Weird serialization error in AppEngine

My previous post discussed a very cool way to do background processing in AppEngine using Deferred.defer. Unfortunately, the AdminEmailTask example I posted results in a cryptic error when the task is dequeued and the Deferred servlet attemps to deserialize it:

com.newatlanta.appengine.taskqueue.Deferred deserialize: invalid type code: 00

After nearly a day’s worth of experimentation, I am really weirded out by what I’ve found: the problem seems to be the name of the private field “msgSubject” in AdminEmailTask. I tried shorter names, longer names, and other names without any problem. I changed the class name, moved it into a different package, and generally pulled my hair out until I simply tried changing the name of the field. At first, I thought it was this JDK bug, but I wasn’t using an array type. Nevertheless, I subclassed ObjectInputStream and overrode the resolveClass method as per the bug’s workaround, only to find out that AppEngine’s implementation of ObjectInputStream doesn’t call the overridden resolveClass and throws a security exception on enableResolveObject(true).

I can only guess that the field name “msgSubject” results in a binary sequence that is somehow special to AppEngine’s implementation of ObjectInputStream, or perhaps it’s a bug in the JDK.

At any rate, if you get the java.io.StreamCorruptedException with invalid type code: 00, try renaming your class fields! I really, really hope I’m mistaken about this.