Different classloaders cause ClassCastException when persisting data via Spring

Question

I'm creating an MVC Spring webapp. Using: Jetty (servlet container), DataNucleus (dao platform), DB4O (embedded datastore).

When I persist an object (done from within a Spring Controller) using JDO from DataNucleus, it stores to the DB fine.

@PersistenceCapable
public class Test {
    @Persistent
    private String testString;
    //getter-setters implemented
}

When I do a simple query for the objects I previously added I get a ClassCastException on my Test class (can't cast a.b.c.Test to a.b.c.Test).

The classloader of Test returned by JDO is (toString) [sun.misc.Launcher$AppClassLoader@5acac268], the classloader of the Test class before I persisted it to the DB is [WebAppClassLoader@1593275665]

I've gotten this far, but I don't really know what to do with a classloader issue like this, I've never spent much thought on classloaders before. Any direction is helpful.

Answer 1

There doesn't need to be two different versions of the class for a class cast exception to appear. Even the same class definition is seen as two different classes when loaded by two distinct classloaders. Which seems to be the case here.

Unfortunately I am not familiar with the platforms you use, so I can't give more concrete advice than this: try to experiment with moving the jar containing your Test class to different places on your web app classpath, and/or reconfiguring the Spring and Jetty classloaders so that both delegate the loading of Test to the same parent classloader.

Answer 2

I think that your problem might be similar to the one described here.

If so, the cure would appear to be to make sure that the jdo jarfile is loaded by a common ancestor of the classloaders.