java 多线程 wait()、notify()和notifyAll()用法

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最近找工作，被问了很多多线程和锁的知识，这里总结一下多线程中wait()与notify()的用法。

直接上代码（生产者/消费者模式：代码来自马士兵老师）：

public class ProducerConsumer {

	public static void main(String[] args) {
		SyncStack ss = new SyncStack();
		Producer p1 = new Producer(ss, "p1");
		Producer p2 = new Producer(ss, "p2");
		Consumer c1 = new Consumer(ss, "c1");
		Consumer c2 = new Consumer(ss, "c2");
		new Thread(p1).start();
		new Thread(p2).start();
		new Thread(c1).start();
		new Thread(c2).start();
	}
}

class WoTou {
	int id;
	public WoTou(int id) {
		this.id = id;
	}

	public String toString() {
		return "WoTou:" + id;
	}

}

//放窝头的篮子
class SyncStack {
	int index = 0;
	WoTou[] arrWoTou = new WoTou[6];

	public synchronized void push(WoTou wt) {
		while (index == arrWoTou.length) {
			try {
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		this.notify();
		arrWoTou[index] = wt;
		index++;
	}

	public synchronized WoTou pop() {
		while (index == 0) {
			try {
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		this.notify();
		index--;
		return arrWoTou[index];
	}
}

class Producer implements Runnable {
	SyncStack ss = null;
	String name;
	Producer(SyncStack ss, String name) {
		this.ss = ss;
		this.name = name;
	}

	public void run() {
		for (int i = 0; i < 20; i++) {
			WoTou wt = new WoTou(i);
			ss.push(wt);
			System.out.println("procude_" + name + " : "+ wt);
			try {
				Thread.sleep((int) (Math.random() * 200));
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
}

class Consumer implements Runnable {
	SyncStack ss = null;
	String name;

	Consumer(SyncStack ss, String name) {
		this.ss = ss;
		this.name = name;
	}

	public void run() {
		for (int i = 0; i < 20; i++) {
			WoTou wt;
			wt = ss.pop();
			System.out.println("consume_" + name + " : "+ wt);
			try {
				Thread.sleep((int) (Math.random() * 1000));
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
}

1、wait() 和 notify()必须在有加锁的时候才能使用；

2、this.wait()不是指当前对象等待，而是指持有当前对象的线程等待！！！

3、notify()是指：调用notify方法的当前线程1，叫醒等待在线程1持有的对象上（上面例子中的ss对象）的另外一个线程；

4、notifyAll()就是叫醒等待ss对象上的其他所有线程，一起去抢对象ss的锁！