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smtJ2EE 13 - EJB Overview

By Darlene Harris,2014-02-13 11:47
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smtJ2EE 13 - EJB Overview

    EJB Overview

    1. EJB is designed

    ; To encapsulate business logic

    ; To provide standard common system services to the developers so that

    developers can concentrate on business logic coding. The key services

    provided by EJB container are:

    i. Life-cycle management

    ii. Client session management

    iii. Persistence management

    iv. Security

    v. Scalability (instance pooling)

    vi. Transaction management

    vii. Locking and concurrency control

    viii. Declarative

    ix. Thread safe (one instance per thread)

; To make the code reusable

    It is a distributed computing and can be accessed by any type of the clients:

    ; Web client using JRMP or RMI-IIOP

    ; Standalone Java client using JRMP or RMI-IIOP

    ; EJB client using JRMP or RMI-IIOP

    ; CORBA client using RMI-IIOP

    2. Some key differences between normal Java Bean and EJB Java Bean EJB

    General purpose Highly specialized business-logic

    component

    Any tier Business logic tier

    No system services provided Several key system services provided

    Local service Distributed computing, remote service

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3. EJB architecture

     Naming

     Service

     Register the home

     when deploying JNDI lookup

     Call Home’s create() method Client Home Return component reference to client Stub create() Bean Pool Network Component Interface

    EJB Container

     Skeleton

    Once component interface receives the method call from skeleton, the container

    will locate a free bean instance from the pool to provide the service. The sequence

    of the service invocations is as follows:

    ; Examine the security credentials of the caller of that method

    ; Start or join with any required transactions

    ; Make any necessary calls to persistence functions

    ; Trigger various callbacks to allow EJB to acquire resources

    ; Make the actual business method call

    ; Do some more works with transactions, persistence, callbacks and so on

    ; Finally, the results of the business method or exception will be sent back to

    client via RMI infrastructure

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    4. Bean Type

    Session Bean: Stateful and Statless

    Entity Bean: BMP vs CMP

    Message Driven Bean (MDB), since EJB 2.0

Local vs Remote

    Session EJBs Implement Business Logic Session beans implement business logic. A session bean instance serves one client at a time. There are two types of session beans: stateful and stateless.

    Stateless Session Beans

    A stateless session bean does not store session or client state information between invocationsthe only state it might contain is not specific to a client, for instance, a cached database connection or a reference to another EJB. At most, a stateless session bean may store state for the duration of a method invocation. When a method completes, state information is not retained. Any instance of a stateless session bean can serve any clientany instance is equivalent. Stateless session beans can provide better performance than stateful session beans, because each stateless session bean instance can support multiple clients, albeit one at a time.

    Figure 2 WebLogic Server Free Pool Showing Stateless Session EJB Life Cycle

Stateful Session Beans

    Stateful session beans maintain state information that reflects the interaction between the bean and a particular client across methods and transactions. A stateful session bean can manage interactions between a client and other enterprise beans, or manage a workflow.

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Figure 3 Stateful Session EJB Life Cycle

    The differences between the stateful and stateless are:

    ; in DD

    ; You must define instance variables to hold the state for Stateful

    Bean

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    Entity EJBs Maintain Persistent Data An entity bean represents a set of persistent data, usually rows in a database, and provides methods for maintaining or reading that data. An entity bean is uniquely identified by a primary key, and can provide services to multiple clients simultaneously. Entity beans can participate in relationships with other entity beans. The relationships between entity beans are a function of the real-world entities that the entity beans model. An entity bean's fields and its relationships to other entity beans are defined in an object schema, which is specified in the bean's ejb-jar.xml

    deployment descriptor.

    An entity bean can have other bean types, such as message-driven or session beans, as clients, or be directly accessed by Web components. The client uses the entity bean to access data in a persistent store. An entity bean encapsulates the mechanics of database access, isolating that complexity from its clients and de-coupling physical database details from business logic.

    Message-Driven Beans Implement Loosely Coupled Business Logic

    A message-driven bean implements loosely coupled or asynchronous business logic in which the response to a request need not be immediate. A message-driven bean receives messages from a JMS Queue or Topic, and performs business logic based on the message contents. It is an asynchronous interface between EJBs and JMS.

    Throughout its lifecycle, an MDB instance can process messages from multiple clients, although not simultaneously. It does not retain state for a specific client. All instances of a message-driven bean are equivalentthe

    EJB container can assign a message to any MDB instance. The container can pool these instances to allow streams of messages to be processed concurrently. It is acceptable or beneficial for customer orders to "stack up" before the associated supplier orders are issued.

    The EJB container interacts directly with a message-driven beancreating

    bean instances and passing JMS messages to those instances as necessary. The container creates bean instances at deployment time, adding and removing instances during operation based on message traffic. Example: In an on-line shopping application, where the process of taking an order from a customer results in a process that issues a purchase order to a supplier, the supplier ordering process could be implemented by a message-driven bean. While taking the customer order always results in placing a supplier order, the steps are loosely coupled because it is not necessary to generate the supplier order before confirming the customer order.

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Table 1 Components of an EJB

    EJB Description Stateless Stateful Entity MDB Component Session Session Remote The remote interface exposes Yes Yes Yes No interface business logic to remote clients

    clients running in a separate

    application from the EJB. It defines

    the business methods a remote

    client can call.

    Local The local interface exposes business Yes Yes Yes No interface logic to local clientsthose running

    in the same application as the EJB.

    It defines the business methods a

    local client can call.

    Local home The local home interface, also Yes Yes Yes No interface referred to as an EJB factory or life-

    cycle interface, provides methods

    that local clientsthose running in

    the same application as the EJB

    can use to create, remove, and in

    the case of an entity bean, find

    instances of the bean.

    The local home interface also has

    "home methods"business logic

    that is not specific to a particular

    bean instance.

    Remote The remote home interface, also Yes Yes Yes No home referred to as an EJB factory, or life-

    interface cycle interface, provides methods

    that remote clientsthose running

    in a separate application from the

    EJBcan use to create, remove, and

    find instances of the bean.

    Bean class The bean class implements business Yes Yes Yes Yes

    logic.

    Primary key Only entity beans have a primary No No Yes No class key class. The primary key class

    maps to one or more fields in a

    databaseidentifying the persistent

    data to which the entity bean

    corresponds.

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