How to fix LazyInitializationException with GWT?

Question

I have:

org.hibernate.LazyInitializationException: could not initialize proxy - no Session

This is my service:

@Service("empService")
public class EmpServiceImpl extends RemoteServiceServlet implements EmpService {
    @Autowired
    EmpHome empHome;

    @Override
    @Transactional
    public Emp findById(short id) {
        return empHome.findById(id);
    }

Im trying to use my service in gwt:

EmpServiceAsync empServiceAsync = GWT.create(EmpService.class);
        AsyncCallback<Emp> callback = new AsyncCallback<Emp>() {
            @Override
            public void onFailure(Throwable caught) {
                Info.display("Failure", "что-то пошло не так");
            }

            @Override
            public void onSuccess(Emp result) {
                Info.display("Succes", result.getEname());
            }
        };

        empServiceAsync.findById((short) 7844, callback);

Answer 1

I would highly discourage using Hibernate mapped object Emp in GWT client side directly. Your Hibernate session will only be available inside findById as it is marked @Transactional, however, GWT will need to traverse the entire Emp object to serialize it for client. That will obviously happen outside findById hence you will get LazyInitializationException if Emp contains any properties that require lazy loading (for example, association lists).

The solution is to use intermediate data transfer object, for example EmpDTO and convert Emp to EmpDTO inside your service transactional block.

Answer 2

I actually got around this issue by creating a CustomFieldSerializer for my domain objects.

Take a look at this file: https://github.com/dartmanx/mapmaker/blob/0.5.2/src/main/java/org/jason/mapmaker/shared/model/FeaturesMetadata_CustomFieldSerializer.java

I've commented out the relevant lines because I ended up not needing it, but here's the code:

public static void serialize(SerializationStreamWriter writer, FeaturesMetadata instance) throws SerializationException {        
    writer.writeInt(instance.getId());
    writer.writeString(instance.getState());
    writer.writeString(instance.getStateAbbr());
    writer.writeString(instance.getUsgsDate());
    writer.writeString(instance.getFilename());
    writer.writeString(instance.getStateGeoId());
    writer.writeString(instance.getCurrentStatus());
    if (instance.getFeatureList().size() == 0) {
        writer.writeObject(new ArrayList<Feature>());
    } else {
        writer.writeObject(new ArrayList<Feature>(instance.getFeatureList()));
    }
}

The final line takes the takes an argument of the instance object's getFeatureList(), which is actually a Hibernate PersistentBag, and writes out an actual ArrayList with the contents of said PersistentBag.