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。（如评论所述，确实有问题，main线程执行完成之后，java虚拟机就退出了，那是不是可以这样猜想，main线程退出，那么Lift off thread也会退出，导致虚拟机中没有线程运行了，于是退出进程？请大神解释）

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