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35 <h1>AWT Threading Issues</h1>
36
37 <a id="ListenersThreads"></a>
38 <h2>Listeners and threads</h2>
39
40 Unless otherwise noted all AWT listeners are notified on the event
41 dispatch thread. It is safe to remove/add listeners from any thread
42 during dispatching, but the changes only effect subsequent notification.
43 <br>For example, if a key listeners is added from another key listener, the
44 newly added listener is only notified on subsequent key events.
45
46 <a id="Autoshutdown"></a>
47 <h2>Auto-shutdown</h2>
48
49 According to
50 <cite>The Java™ Virtual Machine Specification</cite>,
51 sections 2.17.9 and 2.19,
52 the Java virtual machine (JVM) initially starts up with a single non-daemon
53 thread, which typically calls the <code>main</code> method of some class.
54 The virtual machine terminates all its activity and exits when
55 one of two things happens:
56 <ul>
57 <li> All the threads that are not daemon threads terminate.
58 <li> Some thread invokes the <code>exit</code> method of class
59 <code>Runtime</code> or class <code>System</code>, and the exit
60 operation is permitted by the security manager.
61 </ul>
62 <p>
63 This implies that if an application doesn't start any threads itself,
64 the JVM will exit as soon as <code>main</code> terminates.
65 This is not the case, however, for a simple application
66 that creates and displays a <code>java.awt.Frame</code>:
67 <pre>
68 public static void main(String[] args) {
69 Frame frame = new Frame();
70 frame.setVisible(true);
71 }
72 </pre>
73 The reason is that AWT encapsulates asynchronous event dispatch
74 machinery to process events AWT or Swing components can fire. The
75 exact behavior of this machinery is implementation-dependent. In
76 particular, it can start non-daemon helper threads for its internal
77 purposes. In fact, these are the threads that prevent the example
78 above from exiting. The only restrictions imposed on the behavior of
79 this machinery are as follows:
80 <ul>
81 <li> <a href="../EventQueue.html#isDispatchThread()"><code>EventQueue.isDispatchThread</code></a>
82 returns <code>true</code> if and only if the calling thread is the
83 event dispatch thread started by the machinery;
84 <li> <code>AWTEvents</code> which were actually enqueued to a
85 particular <code>EventQueue</code> (note that events being
86 posted to the <code>EventQueue</code> can be coalesced) are
87 dispatched:
88 <ul>
89 <li>
90 <dl><dt>Sequentially.
91 <dd> That is, it is not permitted that several events from
92 this queue are dispatched simultaneously. </dd></dl>
93 <li>
94 <dl><dt>In the same order as they are enqueued.
95 <dd> That is, if <code>AWTEvent</code> A is enqueued
96 to the <code>EventQueue</code> before
97 <code>AWTEvent</code> B then event B will not be
98 dispatched before event A.</dd></dl>
99 </ul>
100 <li> There is at least one alive non-daemon thread while there is at
101 least one displayable AWT or Swing component within the
102 application (see
103 <a href="../Component.html#isDisplayable()"><code>Component.isDisplayable</code></a>).
104 </ul>
105 The implications of the third restriction are as follows:
106 <ul>
107 <li> The JVM will exit if some thread invokes the <code>exit</code>
108 method of class <code>Runtime</code> or class <code>System</code>
109 regardless of the presence of displayable components;
110 <li> Even if the application terminates all non-daemon threads it
111 started, the JVM will not exit while there is at least one
112 displayable component.
113 </ul>
114 It depends on the implementation if and when the non-daemon helper
115 threads are terminated once all components are made undisplayable.
116 The implementation-specific details are given below.
117
118 <h3>
119 Implementation-dependent behavior.
120 </h3>
121
122 Prior to 1.4, the helper threads were never terminated.
123 <p>
124 Starting with 1.4, the behavior has changed as a result of the fix for
125 <a href="https://bugs.java.com/view_bug.do?bug_id=4030718">
126 4030718</a>. With the current implementation, AWT terminates all its
127 helper threads allowing the application to exit cleanly when the
128 following three conditions are true:
129 <ul>
130 <li> There are no displayable AWT or Swing components.
131 <li> There are no native events in the native event queue.
132 <li> There are no AWT events in java EventQueues.
133 </ul>
134 Therefore, a stand-alone AWT application that wishes to exit
135 cleanly without calling <code>System.exit</code> must:
136 <ul>
137 <li> Make sure that all AWT or Swing components are made
138 undisplayable when the application finishes. This can be done
139 by calling
140 <a href="../Window.html#dispose()"><code>Window.dispose</code></a>
141 on all top-level <code>Windows</code>. See
142 <a href="../Frame.html#getFrames()"><code>Frame.getFrames</code></a>.
143 <li> Make sure that no method of AWT event listeners registered by
144 the application with any AWT or Swing component can run into an
145 infinite loop or hang indefinitely. For example, an AWT listener
146 method triggered by some AWT event can post a new AWT event of
147 the same type to the <code>EventQueue</code>.
148 The argument is that methods
149 of AWT event listeners are typically executed on helper
150 threads.
151 </ul>
152 Note, that while an application following these recommendations will
153 exit cleanly under normal conditions, it is not guaranteed that it
154 will exit cleanly in all cases. Two examples:
155 <ul>
156 <li> Other packages can create displayable components for internal
157 needs and never make them undisplayable. See
158 <a href="https://bugs.java.com/view_bug.do?bug_id=4515058">
159 4515058</a>,
160 <a href="https://bugs.java.com/view_bug.do?bug_id=4671025">
161 4671025</a>, and
162 <a href="https://bugs.java.com/view_bug.do?bug_id=4465537">
163 4465537</a>.
164 <li> Both Microsoft Windows and X11 allow an application to send native
165 events to windows that belong to another application. With this
166 feature it is possible to write a malicious program that will
167 continuously send events to all available windows preventing
168 any AWT application from exiting cleanly.
169 </ul>
170 On the other hand, if you require the JVM to continue running even after
171 the application has made all components undisplayable you should start a
172 non-daemon thread that blocks forever.
173
174 <pre>
175 <...>
176 Runnable r = new Runnable() {
177 public void run() {
178 Object o = new Object();
179 try {
180 synchronized (o) {
181 o.wait();
182 }
183 } catch (InterruptedException ie) {
184 }
185 }
186 };
187 Thread t = new Thread(r);
188 t.setDaemon(false);
189 t.start();
190 <...>
191 </pre>
192
193 <cite>The Java™ Virtual Machine Specification</cite>
194 guarantees
195 that the JVM doesn't exit until this thread terminates.
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