Object类的方法：等待wait() ， 唤醒队首线程notify() ， 唤醒全部等待线程notifyAll()。利用等待和唤醒实现Producer线程、Consumer线程的互斥访问Message对象。
代码如下：

package ThreadDemo;

class Message {
    private String msg = "";
    private boolean flag = true; //记录型信号量，规定true表示没有消息（允许生产、不许消费），false表示已有消息（不许生产、允许消费）
    
    public synchronized void setMessage(String msg) {
        if(flag==false) {   
            try {
                super.wait();   //不许生产、让Producer线程等待
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        this.msg=msg;
        flag = false;   //生产完成，改变信号量、唤醒Consumer
        super.notify();
    }
    
    public synchronized String getMessage() {
        if(flag==true) {    
            try {
                super.wait();   //不许消费、让Consumer线程等待
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        try {
        return "msg = "+this.msg;
        }
        finally {       //return后用finally确保指令执行
        flag = true;    //消费完成，改变信号量、唤醒Producer
        super.notify();
        }
    }
}

class Producer implements Runnable{
    private Message msg;
    
    public Producer(Message msg) {
        this.msg = msg;
    }
    @Override
    public void run() {
        for(int x=0;x<30;x++) {
            if(x%2==0) {
                msg.setMessage("夏天，穿短袖短裤");
            }
            else {
                msg.setMessage("冬天，换秋衣秋裤");
            }
        }
    }
}

class Consumer implements Runnable{
    private Message msg;
    
    public Consumer(Message msg) {
        this.msg = msg;
    }
    @Override
    public void run() {
        for(int x=0;x<30;x++) {
            System.out.println(msg.getMessage());
        }
    }
}

public class Main {
    public static void main(String[] args){
        Message msg = new Message();
        new Thread(new Producer(msg)).start();  //启动生产者线程
        new Thread(new Consumer(msg)).start();  //启动消费者线程
    }
}