Skip to main content

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.

Popular posts from this blog

One page Stock

Alright.. That was a long absence. The whole last week I dint blog. I dint go away. I was "occupied". I was learning stock trading. Its very fascinating. I have a good weeeked blog for you all. Here is my experience. I can literally hyper-link every word from the following paragraphs, but I am writing it as simple as I can so you can look up the italicised words in wikipedia . I got a paper trading account from a brokerage firm . You need one brokerage account first. Then it can be an Equity account where all your money is yours or a Margin account , where some of the money is lent by the brokerage firm. Then I get Buying power , which is the dollor value of how much stocks you can buy. I can make profit by simple rules. Buy when Price is low. Sell when price is high. There is another more intersting way of earning money. Selling short . Thats when price is not high, per say, but when are confident that the price WILL go down. then buy back when its lowest. This is what

Appcache manifest file issues/caveats

Application cache (appcache) is a powerful feature in HTML5. However, it does come with baggage. Many (see links below) advocated ferociously against it due to tricky issues it comes with. For someone who is just testing waters, these issues may throw them off grid. Knowing them before hand helps reduce some unpredictable effects.

classpath*: making your Modular Spring Resources

Spring gives multiple options to load XML resources for building contexts. the reference documentation does explain this feature quite well. However, I am taking my shot at explaining the different practical scenarios ( by order of growing modularisation) For Example, A simplest Spring based web Context Loader can be configured with resources like this <context-param> <param-name>contextConfigLocation</param-name> <param-value>applicationContext.xml</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> You just need to put applicationContext.xml in WEB-INF/ folder of your webapp. However, Typically an application is n-tiered. You can also have multiple files setup and in relative paths. like <param-value> context-files/applicationContext.xml context-files/dao.xml context-files/service.xml </param-value>