如何中断线程in Java

如何中断Java线程？查看API，不就是用interrupt()方法么？而线程是否已经中断则用Thread.currentThread().isInterrupted()返回true/false：

public class ThreadDemo extends Thread{
         public static void main(String[] args) throws InterruptedException {
                Thread thread = new ThreadDemo();
                thread.start();
                thread.sleep(100);
                thread.interrupt(); //中断线程
        }

        @Override
         public void run() {
                 while(! Thread.currentThread().isInterrupted()) {
                        System.out.println( "thread running");

                         try {
                                Thread.sleep(100);
                        } catch(InterruptedException e)
                        {
                                System.out.println( "InterruptedException");
                        }
                }

                System.out.println( "thread interrupted");
        }
}

运行结果:
thread running
InterruptedException
thread running
thread running
thread running
thread running
...

 

很快发现这方法似乎有问题，线程没有成功中断，于是在处理InterruptedException异常后调用break跳出while循环：

public class ThreadDemo extends Thread{
         public static void main(String[] args) throws InterruptedException {
                Thread thread = new ThreadDemo();
                thread.start();
                thread.sleep(100);
                thread.interrupt(); //中断线程
        }

        @Override
         public void run() {
                 while(!Thread.currentThread().isInterrupted()) {
                        System.out.println( "thread running");

                         try {
                                Thread.sleep(100);
                        } catch(InterruptedException e)
                        {
                                System.out.println( "InterruptedException");
                                 break;
                        }
                }

                System.out.println( "thread interrupted");
        }
}

运行结果（线程成功中断）:
thread running
InterruptedException
thread interrupted

 

接着在网上搜索了一下，写了一些例子研究后明白如何写一个可中断的线程。
首先发现在调用线程的start()后，立即调用interrupt()中断线程，线程停止了——被挡在while条件的外面：

public class ThreadDemo extends Thread{
         public static void main(String[] args) throws InterruptedException {
                Thread thread = new ThreadDemo();
                thread.start();
                 //thread.sleep(100);
                thread.interrupt(); //中断线程
        }

        @Override
         public void run() {
                System.out.println( "in run()");
                 while(!Thread.currentThread().isInterrupted()) {
                        System.out.println( "thread running");

                         try {
                                Thread.sleep(100);
                        } catch(InterruptedException e)
                        {
                                System.out.println( "InterruptedException");
                                 //break;
                        }
                }

                System.out.println( "thread interrupted");
        }
}

运行结果:
in run()
thread interrupted

 

然后按照网上说的在异常处理时再次中断，原来这样就能成功地用interrupt方法中断线程：

public class ThreadDemo extends Thread{
         public static void main(String[] args) throws InterruptedException {
                Thread thread = new ThreadDemo();
                thread.start();
                thread.sleep(100);
                thread.interrupt(); //中断线程
        }

        @Override
         public void run() {
                 while(!Thread.currentThread().isInterrupted()) {
                        System.out.println( "thread running");

                         try {
                                Thread.sleep(100);
                        } catch(InterruptedException e)
                        {
                                System.out.println( "InterruptedException");
                                Thread.currentThread().interrupt(); //再次中断线程
                                 //break;
                        }
                }

                System.out.println( "thread interrupted");
        }
}

运行结果（线程成功中断）:
thread running
InterruptedException
thread interrupted

 

为什么在调用interrupt()前加上sleep()，导致的结果不同呢？
  我认为Thread对象只有在调用start()时才分配资源创建线程提供给CPU调度。而这过程需要时间，因为时间差，导致了在进入run()方法前，中断标志已经被set on，于是被挡在while条件上。
而再次中断线程，中断标志被set on了，然后循环回到while条件判断，于是退出循环。

而线程被中断后，该对象依然存在的，只是线程不能再次调用start()否则会报异常IllegalThreadStateException。

public class ThreadDemo extends Thread{
         public static void main(String[] args){
           try{
                 ThreadDemo thread = new ThreadDemo();
                 thread.start();
                 thread.sleep(100);

                 thread.interrupt(); //中断线程
                 thread.sleep(100);

                 thread.printStr();
                 thread.interrupt(); //第三次中断线程

                 thread.printStr();
                 thread.join();
          } catch(InterruptedException e){
            e.printStackTrace();
          }
        }

         private void printStr(){
          System.out.println( "thread obj");
        }

        @Override
         public void run() {
                 while(!Thread.currentThread().isInterrupted()) {
                        System.out.println( "thread running");

                         try {
                                Thread.sleep(100);
                        } catch(InterruptedException e)
                        {
                                System.out.println( "InterruptedException");
                                Thread.currentThread().interrupt(); //再次中断线程
                        }
                }

                System.out.println( "thread interrupted");
        }
}

运行结果:
thread running
InterruptedException
thread interrupted
thread obj
thread obj

 

简单总结：
1. 线程start()后马上调用interrupt()，在进入run()时中断标志已经被set on；
2. 在处理sleep()可能抛出的InterruptedException时，再次中断线程即可成功中断；
3. 注意interrupt()方法是set on 中断标志的，interrupted()方法是判断后并清除中断标志的。

 

isInterrupted()和interrputed()方法的区别
public static boolean interrupted()
    测试当前线程是否已经中断。线程的中断状态由该方法清除。换句话说，如果连续两次调用该方法，则第二次调用将返回 false（在第一次调用已清除了其中断状态之后，且第二次调用检验完中断状态前，当前线程再次中断的情况除外）。

 

public boolean isInterrupted()
    测试线程是否已经中断。线程的中断状态不受该方法的影响。

 

两者都是如果该线程已经中断，则返回 true；否则返回 false。

 

 

参考文章 《Java 可中断线程》 http://blog.csdn.net/sapphiron/archive/2008/10/05/3018053.aspx 此文介绍更多的处理其他阻塞情况的中断方法。