java - 为什么 b.wait() 不让主线程等待

Question

这个问题已经有答案了:

Who notifies the Thread when it finishes? (1 个回答)

已关闭 5 年前。

不知道为什么 b.wait() 不等待主线程。如果我们创建一个虚拟对象，则 Object a = new Object();它等待着。如果我用 Thread 扩展 ThreadA 并为 ThreadA 创建一个实例并使用 ThreadA 引用进行锁定，它就可以工作。但为什么下面的 wait() 不起作用

package com.aircell;

public class ThreadA{
    public static void main(String[] args){
        ThreadB b = new ThreadB();
        b.start();
        System.out.println("who is this thread");
        synchronized(b){
            try{
                System.out.println("Waiting for b to complete...");
                b.wait();
            }catch(InterruptedException e){
                e.printStackTrace();
            }

            System.out.println("Total is: " + b.total);
        }
    }
}

class ThreadB extends Thread{
    int total;
    @Override
    public void run(){
        synchronized(this){
            for(int i=0; i<100 ; i++){
                total += i;

            }

        }
        System.out.println("done thread");
    }
}

Answer 1

首先:如果我在这里尝试你的类(class)[TM]，我会得到以下输出:

who is this thread
Waiting for b to complete...
done thread
Total is: 4950

所以它确实有效，这有点令人惊讶，但我认为 wait 的 Javadoc 中的以下句子似乎发生在这里:

As in the one argument version, interrupts and spurious wakeups are possible, and this method should always be used in a loop.

以下更改应该为您提供您想要实现的行为:

public class ThreadA{
    public static void main(String[] args){
        ThreadB b = new ThreadB();
        System.out.println("who is this thread");
        synchronized(b){
            b.start();
            try{
                System.out.println("Waiting for b to complete...");
                while (!b.finished) {
                    b.wait();
                }
            }catch(InterruptedException e){
                e.printStackTrace();
            }

            System.out.println("Total is: " + b.total);
        }
    }
}

class ThreadB extends Thread{
    int total;
    boolean finished;
    @Override
    public void run(){
        synchronized(this){
            try {
                for(int i=0; i<100 ; i++){
                    total += i;
                }
            }
            finally {
                finished = true;
                this.notifyAll();
            }
        }
        System.out.println("done thread");
    }
}