JAVA多线程之中断机制（stop()、interrupted()、isInterrupted()）

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？

 1     /**
 2      * Forces the thread to stop executing.
 3      * <p>
 4      * If there is a security manager installed, its <code>checkAccess</code>
 5      * method is called with <code>this</code>
 6      * as its argument. This may result in a
 7      * <code>SecurityException</code> being raised (in the current thread).
 8      * <p>
 9      * If this thread is different from the current thread (that is, the current
10      * thread is trying to stop a thread other than itself), the
11      * security manager's <code>checkPermission</code> method (with a
12      * <code>RuntimePermission("stopThread")</code> argument) is called in
13      * addition.
14      * Again, this may result in throwing a
15      * <code>SecurityException</code> (in the current thread).
16      * <p>
17      * The thread represented by this thread is forced to stop whatever
18      * it is doing abnormally and to throw a newly created
19      * <code>ThreadDeath</code> object as an exception.
20      * <p>
21      * It is permitted to stop a thread that has not yet been started.
22      * If the thread is eventually started, it immediately terminates.
23      * <p>
24      * An application should not normally try to catch
25      * <code>ThreadDeath</code> unless it must do some extraordinary
26      * cleanup operation (note that the throwing of
27      * <code>ThreadDeath</code> causes <code>finally</code> clauses of
28      * <code>try</code> statements to be executed before the thread
29      * officially dies).  If a <code>catch</code> clause catches a
30      * <code>ThreadDeath</code> object, it is important to rethrow the
31      * object so that the thread actually dies.
32      * <p>
33      * The top-level error handler that reacts to otherwise uncaught
34      * exceptions does not print out a message or otherwise notify the
35      * application if the uncaught exception is an instance of
36      * <code>ThreadDeath</code>.
37      *
38      * @exception  SecurityException  if the current thread cannot
39      *               modify this thread.
40      * @see        #interrupt()
41      * @see        #checkAccess()
42      * @see        #run()
43      * @see        #start()
44      * @see        ThreadDeath
45      * @see        ThreadGroup#uncaughtException(Thread,Throwable)
46      * @see        SecurityManager#checkAccess(Thread)
47      * @see        SecurityManager#checkPermission
48      * @deprecated This method is inherently unsafe.  Stopping a thread with
49      *       Thread.stop causes it to unlock all of the monitors that it
50      *       has locked (as a natural consequence of the unchecked
51      *       <code>ThreadDeath</code> exception propagating up the stack).  If
52      *       any of the objects previously protected by these monitors were in
53      *       an inconsistent state, the damaged objects become visible to
54      *       other threads, potentially resulting in arbitrary behavior.  Many
55      *       uses of <code>stop</code> should be replaced by code that simply
56      *       modifies some variable to indicate that the target thread should
57      *       stop running.  The target thread should check this variable
58      *       regularly, and return from its run method in an orderly fashion
59      *       if the variable indicates that it is to stop running.  If the
60      *       target thread waits for long periods (on a condition variable,
61      *       for example), the <code>interrupt</code> method should be used to
62      *       interrupt the wait.
63      *       For more information, see
64      *       <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why
65      *       are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>.
66      */
67     @Deprecated
68     public final void stop() {
69         stop(new ThreadDeath());
70     }

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：

1     public static void main(String[] args) {
2         MyThread thread = new MyThread...
3         //.....
4         thread.stop();
5         //....
6     }

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()

 1     /**
 2      * Tests whether the current thread has been interrupted.  The
 3      * <i>interrupted status</i> of the thread is cleared by this method.  In
 4      * other words, if this method were to be called twice in succession, the
 5      * second call would return false (unless the current thread were
 6      * interrupted again, after the first call had cleared its interrupted
 7      * status and before the second call had examined it).
 8      *
 9      * <p>A thread interruption ignored because a thread was not alive
10      * at the time of the interrupt will be reflected by this method
11      * returning false.
12      *
13      * @return  <code>true</code> if the current thread has been interrupted;
14      *          <code>false</code> otherwise.
15      * @see #isInterrupted()
16      * @revised 6.0
17      */
18     public static boolean interrupted() {
19         return currentThread().isInterrupted(true);
20     }

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()

 1     /**
 2      * Tests whether this thread has been interrupted.  The <i>interrupted
 3      * status</i> of the thread is unaffected by this method.
 4      *
 5      * <p>A thread interruption ignored because a thread was not alive
 6      * at the time of the interrupt will be reflected by this method
 7      * returning false.
 8      *
 9      * @return  <code>true</code> if this thread has been interrupted;
10      *          <code>false</code> otherwise.
11      * @see     #interrupted()
12      * @revised 6.0
13      */
14     public boolean isInterrupted() {
15         return isInterrupted(false);
16     }

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。

1     /**
2      * Tests if some Thread has been interrupted.  The interrupted state
3      * is reset or not based on the value of ClearInterrupted that is
4      * passed.
5      */
6     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：

    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：

1 public class MyThread extends Thread {
2     @Override
3     public void run() {
4         super.run();
5         for (int i = 0; i < 500000; i++) {
6             System.out.println("i=" + (i + 1));
7         }
8     }
9 }

 

先看interrupted()方法的示例：

 1 public class Run {
 2     public static void main(String[] args) {
 3         try {
 4             MyThread thread = new MyThread();
 5             thread.start();
 6             Thread.sleep(1000);
 7             thread.interrupt();
 8             //Thread.currentThread().interrupt();
 9             System.out.println("是否停止1？="+thread.interrupted());//false
10             System.out.println("是否停止2？="+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：

1 public class Run3 {
2     public static void main(String[] args) {
3         try {
4             MyThread thread = new MyThread();
5             thread.start();
6             Thread.sleep(1000);
7             thread.interrupt();
8             System.out.println("是否停止1？="+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