A bug in wait and notify? -- help

Question

Now I am engaged in a simulation project. The threads are all controlled by the controller program, that is, they are often blocked and unblocked by using wait and notify methods. However, when I was debugging my program, there were always some errors occurring and halting the simulation. And I found that the cause was that the threads sometimes might halt after having executed the wait or notify methods!!
Example,
in thread 1, it would be blocked by a wait method like:
synchronized(lock){
lock.wait();
System.out.println("Thread 1 go on.");
In thread 2, it would unblock thread 1 with the notify method:
synchrnoized(lock){
System.out.println("Thread 2 would notify the threads blocked on lock.");
lock.notifyAll();
System.out.println("Thread 2 go on.");
The simulation program controls the time to execute the thread 2. However, in debugging the program I often encountered the problem that thread 2 did notify thread 1 and thread would go on executing, but thread 2 would not go further. That is, the output is like:
Thread 2 would notify the threads blocked on lock.
Thread 1 go on.
But the String, "Thread 2 go on." would not be printed out.
It was so strange, and I repeated for tens of times, it always occurred here and there.
Is it a bug of java?

Answer 1

There seems to be nothing wrong with this to me:
You should use better debugging messages. I couldn't read it the way you had it.
public class BugThread extends Thread
    Object lock;
    Notifier notifier;
    public static void main(String[] arg)
        for(int i = 0; i < 5; i ++)
            Object lock = "lock " + i;
            Notifier noti = new Notifier(lock);
            BugThread bug = new BugThread(lock, noti);
            bug.start();
            noti.start();
    public BugThread(Object lock, Notifier notifier)
        this.lock = lock;
        this.notifier = notifier;
    public void run()
        int i = 0;
        while( i++ < 100)
            synchronized(lock)
                System.out.println("Bug thread: "
                    + lock + " would block at Object: "
                    + lock);
                try
                    lock.wait();
                catch(InterruptedException ex)
                    ex.printStackTrace();
                System.out.println("Bug thread: "
                    + lock + " unblocked at Object: "
                    + lock);
        notifier.stop();
class Notifier extends Thread
    Object lock;
    public Notifier(Object lock)
        this.lock = lock;
    public void run()
        int i = 0;
        while(true)
            synchronized(lock)
                System.out.println("Notifier: " + lock
                    + " would notify Bugthread blocking at object: "
                    + lock.toString() + ". time no." + i);
                lock.notifyAll();
                System.out.println("Notifier: " + lock
                    + " notified bugthread. time no." + i);
            System.out.println("Notifier: " + lock
                + " go on working. time no." + i);
            i ++;
}