双核CPU上运行Notify 与wait的通知丢失现象

package thread1; // this program is puzzle when run on 2 core ,I do not know why ,it just // do not stop class ThreadA { public static void main(String[] args) { ThreadB b = new ThreadB(); b.start(); System.out.println("b is start...."); synchronized (b)// 括号里的b是什么意思,起什么作用? { try { System.out.println("Waiting for b to complete..."); b.wait();// 这一句是什么意思，究竟让谁wait? System.out.println("Completed.Now back to main thread"); } catch (InterruptedException e) { System.out.println("InterruptedException"); } } System.out.println("main:Total is :" + b.total); } } class ThreadB extends Thread { int total; public void run() { synchronized (this) { System.out.println("ThreadB is running.."); for (int i = 0; i < 10; i++) { total += i; System.out.println("total is " + total); } notify(); /* * try { System.out.println("b is waiting" ); wait(); * } catch (InterruptedException e) { System.out.println("InterruptedException"); } */ } System.out.println("AFTER notify"); System.exit(0); } }

package thread1; // this program is puzzle when run on 2 core ,I do not know why ,it just // do not stop class ThreadA { public static boolean tic = false; public static void main(String[] args) { ThreadB b = new ThreadB(); b.start(); System.out.println("b is start...."); while (true) { while (ThreadA.tic == false) { synchronized (b)// 括号里的b是什么意思,起什么作用? { try { System.out.println("Waiting for b to complete..."); b.wait();// 这一句是什么意思，究竟让谁wait? System.out.println("Completed.Now back to main thread"); } catch (InterruptedException e) { System.out.println("InterruptedException"); } } } if (ThreadA.tic == true) { System.out.println("main:FINISH"); System.exit(4); } } // System.out.println("main:Total is :" + b.total); } } class ThreadB extends Thread { int total; public void run() { System.out.println("ThreadB is running.."); for (int i = 0; i < 100; i++) { total += i; System.out.println("total is " + total); } synchronized (this) { ThreadA.tic = true; notify(); /* * try { System.out.println("b is waiting" ); wait();// 这一句是什么意思，究竟让谁wait? } catch (InterruptedException e) { * System.out.println("InterruptedException"); } */ } System.out.println("AFTER notify"); System.exit(0); } }

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hongke1490
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发表于：2008-05-07 11:22:4315楼得分:40

把原始程序改成如下形式，单CPU机器上运行也不能正常退出了。

Java code


public class ThreadTest
{
    public static void main(String[] args)
    {
        ThreadB b = new ThreadB();
        b.start();
        System.out.println("b is start....");
        try
        {
            Thread.sleep(2000);//没有这个的话，单CPU上运行，成功的可能性很大
        }
        catch (InterruptedException ex)
        {
            ex.printStackTrace();
        }
        synchronized (b)// 括号里的b是什么意思,起什么作用?
        {
            try
            {
                Thread.sleep(2000);
                System.out.println("Waiting for b to complete...");
                b.wait();// 这一句是什么意思，究竟让谁wait?
                System.out.println("Completed.Now back to main thread");
            }
            catch (InterruptedException e)
            {
                System.out.println("InterruptedException");
            }
        }
        System.out.println("main:Total is :" + b.total);
    }
}

class ThreadB extends Thread
{
    int total;

    public void run()
    {
        synchronized (this)
        {
            System.out.println("ThreadB is running..");

            for (int i = 0; i < 100; i++)
            {
                total += i;
                System.out.println("total is " + total);
            }
            notify();
            /*
             * try { System.out.println("b is waiting" ); wait();
             *  } catch (InterruptedException e) {
             * System.out.println("InterruptedException"); }
             */
        }
        System.out.println("AFTER notify");
    }
}

解释：线程的启动需要时间，因此，线程b启动时，主线程很可能已经执行了synchronized (b)
这样的话，线程run方法中的synchronized (this)就必须等待b的对象锁解锁。当主线程
运行b.wait()后，b对象锁解开，run方法执行，同时，主线程等待b的对象锁解锁。当run方法
运行完了后，调用notify()，b对象锁解开，主线程继续执行，程序正常退出。
但是在这个过程中，如果线程b的run比主线程先运行到synchronized块的话，run方法先运行完了后，
主线程才能进入synchronized块，然后就会运行到wait处等待b的对象锁解锁，但这时，b线程已经运行
结束了，没有任何线程会把b解锁，因此程序不会退出。
双核的机器上，这种情况出现的概率更大。
不知道对不对，仅供参考。望高手指正。

修改删除举报引用回复

package thread1; //this program is puzzle when run on 2 core ,I do not know why ,it just // do not stop class ThreadA { public static boolean tic = false; public static void main(String[] args) { ThreadB b = new ThreadB(); b.start(); int i=1; System.out.println("b is start...."); while (true) { while (ThreadA.tic == false) { synchronized (b)// 括号里的b是什么意思,起什么作用? { try { System.out.println("Waiting for b to complete..."); b.wait();// 这一句是什么意思，究竟让谁wait? System.out.println("Completed.Now back to main thread"); } catch (InterruptedException e) { System.out.println("InterruptedException"); } } } System.out.println("loop in while true for"+i+"times"); if (ThreadA.tic == true) { System.out.println("main:FINISH"); System.exit(4); } } // System.out.println("main:Total is :" + b.total); } } class ThreadB extends Thread { int total; public void run() { System.out.println("ThreadB is running.."); for (int i = 0; i < 100; i++) { total += i; System.out.println("total is " + total); } synchronized (this) { ThreadA.tic = true; notify(); /* * try { System.out.println("b is waiting" ); wait();// * 这一句是什么意思，究竟让谁wait? * } catch (InterruptedException e) { * System.out.println("InterruptedException"); } */ } System.out.println("AFTER notify"); System.exit(0); } }

package thread1; //this program is puzzle when run on 2 core ,I do not know why ,it just // do not stop class ThreadA { public static boolean tic = false; public static void main(String[] args) { ThreadBB b = new ThreadBB(); b.start(); int i=1; System.out.println("b is start...."); //while (true) { //while (ThreadA.tic == false) { synchronized (b) { try { System.out.println("father:Waiting for b to complete..."); b.wait(); System.out.println("father:Completed.Now in main thread"); } catch (InterruptedException e) { System.out.println("InterruptedException"); } } } System.out.println("father:loop in while true for"+i+"times"); /* if (ThreadA.tic == true) { System.exit(4); } */ } // System.out.println("main:Total is :" + b.total); System.out.println("father:main:FINISH"); } } class ThreadBB extends Thread { int total; public void run() { System.out.println("ThreadB is running.."); for (int i = 0; i < 10; i++) { total += i; System.out.println("total is " + total); } synchronized (this) { System.out.println("goingto notify"); //ThreadA.tic = true; notify(); /* * try { System.out.println("b is waiting" ); wait();// * 这一句是什么意思，究竟让谁wait? * } catch (InterruptedException e) { * System.out.println("InterruptedException"); } */ } System.out.println("AFTER notify"); System.exit(0); } }