EJB bean生命周期依赖项？

Question

I can't find in the specs any clear statement of how dependency injection with either @EJB or @Inject interact with the container's bean lifecycle management.

I want to inject singleton bean A into singleton bean B. I want the container to guarantee that A exists by the time B's @PostConstruct method is invoked, and continues to exist until after B's @PreDestroy method completes.

1. Will @Inject (CDI injection) accomplish this?
2. What about @EJB?
3. Or do I always need to add @DependsOn?

Answer 1

CDI and EJB are two different specifications. CDI provides integration allowing you to use EJB beans as if they were CDI (but not vice-versa!). So note whether you are talking CDI or EJB as not all annotations are applicable to all beans. For instance @Singleton (EJB) can have @Startup , whereas @ApplicationScoped bean from CDI cannot.

You are mentioning "singleton bean". Just to dispel confusion, in EJB that means @Singleton , in CDI that means @ApplicationScoped bean.

That being said, to the questions you had:

1. Will @Inject (CDI injection) accomplish this?

@Inject should work as well so long as you use it to create a non-circular dependency among those beans - eg if you use @Inject at constructor of one bean, CDI will first initiate the dependency than this bean is trying to use. Keep in mind that CDI is initiating things lazily though - nothing happens until you actually try to use that bean.

2. What about @EJB?

Not sure what you mean here - @EJB is an EJB equivalent of @Inject in CDI.

3. Or do I always need to add @DependsOn?

This would be an option as well but again, note that this can only be used on EJB @Singleton beans (which is perfectly fine, just denoting the CDI vs. EJB difference here).

Answer 2

1. Inject (CDI) is not part of the EJB spec. This is probably vendor specific behavior. Use EJB injection with EJBs. Perhaps this part of the CDI spec may help: cdi spec: relationship to other specifications

2. and 3. See EJB Spec 3.2 chapter 4.8.1: Singleton Session Bean Initialisation. You didn't say anything about the initialisation (eager or at startup), so please refer to the docs for details.

In some cases, explicit initialization ordering dependencies exist between multiple singleton session bean components in an application. The DependsOn annotation is used to express these dependencies. A DependsOn dependency is used in cases where one singleton session bean must initialize before one or more other singleton session beans. The container ensures that all singleton session beans with which a singleton session bean has a DependsOn relationship have been initialized before the PostConstruct method is called.

Note that if one singleton session bean merely needs to invoke another singleton session bean from its PostConstruct method, no explicit ordering metadata is required. In that case, the first singleton session bean would merely use an EJB reference to invoke the target singleton session bean. In this case, the acquisition of the EJB reference (either through injection or lookup) does not necessarily imply the actual creation of the corresponding singleton session bean instance.