could not initialize proxy – no Session

All java web frameworks have one or more servlets that handle the requests. The servlet handles each request (HttpRequest) by creating a new thread that will finally produce the response (HttpResponse). The method that processes each request is executed inside this thread.

At the beginning of the request processing your application should allocate the resources that it needs for processing (Transaction, Hibernate session etc). At the end of the processing cycle these resources are released (Transaction is committed, hibernate session is closed, JDBC connections are released etc). Lifecycle of these resources could be managed by your framework, or could be done by your code.

In order to support application state in a stateless protocol as HTTP, we have the HttpSession object. We (or the frameworks) put on HttpSession the information that remains relevant between different request cycles of the same client.

During the processing of the first request hibernate reads (lazily) an entity from the database. Due to lazy initialization some parts of this object’s structure are hibernate proxy objects. These objects are associated with the hibernate session that created them.

When you try to process the second request, then the framework finds the entity from the previous request in the HttpSession object. Then it is trying to access a property from a child entity that was lazily initialized and now is a hibernate proxy object. The hibernate proxy object is an imitation of the real object that will ask its hibernate session to fill it with information from the database when someone tries to access one of its properties. This what your hibernate proxy is trying to do. But its session was closed at the end of the previous request processing, so now it doesn’t have a hibernate session to use in order to be hydrated (filled with real info).

Note that it is possible that you have already opened a hibernate session at the beginning of the second request, but it isn’t aware of the entity that contains the proxy object because this entity was read by a different hibernate sesion. You should re-attach the entity to the new hibernate session.

There is a lot of discussion about how to re-attach a detached entity, but the simplest approach right now is session.update(entity).

I solved the problem by annotating the method which indirectly uses Hibernate to retrieve data from the database with @Transactional.


Good reference on session and transaction

another post


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