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
	}
}