线程间通讯 wait notify notifyAll简单使用

注意的问题。
为什么这三个方法必须在synchronized方法中使用？
并且调用wait notify和notifyAll的对象必须是锁对象。
可以看一下三个方法的源码实现。
http://blog.youkuaiyun.com/raintungli/article/details/6532784

个人理解如果没有在synchronized修饰的方法中，对象的对象头指向的MonitorObject是null，所以在调用时会出错。

代码如下

package com.example.ahl.one;

public class Test {
    int size;
    public static void main(String[] args) {
        Test test = new Test();
        Object lock = new Object();


        new Thread(test.new Producter(lock)).start();
        new Thread(test.new Consumer(lock)).start();
    }
    class Producter implements Runnable {
        Object lock;

        public Producter(Object lock) {
            this.lock = lock;

        }

        @Override
        public void run() {
                synchronized (lock) {
                    while(true){
                        if (size >= 5) {
                            try {
                                System.out.println("等待消费");
                                lock.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }

                        }
                        try {
                            Thread.sleep(500);
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }
                        size++;
                        System.out.println("正在生产 size = " + size);
                        if (size == 5) {
                            lock.notifyAll();
                        }
                    }

                }
            }


    }

    class Consumer implements Runnable {
        Object lock;

        public Consumer(Object lock) {
            this.lock = lock;

        }

        @Override
        public void run() {

                synchronized (lock) {
                    while (true) {
                    if(size<=0){
                        try {
                            System.out.println("等待生产");
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    size--;
                    System.out.println("消费 size =" + size);
                    if (size == 0) {
                        lock.notifyAll();
                    }
                }
            }
        }
}
}