今天一位同学问我题目中的这个问题，并给了我下面的代码，花了好久才看懂，这里总结一下．
实现代码如下所示：
package com.multithread.wait;
public class MyThreadPrinter2 implements Runnable {
private String name;
private Object prev;
private Object self;
private MyThreadPrinter2(String name, Object prev, Object self) {
this.name = name;
this.prev = prev;
this.self = self;
}
@Override
public void run() {
int count = 10;
while (count > 0) {
synchronized (prev) {
synchronized (self) {
System.out.print(name);
count--;
self.notify();
}
try {
prev.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) throws Exception {
Object a = new Object();
Object b = new Object();
Object c = new Object();
MyThreadPrinter2 pa = new MyThreadPrinter2("A", c, a);
MyThreadPrinter2 pb = new MyThreadPrinter2("B", a, b);
MyThreadPrinter2 pc = new MyThreadPrinter2("C", b, c);
new Thread(pa).start();
Thread.sleep(100); //确保按顺序A、B、C执行
new Thread(pb).start();
Thread.sleep(100);
new Thread(pc).start();
Thread.sleep(100);
}
}
首先，我们提出一个概念模型，就是Java对象中，包含了这么两部分内容：
ArrayList<Thread> threadsWhoWaitForThisObject
Thread threadWhoEnterTheMonitor
在执行synchronized块时，线程会比较相应对象的threadWhoEnterTheMonitor是否是null或者当前线程，如果是的话，就将相应对象的threadWhoEnterTheMonitor设置为当前线程，并且执行synchronized块．当线程调用wait()方法时，会将线程加入到对应对象的threadsWhoWaitForThisObject，并且将对应对象的threadWhoEnterTheMonitor设置为null，让其他的线程可以得到执行．当线程从synchronized块中出来时，也会将对应对象的threadsWhoEnterTheMonitor设置为null.
当然，这只是一个概念模型．Java中对象是否包含上面提到的这两个区域，我还不清楚．
清楚了上面的概念模型后，我们照着上面的代码走一个循环就清楚了．
首先，是线程pa执行，其次是pb，最后是线程pc执行．同时有三个对象，a，b，c，它们和上面的三个线程是一一对应的关系．下面我们将会从这三个执行期间a，b，c中的threadWhoEnterTheMonitor以及threadsWhoWaitForThisObject的变化来解释其实现原理．
在第一次pa执行期间，三个对象的threadWhoEnterTheMonitor以及threadsWhoWaitForThisObject的变化如下：
c's threadWhoEnterTheMonitor: pa -> null
a's threadWhoEnterTheMonitor: pa -> null
b's threadWhoEnterTheMonitor: null
a's threadsWhoWaitForThisObject: null
b's threadsWhoWaitForThisObject: null
c's threadsWhoWaitForThisObject: pa
在第一次pb执行期间，三个对象的threadWhoEnterTheMonitor以及threadsWhoWaitForThisObject的变化如下：
a's threadWhoEnterTheMonitor: pb -> null
b's threadWhoEnterTheMonitor: pb -> null
c's threadWhoEnterTheMonitor: null
a's threadsWhoWaitForThisObject: pb
b's threadsWhoWaitForThisObject: null
c's threadsWhoWaitForThisObject: pa
在第一次pc执行期间，三个对象的threadWhoEnterTheMonitor以及threadsWhoWaitForThisObject的变化如下：
a's threadWhoEnterTheMonitor: null
b's threadWhoEnterTheMonitor: pc -> null
c's threadWhoEnterTheMonitor: pc -> null
a's threadsWhoWaitForThisObject: pb
b's threadsWhoWaitForThisObject: pc
c's threadsWhoWaitForThisObject: pa -> null
在第一次执行pa和pb的期间，由于prev对象对应的threadsWhoWaitForThisObject是null，所以实际上self.notify()是不会起作用的．
而在第一次执行pc的期间，c's threadsWhoWaitForThisObject开始是pa，所以是会唤醒pa的．等到pc的synchronized执行完后，此时尽管三个对象的threadWhoEnterTheMonitor都是null，但是此时pb和pc都没有被唤醒，所以不存在竞争的问题．
后面的迭代就跟第一次基本上差不多了，各位可以自行尝试走一遍．
