一，介紹
本文記錄Java多線程中的中斷機制的一些知識點。主要是stop方法、interrupted()與isInterrupted()方法的區別，並從源代碼的實現上進行簡單分析。
Java中有3種方式可以終止正在運行的線程
①線程正常退出，即run()方法執行完畢了
②使用Thread類中的stop()方法強行終止線程。但stop()方法已經過期了，不推薦使用
③使用中斷機制
線程正常退出沒有什麼東東，中斷機制下面詳細介紹，先看下stop()方法的源代碼，關鍵是源代碼上的注釋。它解釋了為什麼stop()不安全，stop()方法停止的是哪個線程？
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 }
上面注釋，第9行到第16行表明，stop()方法可以停止“其他線程”。執行thread.stop()方法這條語句的線程稱為當前線程，而“其他線程”則是 調用thread.stop()方法的對象thread所代表的線程。
如：
1 public static void main(String[] args) {
2 MyThread thread = new MyThread...
3 //.....
4 thread.stop();
5 //....
6 }
在main方法中，當前線程就是main線程。它執行到第4行，想把“其他線程”thread“ 給停止。這個其他線程就是MyThread類 new 的thread對象所表示的線程。
第21行至23行表明，可以停止一個尚未started(啟動)的線程。它的效果是：當該線程啟動後，就立馬結束了。
第48行以後的注釋，則深刻表明了為什麼stop()方法被棄用！為什麼它是不安全的。
比如說，threadA線程擁有了監視器，這些監視器負責保護某些臨界資源，比如說銀行的轉賬的金額。當正在轉賬過程中，main線程調用 threadA.stop()方法。結果導致監視器被釋放，其保護的資源（轉賬金額）很可能出現不一致性。比如，A賬戶減少了100，而B賬戶卻沒有增加100
二，中斷機制
JAVA中如何正確地使用中斷機制的細節太多了。interrupted()方法與 isInterrupted()方法都是反映當前線程的是否處於中斷狀態的。
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 }
從源碼的注釋中看出，它測試的是當前線程(current thread)的中斷狀態，且這個方法會清除中斷狀態。
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 }
從源碼注釋中可以看出，isInterrupted()方法不會清除中斷狀態。
③interrupted()方法與 isInterrupted()方法的區別
從源代碼可以看出，這兩個方法都是調用的isInterrupted(boolean ClearInterrupted),只不過一個帶的參數是true，另一個帶的參數是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，第一個區別就是，一個會清除中斷標識位，另一個不會清除中斷標識位。
再分析源碼，就可以看出第二個區別在return 語句上：
public static boolean interrupted() {
return currentThread().isInterrupted(true);
}
/************************/
public boolean isInterrupted() {
return isInterrupted(false);
}
interrupted()測試的是當前的線程的中斷狀態。而isInterrupted()測試的是調用該方法的對象所表示的線程。一個是靜態方法（它測試的是當前線程的中斷狀態），一個是實例方法（它測試的是實例對象所表示的線程的中斷狀態）。
下面用個具體的例子來更進一步地闡明這個區別。
有一個自定義的線程類如下：
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線程沒有被中斷!!!
//......
第5行啟動thread線程，第6行使main線程睡眠1秒鐘從而使得thread線程有機會獲得CPU執行。
main線程睡眠1s鐘後，恢復執行到第7行，請求中斷 thread線程。
第9行測試線程是否處於中斷狀態，這裡測試的是哪個線程呢？？？答案是main線程。因為：
（1）interrupted()測試的是當前的線程的中斷狀態
（2）main線程執行了第9行語句，故main線程是當前線程
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對象調用的isInterrupted()方法。因此，測試的是thread對象所代表的線程的中斷狀態。由於在第7行，main線程請求中斷 thread線程，故在第8行的結果為: true
