Java concurrency. Block application from exiting

Question

I am trying to synchronize different threads in my application. Basically there are some threads that should be able to block the application from closing. Here is what I have:

public class Test{
  public static void main(String args[]){
    Syncer syncer = new Syncer();
    Object o = new Object();
    syncer.lockExit(o);
    //.. do some stuff//
    syncer.unlockExit(o);
  }
}

public class Syncer {

    private List<Object> l_Exit;

    public Syncer() {
        l_Exit = new ArrayList<Object>();
    }

    public synchronized void lockExit(Object o) {
        l_Exit.add(o);
    }

    public synchronized void unlockExit(Object o) {
        l_Exit.remove(o);
        if (l_Exit.isEmpty()) {
            l_Exit.notifyAll();
        }
    }

    public synchronized void waitForExit() {
        while (!l_Exit.isEmpty()) {
            try {
                l_Exit.wait();
            } catch (InterruptedException ex) {
                Test.log.log(Level.SEVERE, null, ex);
            }
        }
    }
}

I get IllegalMonitorException when running syncer.unlockExit(o);

Answer 1

public synchronized void unlockExit(Object o) {
    l_Exit.remove(o);
    if (l_Exit.isEmpty()) {
        l_Exit.notifyAll();
    }
}

You need to be in a synchronized block in order for Object.wait(), Object.notify() or Object.notifyAll() to work.

public synchronized void unlockExit(Object o) {
     synchronized (lock) {
        l_Exit.remove(o);
        if (l_Exit.isEmpty()) {
          l_Exit.notifyAll();
        }
     }
}

See this question for clarification so as to why.