How can I call notify() in Java without blocking?

Question

Are there a replacement for the follow Java code that will not block? I want only ping a possible waiting thread. I does not want change any on a possible thread that is already in the monitor.

synchronized( monitor ) {
    monitor.notify();
}

Answer 1

Try park/unpark with LockSupport.getBlocker()

http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/LockSupport.html

Answer 2

No you can not. You will get a "current thread not owner" if you try something like that.

Answer 3

Perhaps a ReentrantLock solve your problem? For example

class X {
   private final ReentrantLock lock = new ReentrantLock();
   // ...

   public void m() {
     lock.lock();  // block until condition holds
     try {
       // ... method body
     } finally {
       lock.unlock()
     }
   }
 }

Additionally, you can do stuff like

lock.isLocked()

and

lock.tryLock()

Answer 4

You could use java.util.concurrent.Semaphore instead of monitor. A binary semaphore can have the same use as a synchronize block:

private final Semaphore sema = new Semaphore(1, true); // binary: only 1 permit
....
try {
    sema.acquire();   // blocks till permit is available
    try {
        // critical section code
        ....
    } finally {
        sema.release();   // release permit
    }
} catch (InterruptedException ie) {
    ....

You can check the state of the semaphore with the following methods:

sema.availablePermits()  // 1 if no threads in critical section
sema.getQueueLength()    // number of threads blocked waiting on acquire
sema.getQueuedThreads()  // list of threads blocked waiting on acquire

Answer 5

The point of the notify is to let other threads that are waiting on the monitor know that they can try to obtain it. Only the current owner of the monitor can send the notify to indicate that they are done with it.

To own the monitor, you are obviously blocking any other thread. That is the purpose of the monitor after all.