停止一個(gè)線程意味著在任務(wù)處理完任務(wù)之前停掉正在做的操作，也就是放棄當(dāng)前的操作。停止一個(gè)線程可以用Thread.stop()方法，但最好不要用它。雖然它確實(shí)可以停止一個(gè)正在運(yùn)行的線程，但是這個(gè)方法是不安全的，而且是已被廢棄的方法。在java中有以下3種方法可以終止正在運(yùn)行的線程：

使用退出標(biāo)志，使線程正常退出，也就是當(dāng)run方法完成后線程終止。

使用stop方法強(qiáng)行終止，但是不推薦這個(gè)方法，因?yàn)閟top和suspend及resume一樣都是過(guò)期作廢的方法。

使用interrupt方法中斷線程。

1. 停止不了的線程

interrupt()方法的使用效果并不像for+break語(yǔ)句那樣，馬上就停止循環(huán)。調(diào)用interrupt方法是在當(dāng)前線程中打了一個(gè)停止標(biāo)志，并不是真的停止線程。

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();
????????for(int?i=0;?i<500000;?i++){
????????????System.out.println("i="+(i+1));
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[]){
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????try?{
????????????Thread.sleep(2000);
????????????thread.interrupt();
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????}
}

輸出結(jié)果：

...
i=499994
i=499995
i=499996
i=499997
i=499998
i=499999
i=500000

2. 判斷線程是否停止?fàn)顟B(tài)

Thread.java類中提供了兩種方法：

this.interrupted(): 測(cè)試當(dāng)前線程是否已經(jīng)中斷；

this.isInterrupted(): 測(cè)試線程是否已經(jīng)中斷；

那么這兩個(gè)方法有什么圖區(qū)別呢？我們先來(lái)看看this.interrupted()方法的解釋：測(cè)試當(dāng)前線程是否已經(jīng)中斷，當(dāng)前線程是指運(yùn)行this.interrupted()方法的線程。

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();
????????for(int?i=0;?i<500000;?i++){
????????????i++;
//????????????System.out.println("i="+(i+1));
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[]){
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????try?{
????????????Thread.sleep(2000);
????????????thread.interrupt();

????????????System.out.println("stop?1??"?+?thread.interrupted());
????????????System.out.println("stop?2??"?+?thread.interrupted());
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????}
}

運(yùn)行結(jié)果：

stop?1??false
stop?2??false

類Run.java中雖然是在thread對(duì)象上調(diào)用以下代碼：thread.interrupt(), 后面又使用

System.out.println("stop?1??"?+?thread.interrupted());
System.out.println("stop?2??"?+?thread.interrupted());

來(lái)判斷thread對(duì)象所代表的線程是否停止，但從控制臺(tái)打印的結(jié)果來(lái)看，線程并未停止，這也證明了interrupted()方法的解釋，測(cè)試當(dāng)前線程是否已經(jīng)中斷。這個(gè)當(dāng)前線程是main，它從未中斷過(guò)，所以打印的結(jié)果是兩個(gè)false.

如何使main線程產(chǎn)生中斷效果呢？

public?class?Run2?{
????public?static?void?main(String?args[]){
????????Thread.currentThread().interrupt();
????????System.out.println("stop?1??"?+?Thread.interrupted());
????????System.out.println("stop?2??"?+?Thread.interrupted());

????????System.out.println("End");
????}
}

運(yùn)行效果為：

stop?1??true
stop?2??false
End

方法interrupted()的確判斷出當(dāng)前線程是否是停止?fàn)顟B(tài)。但為什么第2個(gè)布爾值是false呢？ 官方幫助文檔中對(duì)interrupted方法的解釋：測(cè)試當(dāng)前線程是否已經(jīng)中斷。線程的中斷狀態(tài)由該方法清除。 換句話說(shuō)，如果連續(xù)兩次調(diào)用該方法，則第二次調(diào)用返回false。

下面來(lái)看一下inInterrupted()方法。

public?class?Run3?{
????public?static?void?main(String?args[]){
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????thread.interrupt();
????????System.out.println("stop?1??"?+?thread.isInterrupted());
????????System.out.println("stop?2??"?+?thread.isInterrupted());
????}
}

運(yùn)行結(jié)果：

stop?1??true
stop?2??true

isInterrupted()并為清除狀態(tài)，所以打印了兩個(gè)true。

3. 能停止的線程--異常法

有了前面學(xué)習(xí)過(guò)的知識(shí)點(diǎn)，就可以在線程中用for語(yǔ)句來(lái)判斷一下線程是否是停止?fàn)顟B(tài)，如果是停止?fàn)顟B(tài)，則后面的代碼不再運(yùn)行即可：

public?class?MyThread?extends?Thread?{

????public?void?run(){
????????super.run();
????????for(int?i=0;?i<500000;?i++){
????????????if(this.interrupted())?{
????????????????System.out.println("線程已經(jīng)終止，?for循環(huán)不再執(zhí)行");
????????????????break;
????????????}
????????????System.out.println("i="+(i+1));
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[]){
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????try?{
????????????Thread.sleep(2000);
????????????thread.interrupt();
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????}
}

運(yùn)行結(jié)果：

...
i=202053
i=202054
i=202055
i=202056
線程已經(jīng)終止，?for循環(huán)不再執(zhí)行

上面的示例雖然停止了線程，但如果for語(yǔ)句下面還有語(yǔ)句，還是會(huì)繼續(xù)運(yùn)行的。看下面的例子：

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();
????????for(int?i=0;?i<500000;?i++){
????????????if(this.interrupted())?{
????????????????System.out.println("線程已經(jīng)終止，?for循環(huán)不再執(zhí)行");
????????????????break;
????????????}
????????????System.out.println("i="+(i+1));
????????}

????????System.out.println("這是for循環(huán)外面的語(yǔ)句，也會(huì)被執(zhí)行");
????}
}

使用Run.java執(zhí)行的結(jié)果是：

...
i=180136
i=180137
i=180138
i=180139
線程已經(jīng)終止，?for循環(huán)不再執(zhí)行
這是for循環(huán)外面的語(yǔ)句，也會(huì)被執(zhí)行

如何解決語(yǔ)句繼續(xù)運(yùn)行的問(wèn)題呢？ 看一下更新后的代碼：

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();
????????try?{
????????????for(int?i=0;?i<500000;?i++){
????????????????if(this.interrupted())?{
????????????????????System.out.println("線程已經(jīng)終止，?for循環(huán)不再執(zhí)行");
????????????????????????throw?new?InterruptedException();
????????????????}
????????????????System.out.println("i="+(i+1));
????????????}

????????????System.out.println("這是for循環(huán)外面的語(yǔ)句，也會(huì)被執(zhí)行");
????????}?catch?(InterruptedException?e)?{
????????????System.out.println("進(jìn)入MyThread.java類中的catch了。。。");
????????????e.printStackTrace();
????????}
????}
}

使用Run.java運(yùn)行的結(jié)果如下：

...
i=203798
i=203799
i=203800
線程已經(jīng)終止，?for循環(huán)不再執(zhí)行
進(jìn)入MyThread.java類中的catch了。。。
java.lang.InterruptedException
?at?thread.MyThread.run(MyThread.java:13)

4. 在沉睡中停止

如果線程在sleep()狀態(tài)下停止線程，會(huì)是什么效果呢？

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();

????????try?{
????????????System.out.println("線程開(kāi)始。。。");
????????????Thread.sleep(200000);
????????????System.out.println("線程結(jié)束。");
????????}?catch?(InterruptedException?e)?{
????????????System.out.println("在沉睡中被停止,?進(jìn)入catch，?調(diào)用isInterrupted()方法的結(jié)果是："?+?this.isInterrupted());
????????????e.printStackTrace();
????????}

????}
}

使用Run.java運(yùn)行的結(jié)果是：

線程開(kāi)始。。。
在沉睡中被停止,?進(jìn)入catch，?調(diào)用isInterrupted()方法的結(jié)果是：false
java.lang.InterruptedException:?sleep?interrupted
?at?java.lang.Thread.sleep(Native?Method)
?at?thread.MyThread.run(MyThread.java:12)

從打印的結(jié)果來(lái)看， 如果在sleep狀態(tài)下停止某一線程，會(huì)進(jìn)入catch語(yǔ)句，并且清除停止?fàn)顟B(tài)值，使之變?yōu)閒alse。

前一個(gè)實(shí)驗(yàn)是先sleep然后再用interrupt()停止，與之相反的操作在學(xué)習(xí)過(guò)程中也要注意：

public?class?MyThread?extends?Thread?{
????public?void?run(){
????????super.run();
????????try?{
????????????System.out.println("線程開(kāi)始。。。");
????????????for(int?i=0;?i<10000;?i++){
????????????????System.out.println("i="?+?i);
????????????}
????????????Thread.sleep(200000);
????????????System.out.println("線程結(jié)束。");
????????}?catch?(InterruptedException?e)?{
?????????????System.out.println("先停止，再遇到sleep，進(jìn)入catch異常");
????????????e.printStackTrace();
????????}

????}
}

public?class?Run?{
????public?static?void?main(String?args[]){
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????thread.interrupt();
????}
}

運(yùn)行結(jié)果：

i=9998
i=9999
先停止，再遇到sleep，進(jìn)入catch異常
java.lang.InterruptedException:?sleep?interrupted
?at?java.lang.Thread.sleep(Native?Method)
?at?thread.MyThread.run(MyThread.java:15)

5. 能停止的線程---暴力停止

使用stop()方法停止線程則是非常暴力的。

public?class?MyThread?extends?Thread?{

????private?int?i?=?0;
????public?void?run(){
????????super.run();
????????try?{
????????????while?(true){
????????????????System.out.println("i="?+?i);
????????????????i++;
????????????????Thread.sleep(200);
????????????}
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[])?throws?InterruptedException?{
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????Thread.sleep(2000);
????????thread.stop();
????}
}

運(yùn)行結(jié)果：

i=0
i=1
i=2
i=3
i=4
i=5
i=6
i=7
i=8
i=9

Process?finished?with?exit?code?0

6.方法stop()與java.lang.ThreadDeath異常

調(diào)用stop()方法時(shí)會(huì)拋出java.lang.ThreadDeath異常，但是通常情況下，此異常不需要顯示地捕捉。

public?class?MyThread?extends?Thread?{

????private?int?i?=?0;
????public?void?run(){
????????super.run();
????????try?{
????????????this.stop();
????????}?catch?(ThreadDeath?e)?{
????????????System.out.println("進(jìn)入異常catch");
????????????e.printStackTrace();
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[])?throws?InterruptedException?{
????????Thread?thread?=?new?MyThread();
????????thread.start();
????}
}

stop()方法以及作廢，因?yàn)槿绻麖?qiáng)制讓線程停止有可能使一些清理性的工作得不到完成。另外一個(gè)情況就是對(duì)鎖定的對(duì)象進(jìn)行了解鎖，導(dǎo)致數(shù)據(jù)得不到同步的處理，出現(xiàn)數(shù)據(jù)不一致的問(wèn)題。

7. 釋放鎖的不良后果

使用stop()釋放鎖將會(huì)給數(shù)據(jù)造成不一致性的結(jié)果。如果出現(xiàn)這樣的情況，程序處理的數(shù)據(jù)就有可能遭到破壞，最終導(dǎo)致程序執(zhí)行的流程錯(cuò)誤，一定要特別注意：

public?class?SynchronizedObject?{

????private?String?name?=?"a";
????private?String?password?=?"aa";

????public?synchronized?void?printString(String?name,?String?password){
????????try?{
????????????this.name?=?name;
????????????Thread.sleep(100000);
????????????this.password?=?password;
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????}

????public?String?getName()?{
????????return?name;
????}

????public?void?setName(String?name)?{
????????this.name?=?name;
????}

????public?String?getPassword()?{
????????return?password;
????}

????public?void?setPassword(String?password)?{
????????this.password?=?password;
????}
}

public?class?MyThread?extends?Thread?{
????private?SynchronizedObject?synchronizedObject;
????public?MyThread(SynchronizedObject?synchronizedObject){
????????this.synchronizedObject?=?synchronizedObject;
????}

????public?void?run(){
????????synchronizedObject.printString("b",?"bb");
????}
}

public?class?Run?{
????public?static?void?main(String?args[])?throws?InterruptedException?{
????????SynchronizedObject?synchronizedObject?=?new?SynchronizedObject();
????????Thread?thread?=?new?MyThread(synchronizedObject);
????????thread.start();
????????Thread.sleep(500);
????????thread.stop();
????????System.out.println(synchronizedObject.getName()?+?"??"?+?synchronizedObject.getPassword());
????}
}

輸出結(jié)果：b??aa

由于stop()方法以及在JDK中被標(biāo)明為“過(guò)期/作廢”的方法，顯然它在功能上具有缺陷，所以不建議在程序張使用stop()方法。

8. 使用return停止線程

將方法interrupt()與return結(jié)合使用也能實(shí)現(xiàn)停止線程的效果：

public?class?MyThread?extends?Thread?{

????public?void?run(){
????????while?(true){
????????????if(this.isInterrupted()){
????????????????System.out.println("線程被停止了！");
????????????????return;
????????????}
????????????System.out.println("Time:?"?+?System.currentTimeMillis());
????????}
????}
}

public?class?Run?{
????public?static?void?main(String?args[])?throws?InterruptedException?{
????????Thread?thread?=?new?MyThread();
????????thread.start();
????????Thread.sleep(2000);
????????thread.interrupt();
????}
}

輸出結(jié)果：

...
Time:?1467072288503
Time:?1467072288503
Time:?1467072288503
線程被停止了！

不過(guò)還是建議使用“拋異?！钡姆椒▉?lái)實(shí)現(xiàn)線程的停止，因?yàn)樵赾atch塊中還可以將異常向上拋，使線程停止事件得以傳播。

編輯：黃飛