Question

我正在尝试为线程运行以下程序，并期望变量对象应该由两个不同的线程更新。非常奇怪的是，它只是显示值'加-1'和'乘-2'。但是当我使用调试器并逐步调试它时，'添加'和乘法正如预期的那样发生。如果我将'Thread.sleep'放在'P'的'run'方法中1000毫秒。它工作正常。我已经注释掉了'Thread.sleep'的代码，但如果你取消注释它并运行，你会发现预期的结果。有人可以向我澄清这个吗？

public class TestingThreads {   

    public static void main(String[] args) {

        Variable b = new Variable();
        Thread tp = new Thread(new P(b));
        Thread tq = new Thread(new Q(b));
        tp.start();
        tq.start();
        try {
            tp.join();
            tq.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

class Variable {
    int i = 0;
    boolean on = true;

    public synchronized void addition() {
        if (!on) {
            try {
                wait();
            } catch (InterruptedException e) {

                e.printStackTrace();
            }

        }
        i = i + 1;
        on=false;
        notify();
        System.out.println("addition " + i);



    }

    public synchronized void multiply() {

        if (on) {
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
        i = i * 2;
        on=true;
        notify();
        System.out.println("multiply " + i);

    }

}

class P implements Runnable {

    Variable b;

    P(Variable b) {
        this.b = b;
    }


    @Override
    public void run() {


        while (true) {
            /*uncomment it and it will work fine.*/

            /*try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }*/

            b.addition();

        }
    }

}

class Q implements Runnable {

    Variable b;

    Q(Variable b) {
        this.b = b;
    }

    @Override
    public void run() {

        while (true) {

            b.multiply();
        }
    }
}

Answer 1

你正在击中整数的顶部并环绕。查看每个操作的值之前和之后：

-1

...

addition pre: 0
addition post: 1
multiply pre: 1
multiply post: 2
addition pre: 2
addition post: 3
multiply pre: 3
multiply post: 6
addition pre: 6
addition post: 7
multiply pre: 7
multiply post: 14
addition pre: 14
addition post: 15
multiply pre: 15
multiply post: 30
addition pre: 30
addition post: 31
multiply pre: 31
multiply post: 62
addition pre: 62
addition post: 63
multiply pre: 63
multiply post: 126
addition pre: 126
addition post: 127
multiply pre: 127
multiply post: 254
addition pre: 254
addition post: 255
multiply pre: 255
multiply post: 510
addition pre: 510
addition post: 511
multiply pre: 511
multiply post: 1022
addition pre: 1022
addition post: 1023
multiply pre: 1023
multiply post: 2046
addition pre: 2046
addition post: 2047
multiply pre: 2047
multiply post: 4094
addition pre: 4094
addition post: 4095
multiply pre: 4095
multiply post: 8190
addition pre: 8190
addition post: 8191
multiply pre: 8191
multiply post: 16382
addition pre: 16382
addition post: 16383
multiply pre: 16383
multiply post: 32766
addition pre: 32766
addition post: 32767
multiply pre: 32767
multiply post: 65534
addition pre: 65534
addition post: 65535
multiply pre: 65535
multiply post: 131070
addition pre: 131070
addition post: 131071
multiply pre: 131071
multiply post: 262142
addition pre: 262142

同样的事情会在睡眠中发生，只是慢得多。：）

线程相关程序无法正常工作

1 个答案: