I would like to create a REST interface for my Java Google App Engine program. I figured I would start by generating some XML from my POJOS. However, it seems that XStream is bitten by GAE's restrictions.

What can I use to generate an XML string in Google App Engine?

Thanks.

Edit 1:
Here is the beginning of the exception:

```
javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract    java.lang.String com.mydomain.client.ObjectService.sendObject(com.mydomain.client.models.myobject)' threw an unexpected exception: java.security.AccessControlException: access denied (java.io.SerializablePermission enableSubclassImplementation)

```