EJB
Enterprise Java Beans (EJB) is architecture for
server-side component based distributed applications written in Java.
EJB defines how server-side components are
written and provides a standard architectural contract between the components
and the application servers and containers that manage them.
Power of EJB
EJB component model simplifies development of business components
that are transactional, scalable, and portable. Enterprise JavaBean servers
reduce the complexity of developing business components by providing automatic
support for system-level services, such as transactions, security, and database
connectivity, thus allowing the developers to concentrate on developing the
business logic.
Why EJBs?
è Numerous Web sites are up and running
using Java without any EJB technology.
è Developers have been using
servlet/JSP models and managing transactions themselves using commit and
rollback functionality that is built into JDBC without the help of application
servers.
è The application developers are
confronted with many challenges such as managing
concurrency, persistence and transactions.
These problems are solved by using enterprise beans
The use of enterprise beans allow developers to focus on the business
logic and release them from coding infrastructure and middle-ware logic, and
developers become more productive and efficient.
As with most other technologies, enterprise beans do not provide the unique
solution to all problems. Using enterprise beans has advantages and disadvantages.
However, the advantages outweigh the disadvantages, especially for more complex
applications that require a sophisticated robust and distributed persistent
model.
Main EJB framework
components
There are six main framework components of EJB technology:
·
EJB server—provides the actual primary services to all EJBs. An EJB server may
host one or more EJB containers. It is also called generically an Enterprise
Java Server (EJS). Jboss Application Server is an EJS. Containers are
transparent to the client—there is no client API to manipulate the container,
and there is no way for a client to tell in which container an enterprise bean
is deployed.
The EJB specification outlines seven services
that must be provided by an EJB server: Primary Services
_ Naming A client can invoke an
enterprise bean by looking up the name of the bean in a centralized naming
space called a naming service, and this is accessed via the Java Naming and
Directory Interface (JNDI) API. The container is responsible for registering
the (unique) lookup name in the JNDI namespace when the server starts up, and
binding the appropriate object type into the JNDI namespace.
_ Transaction A
transaction is defined as a set of tasks that must execute together; either all
must work or all must be undone. The EJB container handles the transaction
operations, as well as the coordination activities between the transactions of
the beans. The EJBs transaction behavior is described in the deployment
descriptor
_ Security The EJB container
provides a security domain for enterprise beans. The container is responsible
for enforcing the security policies defined at deployment time whenever there
is a method call, through access control lists (ACL). An ACL is a list of
users, the groups they belong to, and their rights, and it ensures that users
access only those resources and perform only those tasks for which they have
been given permission.
_ Persistence The
container is also responsible for managing the persistence of a bean (all
storage and retrieval of data) by synchronizing the state of the bean instance
in memory with the respective record in the data source. Concurrent access to
the data from multiple clients is managed through the concurrency and
transaction services (entity beans only).
_ Concurrency Concurrency
is defined as access by two or more clients to the same bean, and the container
manages concurrency according to the rules of the bean type.
_ Life cycle the
container is responsible for controlling the life cycle of the deployed
components. As EJB clients start giving requests to the container, the
container dynamically instantiates, destroys, and reuses the beans as
appropriate.
_ Messaging The container must provide for asynchronous messaging.
EJBs do not interact directly with the EJB
server, but rather, do so
through the
EJB container, which acts as a broker to the services provided by the EJB
server. So, from the EJB’s perspective, it appears that the EJB container is in
fact providing those services.
·
EJB container—The EJB container is a
system that functions as a runtime environment for enterprise beans by managing
and applying the primary services that are needed for bean management at
runtime. In addition to being an intermediary to the seven services above
provided by the EJB server, the EJB container will also provide for EJB
instance life-cycle management, and EJB instance identification. EJB containers
create bean instances, manage pools of instances, and destroy them.
·
EJB component—The EJB components
refer to the beans themselves, and include all the classes, interfaces, and
constructs that make up the enterprise bean. There are three types of
enterprise beans: entity, session, and message-driven beans.
·
EJB interfaces and EJB bean—the
interfaces for client access (EJB home and EJB object) and the EJB bean class.
·
EJB deployment descriptor—the
descriptor defines the runtime quality of service settings for the bean when it
is deployed. Many of the settings of an EJB, such as the transactional
settings, are defined in the deployment descriptor. The deployment descriptor
is an external entity from the bean, and therefore decouples the bean component
itself from the runtime characteristics.
EJB
client—A client that accesses EJBs.
