Unit of work by R4R Team

The challenge is the implementation of this (and other) behavior outside an EJB3 container: not only has the EntityManager and resource-local transaction to be started and ended correctly, but they also have to be accessible for data access operations. The demarcation of a unit of work is ideally implemented using an interceptor that runs when a request hits the non-EJB3 container server and before the response will be send (ServletFilter if you are using a standalone servlet container).

This is the default JPA persistence model in a Java EE environment (JTA bounded, transaction-scoped persistence context); injected (looked up) entity managers share the same persistence context for a particular JTA transaction. The beauty of JPA is that you don't have to care about that anymore and just see data access through entity manager and demarcation of transaction scope on session beans as completely orthogonal.

We recommend to bind the EntityManager to the thread that serves the request, using a ThreadLocal variable. This allows easy access (like accessing a static variable) in all code that runs in this thread. Depending on the database transaction demarcation mechanism you chose, you might also keep the transaction context in a ThreadLocal variable. The implementation patterns for this are known as ThreadLocal Session and Open Session in View in the Hibernate community. 

Than we can easily extend the HibernateUtil shown in the Hibernate reference documentation to implement this pattern, you don't need any external software (it's in fact very trivial). Of course, you'd have to find a way to implement an interceptor and set it up in your environment.

The most common pattern in a multi-user client/server application is entitymanager-per-request. In this model, a request from the client is send to the server (where the JPA persistence layer runs), a new EntityManager is opened, and all database operations are executed in this unit of work. Once the work has been completed (and the response for the client has been prepared), the persistence context is flushed and closed, as well as the entity manager object. 

Then we also use a single database transaction to serve the clients request. The relationship between the two is one-to-one and this model is a perfect fit for many applications.

First, don't use the entitymanager-per-operation antipattern, that is, don't open and close an EntityManager for every simple database call in a single thread! Of course, the same is true for database transactions. Database calls in an application are made using a planned sequence, they are grouped into atomic units of work. (Note that this also means that auto-commit after every single SQL statement is useless in an application, this mode is intended for ad-hoc SQL console work.)
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