Technical Blog Post

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Omnibus WebGUI - The what, when, where and how of Java heap dumps

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Welcome

A fine afternoon to you, the Developer Works reader, I'm James Grafton a developer on the Omnibus Web GUI team and I'm here today to blog to you about Java Heap Dumps!

Java heap huh?

A Java heap dump is a snapshot of the entire memory structure of your Java program at a specific point in time.

Sounds useful, how do I get one?

As long as you're using an IBM based JDK there is a rather handy guide here. Essentially it's just a case of setting an environmental variable at launch, then, at some later point, sending the running JVM a specific signal indicating that you'd like to generate a heap dump - simples huh?

Fantasmagorical! Now, to make the most of it...

Hot off the press! A graphical heap analyser able to run across platforms, suggest likely memory leaks, performance faux pars - a most useful tool indeed! But wait, even better, it comes with a full overview tutorial well worth the download then!

And here's one I made earlier...

Based on the above, an example was produced representing end to end production and analysis of a simplistic memory leak scenario - hopefully it should help clarify everything.

Tools used:

Eclipse Indigo
IBM Heap Analyser 4.1.4
IBM Java 1.6.0

1. Step one

Create a new Java project in Eclipse, adding a single main class named HeapDump.java with the following code:

     public class Heapdump   {       @Override  protected void finalize() throws Throwable       {           while (true)           {               Thread.yield();           }       }       /**  * @param args  */  public static void main(String[] args)       {           while (true)           {               for (int i = 0;i > 100000; i++)               {                   Heapdump hd = new Heapdump();               }               System.out.println("" + Runtime.getRuntime().freeMemory() + " bytes free!");           }       }   }

This is an example of one of the only conditions in Java where it's possible to create a true memory leak - disrupting the garbage collector during the finalize() call.

2. Step two

Create a run time configuration as detailed in the Alpha Works article:

3. Step three

Watch the program spectacularly fail!

4. Step four

Start HeapAnalyser, via [java -Xmx1000m jar ha414.jar <path to heapdump.phd>] and wait a few minutes for it to load the heap dump in.

5. Step five

Observe the correctly identified diagnostic error:

A whistle-stop tour of Java heap dumps and their associated analysis, hopefully enough insight here to help identify their potential value.

Always interested to hear any questions on the topic - feel free to give me a shout whenever.

-James Grafton

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