I have some time this days and I’m trying to fly some clouds with Google App Engine (GAE), Now when they have released Java support I can try to use Groovy. Today I tried to implement some simple storage using JDO. GAE team supports some of its features via  DataNucleus. DataNucleus is using a post compilation hook to enhance any persistable classes.

Having Groovy in a lib folder of your war (GAE runs your application from this folder) I need to put asm library as well. Latest version of Groovy (1.6.1) has a dependency on asm library in version 2.2.3. On the other hand DataNucleus enhancer has dependency on asm library in version 3.1.

GAE Java SDK is shipped with Ant task for enhancing persistable classes. It also contains Ant macro definition file that makes it simple for usage. Unfortunately this macro includes all libraries from application lib folder. This is causing Enhancer to use asm library in a version that causes it to fail massively.

[enhance] SEVERE: An error occured for ClassEnhancer "ASM" when trying to call the method "org.datanucleus.enhancer.as
m.ASMClassEnhancer" on class "getClassNameForFileName" : org.objectweb.asm.ClassReader.accept(Lorg/objectweb/asm/ClassVi
sitor;I)V

Number of this error messages is as big as the number of classes in my application.

Solution is to exclude asm library from the macro that Google shipped. Easiest way is to edit, or copy to your local application folder and use it there. The line problematic line of code is in classpath element.

<classpath>
          <pathelement path="${appengine.tools.classpath}"/>
          <pathelement path="@{war}/WEB-INF/classes"/>
          <fileset dir="@{war}/WEB-INF/lib" includes="*.jar">
          </fileset>
</classpath>

The easiest way to fix it is to exclude asm library in fileset element, just like this:

<classpath>
          <pathelement path="${appengine.tools.classpath}"/>
          <pathelement path="@{war}/WEB-INF/classes"/>
          <fileset dir="@{war}/WEB-INF/lib" includes="*.jar" excludes="asm*.jar">
          </fileset>
</classpath>

Hope that helps anyone with similar problem.

Cheers, Greg