java - 为什么主线程在启动第一个线程后等待？

Question

我试图了解 CountDownLatch 的用法，下面是我在这里使用的代码，

DecrementRunnable.java

    package com.nirjhar.java.countdownlatchexample;

    import java.util.concurrent.CountDownLatch;

    public class DecrementRunnable implements Runnable {

    private String name;
    private CountDownLatch cdl;

    public DecrementRunnable(String name, CountDownLatch cdl) {
    this.name=name;
    this.cdl=cdl;
    }

    @Override
    public void run() {
        System.out.println("in run method");
    for (int i = 0; i < 6; i++) {
        cdl.countDown();
        System.out.println("counting down "+cdl.getCount());
    }
    //cdl.notifyAll();
    System.out.println("Notified to all");
    }
}

CountDownDemoThread.java

package com.nirjhar.java.countdownlatchexample;

import java.util.concurrent.CountDownLatch;

public class CountDownDemoThread extends Thread {

    private CountDownLatch cdl;

    public CountDownDemoThread(String name, CountDownLatch cdl) {
        this.cdl=cdl;
        setName(name);
    }

    @Override
    public synchronized void start() {

        System.out.println("Task completed ... waiting for other thread to complete their task");
        try {
            cdl.await();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }   
        System.out.println("Continuing my tasks ..");
    }
}

主程序，

package com.nirjhar.java.countdownlatchexample;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchDemo {
    public static void main(String[] args)
    {
        CountDownLatch cdl=new CountDownLatch(6);
        Thread tDecrementThread=new Thread(new DecrementRunnable("Runnable", cdl));
        CountDownDemoThread cddt=new CountDownDemoThread("thread", cdl);        
        cddt.start();   
        System.out.println("thread 1 started");     
        tDecrementThread.start();
    }
}

在这个主程序中，我希望在线程一启动后打印“线程 1 已启动”这一行，但由于 cddt 线程中的 cdl.await() 语句中的等待语句，这里主线程被阻塞。只是想知道这背后的原因是什么？

Answer 1

在您的 CountDownThread 类中，我认为您想要覆盖 run() 方法而不是 start

代替

public synchronized void start() {

试试

public void run() {

---

原因

wait statement in cdl.await() statement in cddt thread. Just wanted to know what is the reason behind this?

在这种情况下，您已经覆盖了 Thread 的 start 方法，因此它不再像往常一样生成线程来执行代码。

因此没有生成线程，事实上 main 线程正在调用 cd1.await() 而不是 cd1 线程(你想要的)。这就是 main 线程被阻塞的原因。