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Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA or visit www.oracle.com if you need additional information or have any questions.--> <html><body bgcolor=white> <h1 align=center>AWT Threading Issues</h1> <a name="ListenersThreads"></a><h2>Listeners and threads</h2> Unless otherwise noted all AWT listeners are notified on the eventdispatch thread. It is safe to remove/add listeners from any threadduring dispatching, but the changes only effect subsequent notification.<br>For example, if a key listeners is added from another key listener, thenewly added listener is only notified on subsequent key events. <a name="Autoshutdown"></a><h2>Auto-shutdown</h2> According to<cite>The Java&trade; Virtual Machine Specification</cite>,sections 2.17.9 and 2.19,the Java virtual machine (JVM) initially starts up with a single non-daemonthread, which typically calls the <code>main</code> method of some class.The virtual machine terminates all its activity and exits whenone of two things happens:<ul>  <li> All the threads that are not daemon threads terminate.  <li> Some thread invokes the <code>exit</code> method of class   <code>Runtime</code> or class <code>System</code>, and the exit  operation is permitted by the security manager.</ul><p>This implies that if an application doesn't start any threads itself,the JVM will exit as soon as <code>main</code> terminates.This is not the case, however, for a simple applicationthat creates and displays a <code>java.awt.Frame</code>:<pre>        public static void main(String[] args) {            Frame frame = new Frame();            frame.setVisible(true);         }</pre>The reason is that AWT encapsulates asynchronous event dispatchmachinery to process events AWT or Swing components can fire. Theexact behavior of this machinery is implementation-dependent. Inparticular, it can start non-daemon helper threads for its internalpurposes. In fact, these are the threads that prevent the exampleabove from exiting. The only restrictions imposed on the behavior ofthis machinery are as follows:<ul>  <li> <a href="../EventQueue.html#isDispatchThread()"><code>EventQueue.isDispatchThread</code></a>       returns <code>true</code> if and only if the calling thread is the       event dispatch thread started by the machinery;  <li> <code>AWTEvents</code> which were actually enqueued to a       particular <code>EventQueue</code> (note that events being       posted to the <code>EventQueue</code> can be coalesced) are       dispatched:       <ul>           <li> Sequentially.           <dd> That is, it is not permitted that several events from	        this queue are dispatched simultaneously.            <li> In the same order as they are enqueued.           <dd> That is, if <code>AWTEvent</code>&nbsp;A is enqueued	        to the <code>EventQueue</code> before		<code>AWTEvent</code>&nbsp;B then event B will not be                 dispatched before event A.       </ul>  <li> There is at least one alive non-daemon thread while there is at       least one displayable AWT or Swing component within the       application (see       <a href="../Component.html#isDisplayable()"><code>Component.isDisplayable</code></a>).</ul>The implications of the third restriction are as follows: <ul>  <li> The JVM will exit if some thread invokes the <code>exit</code>  method of class <code>Runtime</code> or class <code>System</code>  regardless of the presence of displayable components;  <li> Even if the application terminates all non-daemon threads it  started, the JVM will not exit while there is at least one  displayable component.</ul>It depends on the implementation if and when the non-daemon helperthreads are terminated once all components are made undisplayable. The implementation-specific details are given below.  <h3>Implementation-dependent behavior.</h3> Prior to 1.4, the helper threads were never terminated.<p>Starting with 1.4, the behavior has changed as a result of the fix for<a href="http://bugs.sun.com/view_bug.do?bug_id=4030718">4030718</a>. With the current implementation, AWT terminates all itshelper threads allowing the application to exit cleanly when thefollowing three conditions are true:<ul>  <li> There are no displayable AWT or Swing components.  <li> There are no native events in the native event queue.  <li> There are no AWT events in java EventQueues.</ul>Therefore, a stand-alone AWT application that wishes to exitcleanly without calling <code>System.exit</code> must:<ul>  <li> Make sure that all AWT or Swing components are made       undisplayable when the application finishes. This can be done       by calling<a href="../Window.html#dispose()"><code>Window.dispose</code></a>       on all top-level <code>Windows</code>. See<a href="../Frame.html#getFrames()"><code>Frame.getFrames</code></a>.  <li> 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 <code>EventQueue</code>.       The argument is that methods       of AWT event listeners are typically executed on helper       threads.</ul>Note, that while an application following these recommendations willexit cleanly under normal conditions, it is not guaranteed that itwill exit cleanly in all cases. Two examples: <ul>  <li> Other packages can create displayable components for internal       needs and never make them undisplayable. See<a href="http://bugs.sun.com/view_bug.do?bug_id=4515058">4515058</a>,<a href="http://bugs.sun.com/view_bug.do?bug_id=4671025">4671025</a>, and<a href="http://bugs.sun.com/view_bug.do?bug_id=4465537">4465537</a>.   <li> Both Microsoft Windows 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.</ul>On the other hand, if you require the JVM to continue running even afterthe application has made all components undisplayable you should start anon-daemon thread that blocks forever.  <pre>        <...>        Runnable r = new Runnable() {            public void run() {                Object o = new Object();                try {                    synchronized (o) {                        o.wait();                    }                } catch (InterruptedException ie) {                }            }        };        Thread t = new Thread(r);        t.setDaemon(false);        t.start();        <...></pre> <cite>The Java&trade; Virtual Machine Specification</cite> guaranteesthat the JVM doesn't exit until this thread terminates.</body></html>