EJB

References

Java EE 7 tutorial

EJB 3.2 specification

EJB

Note, according to some Internet articles and my understanding, CDI is in favour of local session bean. Use annotation stereotypes for transaction / security.

• implement business logic

• enjoy services provided by container

• types:

  • session

    • stateful

    • stateless

    • singleton

  • message-driven

Session Bean

• can be invoked over

  • local

  • remote

  • web service

• @stateful

  • contains state represents conversational state

  • not shared, have only one client

  • client terminate, its beans appears to terminate and no longer associates with client

  • client removes bean, session ends and state disappears

  • can NOT implement web service

• @stateless

  • state only for the duration of invocation

  • can be pooled, all instances are equivalent

  • better scalability

  • can implement a web service

  • choose if

    • no data for specific client

    • in a single method invocation performs a generic task for all clients

• @singleton

  • instanced per application

  • can implement web service endpoint

  • state not across server crashes or shutdowns

  • can be created upon server @Startup

  • manage concurrency with @ConcurrencyManagement

  • see more on container managed concurrency

  • choose if

    • state needs to be shared across application

    • needs to be concurrently accessed by multiple threads (thread safety)

    • to perform tasks on application startup / shutdown

Accessing Session Bean

• no-interface view:

  • annotate class that DO NOT IMPLEMENT ANY INTERFACE (otherwise confuses server)

  • exposes public method to clients

• business interface

  • one bean can have more than one

  • bean should (not required) to implement business interface(s)

• client access

  • injection

    • within container

    • @EJB annotation, more see "EJB Reference"

  • JNDI lookup

    • can be used outside container

    • three namespaces

      • java:global - finding remote EJB "java:global[/application name]/module name /enterprise bean name[/interface name ]

      • java:module - find local EJB within same module "java:module/enterprise bean name/[interface name]"

      • java:app - find EJB within same application (within EAR) "java:app[/module name]/enterprise bean name [/interface name]"

• define local access

  • no-interface view is local

  • @Session

  • @Local interface, or no decoration (default), is local

  • Examples:

    • @Stateful public class MyBean {...} - local through no-interface

    • @Local public interface InterfaceName {...} - annotate interface

    • @Local(InterfaceName.class) public class BeanName implements InterfaceName - annotate implementation

• client local access

  • @EJB ExampleBean exampleBean

  • ExampleBean exampleBean = (ExampleBean)InitialContext.lookup("java:module/ExampleBean")

• define remote access

  • @Remote public interface InterfaceName{...}

  • @Remote(InterfaceName.class) public BeanName implements InterfaceName{...}

• client remote access

  • @EJB Example example;

  • ExampleRemote example = (ExampleRemote)InitialContext.lookup("java:global/myApp/ExampleRemote");

• web service clients (skipped for now)

EJB References (see specification 11.5)

Bean Provider:

@EJB annotation contains following elements:

• name - name by which resource is to be looked up in environment (logical name, e.g. "ejb/EmplRecord")

• beanInterface - referenced interface type; can be business interface, session bean's no-interface view, or local home / remote home interface

• beanName - references the value of the name element of the Stateful or Stateless annotation (or ejb-name element); allows disambiguation if several session beans implement the same interface

• mappedName - product specific; using may not be portable

• lookup - JNDI name for the target component (beanName or lookup cannot be both used)

If no explicit linking information provided and there is only one session bean within the same application that exposes the matching client view type, by default the EJB dependency resolves to that session bean.

Message Driven Bean

• process messages asynchronously

• normally a JMS message listener

• resembles a stateless session bean

  • retain no client data

  • all instances are equivalent

• can contain some state such as

  • JMS API connection

  • open database connection

  • reference to EJB

• invoked asynchronously

• relatively short-lived

• can be transaction-aware

• stateless

• message can be delivered in a transaction context: if operation in onMessage rolled back, message will be redelivered

Method Parameters (include return values) and Access

Isolation

• remote calls work on different copies of parameter object (by copy, not by reference)

• local call work by reference

Granularity

• remote calls are likely slower, should use relatively coarse-grained parameters

• local calls are ok for fine-grained

Contents of EJB

• Bean class: implement business methods and callback methods (naming convention: nameBean, also for bean name)

• Business interfaces (naming convention: name)

• Helper classes

Lifecycle

Stateful bean:

None-existence <-> Ready <-> Passive

• @PostConstruct

• @PrePassive

• @PostActive

• @PreDestroy

• @Remove - client remove bean instance

Stateless bean / Singleton:

None-existence <-> Ready

• @PostConstruct

• @PreDestroy

Message Driven Bean:

None-existence <-> Ready (onMessage)

• @PostConstruct

• @PreDestroy

Bean Dependency

@DependsOn("AnotherBean","YetAnotherBean")

Exceptions

• system exception

  • bean throw a javax.ejb.EJBException (need not declare in "throws")

  • container might destroy bean instance

  • cannot be handled by client

  • container rolls back transaction

• application exeption

  • client should be able to handle

  • container does NOT roll back transaction

Timer and Schedule Service

Skipped

Do not confuse "this" to the bean

The bean is a proxy and is not "this". Using "this" will not have added features such as transaction, because "this" is the not decorated bare class. Obtain reference to a business object with SessionContext