Java wait和notify

wait和notify作用

wait和notify是定义在Object类中的，而且是final的。因此会被所有的Java类继承并且无法重写。这两个方法要求在调用时所处的线程已经获取了对象monitor锁，因此对这两个方法的调用需要在synchronized方法或者代码块中。比如wait方法的Java Doc中也有对此的说明：

当线程执行了wait方法时，会释放掉前面synchronized获取的monitor锁。

举例说明

下面举个两个线程通过wait和notify进行通信的例子：通过两个线程，实现一个将int num在0-1-0-1之间不断加1和减1的功能。

Sample类

public class Sample {
    // 成员变量 持有num
    int num = 0;
    
    // 加1
    synchronized public void increase() {
        while (num != 0) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        this.num ++;
        notify();
        System.out.println(Thread.currentThread().getName() + "--" + this.num);
    }
    
    // 减1
    synchronized public void decrease() {
        while (num == 0) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        this.num --;
        notify();
        System.out.println(Thread.currentThread().getName() + "--" + this.num);
    }
}

IncreaseRunnable类

public class IncreaseRunnable implements Runnable {
    Sample sample;
    public IncreaseRunnable(Sample sample) {
        this.sample = sample;
    }
    @Override
    public void run() {
        for (int i = 0; i < 50; i++) {
            sample.increase();
        }
    }
}

DecreaseRunnable类

public class DecreaseRunnable implements Runnable{
    Sample sample;
    public DecreaseRunnable(Sample sample) {
        this.sample = sample;
    }
    @Override
    public void run() {
        for (int i = 0; i < 50; i++) {
            sample.decrease();
        }
    }
}

主测试类

public class TestMain {
    public static void main(String[] args) {
        Sample sample = new Sample();
        IncreaseRunnable increaseRunnable = new IncreaseRunnable(sample);
        DecreaseRunnable decreaseRunnable = new DecreaseRunnable(sample);

        Thread t1 = new Thread(increaseRunnable);
        t1.setName("increase-thread-1");
        Thread t2 = new Thread(increaseRunnable);
        t2.setName("increase-thread-2");
        Thread t3 = new Thread(decreaseRunnable);
        t3.setName("decrease-thread-1");
        Thread t4 = new Thread(decreaseRunnable);
        t4.setName("decrease-thread-2");

        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}