如何在Java中使用wait()和notify()？

Question

据我所知,我想在互斥锁上调用wait(),当我希望当前线程停止工作,直到另一个线程在同一个互斥对象上调用notify().这似乎不起作用.

我正在尝试打印线程1-10.然后等待另一个线程打印11-20.然后第一个线程将再次打印21-30

Main.java

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Object mutex = 1;

        Thread child1 = new Thread(new Child1(mutex));
        Thread child2 = new Thread(new Child2(mutex));

        child1.start();
        child2.start();

    }

}

Child1.java

public class Child1 implements Runnable {
    Object mutex;

    public Child1(Object mutex){
        this.mutex = mutex;
    }

    public void run() {
        synchronized (mutex) {
            for(int c = 0; c < 10; c++){
                System.out.println(c+1);
            }

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


        for(int c = 20; c < 31; c++){
            System.out.println(c+1);
        }
    }
}

Child2.java

public class Child2 implements Runnable {
    Object mutex;

    public Child2(Object mutex) {
        this.mutex = mutex;
    }

    public void run() {
        synchronized (mutex) {
            for (int c = 11; c < 21; c++) {
                System.out.println(c);
            }
            notify();
        }

    }
}

产量

1
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17Exception in thread "Thread-0" 
18
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20
Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at task42.Child1.run(Child1.java:18)
    at java.lang.Thread.run(Thread.java:745)
java.lang.IllegalMonitorStateException
    at java.lang.Object.notify(Native Method)
    at task42.Child2.run(Child2.java:15)
    at java.lang.Thread.run(Thread.java:745)

我错过了什么？

Answer 1

您必须添加mutex对wait()和的引用notify(); 也就是说,更改wait()到mutex.wait()和notify()到mutex.notify().

没有这个,你正在呼叫等待/通知this(method()相当于this.method())

以下是您的代码,并进行了相应的更改:

Child1.java

public class Child1 implements Runnable {
    Object mutex;

    public Child1(Object mutex){
        this.mutex = mutex;
    }

    public void run() {
        synchronized (mutex) {
            for(int c = 0; c < 10; c++){
                System.out.println(c+1);
            }

            try {
                mutex.wait(); // Changed here
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }


        for(int c = 20; c < 31; c++){
            System.out.println(c+1);
        }
    }
}

Child2.java

public class Child2 implements Runnable {
    Object mutex;

    public Child2(Object mutex) {
        this.mutex = mutex;
    }

    public void run() {
        synchronized (mutex) {
            for (int c = 11; c < 21; c++) {
                System.out.println(c);
            }
            mutex.notify(); // Changed here
        }

    }
}