Annoying Hibernate serialization

[Mathias Nilsson]

I have a very annoying hibernate serialization problem.

On the server side I have servlets and Hibernate to handle the persitence. The hibernate mapping uses annotation and plain POJO objects.

  LanguageDAO dao = new LanguageDAO();
  dao.persist( new Language( "GB" ) );

Many of these POJO objects must be used in my applet as well. It's the exact same functionality in the applet as in the server and they need to be serialized from and to the applet.

ex

 a new Block is created in the applet. Draged around and placed with pixels 
by the user. Upon save the Block class gets serialized to the servlet 
persisted and responded with and id from the database.

How do you handle this situation. I want to load with lazy because there can be very much data on some of the object. If I use eager then the whole database will be serialized.

ex.

1. Applet init and sends an Id to the servlet requesting a Document.
 
2. Servlet checks the id and respond with a Template wich has Customer, 
User, GridLines, Colors and pages connected.
 
3. Applet paints the template, gridlines and uses the colors for changing 
background.
 
4. When user clicks on a page in a JTree applets sends request to server to 
get all blocks connected to a page. A page can be created in the applet and 
saved as well.
 
5. Servlet checks the page and responds with all blocks.

Do I have to create DTO that are not Hibernate annotated for this? There will be the exakt same classes in the servlet as in the applet and it seems like duplicating code to me. I assume that many has done this before, but when I search google, Javaranch and Hibernate forum I can't get a satisfying answer.

Hope you can help me.
[ August 27, 2007: Message edited by: Mark Spritzler ]

[Mark Spritzler]

So you just want to use detached objects. The Hibernate Annotated objects are just POJOs and if you have them implement Serializable, and I recommend highly that you implement the equals and hashcode methods, then you can send them down to your client and re-attach them when you send them back to the server. Now note that if you lazy load one of these objects, send it to the client, and try to access data that is not there because of lazy, you will get a LazyInitializationException. But there are also solutions for that. Basically you want to make sure you bring down just the exact amount of data that you need for that request. I think almost all applications need to do something like this.

Mark

[Mathias Nilsson]

Thanks Mark!

I have done exactly this overloaded equals but get an error in my applet.
I get InvalidClassException: org.hibernate.AbstractPersitentCollection; local class incompatible.

I've added my jar and hibernate jars in the client for test. When I try to read the object in my client I get this error. Any ideas?

[Mark Spritzler]

I'm not positive you need the Hibernate jars on your client side. At least you shouldn't need to have them.

Mark

[Mathias Nilsson]

It's because the DTO's are Hibernate annotated. If I don't have hibernate on the client side the compiler complains about CascadeType and other types.

// Mathias