IllegalMonitorStateException 当使用 .class 作为监视器而 "this" 不是

Question

我刚写了一个程序，用两个线程轮流打印0-99。我使用同步块来完成这项工作。在我的代码中，当我使用“this”作为同步监视器时，程序运行良好。但是当我使用“.class”作为同步监视器时，我得到了IllegalMonitorStateException。谁能告诉我发生了什么事？

这是我的代码，运行良好

public class WaitTest implements Runnable {
    private int n = 0;

    @Override
    public void run() {
        while (true) {
            synchronized (this){
                notify();
                if (n < 100) {

                    System.out.println(Thread.currentThread().getName() + " " + Integer.toString(n));
                    ++n;
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    try {
                        wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                } else
                    break;
            }
        }
    }

    public static void main(String[] args) {
        WaitTest waitTest = new WaitTest();
        Thread threadA = new Thread(waitTest);
        Thread threadB = new Thread(waitTest);
        threadA.start();
        threadB.start();
    }
}

这是我遇到异常的代码

public class WaitTest implements Runnable {
    private int n = 0;

    @Override
    public void run() {
        while (true) {
            synchronized (WaitTest.class){
                notify();
                if (n < 100) {

                    System.out.println(Thread.currentThread().getName() + " " + Integer.toString(n));
                    ++n;
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    try {
                        wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                } else
                    break;
            }
        }
    }

    public static void main(String[] args) {
        WaitTest waitTest = new WaitTest();
        Thread threadA = new Thread(waitTest);
        Thread threadB = new Thread(waitTest);
        threadA.start();
        threadB.start();
    }
}

它们之间唯一的区别是synchronized

后大括号中的内容

Answer 1

In my code, when I use "this" as synchronized monitor, the program works well.

这是因为当您调用方法 wait() 和 notify() 时，您是从实例 return by this 隐式调用它们，这与您的同步子句（即 synchronized (this)）。

But when I use ".class" as synchronized monitor, I got IllegalMonitorStateException.

发生这种情况是因为您在 WaitTest.class class 上进行了同步，但您再次为 this 实例调用了 wait 和 notify 方法。因此，为避免该异常，将 wait(); and notify(); 的调用分别更改为 WaitTest.class.wait(); 和 WaitTest.class.notify();。

固定代码：

public class WaitTest implements Runnable {
    private int n = 0;

    @Override
    public void run() {
        while (true) {
            synchronized (WaitTest.class){
                WaitTest.class.notify();
                if (n < 100) {

                    System.out.println(Thread.currentThread().getName() + " " + Integer.toString(n));
                    ++n;
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    try {
                        WaitTest.class.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                } else
                    break;
            }
        }
    }

    public static void main(String[] args) {
        WaitTest waitTest = new WaitTest();
        Thread threadA = new Thread(waitTest);
        Thread threadB = new Thread(waitTest);
        threadA.start();
        threadB.start();
    }
}

IllegalMonitorStateException 当使用 .class 作为监视器而 "this" 不是

IllegalMonitorStateException when use .class as monitor while "this" not

java

parallel-processing

multithreading

thread-safety