Java Object - wait - notify - a practical example

Object is the super class of anything in Java. But, Most people fear the wait and notify methods in Object. The common java practitioner might not need to use these for all practical purposes. However, the fear is not because they are hard to understand, but because of the assumption and false notion (generally pushed from the *experts* to the newbies) that these are for *super users*. The easiest way to understand this is by example. Most examples include Produce Consumer, But in practice where is this applied?

Some time back I wrote about Commons Pooling. Ever wonder how an Object pool Implemention, like this one, would time out on a blocking call without creating a new thread on its own? It uses a wait - notify[All] paradigm.

To understand that Look at the source code of the said GenericObjectPool.

The borrowObject method (line 911 - so apt??) which optionally waits for a maxWait long secs, is used like

for (;;) {
 synchronized (this) {
  // Some Logic
  try {
   if (_maxWait <= 0) {
    wait();
   } else {
    final long elapsed = (System.currentTimeMillis() - starttime);
    final long waitTime = _maxWait - elapsed;
    if (waitTime > 0) {
     wait(waitTime);
    }
   }
  } catch (InterruptedException e) {
   Thread.currentThread().interrupt();
   throw e;
  }
  if (_maxWait > 0
    && ((System.currentTimeMillis() - starttime) >= _maxWait)) {
   throw new NoSuchElementException(
     "Timeout waiting for idle object");
  } else {
   continue; // keep looping
  }

 }
}

And the addObjectToPool method does the notify

if (decrementNumActive) { //Some condition
       synchronized(this) { //Sync the same object
           _numActive--;
           notifyAll(); // wakeup all threads waiting on *this*
       }
   }

So one method will call wait, (optionally with a with a wait time) and the other will notify. Note that it is the same object used (*this*). Here the producer is the addObjectToPool method and consumer is the borrowObject. Generally wait(*no time out*) is in a loop, wait with time out is a single flow. Both these are demostrated by the above example.