1 <!doctype html>
2 <html lang="en">
3 <head>
4 <meta charset="utf-8"/>
5 <title>AWT Threading Issues</title>
6 </head>
7 <!--
8 Copyright (c) 2002, 2017, Oracle and/or its affiliates. All rights reserved.
9 DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
10
11 This code is free software; you can redistribute it and/or modify it
12 under the terms of the GNU General Public License version 2 only, as
13 published by the Free Software Foundation. Oracle designates this
14 particular file as subject to the "Classpath" exception as provided
15 by Oracle in the LICENSE file that accompanied this code.
16
17 This code is distributed in the hope that it will be useful, but WITHOUT
18 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
19 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
20 version 2 for more details (a copy is included in the LICENSE file that
21 accompanied this code).
22
23 You should have received a copy of the GNU General Public License version
24 2 along with this work; if not, write to the Free Software Foundation,
25 Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
26
27 Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
28 or visit www.oracle.com if you need additional information or have any
29 questions.
30 -->
31
32 <body>
33
34 <h1>AWT Threading Issues</h1>
35
36 <a id="ListenersThreads"></a>
37 <h2>Listeners and threads</h2>
38
39 Unless otherwise noted all AWT listeners are notified on the event
40 dispatch thread. It is safe to remove/add listeners from any thread
41 during dispatching, but the changes only effect subsequent notification.
42 <br>For example, if a key listeners is added from another key listener, the
43 newly added listener is only notified on subsequent key events.
44
45 <a id="Autoshutdown"></a>
46 <h2>Auto-shutdown</h2>
47
48 According to
49 <cite>The Java™ Virtual Machine Specification</cite>,
50 sections 2.17.9 and 2.19,
51 the Java virtual machine (JVM) initially starts up with a single non-daemon
52 thread, which typically calls the <code>main</code> method of some class.
53 The virtual machine terminates all its activity and exits when
54 one of two things happens:
55 <ul>
56 <li> All the threads that are not daemon threads terminate.
57 <li> Some thread invokes the <code>exit</code> method of class
58 <code>Runtime</code> or class <code>System</code>, and the exit
59 operation is permitted by the security manager.
60 </ul>
61 <p>
62 This implies that if an application doesn't start any threads itself,
63 the JVM will exit as soon as <code>main</code> terminates.
64 This is not the case, however, for a simple application
65 that creates and displays a <code>java.awt.Frame</code>:
66 <pre>
67 public static void main(String[] args) {
68 Frame frame = new Frame();
69 frame.setVisible(true);
70 }
71 </pre>
72 The reason is that AWT encapsulates asynchronous event dispatch
73 machinery to process events AWT or Swing components can fire. The
74 exact behavior of this machinery is implementation-dependent. In
75 particular, it can start non-daemon helper threads for its internal
76 purposes. In fact, these are the threads that prevent the example
77 above from exiting. The only restrictions imposed on the behavior of
78 this machinery are as follows:
79 <ul>
80 <li> <a href="../EventQueue.html#isDispatchThread()"><code>EventQueue.isDispatchThread</code></a>
81 returns <code>true</code> if and only if the calling thread is the
82 event dispatch thread started by the machinery;
83 <li> <code>AWTEvents</code> which were actually enqueued to a
84 particular <code>EventQueue</code> (note that events being
85 posted to the <code>EventQueue</code> can be coalesced) are
86 dispatched:
87 <ul>
88 <li>
89 <dl><dt>Sequentially.
90 <dd> That is, it is not permitted that several events from
91 this queue are dispatched simultaneously. </dd></dl>
92 <li>
93 <dl><dt>In the same order as they are enqueued.
94 <dd> That is, if <code>AWTEvent</code> A is enqueued
95 to the <code>EventQueue</code> before
96 <code>AWTEvent</code> B then event B will not be
97 dispatched before event A.</dd></dl>
98 </ul>
99 <li> There is at least one alive non-daemon thread while there is at
100 least one displayable AWT or Swing component within the
101 application (see
102 <a href="../Component.html#isDisplayable()"><code>Component.isDisplayable</code></a>).
103 </ul>
104 The implications of the third restriction are as follows:
105 <ul>
106 <li> The JVM will exit if some thread invokes the <code>exit</code>
107 method of class <code>Runtime</code> or class <code>System</code>
108 regardless of the presence of displayable components;
109 <li> Even if the application terminates all non-daemon threads it
110 started, the JVM will not exit while there is at least one
111 displayable component.
112 </ul>
113 It depends on the implementation if and when the non-daemon helper
114 threads are terminated once all components are made undisplayable.
115 The implementation-specific details are given below.
116
117 <h3>
118 Implementation-dependent behavior.
119 </h3>
120
121 Prior to 1.4, the helper threads were never terminated.
122 <p>
123 Starting with 1.4, the behavior has changed as a result of the fix for
124 <a href="http://bugs.sun.com/view_bug.do?bug_id=4030718">
125 4030718</a>. With the current implementation, AWT terminates all its
126 helper threads allowing the application to exit cleanly when the
127 following three conditions are true:
128 <ul>
129 <li> There are no displayable AWT or Swing components.
130 <li> There are no native events in the native event queue.
131 <li> There are no AWT events in java EventQueues.
132 </ul>
133 Therefore, a stand-alone AWT application that wishes to exit
134 cleanly without calling <code>System.exit</code> must:
135 <ul>
136 <li> Make sure that all AWT or Swing components are made
137 undisplayable when the application finishes. This can be done
138 by calling
139 <a href="../Window.html#dispose()"><code>Window.dispose</code></a>
140 on all top-level <code>Windows</code>. See
141 <a href="../Frame.html#getFrames()"><code>Frame.getFrames</code></a>.
142 <li> Make sure that no method of AWT event listeners registered by
143 the application with any AWT or Swing component can run into an
144 infinite loop or hang indefinitely. For example, an AWT listener
145 method triggered by some AWT event can post a new AWT event of
146 the same type to the <code>EventQueue</code>.
147 The argument is that methods
148 of AWT event listeners are typically executed on helper
149 threads.
150 </ul>
151 Note, that while an application following these recommendations will
152 exit cleanly under normal conditions, it is not guaranteed that it
153 will exit cleanly in all cases. Two examples:
154 <ul>
155 <li> Other packages can create displayable components for internal
156 needs and never make them undisplayable. See
157 <a href="http://bugs.sun.com/view_bug.do?bug_id=4515058">
158 4515058</a>,
159 <a href="http://bugs.sun.com/view_bug.do?bug_id=4671025">
160 4671025</a>, and
161 <a href="http://bugs.sun.com/view_bug.do?bug_id=4465537">
162 4465537</a>.
163 <li> Both Microsoft Windows and X11 allow an application to send native
164 events to windows that belong to another application. With this
165 feature it is possible to write a malicious program that will
166 continuously send events to all available windows preventing
167 any AWT application from exiting cleanly.
168 </ul>
169 On the other hand, if you require the JVM to continue running even after
170 the application has made all components undisplayable you should start a
171 non-daemon thread that blocks forever.
172
173 <pre>
174 <...>
175 Runnable r = new Runnable() {
176 public void run() {
177 Object o = new Object();
178 try {
179 synchronized (o) {
180 o.wait();
181 }
182 } catch (InterruptedException ie) {
183 }
184 }
185 };
186 Thread t = new Thread(r);
187 t.setDaemon(false);
188 t.start();
189 <...>
190 </pre>
191
192 <cite>The Java™ Virtual Machine Specification</cite>
193 guarantees
194 that the JVM doesn't exit until this thread terminates.
195 </body>
196 </html>