Java wait without lock release

Question

How can I make a thread wait without releasing the lock ? If this is not possible, then how can I pause a thread while a certain condition is not met and unpause it as soon the condition is met or when I notify it

Answer 1

Concept of wait/notify

Waiting/Notifying can only be done in the scope of a lock (eg a synchronized method or synchronized block).

Both the wait and notify should synchronize on the same object.

The waiting thread:

synchronized(lockObject) {
  lockObject.wait(); <-- it will wait here until the notify is called.
}

The notifying thread:

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

The lockObject can be anything. It could just be a new Object() , but very often it could be some collection, a logical object that represents a printer, ... anything.

Hint: Judging from the comments section, you may need the following: The notify releases just one waiting thread. But you could actually have multiple threads waiting. If you want to release all of them, use notifyAll .

Same thing, but with synchronized methods

Alternatively, you could create a class, and use method level locking.

class LockObject {
  public synchronized void waitMethod() {
    wait();
  }

  public synchronized void notifyMethod() {
    notify();
  }
}

Again both have to use the same object to lock on.

LockObject instance = new LockObject();

Then the waiting thread:

instance.waitMethod();

And the notifying thread:

instance.notifyMethod();

If you are serious about this

Also take a look inside the concurrency packages and tutorials of the JDK , and you will find more advanced locking objects. Just to name one, countdown latches are powerful.