求多线程调度问题调试

Question

目标是建立三个线程 th1，th2，th3，希望其能按照顺序th1->th2->th3的顺序执行。

下面贴出我的代码，运行时总是不能得到正确的结果，求大神指点~~~~

package threadSeq;

public class testThSeq {

    public volatile static int state = 1;

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Thread th1 = new Thread(new Runnable() {
            public void run() {
                try {
                    increaseAndNotifyAll(1);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        });

        Thread th2 = new Thread(new Runnable() {
            public void run() {
                try {
                    increaseAndNotifyAll(2);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }

        });
        Thread th3 = new Thread(new Runnable() {
            public void run() {
                try {
                    increaseAndNotifyAll(3);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });

        th3.start();
        th2.start();
        th1.start();
    }

    public synchronized static void increaseAndNotifyAll(int wantedState) throws InterruptedException {
        System.out.println("state:"+state+";"+Thread.currentThread()+wantedState);
        while(state!=wantedState){
            Thread.currentThread().wait();
            Thread.currentThread().notifyAll();
        }
        System.out.println(Thread.currentThread() + " finished");
        state++;
    }
}

Answer 1

有两个问题。
第一：获取哪个对象的锁。都只能对那个对象wait和notify。你现在通过synchronized获得的是testThSeq.class的锁。（synchronized一个方法，获取的是方法所属对象的锁，synchronized一个static方法，获取的是方法所属类的class的锁）所以你不能Thread.currentThread().wait();应该testThSeq.class.wait()。notifyAll也一样。

第二：notifyAll紧跟着wait后面，并且都在条件判断state!=wantedState里面，必将产生死锁。线程要么state!=wantedState为true，进入wait等待。要么state!=wantedState为false。永远无法执行到notifyAll这条语句。
改为：

[code="java"]
while (state != wantedState) {
testThSeq.class.wait();
}
testThSeq.class.notifyAll();
[/code]