Weird thread sync behavior (only every other notify works...)

Question

So what's up with this? Why does only every other call to notify() within SyncThread work? I've tested this app in 1.4.2_03, 1.4.2_08 and 1.5.0_04 and they all report the following output:
Count is 2
Count is 4
Count is 6
Count is 8
Count is 10
Count is 12
Count is 14
Count is 15
:public class JavaThreadSyncBug {
:    public static void main(String args[]) {
:        try {
:            SyncThread s = new SyncThread();
:            CallerThread c = new CallerThread(s);
:            s.start();
:            c.start();
:            s.join();
:        catch(Exception e) {
:            System.out.println(e.getMessage());
:public class SyncThread extends Thread {
:    boolean terminated = false;
:    int count = 0;
:    public SyncThread() {
:        super("SyncThread");
:    synchronized public void run() {
:        while (!terminated) {
:            try {
:                wait();
:                System.out.println("Count is " + count);
:            catch(InterruptedException e) {
:                System.out.println(e.getMessage());
:    synchronized public void touch() {
:        ++count;
:        notify();
:    synchronized public void terminate() {
:        terminated = true;
:        if (isAlive()) {
:            notify();
:public class CallerThread extends Thread {
:    SyncThread s = null;
:    CallerThread(SyncThread _s) {
:        super("CallerThread");
:        s = _s;
:    public void run() {
:        try {
:            for (int i = 0; i < 15; ++i) {
:                sleep(1000);
:                s.touch();
:            s.terminate();
:        catch(InterruptedException e) {
:            System.out.println(e.getMessage());
:}I should point out that I can work around this by doing the following in SyncThread's touch function:
notify();
notify();
or by calling
notifyAll();
But I am really curious about what's going on here...
Thanks,
Ken

Answer 1

>
Rhetorical question:
Is there an underlying reason why a joining thread is
eligible to be scheduled via a call to notify()? It
seems to me that the joining thread (by definition)
wouldn't care about anything until the thread it's
joined to has terminated...Even 'Rhetoricallly' I don't know. I have just taken your example a bit further so that 2 threads are joined and then you only get every 3rd notify()! If I use notifyAll() I see everyone!
public class JavaThreadSyncBug
    public static void main(String args[])
        try
            final SyncThread s = new SyncThread();
            CallerThread c = new CallerThread(s);
            s.start();
            c.start();
            new Thread()
                public void run()
                    try
                        s.join();
                        System.out.println("XXXX");
                    catch(InterruptedException e)
                        System.out.println(e.getMessage());
            }.start();
            s.join();
        catch(Exception e)
            System.out.println(e.getMessage());
class SyncThread extends Thread
    boolean terminated = false;
    int count = 0;
    public SyncThread()
        super("SyncThread");
    public void run()
        while (!terminated)
            synchronized(this)
                try
                    wait();
                    System.out.println("Count is " + count);
                catch(InterruptedException e)
                    System.out.println(e.getMessage());
    synchronized public void touch()
        ++count;
        System.out.println(count);
        notify();
    synchronized public void terminate()
        terminated = true;
        if (isAlive())
            notify();
class CallerThread extends Thread
    SyncThread s = null;
    CallerThread(SyncThread _s)
        super("CallerThread");
        s = _s;
    public void run()
        try
            for (int i = 0; i < 15; ++i)
                sleep(1000);
                s.touch();
            s.terminate();
        catch(InterruptedException e)
            System.out.println(e.getMessage());
}