Java EE - EJB

Enterprise Java Beans - EJB

EJBs are the Java EE server side components which implement app's business logic. They normally be deployed in EJB containers provided by the app servers. Implementing EJBs can provide scalability to the application and better handling of security and transactions. EJBs can also implement webservices.

Types of EJB:
Session - To implement user actions
- stateful: maintains a state for the client and this bean can not be shared.
- stateless: does not maintain state so can be allocated to any clients. They are reusable. So this
        pool will be less compared to stateful.
- singletone session beans: have only one instance for the whole time.  They can get initiated when the app starts. Although they act as stateless but there is no pool because of single existence

  Message Driven Beans (MDB): for listening to the messages either from queues or JMS
(if someone read about entity beans, they are now part of persistence API..(referring to Java EE7)

Implementation is simple.. infact, the annotations made it so..lets says to write a simple stateless EJB, just annotate the class with @Stateless and implement the business logic.. and to call this EJB, a servlet can do ..ofcourse, you need to annotate with @WebServlet(urlPatterns="/") the url pattern says the context root. And, as usual extend the class with HttpServlet. And, to access the EJB, just annotate with @EJB and then the declare the EJB instance.

However, in a typical EJB implementation, there could be all type of session beans - stateless, stateless bean implementing a service, stateful bean accessed remotely etc.,

A remote interface is required for the beans which allow remote access. This remote business interface defines all the business methods of a bean and annotated with Remote from javax.ejb pacakge and it gets implemented by the session bean. A session bean can be an end point for a web service.

A stateful session bean can have methods annotated with Remove which can be invoked by client to remove the instance.

In the case of singleton bean, the concurrent access from the clients can be controlled in two ways - container managed or bean managed by annotating accordingly. And, the methods must be annotated with the locktype i.e. read or write so that concurrent access can allowed or provided with a synchronous mechanism respectively..

For stateless session beans to implement service end points, they must be annotated with @Webservice and the business methods that are exposed must be annotated with @Webmethods. They can also implement async methods so that clients no need to wait for response from long running methods.

Coming to EJB pools - in weblogic, there is an element called max-beans-in-free-pool in weblogic-ejb-jar.xml. This determines how many EJBs must be made available in free pool. max-beans-in-pool will put a cap on the pool limit. For MDBs, the container will create as many instances required based on the size limited to max-beans-in-free-pool. Default MDB threads are 16 but this can be changed by having custom queue or workmanagers