LOTD #22: How to inject JPA resources ? - PersistenceUnit vs PersistenceContext


Java Persistence API defines a standard object/relational mapping using POJOs. In JPA, a persistence unit is described using "persistence.xml", bundled with the web application, injected into your web application and then POJOs are used to access all the information from the underlying persistence mechanism such as a database.

JPA can injected into your application couple of different ways as shown below:

@PersistenceUnit
EntityManagerFactory emf;

and

@PersistenceContext
EntityManager manager;

Which one is preferred, why, and pros/cons are very clearly explained in (slightly old but very relevant) this blog. It also discusses a JNDI approach.

In case you are interested in the summary:
  • Use "@PersistenceUnit EntityManagerFactory" for Servlets because of thread safety
  • Use "@PersistenceContext EntityManager" in EJBs for simpler/cleaner code
Read other JPA related entries.
All previous entries in this series are archived at LOTD.

Technorati: lotd glassfish jpa javaee persistence
Comments:

Arun,

Maybe a tip for a follow-up TOTD: how to combine OSGi and JPA in GlassFish v3?
Example:
Bundle A is a WAB with a PU and a class Foo, Bundle B contains class Bar extends Foo. How can B persist instances of Bar using JPA? (the PU in Bundle A doesn't know how to persist class BAR).
I can come up with a number of solutions that all have drawbacks.

- Johan

Posted by Johan Vos on August 17, 2009 at 10:19 PM PDT #

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Arun Gupta is a technology enthusiast, a passionate runner, author, and a community guy who works for Oracle Corp.


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