Integer and int are not equal... Especially if received via JSNI

I've spent a huge amount of time trying to find the source of one strange bug in my app, so I want to share what I've found.

First I should say I'm using GWT 2.3 currently, so i don't know if the described behavior changed in latest version.

My app is receiving some data from my server, displays it and after some user actions send something from this data back to the server.

I've used standart GWT RPC for this data exchange and everything was fine.

Then we decided to receive that data via JSON requests and here my trubles started: trying to send data back to the server (with the old GWT RPC method) throwed the com.google.gwt.user.client.rpc.SerializationException: undefined at Unknown.anonymous(Unknown Source).

Well, I'm not going to tell all the steps how I've found the problem, just the result.

My data class looks like this: 

public final class MyDataObj  extends JavaScriptObject {

//… NoArgs constructor and other stuff

public final native Ineger getMyDataVal() /*-{ return this.myDataVal; }-*/;

}

I've used Integer and not int here because null value is perfectly valid here and has its own meaning.

But in JSON this was a normal JavaScript integer or the absence of the property.

It was fine. Just until GWT had to serialize this Integer for RPC. Or until I've tried to use toString() or intValue() on it. That fails, since this was not an Integer.

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