Wait（）/ notify（）同步

Question

我正在尝试检查java中的wait / notify如何工作。

码：

public class Tester {
    public static void main(String[] args) {
        MyRunnable r = new MyRunnable();
        Thread t = new Thread(r);
        t.start();
        synchronized (t) {
            try {
                System.out.println("wating for t to complete");
                t.wait();
                System.out.println("wait over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

class MyRunnable implements Runnable {
    public void run() {
        System.out.println("entering run method");
        synchronized (this) {
            System.out.println("entering syncronised block");
            notify();
            try {
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("leaving syncronized block");
        }
        System.out.println("leaving run method");
    }
}

输出返回

wating for t to complete
entering run method
entering syncronised block
//sleep called
leaving syncronized block
leaving run method
wait over

我期望在执行notify（）时，等待将结束并被System.out.println("wait over");打印。但似乎只有t完成打印后才能打印run()。

Answer 1

对象监视器锁需要执行相同锁的单个引用。

在你的榜样，你是waiting对的一个实例Thread，但使用notify从Runnable。相反，您应该使用单个通用锁定对象…例如

public class Tester {

    public static final Object LOCK = new Object();

    public static void main(String[] args) {
        MyRunnable r = new MyRunnable();
        Thread t = new Thread(r);
        t.start();
        synchronized (LOCK) {
            try {
                System.out.println("wating for t to complete");
                LOCK.wait();
                System.out.println("wait over");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static class MyRunnable implements Runnable {

        public void run() {
            System.out.println("entering run method");
            synchronized (LOCK) {
                System.out.println("entering syncronised block");
                LOCK.notify();
                try {
                    Thread.currentThread().sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("leaving syncronized block");
            }
            System.out.println("leaving run method");
        }
    }
}

输出…

wating for t to complete
entering run method
entering syncronised block
leaving syncronized block
wait over
leaving run method

wait over并leaving run method可能根据线程调度更改位置。

您可以尝试将睡眠排除在synchronized障碍之外。这将释放监视器锁定，从而允许该wait部分继续运行（因为在锁定释放之前它无法启动）

    public static class MyRunnable implements Runnable {

        public void run() {
            System.out.println("entering run method");
            synchronized (LOCK) {
                System.out.println("entering syncronised block");
                LOCK.notify();
                System.out.println("leaving syncronized block");
            }
            try {
                Thread.currentThread().sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("leaving run method");
        }
    }