.isDispatchThread returns true if and only if the calling thread is the event dispatch thread started by the machinery;
• AWTEvents which were actually enqueued to a particular EventQueue (note that events being posted to the EventQueue can be coalesced) are dispatched:
  • Sequentially.
    That is, it is not permitted that several events from this queue are dispatched simultaneously.

    In the same order as they are enqueued.

    That is, if AWTEvent A is enqueued to the EventQueue before AWTEvent B then event B will not be dispatched before event A.
• There is at least one alive non-daemon thread while there is at least one displayable AWT or Swing component within the application (see Component.isDisplayable ).

• The JVM will exit if some thread invokes the exit method of class Runtime or class System regardless of the presence of displayable components;
• Even if the application terminates all non-daemon threads it started, the JVM will not exit while there is at least one displayable component.

Implementation-dependent behavior.

Starting with 1.4, the behavior has changed as a result of the fix for 4030718 . With the current implementation, AWT terminates all itshelper threads allowing the application to exit cleanly when thefollowing three conditions are true:

• There are no displayable AWT or Swing components.
• There are no native events in the native event queue.
• There are no AWT events in java EventQueues.

• Make sure that all AWT or Swing components are made undisplayable when the application finishes. This can be done by calling Window.dispose on all top-level Windows. See Frame.getFrames .
• Make sure that no method of AWT event listeners registered by the application with any AWT or Swing component can run into an infinite loop or hang indefinitely. For example, an AWT listener method triggered by some AWT event can post a new AWT event of the same type to the EventQueue. The argument is that methods of AWT event listeners are typically executed on helper threads.

• Other packages can create displayable components for internal needs and never make them undisplayable. See 4515058 , 4671025 , and 4465537 .
• Both Microsoft Windows Win32 and X11 allow an application to send native events to windows that belong to another application. With this feature it is possible to write a malicious program that will continuously send events to all available windows preventing any AWT application from exiting cleanly.

	<...>
	Runnable r = new Runnable() {
	    public void run() {
	        Object o = new Object();
		try {
		    o.wait();
		} catch (InterruptedException ie) {
		}
            }
	};
	Thread t = new Thread(r);
	t.setDaemon(false);
	t.start();
	<...>