Question

根据我对join（）方法的理解，它允许一个线程等待另一个线程完成。

然后，根据我的代码：一旦线程（t0）结束，线程（t3）就应该启动，这不会发生

public class Threaded extends Thread {

     @Override
     public void run() {

      for (int i=0; i<5; i++) {

           System.out.println(Thread.currentThread().getName() + ": " + i);

       }

       }

}

public class Demo {

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

        Thread main = Thread.currentThread();

        Threaded t0 = new Threaded();
        Threaded t1 = new Threaded();
        Threaded t2 = new Threaded();
        Threaded t3= new Threaded();

        t0.start();
        t1.start();
        t2.start();

        t0.join();

        t3.start();

    }

}

输出为

main 5  
Thread-1: 0  
Thread-2: 0  
Thread-0: 0  
Thread-2: 1  
Thread-2: 2  
Thread-1: 1  
Thread-2: 3  
Thread-2: 4  
Thread-0: 1  
Thread-0: 2  
Thread-0: 3  
Thread-0: 4  
Thread-1: 2  
Thread-1: 3  
Thread-1: 4  
Thread-3: 0  
Thread-3: 1  
Thread-3: 2  
Thread-3: 3  
Thread-3: 4

在此输出中，线程3在线程0，线程1和线程2结束之后开始。但是，据我所知，线程0结束后，线程3即开始启动。

Answer 1

线程3结束时，线程3就会启动。

线程3完成后，线程3即可运行。但这并不意味着立即安排。可能同时有3个可运行线程，并且不确定哪个线程先打印。

Answer 2

Thread.join()等待该线程死亡。基本上所有线程都在主线程上运行。因此，让我尝试解释您的代码流程。

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

    Thread main = Thread.currentThread();

    // you have created 4 Thread instances here...
    Threaded t0 = new Threaded();
    Threaded t1 = new Threaded();
    Threaded t2 = new Threaded();
    Threaded t3= new Threaded();

    // you have started t0, t1 and t2 to run on top of main thread
    t0.start();
    t1.start();
    t2.start();

    // here you have used Thread.join()
    // so your main thread will wait here, 
    // it will wait for the completion of t0 
    t0.join();

    // so after the completion of t0, t3 will start
    t3.start();

}

因此，对于您的问题，您的线程t1，t2和t3都在可运行的线程池中。因此，现在选择哪个线程并执行完全由Thread Scheduler负责。

join（）方法如何在多线程中工作

2 个答案: