Java 多线程 --- 终止线程 Terminate Threads

Java 多线程 --- 终止线程 Terminate Threads

• 为什么要终止线程
• 终止线程的方法
• • return()
  • stop()
  • interrupt()
• InterruptedException

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

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;}@Overridepublic 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;}}
}

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 workThread.sleep(delay);}}catch(InterruptedException e) {// thread was interrupted during sleep}finally {//cleanup, if required}// exiting the run method terminates the thread
};

• 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();
}

• 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();
}

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

public class PassInterrupt implements Runnable{@Overridepublic 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();}
}

• 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{@Overridepublic 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();}
}

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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