• Java基础 --- 终止线程 Terminate Threads

    Java基础 --- 终止线程 Terminate Threads

    为什么要终止线程

    • 线程消耗资源(包括内存,内核, CPU等资源).
    • 只有当一个Java程序的所有线程都运行结束的时候，一个 Java 程序才算运行结束.
    • 所以当一个线程不再被需要使用时或者运行不正常时需要清理掉.
    • 下面代码中, 即使main方法已经运行完毕, 但是整个程序还是等待 BlockingTask运行结束.
    public class Main1 {
    public static void main(String [] args) {
        Thread thread = new Thread(new BlockingTask());

        thread.start();
    }

    private static class BlockingTask implements Runnable {

        @Override
        public void run() {
            //do things
            try {
                Thread.sleep(500000);
            } catch (InterruptedException e) {
                System.out.println("Existing blocking thread");
            }
        }
    }
}

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    终止线程的方法

    return()

    • 当一个线程的run方法执行return语句时, 线程会自动结束

    stop()

    • 再以前的版本中, 使用stop和suspend可以立即结束一个线程
    • 但是已经被废弃, 因为直接中断的方式，并没有让线程留下存储数据的时间，这也极容易导致线程的数据丢失或不一致性的问题

    interrupt()

    • java并没有提供任何机制来安全的停止线程，只提供了中断（interruption），这其实是一种协作机制，让一个线程去通知另外一个线程停止当前的工作。
    • interrupt方法不是直接终止线程, 而是给目标线程发送一个中断信号，如果目标线程没有接收线程中断的信号并结束线程，线程则不会终止，具体是否退出或者执行其他逻辑由目标线程决定.
    • When the interrupt method is called on a thread, the interrupted status of the thread is set. This is a boolean flag that is present in every thread.

    interrupt() 和 isInterrupted()

    • 可以使用interrupt() 和 isInterrupted()从一个线程中断另一个线程并查看目前线程是否被中断
    import java.math.BigInteger;
public class Main2 {

    public static void main(String[] args) {
        Thread thread = new Thread(new LongComputationTask(new BigInteger("200000"), new BigInteger("100000000")));

        thread.start();
        //中断thread线程
        thread.interrupt();
    }

    private static class LongComputationTask implements Runnable {
        private BigInteger base;
        private BigInteger power;

        public LongComputationTask(BigInteger base, BigInteger power) {
            this.base = base;
            this.power = power;
        }

        @Override
        public void run() {
            System.out.println(base + "^" + power + " = " + pow(base, power));
        }

        private BigInteger pow(BigInteger base, BigInteger power) {
            BigInteger result = BigInteger.ONE;

            for (BigInteger i = BigInteger.ZERO; i.compareTo(power) != 0; i = i.add(BigInteger.ONE)) {
                
                // 查看目前线程是否被中断, 以及做出相应处理
                if (Thread.currentThread().isInterrupted()) {
                    System.out.println("Prematurely interrupted computation");
                    return BigInteger.ZERO;
                }
                result = result.multiply(base);
            }

            return result;
        }
    }
}

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    InterruptedException

    • 当线程处于阻塞状态时(比如sleep和wait), 线程无法使用isInterrupted()查看是否收到中断信号, 此时需要InterruptedException
    • InterruptedException会结束线程的阻塞状态
    Runnable r = () -> {
 try
 {
 	 . . .
	 while (more work to do)
	 {
		//do more work
	 	Thread.sleep(delay);
	 }
 }
 catch(InterruptedException e) {
 	// thread was interrupted during sleep
 }
 finally {
	//cleanup, if required
 }
 // exiting the run method terminates the thread
};

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    • 当InterruptedException被抛出后, Interrupt status会被清除
    • 下面代码就中断失败, 因为当InterruptedException被抛出后, Interrupt status被清除. 代码进入不到if语句
    private static void test3() throws InterruptedException {
    Thread thread = new Thread(() -> {
        while (true) {
            // 响应中断
            if (Thread.currentThread().isInterrupted()) {
                System.out.println("青秧线程被中断，程序退出。");
                return;
            }

            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                System.out.println("青秧线程休眠被中断，程序退出。");
            }
        }
    });
    thread.start();
    Thread.sleep(2000);
    thread.interrupt();
}

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    • 中断成功: 在catch中加入Thread.currentThread().interrupt(). 重新设置中断状态.
    private static void test4() throws InterruptedException {
    Thread thread = new Thread(() -> {
        while (true) {
            // 响应中断
            if (Thread.currentThread().isInterrupted()) {
                System.out.println("青秧线程被中断，程序退出。");
                return;
            }

            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                System.out.println("青秧线程休眠被中断，程序退出。");
                Thread.currentThread().interrupt();
            }
        }
    });
    thread.start();
    Thread.sleep(2000);
    thread.interrupt();
}

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    使用sleep时, 不要在底层代码中处理InterruptException

    public class PassInterrupt implements Runnable{
    @Override
    public void run() {
        while(true){
            System.out.println("go");
            throwInMethod();
        }
    }
    private void throwInMethod()  {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Thread thread=new Thread (new PassInterrupt());
        thread.start();
        Thread.sleep(1000);
        thread.interrupt();
    }
}

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    在这里插入图片描述

    • 因为throwInMethod存在于底层方法中，如果在其中进行try/catch顶层方法可能无法感知，导致该中断被遗漏.

    正确方式: 将exception抛给顶层方法处理

    public class PassInterrupt implements Runnable{
    @Override
    public void run() {
        while(true){
            System.out.println("go");
            try {
                throwInMethod();
            } catch (InterruptedException e) {
                //保存日志/停止程序等操作
                System.out.println("日志已经保存");
                e.printStackTrace();
            }
        }
    }
    private void throwInMethod() throws InterruptedException {
         Thread.sleep(2000);
        //不在这里try/catch 是因为在这里的代码层次很深，run在较高层次，而throwInMethod较低层次
    }
    public static void main(String[] args) throws InterruptedException {
        Thread thread=new Thread (new PassInterrupt());
        thread.start();
        Thread.sleep(1000);
        thread.interrupt();
    }
}

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    isInterrupt() 和 interrupted的区别 (注意是interrupted不是interrupt)

    • The interrupted method is a static method that checks whether the current thread has
      been interrupted. Furthermore, calling the interrupted method clears the interrupted
      status of the thread.
    • isInterrupted method is an instance method that you can use to check whether any thread has been interrupted.
      Calling it does not change the interrupted status.
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  • 原文地址：https://blog.csdn.net/weixin_38803409/article/details/125467641