Contexts & Dependency Injection (CDI) in Java EE 6 provides type-safe dependency injection. The type-safety part comes from the fact that no String-based identifiers are used for dependency injection. Instead CDI runtime uses the typing information that is already available in the Java object model.
Java EE 5 already had resource injection available in terms of PersistenceContext, PersistenceUnit, Resource, and others. But they require String-based identifiers to identify the resource to be injected. For example:
- @PersistenceUnit(unitName="SOME_NAME")
- @Resource(name="JNDI_NAME")
- @WebServiceRefs(lookup="JNDI_NAME_OF_WEB_SERVICE_REF")
The main proposition of CDI is type-safety. This Tip Of The Day explains how @Produces annotation provided by CDI can be used to centralize all these String-based resource injection and add a facade of type-safety on them. Specifically, it shows how type-safety can be achieved for @PersistenceUnit. A similar approach can be taken for other String-based resource injections as well.
- Create a Singleton-scoped bean or Application-scoped bean as:
import javax.inject.Singleton; @Singleton public class ApplicationResources { }All the Java EE component environment references can be centralized in this bean.
- If the PersistenceUnit is currently initialized as:
@PersistenceUnit(unitName="StatesPU") EntityManagerFactory statesEMF;in other Java EE components, such as Servlet, then it can be alternatively defined in the type-safe manner using the following steps:
- Define a new Qualifier as:
import static java.lang.annotation.ElementType.TYPE; import static java.lang.annotation.ElementType.FIELD; import static java.lang.annotation.ElementType.PARAMETER; import static java.lang.annotation.ElementType.METHOD; import static java.lang.annotation.RetentionPolicy.RUNTIME; import java.lang.annotation.Retention; import java.lang.annotation.Target; import javax.inject.Qualifier; @Qualifier @Retention(RUNTIME) @Target({METHOD, FIELD, PARAMETER, TYPE}) public @interface StatesDatabase { } - Add the type-safe definition of "EntityManagerFactory" in "ApplicationResources" bean (defined above) as:
@Produces @PersistenceUnit(unitName="StatesPU") @StatesDatabase EntityManagerFactory statesEMF; - The "EntityManagerFactory" can now be injected in the Servlet in a type-safe manner as:
@Inject @StatesDatabase EntityManagerFactory emf;
- Define a new Qualifier as:
This procedure can be repeated for other String-based resources as well and thus centralize all of them at one place. And now your application becomes more type-safe! With this TOTD, you can use @Inject for injecting your container- and application-managed resources easily.
Read the latest documentation on Weld (Reference Implementation for CDI and included in GlassFish) for more details.
Technorati: totd cdi javaee6 glassfish weld produces typesafety
- TOTD #145: CDI Events – a light-weight producer/consumer in Java EE 6
- TOTD #109: How to convert a JSF managed bean to JSR 299 bean (Web Beans) ?
- TOTD #151: Transactional Interceptors using CDI – Extensible Java EE 6
- TOTD #134: Interceptors 1.1 in Java EE 6 – What and How ?
- TOTD #129: Managed Beans 1.0 in Java EE 6 – What and How ?
Hi Arun,
I tried your approach with @PersistenceContext and EntityManager and it failed.
Is the TOTD only applicable to @PersistenceUnit? Is there any way to centralised @EntityManager injected in various EJBs?
Comment by DWuysan — October 4, 2010 @ 3:17 pm
DWusyan,
Should work for both. What is the exact syntax you are using ?
Comment by Arun Gupta — October 5, 2010 @ 7:08 am
Hi Arun,
I finally got it working with @PersistenceContext.
As far as I can see, there are a number differences between your example and mine actually lies on the use of the producer method.
Please refer to the post by John Ament <http://seamframework.org/Community/CDISpec351DeclaringAResourceDoesNotSeemToWorkInRI#comment114350>
So, on the ApplicationResources.java will be as follows:
@ApplicationScoped
@Singleton
public class ApplicationResources {
@PersistenceContext(unitName = "mck-ejbPU")
EntityManager em;
@Produces
@OnlineDatasource
public EntityManager produce() { return em; }
}
If I use field producer (see below), it does not work:
@Produces @PersistenceContext(unitName = "mck-ejbPU") @OnlineDatasource EntityManager em;
Comment by DWuysan — October 5, 2010 @ 3:47 pm
DWuysan, I was able to run the sample without problems and field producer worked for me.
Arun, I was wondering whether some NetBeans code wizards should utilize field producers but so far I do not see much benefit for user. If type safety is a concern then user could stick with EJBs and would get the same result simpler way, no? For example:
@javax.ejb.Singleton
public class ApplicationResources {
private @PersistenceContext(unitName="StatesPU") EntityManager em;
public EntityManager getStatesDatabase() {
return em;
}
}
and in order to use it you inject singleton bean:
private @EJB ApplicationResources ar;
and use it:
ar.getStatesDatabase();
Or is it a matter of finding more suitable case, for example a unit testing one where you want to inject different persistence context from unit tests?
Comment by David — October 21, 2010 @ 6:59 am