Stumped with registering objects

Question

I have tried mightily, but am not getting behaviour I desire.
I would like to eventually move to using DTO's to move data in and out of my session tier, but for the short term, I am trying to get the 'quick and dirty' approach to work.
For instance,
I query say 40 objects. I store a list of them in session scope in the servlet container. The gui then will allow a user to make some changes to one of these objects, and then resubmit it to the session tier to be saved.
The problem seems to be I am not starting a unit of work before the changes start, and making the changes to clones. The servlet code does not know about toplink, it just reads a list of objects, picks an arbitrary one to edit, and then resubmits it.
I was using deepMergeClone on the object to be saved to allow toplink to get a list of differences, and this seemed to work for most scenarios, but not all. I have heard that the merge api is more for objects which have been disconnected from the session.
I have attached the methods i have written for load/save. Any feedback would be very much appreciated...espicially if it explains that this interm method will work, acknowledging I will implement a better facade pattern with DTO's in the future. (I really need demo quality soon)
Here is the load method:
public static Collection returnQueryResult(String queryBarId) {
          UnitOfWork uow= getSession().acquireUnitOfWork();
          Vector facilities= null;
          try {
               Vector results=
                    (Vector) getSession().readAllObjects(
                         QueryBarResult.class,
                         new ExpressionBuilder().get("id").equal(queryBarId));
               facilities= new Vector(results.size());
               Iterator iter= results.iterator();
               IdentifiableObject theFacility;
               while (iter.hasNext()) {
                    theFacility= ((QueryBarResult) iter.next()).getQueriedObject();
                    facilities.add(uow.registerExistingObject(theFacility));
               } catch (Throwable th) {
               th.printStackTrace();
          return facilities;
Here is the save:
     public static Object save(IdentifiableValue obj, String sessionName) {
          try {
               UnitOfWork uow= getSession(sessionName).acquireUnitOfWork();
               uow.registerObject(obj);
               uow.deepMergeClone(obj);
               uow.commit();               
          } catch (DatabaseException e) {
               e.printStackTrace();
          return getSession(sessionName).readObject(obj);

Answer 1

Craig,
Sorry to hear you are having such difficulty getting your demo going. Have you looked at the session-bean and jsp-Servlet demos that are shipped with TopLink. They may help you get something running quickly.
I would like to clarify some issues that you have raised.
1. The instances in the shared cache, those returned from a query against a client session, are in fact the original objects. You need to be careful not to modify these outside the context of a UnitOfWork/TX.
2. The use of the merge API is typically used when the objects are detached from the session through serialization. This means that you architecture has a serialized link between the web tier and the server tier. This is more often accomplished by wrapping your server logic with a session beans. You will also need to ensure that if the web tier and server/ejb tier are in the same JVM that the app server is not passing the object by reference instead of by value.
3. If the objects you are changing in the web-tier are not separated by a serialized link such as a session-bean then you must register the objects prior to making any changes to them.
The code you have shown seems to assume that the web application will be in the same JVM and not have a serialized link between it and the persistence layer. Then to ensure you have a copy of the object you use one UnitOfWork to copy the objects and another to persist the changes. If you were willing to have a slightly more stateful system you could just maintain the UnitOfWork and avoid having to register or merge on the save, just call UnitOfWork.commit().
To address your specific code without writing a complete novel I would suggest the following:
Here is the load method:
public static Collection returnQueryResult(String queryBarId) {
UnitOfWork uow= getSession().acquireUnitOfWork();
Vector facilities= null;
try {
// Note: reading through the UOW will register results
Vector facilities = (Vector) uow.readAllObjects(QueryBarResult.class, new ExpressionBuilder().get("id").equal(queryBarId));
uow.release(); // not going to need it any more
} catch (Throwable th) {
th.printStackTrace();
return facilities;
Here is the save:
public static Object save(IdentifiableValue obj, String sessionName) {
try {
UnitOfWork uow= getSession(sessionName).acquireUnitOfWork();
uow.readObject(obj); //ensure its cached and registered
uow.deepMergeClone(obj);
uow.commit();
} catch (DatabaseException e) {
e.printStackTrace();
return getSession(sessionName).readObject(obj);
Although this should work for all cases there will be issues if you attempt to instantiate any indirect relationships on the copies returned from the query method. These objects have been copied through a now inactive UOW. If you require lazy loading after the query I would recommend using a more stateful solution an maintaining the UOW.
I hope this helps you get your demo going quickly. Please let me know if you run into any further complications.
Doug