XStream - treat objects with the same identifier as one

Question

I am working on a legacy system where XStream is being used to serialize objects in order to keep two databases in sync. A new object is first stored in one database, then the stored object is serialized and sent to be stored in the other database.

Up until recently, the structure of the object in question was like this:

public class Project {
    List<Milestone> milestones;
    [...]
}

But, after changes to the requirements, the structure is supposed to be like this:

public class Project {
    List<Goal> goals;
}

public class Goal {
    List<Milestone> milestones;
}

In order to keep the milestones of legacy data, which knew nothing about goals, the final structure of project was this:

public class Project {
    List<Goal> goals;
    List<Milestone> milestones;
}

So, there are two paths from a Project, to a Milestone, one directly and one through a Goal. The problem occurs when this structure is deserialized and stored. When it is being deserialized by XStream, the objects for the Milestones connected to the Project directly becomes different objects from the ones connected through Goals, even though they have the same id.

As long as Hibernate's Session#merge() was used to persist this object, it was no problem, since merge() doesn't care about the object identifiers as long as the db identifiers are the same.

But, I can no longer use merge() for this purpose, and have to rely on Session#save() instead. And save() DO care about the object identifiers! So now I get a org.hibernate.NonUniqueObjectException when trying to store the deserialized object.

I figure the least intrusive way to solve this is, if it's possible, to make XStream create 1 object per database id. But is this possible?

Answer 1

After some consideration, it is appearant to me that the problem is not XStream, as it has mechanisms for object references. The problem is another nifty "feature" of the project I'm working on - it has 2 versions of each domain class, one for commmunication with Hibernate, and one for "logic use" (don't ask me why...) In the conversion between these two versions (which basically moves values from one object to another), objects are new-ed uncritically, resulting in the same "Hibernate-object" being transformed into multiple "Java-objects". Then, I can't really blame XStream for not understanding that these should be the same :)