Ivan’s agile thinkings

A while back I attempted to create a visualization of the relations that OpenJPA creates between the Java classes that it enhances - the intent was to help better understand a particularly intertwined persistent object graph and see if I couldn’t figure out why we were getting a few strange problems.

The initial approach I took was a hilarious trip into the land of sed, perl and graphviz.  It kinda looked like it worked, but didn’t actually

Time passed……more time passed……I learned more……more time passed…….and then I had the wacky notion of tying directly into the Enhancer that OpenJPA uses to create the persistence enabled classes.  I haven’t done the visualization yet, but this is what I learned about actually tying in an AuxiliaryEnhancer.

1. Create the AuxiliaryEnhancer

The following code is the minimal you will need.  Enter this and then compile - you will need OpenJPA and serp on your classpath.

package com.ij;

import org.apache.openjpa.enhance.PCEnhancer.AuxiliaryEnhancer;
import serp.bytecode.BCClass;
import serp.bytecode.BCMethod;
import org.apache.openjpa.meta.ClassMetaData;

public class Grapher implements AuxiliaryEnhancer {

    public void run(BCClass bc, ClassMetaData meta) {
        System.out.println("Running Enhancement");
    }

    public boolean skipEnhance(BCMethod m) {
         System.out.println("skipEnhance?");
         return false;
    }
}

2. Register the enhancer using jar based services

This is something I was only barely aware off - registering concrete implementations of services through jar file meta-data.

• Create a file in a META-INF/services/ directory called org.apache.openjpa.enhance.PCEnhancer$AuxiliaryEnhancer
  • Note the dollar sign! That one caught me out for a while: I couldn’t decide whether the dot notation or the dollar notation should be used to reference the nested class.
• This file should simply contain the fully qualified name of the Enhancer you wrote: com.ij.Grapher
• Jar this directory and the class file up and you should end up with a jar listing like this

ivan-jensens-computer:jar ivanjensen$ jar tvf ../grapher.jar
     0 Tue Dec 02 21:56:42 PST 2008 META-INF/
    60 Tue Dec 02 21:56:42 PST 2008 META-INF/MANIFEST.MF
     0 Tue Dec 02 21:54:28 PST 2008 META-INF/services/
    16 Tue Dec 02 21:30:44 PST 2008 META-INF/services/org.apache.renamed.openjpa.enhance.PCEnhancer$AuxiliaryEnhancer
     0 Tue Dec 02 21:55:16 PST 2008 com/
     0 Tue Dec 02 21:55:38 PST 2008 com/ij/
   771 Tue Dec 02 21:55:38 PST 2008 com/ij/Grapher.class
   465 Tue Dec 02 21:54:46 PST 2008 com/ij/Grapher.java

3. Add the new jar file to the enhancer classpath

I did this by adding my jar file to the classpath element of my ant script - pretty simple.

4. Re-enhance your classes

Re-enhance your classes and you will see the System.out.println statements all over your console.

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] Running Enhancement

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

 [openjpac] skipEnhance?

One thing I noticed about this: I had to delete my persistent classes and re-enhance, only then would the ‘run’ method be called.  The skipMethod is called whether I delete the classes or not.  I don’t know what that means yet, but I’m sure I am about to find out.

Notes:

I was actually using a patched version of OpenJpa 1.0.1 (for various, non-amusing, reasons), but the patches shouldn’t affect this tutorial.