Java Wait 与 Sleep

首先，这两个都可以暂停当前线程的后续执行。但他们也有很明显的区别：

• sleep 可以用在代码中的任何地方， Thread.sleep(xxx).
• wait 是Object类型的对象的， 用在synchronized 包装的代码中。如果不在synchronized 中，会报 IllegalMonitorStateException 异常
• sleep 不会让出CPU时间片
• wait会直接让出CPU

下面通过一个例子来说明，下面是一个完整的测试代码：

1，请看下面的代码, 如果直接用lock.wait(), 会直接报 IllegalMonitorStateException

        try {
            System.out.println("test2");
          //  lock.wait(1000);  IllegalMonitorStateException
            System.out.println("test");
        } catch (Exception e){
            e.printStackTrace();
        }

2，创建一个线程A, 用wait() 方法来暂停线程A:

     new Thread(()->{
            System.out.println("Thread A is waiting to get lock");
                synchronized (lock){
                    System.out.println("Thead A get the lock");
                    try {
                        //object.wait 会让出时间片
                        lock.wait(1000);
                        System.out.println("Thread A done");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
        }).start();

3，主线程暂停200毫秒：

  //为了让线程A 和线程B是按照 A->B 的顺序执行的
        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

4,创建一个线程B用sleep方法来暂停线程B:

    new Thread(()->{
            System.out.println("Thread B is waiting to get lock");
            synchronized (lock){
                System.out.println("Thead B get the lock");
                try {
                    //lock.wait(1000);
                    //lock.notifyAll();
                    Thread.sleep(3000);
                    System.out.println("Thread B done");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();

该测试程序执行流程：

首先线程A启动并调用wait方法，然后会让出CPU , 200毫秒后 B线程启动，并sleep 3秒，然后B线程执行完成后归还CPU, A线程获得CPU继续执行.
打印结果如下：

test2
test
Thread A is waiting to get lock
Thead A get the lock
Thread B is waiting to get lock
Thead B get the lock
Thread B done
Thread A done

完整代码如下：

public class WaitAndSleep {

    public static void main(String[] args){
        Object lock = new Object();
        try {
            System.out.println("test2");
          //  lock.wait(1000);  IllegalMonitorStateException
            System.out.println("test");
        } catch (Exception e){
            e.printStackTrace();
        }

        new Thread(()->{
            System.out.println("Thread A is waiting to get lock");
                synchronized (lock){
                    System.out.println("Thead A get the lock");
                    try {
                        //object.wait 会让出时间片
                        lock.wait(1000);
                        System.out.println("Thread A done");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
        }).start();

        //为了让线程A 和线程B是按照 A->B 的顺序执行的
        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread(()->{
            System.out.println("Thread B is waiting to get lock");
            synchronized (lock){
                System.out.println("Thead B get the lock");
                try {
                    //lock.wait(1000);
                    //lock.notifyAll();
                    Thread.sleep(3000);
                    System.out.println("Thread B done");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }


}

class MyThread implements  Runnable{
    public void run() {
        int i = 0;

        while (i++ < 10){
            try {
                Thread.sleep(1000);

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}