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

Using Equinox CommandProvider to make OSGi console interactive.

After fiddling with the First Bundles that "Hello World"-ed upon Activation, You want to see more interactivity in OSGi. Although Using OSGi for an interactive Command Line Application would be like this one would be, well, a callable over-kill, I am going to start with an example and Expand it in later posts. So, please Welcome CommandProvider. CommandProvider is an EQUINOX specific API for extending the Console. This basic Example illustrates how to get a command from console and do something in java and also gets your feet wet on Service Registry package com.so.examples.commandconsole; import org. eclipse .osgi.framework.console .CommandInterpreter; import org.eclipse.osgi.framework.console.CommandProvider; public class DisplayMessageCommand implements CommandProvider { public void _say(CommandInterpreter ci) { ci.print("You said:" + ci.nextArgument()); } @Override public String getHelp() { return "\tsay - repeats what you say\n"; } }

javascript maxlength for textarea with \r\n breaks in java (esp Firefox)

Textareas allow new lines to enter. These are represented by \n (1) or \r\n (2) characters. But when you save to DB you have a limit to certain length of chars. There is no maxlength attribute in HTML that will stop you from entering data. This is generally acomplished by Javascript. You do a onkeyup hook and stop event or trim after textarea.value.length > maxlength. There are many other solutions out there.. But.. Here is the problem that most of those solutions overlook, How do you deal with the count on \n and \r\n representations. Lets first see how it matters. If the text entered has new lines, the length is calculated differently in Firefox and IE. When you enter a Text like 01234 567890 You expect the textarea.value.length to be 11. (10 chars + new line).On the backend, however, java would recieve it as 12 chars (10 chars + \r\n) (this is irrespective of FF or IE). So you are effectively saving 12 chars to DB. Worse yet, IE seems to figure textarea.value.length as 12 (

How to Make a Local (Offline) Repository in Ubuntu / Debian

If you are in a place where you dont have internet (or have a bad one) You want to download .deb packages and install them offline. Each deb file is packaged as a seperate unit but may contain dependencies (recursively). apt-get automagically solves all the dependencies and installs all that are necessary. Manually install deb files one by one resolving each dependency would be tedious. A better approach is to make your own local repository. Before you actually make a repo, You need *all* deb files. You dont practically have to mirror all of the packages from the internet, but enough to resolve all dependencies. Also, You have to make sure, you are getting debs of the correct architecture of your system (i386 etc) # 1. make a dir accessible (atleast by root) sudo mkdir /var/my-local-repo # 2. copy all the deb files to this directory. # 3. make the directory as a sudo dpkg-scanpackages /var/my-local-repo /dev/null > \ /var/my-local-repo/Packages # 4. add the local repo to sour