study notes for EJB 3 in action (III)

EJB3 in Action

1.3 Getting inside EJB

1.3.1 Accessing EJB services: the EJB container.
Just as the JVM transparently manages memory on your behalf, the container transparently provides EJB component services such as transactions, security management, remoting, and web services support. In EJB 3, the container provides services applicable to session beans and MDBs only.

1.3.2 Accessing JPA services: the persistence provider
Instead of following the JVM-like container model, JPA follows a model similar to APIs, like JDBC. JPA provides persistence services such as retrieving, adding, modifying, and deleting JPA entities when you explicitly ask for them by invoking EntityManager API methods.

The "provider" terminology comes from APIs such as JDBC and JNDI too. If you've worked with JDBC, you know that a "provider" is essentially the vendor implementation that the JDBC API uses under the covers.

1.3.3 Gaining functionality with EJB services
The first thing that should cross your mind while evaluating any technology is what it really gives you.

Integration - Session beans and MDBs
Helps glue together components, ideally through simple configuration instead of code. In EJB 3, this is done through dependency injection (DI) as well as lookup.

Pooling Stateless - session beans, MDBs
For each EJB component, the EJB platform creates a pool of component instances that are shared by clients. At any point in time, each pooled instance is only allowed to be used by a single client. As soon as an instance is finished servicing a client, it is returned to the pool for reuse instead of being frivolously discarded for the garbage collector to reclaim.

Thread-safety - Session beans and MDBs
EJB makes all components thread-safe and highly performant in ways that are completely invisible. This means that you can write your server components as if you were developing a single-threaded desktop application. It doesn't matter how complex the component itself is; EJB will make sure it is thread-safe.

State management - Stateful session beans
The EJB container manages state transparently for stateful components instead of having you write verbose and error-prone code for state management. This means that you can maintain state in instance variables as if you were developing a desktop application. EJB takes care of all the details of session maintenance behind the scenes.

Messaging - MDBs
EJB 3 allows you to write messaging-aware components without having to deal with a lot of the mechanical details of the Java Messaging Service (JMS) API.

Transactions - Session beans and MDB
EJB supports declarative transaction management that helps you add transactional behavior to components using simple configuration instead of code. In effect, you can designate any component method to be transactional. If the method completes normally, EJB commits the transaction and makes the data changes made by the method permanent.
Otherwise the transaction is rolled back.

Security - Session beans
EJB supports integration with the Java Authentication and Authorization Service (JAAS) API, so it is very easy to completely externalize security and secure an application using simple configuration instead of cluttering up your application with security code.

Interceptors - Session beans and MDBs
EJB 3 introduces AOP in a very lightweight, accessible manner using interceptors. This allows you to easily separate out crosscutting concerns such as logging, auditing, and so on in a configurable way.

Remote access - Session beans
In EJB 3, you can make components remotely accessible without writing any code. In addition, EJB 3 enables client code to access remote components as if they were local components using DI.

Technorati : EJB