Arun Gupta, Miles to go ...
Arun Gupta is a technology enthusiast, a passionate runner, and a community guy who works for Sun Microsystems.
Tuesday August 18, 2009
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
All previous entries in this series are archived at LOTD.
Technorati: lotd glassfish jpa javaee persistence
Posted by Arun Gupta in General | Comments[1]
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This is a personal weblog, I do not speak for my employer.
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 18, 2009 at 05:19 AM PDT #