一，介紹
本文記錄JAVA多線程中的中斷機(jī)制的一些知識(shí)點(diǎn)。主要是stop方法、interrupted()與isInterrupted()方法的區(qū)別，并從源代碼的實(shí)現(xiàn)上進(jìn)行簡(jiǎn)單分析。
JAVA中有3種方式可以終止正在運(yùn)行的線程
①線程正常退出，即run()方法執(zhí)行完畢了
②使用Thread類(lèi)中的stop()方法強(qiáng)行終止線程。但stop()方法已經(jīng)過(guò)期了，不推薦使用
③使用中斷機(jī)制
線程正常退出沒(méi)有什么東東，中斷機(jī)制下面詳細(xì)介紹，先看下stop()方法的源代碼，關(guān)鍵是源代碼上的注釋。它解釋了為什么stop()不安全，stop()方法停止的是哪個(gè)線程？
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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());} |
上面注釋，第9行到第16行表明，stop()方法可以停止“其他線程”。執(zhí)行thread.stop()方法這條語(yǔ)句的線程稱為當(dāng)前線程，而“其他線程”則是 調(diào)用thread.stop()方法的對(duì)象thread所代表的線程。
如：
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public static void main(String[] args) {MyThread thread = new MyThread...//.....thread.stop();//....} |
在main方法中，當(dāng)前線程就是main線程。它執(zhí)行到第4行，想把“其他線程”thread“ 給停止。這個(gè)其他線程就是MyThread類(lèi) new 的thread對(duì)象所表示的線程。
第21行至23行表明，可以停止一個(gè)尚未started(啟動(dòng))的線程。它的效果是：當(dāng)該線程啟動(dòng)后，就立馬結(jié)束了。
第48行以后的注釋，則深刻表明了為什么stop()方法被棄用！為什么它是不安全的。
比如說(shuō)，threadA線程擁有了監(jiān)視器，這些監(jiān)視器負(fù)責(zé)保護(hù)某些臨界資源，比如說(shuō)銀行的轉(zhuǎn)賬的金額。當(dāng)正在轉(zhuǎn)賬過(guò)程中，main線程調(diào)用 threadA.stop()方法。結(jié)果導(dǎo)致監(jiān)視器被釋放，其保護(hù)的資源（轉(zhuǎn)賬金額）很可能出現(xiàn)不一致性。比如，A賬戶減少了100，而B(niǎo)賬戶卻沒(méi)有增加100
二，中斷機(jī)制
JAVA中如何正確地使用中斷機(jī)制的細(xì)節(jié)太多了。interrupted()方法與 isInterrupted()方法都是反映當(dāng)前線程的是否處于中斷狀態(tài)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);} |
從源碼的注釋中看出，它測(cè)試的是當(dāng)前線程(current thread)的中斷狀態(tài)，且這個(gè)方法會(huì)清除中斷狀態(tài)。
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);} |
從源碼注釋中可以看出，isInterrupted()方法不會(huì)清除中斷狀態(tài)。
③interrupted()方法與 isInterrupted()方法的區(qū)別
從源代碼可以看出，這兩個(gè)方法都是調(diào)用的isInterrupted(boolean ClearInterrupted),只不過(guò)一個(gè)帶的參數(shù)是true，另一個(gè)帶的參數(shù)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，第一個(gè)區(qū)別就是，一個(gè)會(huì)清除中斷標(biāo)識(shí)位，另一個(gè)不會(huì)清除中斷標(biāo)識(shí)位。
再分析源碼，就可以看出第二個(gè)區(qū)別在return 語(yǔ)句上：
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public static boolean interrupted() {return currentThread().isInterrupted(true);}/************************/public boolean isInterrupted() {return isInterrupted(false);} |
interrupted()測(cè)試的是當(dāng)前的線程的中斷狀態(tài)。而isInterrupted()測(cè)試的是調(diào)用該方法的對(duì)象所表示的線程。一個(gè)是靜態(tài)方法（它測(cè)試的是當(dāng)前線程的中斷狀態(tài)），一個(gè)是實(shí)例方法（它測(cè)試的是實(shí)例對(duì)象所表示的線程的中斷狀態(tài)）。
下面用個(gè)具體的例子來(lái)更進(jìn)一步地闡明這個(gè)區(qū)別。
有一個(gè)自定義的線程類(lèi)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線程沒(méi)有被中斷!!! //...... |
第5行啟動(dòng)thread線程，第6行使main線程睡眠1秒鐘從而使得thread線程有機(jī)會(huì)獲得CPU執(zhí)行。
main線程睡眠1s鐘后，恢復(fù)執(zhí)行到第7行，請(qǐng)求中斷 thread線程。
第9行測(cè)試線程是否處于中斷狀態(tài)，這里測(cè)試的是哪個(gè)線程呢？？？答案是main線程。因?yàn)椋?/p>
（1）interrupted()測(cè)試的是當(dāng)前的線程的中斷狀態(tài)
（2）main線程執(zhí)行了第9行語(yǔ)句，故main線程是當(dāng)前線程
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對(duì)象調(diào)用的isInterrupted()方法。因此，測(cè)試的是thread對(duì)象所代表的線程的中斷狀態(tài)。由于在第7行，main線程請(qǐng)求中斷 thread線程，故在第8行的結(jié)果為: true
