线程的阻塞和唤醒之wait()和notify()

Object对象的wait()方法和notify()方法通常用于线程之间的阻塞和唤醒，但是这两个方法必须在同步代码块synchronized里面，否则会报错，也不能先执行notify()方法后执行wait()方法，这样wait()方法执行完后就一直阻塞了，下面是例子。

1、正常使用的代码

public class WaitAndNotify {
    public static void main(String[] args) {
        Object lock = new Object();
        //wait()和notify()的使用必须要有锁,没有锁的话会报错
        new Thread(()->{
            synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"进来啦，然后被wait阻塞");
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
            }
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"notify唤醒了t1");
                lock.notify();
            }
        },"t2").start();
    }
}

输出结果

t1进来啦，然后被wait阻塞
t2notify唤醒了t1
t1被唤醒啦

跟预想的一致。

2、wait()和notify()的使用必须要有锁,没有锁的话会报错

public class WaitAndNotify {
    public static void main(String[] args) {
        Object lock = new Object();
        //wait()和notify()的使用必须要有锁,没有锁的话会报错
        new Thread(()->{
            //synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"进来啦，然后被wait阻塞");
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
            //}
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            //synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"notify唤醒了t1");
                lock.notify();
            //}
        },"t2").start();
    }
}

t1进来啦，然后被wait阻塞
Exception in thread "t1" java.lang.IllegalMonitorStateException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:502)
    at mydemo.jdk.juc.wait.WaitAndNotify.lambda$0(WaitAndNotify.java:16)
    at mydemo.jdk.juc.wait.WaitAndNotify$$Lambda$1/989110044.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:745)

3、如果先执行notify()后执行wait()则唤醒失败，会阻塞下去

public class WaitAndNotify {
    public static void main(String[] args) {
        Object lock = new Object();
        //wait()和notify()的使用必须要有锁,没有锁的话会报错
        new Thread(()->{
            try {
                //这里休息三秒，那么就是她唤醒线程先执行才能到这里的阻塞执行了
                Thread.sleep(3000);
            } catch (InterruptedException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"进来啦，然后被wait阻塞");
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"被唤醒啦");
            }
        },"t1").start();
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        new Thread(()->{
            synchronized (lock) {
                System.out.println(Thread.currentThread().getName()+"notify唤醒了t1");
                lock.notify();
            }
        },"t2").start();
    }
}

t2notify唤醒了t1
t1进来啦，然后被wait阻塞