一，介绍
本文记录JAVA多线程中的中断机制的一些知识点。主要是stop方法、interrupted()与isInterrupted()方法的区别，并从源代码的实现上进行简单分析。
JAVA中有3种方式可以终止正在运行的线程
①线程正常退出，即run()方法执行完毕了
②使用Thread类中的stop()方法强行终止线程。但stop()方法已经过期了，不推荐使用
③使用中断机制
线程正常退出没有什么东东，中断机制下面详细介绍，先看下stop()方法的源代码，关键是源代码上的注释。它解释了为什么stop()不安全，stop()方法停止的是哪个线程？
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/*** Forces the thread to stop executing.* <p>* If there is a security manager installed, its <code>checkAccess</code>* method is called with <code>this</code>* as its argument. This may result in a* <code>SecurityException</code> being raised (in the current thread).* <p>* If this thread is different from the current thread (that is, the current* thread is trying to stop a thread other than itself), the* security manager's <code>checkPermission</code> method (with a* <code>RuntimePermission("stopThread")</code> argument) is called in* addition.* Again, this may result in throwing a* <code>SecurityException</code> (in the current thread).* <p>* The thread represented by this thread is forced to stop whatever* it is doing abnormally and to throw a newly created* <code>ThreadDeath</code> object as an exception.* <p>* It is permitted to stop a thread that has not yet been started.* If the thread is eventually started, it immediately terminates.* <p>* An application should not normally try to catch* <code>ThreadDeath</code> unless it must do some extraordinary* cleanup operation (note that the throwing of* <code>ThreadDeath</code> causes <code>finally</code> clauses of* <code>try</code> statements to be executed before the thread* officially dies). If a <code>catch</code> clause catches a* <code>ThreadDeath</code> object, it is important to rethrow the* object so that the thread actually dies.* <p>* The top-level error handler that reacts to otherwise uncaught* exceptions does not print out a message or otherwise notify the* application if the uncaught exception is an instance of* <code>ThreadDeath</code>.** @exception SecurityException if the current thread cannot* modify this thread.* @see #interrupt()* @see #checkAccess()* @see #run()* @see #start()* @see ThreadDeath* @see ThreadGroup#uncaughtException(Thread,Throwable)* @see SecurityManager#checkAccess(Thread)* @see SecurityManager#checkPermission* @deprecated This method is inherently unsafe. Stopping a thread with* Thread.stop causes it to unlock all of the monitors that it* has locked (as a natural consequence of the unchecked* <code>ThreadDeath</code> exception propagating up the stack). If* any of the objects previously protected by these monitors were in* an inconsistent state, the damaged objects become visible to* other threads, potentially resulting in arbitrary behavior. Many* uses of <code>stop</code> should be replaced by code that simply* modifies some variable to indicate that the target thread should* stop running. The target thread should check this variable* regularly, and return from its run method in an orderly fashion* if the variable indicates that it is to stop running. If the* target thread waits for long periods (on a condition variable,* for example), the <code>interrupt</code> method should be used to* interrupt the wait.* For more information, see* <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why* are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.*/@Deprecatedpublic final void stop() {stop(new ThreadDeath());} |
上面注释，第9行到第16行表明，stop()方法可以停止“其他线程”。执行thread.stop()方法这条语句的线程称为当前线程，而“其他线程”则是 调用thread.stop()方法的对象thread所代表的线程。
如：
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public static void main(String[] args) {MyThread thread = new MyThread...//.....thread.stop();//....} |
在main方法中，当前线程就是main线程。它执行到第4行，想把“其他线程”thread“ 给停止。这个其他线程就是MyThread类 new 的thread对象所表示的线程。
第21行至23行表明，可以停止一个尚未started(启动)的线程。它的效果是：当该线程启动后，就立马结束了。
第48行以后的注释，则深刻表明了为什么stop()方法被弃用！为什么它是不安全的。
比如说，threadA线程拥有了监视器，这些监视器负责保护某些临界资源，比如说银行的转账的金额。当正在转账过程中，main线程调用 threadA.stop()方法。结果导致监视器被释放，其保护的资源（转账金额）很可能出现不一致性。比如，A账户减少了100，而B账户却没有增加100
二，中断机制
JAVA中如何正确地使用中断机制的细节太多了。interrupted()方法与 isInterrupted()方法都是反映当前线程的是否处于中断状态的。
①interrupted()
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/*** Tests whether the current thread has been interrupted. The* <i>interrupted status</i> of the thread is cleared by this method. In* other words, if this method were to be called twice in succession, the* second call would return false (unless the current thread were* interrupted again, after the first call had cleared its interrupted* status and before the second call had examined it).** <p>A thread interruption ignored because a thread was not alive* at the time of the interrupt will be reflected by this method* returning false.** @return <code>true</code> if the current thread has been interrupted;* <code>false</code> otherwise.* @see #isInterrupted()* @revised .*/public static boolean interrupted() {return currentThread().isInterrupted(true);} |
从源码的注释中看出，它测试的是当前线程(current thread)的中断状态，且这个方法会清除中断状态。
②isInterrupted()
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/*** Tests whether this thread has been interrupted. The <i>interrupted* status</i> of the thread is unaffected by this method.** <p>A thread interruption ignored because a thread was not alive* at the time of the interrupt will be reflected by this method* returning false.** @return <code>true</code> if this thread has been interrupted;* <code>false</code> otherwise.* @see #interrupted()* @revised .*/public boolean isInterrupted() {return isInterrupted(false);} |
从源码注释中可以看出，isInterrupted()方法不会清除中断状态。
③interrupted()方法与 isInterrupted()方法的区别
从源代码可以看出，这两个方法都是调用的isInterrupted(boolean ClearInterrupted),只不过一个带的参数是true，另一个带的参数是false。
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/*** Tests if some Thread has been interrupted. The interrupted state* is reset or not based on the value of ClearInterrupted that is* passed.*/private native boolean isInterrupted(boolean ClearInterrupted); |
因此，第一个区别就是，一个会清除中断标识位，另一个不会清除中断标识位。
再分析源码，就可以看出第二个区别在return 语句上：
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public static boolean interrupted() {return currentThread().isInterrupted(true);}/************************/public boolean isInterrupted() {return isInterrupted(false);} |
interrupted()测试的是当前的线程的中断状态。而isInterrupted()测试的是调用该方法的对象所表示的线程。一个是静态方法（它测试的是当前线程的中断状态），一个是实例方法（它测试的是实例对象所表示的线程的中断状态）。
下面用个具体的例子来更进一步地阐明这个区别。
有一个自定义的线程类如下：
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public class MyThread extends Thread {@Overridepublic void run() {super.run();for (int i = ; i < ; i++) {System.out.println("i=" + (i + ));}}} |
先看interrupted()方法的示例：
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public class Run {public static void main(String[] args) {try {MyThread thread = new MyThread();thread.start();Thread.sleep();thread.interrupt();//Thread.currentThread().interrupt();System.out.println("是否停止？="+thread.interrupted());//falseSystem.out.println("是否停止？="+thread.interrupted());//false main线程没有被中断!!! //...... |
第5行启动thread线程，第6行使main线程睡眠1秒钟从而使得thread线程有机会获得CPU执行。
main线程睡眠1s钟后，恢复执行到第7行，请求中断 thread线程。
第9行测试线程是否处于中断状态，这里测试的是哪个线程呢？？？答案是main线程。因为：
（1）interrupted()测试的是当前的线程的中断状态
（2）main线程执行了第9行语句，故main线程是当前线程
再看isInterrupted()方法的示例：
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public class Run {public static void main(String[] args) {try {MyThread thread = new MyThread();thread.start();Thread.sleep();thread.interrupt();System.out.println("是否停止？="+thread.isInterrupted());//true |
在第8行，是thread对象调用的isInterrupted()方法。因此，测试的是thread对象所代表的线程的中断状态。由于在第7行，main线程请求中断 thread线程，故在第8行的结果为: true
