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<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 event
during dispatching, but the changes only effect subsequent notification.
<br>For example, if a key listeners is added from another key listener, the
newly added listener is only notified on subsequent key events.
<a name="Autoshutdown"></a>
<h2>Auto-shutdown</h2>
According to
<cite>The Java™ Virtual Machine Specification</cite>,
sections 2.17.9 and 2.19,
the Java virtual machine (JVM) initially starts up with a single non-daemon
thread, which typically calls the <code>main</code> method of some class.
The virtual machine terminates all its activity and exits when
one 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 application
<pre>
public static void main(String[] args) {
Frame frame = new Frame();
frame.setVisible(true);
}
</pre>
The reason is that AWT encapsulates asynchronous event dispatch
machinery to process events AWT or Swing components can fire. The
exact behavior of this machinery is implementation-dependent. In
particular, it can start non-daemon helper threads for its internal
purposes. In fact, these are the threads that prevent the example
above from exiting. The only restrictions imposed on the behavior of
this machinery are as follows:
<ul>
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> A is enqueued
to the <code>EventQueue</code> before
<code>AWTEvent</code> 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
</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 helper
threads 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
4030718</a>. With the current implementation, AWT terminates all its
helper threads allowing the application to exit cleanly when the
following 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 exit
<ul>
<li> Make sure that all AWT or Swing components are made
undisplayable when the application finishes. This can be done
by calling
on all top-level <code>Windows</code>. See
<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 will
exit cleanly under normal conditions, it is not guaranteed that it
will exit cleanly in all cases. Two examples:
<ul>
<li> Other packages can create displayable components for internal
needs and never make them undisplayable. See
4515058</a>,
4671025</a>, and
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 after
the application has made all components undisplayable you should start a
non-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™ Virtual Machine Specification</cite>
guarantees
that the JVM doesn't exit until this thread terminates.
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