| author | smarks |
| Tue, 12 Jun 2018 13:24:37 -0700 | |
| changeset 50530 | 7e3aa681a484 |
| parent 48942 | a6c4b85163c1 |
| child 52116 | f0340f0ea249 |
| permissions | -rw-r--r-- |
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<!doctype html> |
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<!-- |
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Copyright (c) 2005, 2018, Oracle and/or its affiliates. All rights reserved. |
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DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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This code is free software; you can redistribute it and/or modify it |
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under the terms of the GNU General Public License version 2 only, as |
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published by the Free Software Foundation. Oracle designates this |
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particular file as subject to the "Classpath" exception as provided |
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by Oracle in the LICENSE file that accompanied this code. |
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This code is distributed in the hope that it will be useful, but WITHOUT |
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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version 2 for more details (a copy is included in the LICENSE file that |
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accompanied this code). |
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You should have received a copy of the GNU General Public License version |
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2 along with this work; if not, write to the Free Software Foundation, |
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Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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or visit www.oracle.com if you need additional information or have any |
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questions. |
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--> |
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<html lang="en"> |
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<head> |
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<title>Java Thread Primitive Deprecation</title> |
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<link rel="stylesheet" type="text/css" href="../../../stylesheet.css" title="Style"> |
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</head> |
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<body> |
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<h2>Java Thread Primitive Deprecation</h2> |
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<hr> |
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<h3>Why is <code>Thread.stop</code> deprecated?</h3> |
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<p>Because it is inherently unsafe. Stopping a thread causes it to |
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unlock all the monitors that it has locked. (The monitors are |
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unlocked as the <code>ThreadDeath</code> exception propagates up |
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the stack.) If any of the objects previously protected by these |
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monitors were in an inconsistent state, other threads may now view |
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these objects in an inconsistent state. Such objects are said to be |
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<i>damaged</i>. When threads operate on damaged objects, arbitrary |
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behavior can result. This behavior may be subtle and difficult to |
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detect, or it may be pronounced. Unlike other unchecked exceptions, |
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<code>ThreadDeath</code> kills threads silently; thus, the user has |
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no warning that his program may be corrupted. The corruption can |
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manifest itself at any time after the actual damage occurs, even |
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hours or days in the future.</p> |
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<hr> |
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<h3>Couldn't I just catch the <code>ThreadDeath</code> exception |
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and fix the damaged object?</h3> |
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<p>In theory, perhaps, but it would <em>vastly</em> complicate the |
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task of writing correct multithreaded code. The task would be |
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nearly insurmountable for two reasons:</p> |
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<ol> |
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<li>A thread can throw a <code>ThreadDeath</code> exception |
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<i>almost anywhere</i>. All synchronized methods and blocks would |
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have to be studied in great detail, with this in mind.</li> |
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<li>A thread can throw a second <code>ThreadDeath</code> exception |
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while cleaning up from the first (in the <code>catch</code> or |
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<code>finally</code> clause). Cleanup would have to be repeated till |
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it succeeded. The code to ensure this would be quite complex.</li> |
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</ol> |
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In sum, it just isn't practical. |
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<hr> |
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<h3>What should I use instead of <code>Thread.stop</code>?</h3> |
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<p>Most uses of <code>stop</code> should be replaced by code that |
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simply modifies some variable to indicate that the target thread |
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should stop running. The target thread should check this variable |
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regularly, and return from its run method in an orderly fashion if |
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the variable indicates that it is to stop running. To ensure prompt |
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communication of the stop-request, the variable must be |
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<code>volatile</code> (or access to the variable must be |
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synchronized).</p> |
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<p>For example, suppose your applet contains the following |
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<code>start</code>, <code>stop</code> and <code>run</code> |
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methods:</p> |
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<pre> |
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private Thread blinker; |
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public void start() {
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blinker = new Thread(this); |
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blinker.start(); |
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} |
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public void stop() {
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blinker.stop(); // UNSAFE! |
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} |
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public void run() {
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while (true) {
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try {
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Thread.sleep(interval); |
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} catch (InterruptedException e){
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} |
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repaint(); |
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} |
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} |
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</pre> |
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You can avoid the use of <code>Thread.stop</code> by replacing the |
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applet's <code>stop</code> and <code>run</code> methods with: |
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<pre> |
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private volatile Thread blinker; |
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public void stop() {
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blinker = null; |
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} |
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public void run() {
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Thread thisThread = Thread.currentThread(); |
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while (blinker == thisThread) {
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try {
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Thread.sleep(interval); |
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} catch (InterruptedException e){
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} |
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repaint(); |
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} |
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} |
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</pre> |
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<hr> |
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<h3>How do I stop a thread that waits for long periods (e.g., for |
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input)?</h3> |
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<p>That's what the <code>Thread.interrupt</code> method is for. The |
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same "state based" signaling mechanism shown above can be used, but |
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the state change (<code>blinker = null</code>, in the previous |
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example) can be followed by a call to |
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<code>Thread.interrupt</code>, to interrupt the wait:</p> |
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<pre> |
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public void stop() {
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Thread moribund = waiter; |
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waiter = null; |
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moribund.interrupt(); |
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} |
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</pre> |
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For this technique to work, it's critical that any method that |
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catches an interrupt exception and is not prepared to deal with it |
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immediately reasserts the exception. We say <em>reasserts</em> |
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rather than <em>rethrows</em>, because it is not always possible to |
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rethrow the exception. If the method that catches the |
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<code>InterruptedException</code> is not declared to throw this |
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(checked) exception, then it should "reinterrupt itself" with the |
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following incantation: |
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<pre> |
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Thread.currentThread().interrupt(); |
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</pre> |
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This ensures that the Thread will reraise the |
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<code>InterruptedException</code> as soon as it is able. |
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<hr> |
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<h3>What if a thread doesn't respond to |
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<code>Thread.interrupt</code>?</h3> |
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<p>In some cases, you can use application specific tricks. For |
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example, if a thread is waiting on a known socket, you can close |
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the socket to cause the thread to return immediately. |
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Unfortunately, there really isn't any technique that works in |
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general. <em>It should be noted that in all situations where a |
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waiting thread doesn't respond to <code>Thread.interrupt</code>, it |
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wouldn't respond to <code>Thread.stop</code> either.</em> Such |
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cases include deliberate denial-of-service attacks, and I/O |
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operations for which thread.stop and thread.interrupt do not work |
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properly.</p> |
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<hr> |
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<h3>Why are <code>Thread.suspend</code> and |
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<code>Thread.resume</code> deprecated?</h3> |
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<p><code>Thread.suspend</code> is inherently deadlock-prone. If the |
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target thread holds a lock on the monitor protecting a critical |
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system resource when it is suspended, no thread can access this |
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resource until the target thread is resumed. If the thread that |
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would resume the target thread attempts to lock this monitor prior |
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to calling <code>resume</code>, deadlock results. Such deadlocks |
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typically manifest themselves as "frozen" processes.</p> |
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<hr> |
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<h3>What should I use instead of <code>Thread.suspend</code> and |
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<code>Thread.resume</code>?</h3> |
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<p>As with <code>Thread.stop</code>, the prudent approach is to |
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have the "target thread" poll a variable indicating the desired |
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state of the thread (active or suspended). When the desired state |
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is suspended, the thread waits using <code>Object.wait</code>. When |
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the thread is resumed, the target thread is notified using |
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<code>Object.notify</code>.</p> |
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<p>For example, suppose your applet contains the following |
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mousePressed event handler, which toggles the state of a thread |
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called <code>blinker</code>:</p> |
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<pre> |
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private boolean threadSuspended; |
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184 |
|
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Public void mousePressed(MouseEvent e) {
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e.consume(); |
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187 |
|
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if (threadSuspended) |
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blinker.resume(); |
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else |
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blinker.suspend(); // DEADLOCK-PRONE! |
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192 |
|
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threadSuspended = !threadSuspended; |
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} |
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</pre> |
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You can avoid the use of <code>Thread.suspend</code> and |
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<code>Thread.resume</code> by replacing the event handler above |
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with: |
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<pre> |
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public synchronized void mousePressed(MouseEvent e) {
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e.consume(); |
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202 |
|
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threadSuspended = !threadSuspended; |
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204 |
|
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if (!threadSuspended) |
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notify(); |
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} |
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</pre> |
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and adding the following code to the "run loop": |
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<pre> |
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synchronized(this) {
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while (threadSuspended) |
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wait(); |
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} |
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</pre> |
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The <code>wait</code> method throws the |
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<code>InterruptedException</code>, so it must be inside a <code>try |
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... catch</code> clause. It's fine to put it in the same clause as |
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the <code>sleep</code>. The check should follow (rather than |
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precede) the <code>sleep</code> so the window is immediately |
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repainted when the thread is "resumed." The resulting |
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<code>run</code> method follows: |
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<pre> |
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public void run() {
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while (true) {
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try {
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Thread.sleep(interval); |
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228 |
|
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synchronized(this) {
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while (threadSuspended) |
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wait(); |
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} |
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} catch (InterruptedException e){
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} |
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repaint(); |
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} |
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} |
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</pre> |
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Note that the <code>notify</code> in the <code>mousePressed</code> |
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method and the <code>wait</code> in the <code>run</code> method are |
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inside <code>synchronized</code> blocks. This is required by the |
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language, and ensures that <code>wait</code> and |
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<code>notify</code> are properly serialized. In practical terms, |
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this eliminates race conditions that could cause the "suspended" |
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thread to miss a <code>notify</code> and remain suspended |
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indefinitely. |
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<p>While the cost of synchronization in Java is decreasing as the |
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platform matures, it will never be free. A simple trick can be used |
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to remove the synchronization that we've added to each iteration of |
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the "run loop." The synchronized block that was added is replaced |
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by a slightly more complex piece of code that enters a synchronized |
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block only if the thread has actually been suspended:</p> |
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<pre> |
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if (threadSuspended) {
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synchronized(this) {
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while (threadSuspended) |
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wait(); |
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} |
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} |
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</pre> |
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<p>In the absence of explicit synchronization, |
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<code>threadSuspended</code> must be made <code>volatile</code> to ensure |
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prompt communication of the suspend-request.</p> |
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The resulting <code>run</code> method is: |
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<pre> |
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private volatile boolean threadSuspended; |
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public void run() {
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while (true) {
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try {
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Thread.sleep(interval); |
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|
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if (threadSuspended) {
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synchronized(this) {
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while (threadSuspended) |
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wait(); |
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} |
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} |
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} catch (InterruptedException e){
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} |
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repaint(); |
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} |
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} |
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</pre> |
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<hr size="3" noshade="noshade" /> |
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<h3>Can I combine the two techniques to produce a thread that may |
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be safely "stopped" or "suspended"?</h3> |
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Yes, it's reasonably straightforward. The one subtlety is that the |
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target thread may already be suspended at the time that another |
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thread tries to stop it. If the <code>stop</code> method merely sets |
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the state variable (<code>blinker</code>) to null, the target thread |
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will remain suspended (waiting on the monitor), rather than exiting |
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gracefully as it should. If the applet is restarted, multiple |
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threads could end up waiting on the monitor at the same time, |
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resulting in erratic behavior. |
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<p>To rectify this situation, the <code>stop</code> method must ensure |
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that the target thread resumes immediately if it is suspended. Once |
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the target thread resumes, it must recognize immediately that it |
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has been stopped, and exit gracefully. Here's how the resulting |
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<code>run</code> and <code>stop</code> methods look:</p> |
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<pre> |
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public void run() {
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Thread thisThread = Thread.currentThread(); |
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while (blinker == thisThread) {
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try {
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Thread.sleep(interval); |
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|
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synchronized(this) {
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while (threadSuspended && blinker==thisThread) |
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wait(); |
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} |
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} catch (InterruptedException e){
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} |
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repaint(); |
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} |
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} |
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public synchronized void stop() {
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blinker = null; |
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notify(); |
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} |
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</pre> |
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If the <code>stop</code> method calls <code>Thread.interrupt</code>, as |
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described above, it needn't call <code>notify</code> as well, but it |
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still must be synchronized. This ensures that the target thread |
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won't miss an interrupt due to a race condition. |
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<p><!-- Body text ends here --></p> |
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</body> |
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</html> |