为什么synchronized不能同步线程？

Question

我已经设置了标志的值，但结果不是'add'和'sub'交替出现。为什么？当我查看结果时，它执行了两次“sub”方法。但是当 'sub' 方法结束时，标志的值将被设置为 'false'。但是结果却连续打印了两次“subxxxxxx”。

class Resource {
    private boolean flag = true;
    private int num = 0;

// At here I have declared an add()
    public synchronized void add() throws InterruptedException {
        if (this.flag == false) {
            super.wait();
        }
        Thread.sleep(100);
        this.num++;
        System.out.println("addition:"+Thread.currentThread().getName() + this.num);
        this.flag = false;
        super.notifyAll();
    }

// At here I have declared an sub()
    public synchronized void sub() throws InterruptedException {
        if (this.flag == true) {
            super.wait();
        }
        Thread.sleep(200);
        this.num--;
        System.out.println("subtraction:"+Thread.currentThread().getName() + this.num);
        this.flag = true;
        super.notifyAll();
    }
}

/*
* I will test it with multiple threads. For example:
*new Thread(ad, "add").start();
*new Thread(ad, "add").start();
*new Thread(sub, "sub").start();
*new Thread(sub, "sub").start();
*When threads start. it will execute alternately. For example:
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
Thread add:0
Thread sub:-1
But the result is like this:
Thread add:0
Thread sub:-1
Thread sub:-2
Thread add:-1
Thread sub:-3
Thread sub:-4
Why?Why?Why?
*/
        new Thread(ad, "add").start();
        new Thread(ad, "add").start();
        new Thread(sub, "sub").start();
        new Thread(sub, "sub").start();
    }
}

Answer 1

您似乎假设当您的wait() 调用结束时，flag 已更改为您调用wait() 之前想要的。没有这样的保证，特别是因为您涉及两个以上的线程。您应该检查是否需要继续等待。另见Wait until boolean value changes it state

但总的来说，这些构造太低级而无法使用（除非您想了解细节），您应该查看 concurreny utils 包以获得更简单的高级构造（如队列、锁存器、条件） .