连接()方法在多线程中的工作原理

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:线程(t0)一结束，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();

    }

}

输出是

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结束后启动.但是，据我所知，线程3在线程0结束时就开始了。

Answer 1

Thread.join()等待这个线程死掉。基本上，所有线程都运行在主线程之上。让我试着解释一下你的代码的流程。

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();

}

现在，对于您的问题，您的线程t1、t2和t3都位于可运行线程池中。因此，现在选择哪个线程并执行完全掌握在线程调度器的手中。

Answer 2

线程-3在线程-0结束后立即启动。

线程-3在线程-0完成后成为可运行的.但这并不意味着它是立即安排的。同时可能有3个可运行的线程，而且先打印哪个线程是不可预测的。