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  26 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
  27 <html>
  28 <head>
  29 <title></title>
  30 </head>
  31 <body bgcolor=white>
  32 
  33 <h1 align=center>AWT Threading Issues</h1>
  34 
  35 <a name="ListenersThreads"></a>
  36 <h2>Listeners and threads</h2>
  37 
  38 Unless otherwise noted all AWT listeners are notified on the event
  39 dispatch thread. It is safe to remove/add listeners from any thread
  40 during dispatching, but the changes only effect subsequent notification.
  41 <br>For example, if a key listeners is added from another key listener, the
  42 newly added listener is only notified on subsequent key events.
  43 
  44 <a name="Autoshutdown"></a>
  45 <h2>Auto-shutdown</h2>
  46 
  47 According to
  48 <cite>The Java&trade; Virtual Machine Specification</cite>,
  49 sections 2.17.9 and 2.19,
  50 the Java virtual machine (JVM) initially starts up with a single non-daemon
  51 thread, which typically calls the <code>main</code> method of some class.
  52 The virtual machine terminates all its activity and exits when
  53 one of two things happens:
  54 <ul>
  55   <li> All the threads that are not daemon threads terminate.
  56   <li> Some thread invokes the <code>exit</code> method of class 
  57   <code>Runtime</code> or class <code>System</code>, and the exit
  58   operation is permitted by the security manager.
  59 </ul>
  60 <p>
  61 This implies that if an application doesn't start any threads itself,
  62 the JVM will exit as soon as <code>main</code> terminates.
  63 This is not the case, however, for a simple application
  64 that creates and displays a <code>java.awt.Frame</code>:
  65 <pre>
  66         public static void main(String[] args) {
  67             Frame frame = new Frame();
  68             frame.setVisible(true);
  69          }
  70 </pre>
  71 The reason is that AWT encapsulates asynchronous event dispatch
  72 machinery to process events AWT or Swing components can fire. The
  73 exact behavior of this machinery is implementation-dependent. In
  74 particular, it can start non-daemon helper threads for its internal
  75 purposes. In fact, these are the threads that prevent the example
  76 above from exiting. The only restrictions imposed on the behavior of
  77 this machinery are as follows:
  78 <ul>
  79   <li> <a href="../EventQueue.html#isDispatchThread()"><code>EventQueue.isDispatchThread</code></a>
  80        returns <code>true</code> if and only if the calling thread is the
  81        event dispatch thread started by the machinery;
  82   <li> <code>AWTEvents</code> which were actually enqueued to a
  83        particular <code>EventQueue</code> (note that events being
  84        posted to the <code>EventQueue</code> can be coalesced) are
  85        dispatched:
  86        <ul>
  87            <li> Sequentially.
  88            <dl><dd> That is, it is not permitted that several events from

  89                 this queue are dispatched simultaneously. </dd></dl>
  90            <li> In the same order as they are enqueued.
  91            <dl><dd> That is, if <code>AWTEvent</code>&nbsp;A is enqueued

  92                 to the <code>EventQueue</code> before
  93                 <code>AWTEvent</code>&nbsp;B then event B will not be 
  94                 dispatched before event A.</dd></dl>
  95        </ul>
  96   <li> There is at least one alive non-daemon thread while there is at
  97        least one displayable AWT or Swing component within the
  98        application (see
  99        <a href="../Component.html#isDisplayable()"><code>Component.isDisplayable</code></a>).
 100 </ul>
 101 The implications of the third restriction are as follows: 
 102 <ul>
 103   <li> The JVM will exit if some thread invokes the <code>exit</code>
 104   method of class <code>Runtime</code> or class <code>System</code>
 105   regardless of the presence of displayable components;
 106   <li> Even if the application terminates all non-daemon threads it
 107   started, the JVM will not exit while there is at least one
 108   displayable component.
 109 </ul>
 110 It depends on the implementation if and when the non-daemon helper
 111 threads are terminated once all components are made undisplayable. 
 112 The implementation-specific details are given below. 
 113 
 114 <h3>
 115 Implementation-dependent behavior.
 116 </h3>
 117 
 118 Prior to 1.4, the helper threads were never terminated.
 119 <p>
 120 Starting with 1.4, the behavior has changed as a result of the fix for
 121 <a href="http://bugs.sun.com/view_bug.do?bug_id=4030718">
 122 4030718</a>. With the current implementation, AWT terminates all its
 123 helper threads allowing the application to exit cleanly when the
 124 following three conditions are true:
 125 <ul>
 126   <li> There are no displayable AWT or Swing components.
 127   <li> There are no native events in the native event queue.
 128   <li> There are no AWT events in java EventQueues.
 129 </ul>
 130 Therefore, a stand-alone AWT application that wishes to exit
 131 cleanly without calling <code>System.exit</code> must:
 132 <ul>
 133   <li> Make sure that all AWT or Swing components are made
 134        undisplayable when the application finishes. This can be done
 135        by calling
 136 <a href="../Window.html#dispose()"><code>Window.dispose</code></a>
 137        on all top-level <code>Windows</code>. See
 138 <a href="../Frame.html#getFrames()"><code>Frame.getFrames</code></a>.
 139   <li> Make sure that no method of AWT event listeners registered by
 140        the application with any AWT or Swing component can run into an
 141        infinite loop or hang indefinitely. For example, an AWT listener
 142        method triggered by some AWT event can post a new AWT event of
 143        the same type to the <code>EventQueue</code>.
 144        The argument is that methods
 145        of AWT event listeners are typically executed on helper
 146        threads.
 147 </ul>
 148 Note, that while an application following these recommendations will
 149 exit cleanly under normal conditions, it is not guaranteed that it
 150 will exit cleanly in all cases. Two examples: 
 151 <ul>
 152   <li> Other packages can create displayable components for internal
 153        needs and never make them undisplayable. See
 154 <a href="http://bugs.sun.com/view_bug.do?bug_id=4515058">
 155 4515058</a>,
 156 <a href="http://bugs.sun.com/view_bug.do?bug_id=4671025">
 157 4671025</a>, and
 158 <a href="http://bugs.sun.com/view_bug.do?bug_id=4465537">
 159 4465537</a>. 
 160   <li> Both Microsoft Windows and X11 allow an application to send native
 161        events to windows that belong to another application. With this
 162        feature it is possible to write a malicious program that will
 163        continuously send events to all available windows preventing
 164        any AWT application from exiting cleanly.
 165 </ul>
 166 On the other hand, if you require the JVM to continue running even after
 167 the application has made all components undisplayable you should start a
 168 non-daemon thread that blocks forever. 
 169 
 170 <pre>
 171         <...>
 172         Runnable r = new Runnable() {
 173             public void run() {
 174                 Object o = new Object();
 175                 try {
 176                     synchronized (o) {
 177                         o.wait();
 178                     }
 179                 } catch (InterruptedException ie) {
 180                 }
 181             }
 182         };
 183         Thread t = new Thread(r);
 184         t.setDaemon(false);
 185         t.start();
 186         <...>
 187 </pre>
 188 
 189 <cite>The Java&trade; Virtual Machine Specification</cite>
 190  guarantees
 191 that the JVM doesn't exit until this thread terminates.
 192 </body>
 193 </html>
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