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java主线程结束和子线程结束之间的关系

java主线程结束和子线程结束之间的关系

作者: 幽澜先生 | 来源:发表于2019-01-22 19:22 被阅读0次

情况1:正常情况下,主线程启动了子线程,主线程、子线程各自执行,彼此不受影响。

当你在run一个Java application的时候,这个时候系统会开一个进程。然后这个进程启动了Main线程。Java进程确定虚拟机中没有线程运行的时候,退出进程。或者也可以用System.exit(0);强制退出进程

代码示例如下:参考Thinkingin java代码

class LiftOff implements Runnable {

    Logger logger = LoggerFactory.getLogger(LiftOff.class);

    protected int countDown = 10; // Default

    private static int taskCount = 0;

    private final int id = taskCount++;

    public LiftOff() {}

    public LiftOff(int countDown) {

        this.countDown = countDown;

    }

    public String status() {

        return "#" + id + "(" +

                (countDown > 0 ? countDown : "Liftoff!") + "), ";

    }

    @Override

    public void run() {

        while(countDown-- > 0) {

            logger.info(status());

            Thread.yield();

        }

    }

}

public class MainThread{

    Logger logger = LoggerFactory.getLogger(MainClass.class);

    @Test

    public void test(){

        Thread t = new Thread(new LiftOff());

        t.start();

        t.setName("lift off thread");

        logger.info("waiting for liftoff");

    }

}

显示结果:

情况2:需求是主线程执行结束,由主线程启动的子线程都结束

代码如下

class SimpleDaemons implements Runnable {

    Logger logger = LoggerFactory.getLogger(SimpleDaemons.class);

    public void run() {

        try {

            while (true) {

                TimeUnit.MILLISECONDS.sleep(100);

                logger.info("run..");

            }

        } catch (InterruptedException e) {

            logger.info("sleep() interrupted");

        }

    }

}

public class MainThread {

    Logger logger = LoggerFactory.getLogger(MainThread.class);

    @Test

    public void test() {

        for(int i = 0; i < 5; i++) {

            Thread daemon = new Thread(new SimpleDaemons());

            daemon.setDaemon(true); // Must call before start()

            daemon.start();

        }

        logger.info("All daemons started");

        try {

            TimeUnit.MILLISECONDS.sleep(175);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

    }

}

运行结果:

情况3:需求是子线程执行结束,主线程等待启动的子线程都结束之后再结束

代码:

public class IsAliveTest {

    public static void main(String args[]) throws Exception {

        Thread t = new Thread(new ThreadDemo());

        // this will call run() function

        t.start();

        // waits for this thread to die

        t.join();

        // tests if this thread is alive

        System.out.println("thread t status = " + t.isAlive());

        System.out.println("thread main status = " + Thread.currentThread().isAlive());

    }

}

class ThreadDemo implements Runnable {

    public void run() {

        Thread t = Thread.currentThread();

        // tests if this thread is alive

        System.out.println("status = " + t.isAlive());

        try {

            t.sleep(3000L);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

    }

}

测试结果

什么情况下一个java thread reach the ‘Die’ state?

From the ThreadAPI, here is a complete list:

  1. If the run() method returns. 例如join()之后

  2. If an exception is thrown that propagates beyond the run method.

  3. If it is a daemon thread and all non-daemonthreads have 'died' 非后台线程都结束

  4. If the exit method of class Runtime has been called (even at another thread).

实际上,我们对Thread类没有什么控制权,我们几乎不能设置线程的任何状态,我们只能创建任务,并通过某种方式使用线程驱动这个任务

所以,在编写多线程代码的时候,遵循规则就变得非常重要

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