EJB Entity Bean trong Lập trình mạng nâng cao: Bài giảng Nguyễn Xuân Vinh

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EJB: Entity Bean

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Presenter: Nguy n Xuân Vinh Information Technology Faculty Nong Lam University

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Session Objectives

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Describe the concept of persistence. Define entity beans. List the features offered by entity beans. Compare entity and session beans. Explain MessageDriven Beans.

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Review of Session 4

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In Session 4 we discussed:  Definition of a Stateful Session Bean.  The characteristics of a Stateful Session Bean.  How to program Stateful session beans.  Difference between Stateless and Stateful Session beans.

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Persistence

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Storage

Java Object Serialization

Persistence Objects

Database

Obj 1

Storage

Object database Persistence

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Persistence

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Obj 2

Storage

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Object Relational Mapping

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Three ways to make an object persistent

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Java Object Serialization

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Marshall an object graph into a compact representation

Object 1

Compact representation

State of the object

Object 2

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Serialization of object graph into byte stream

Object n

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Developer pushes data over the network or saves the stream to a storage

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Byte Stream

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Object Relational Mapping

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Travel Account

Manual Mapping

Database API

String Name String TktNo Double Amount

Use an Object- Relational Mapping Product

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Object

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Relational Database

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Object Database Persistence

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 Object database persistence is designed to store Java Objects as whole objects which means that there is no need to program a relational database.

 The Object Query Language (OQL) provides the facility to

query the persisted objects.

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 This language adds a layer of abstraction from the

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relational database queries.

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 The EJB QL eliminates the inconsistencies caused due to the use of the different Query languages used by different application servers.

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Components deployed in Multi-tier Deployment- Application logic components

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Application logic components are components that provide methods which perform common tasks

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Billing of products

Computing the price of a ticket

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Components deployed in Multi-tier Deployment - Persistent Data Components

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Object

Database

Serialization of data

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Employee data such as salaries and place of work

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Examples are air ticket information such as ticket number and amount

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Files in an Entity bean

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Entity Bean Class

Remote Interface

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Entity Bean

Deployment Descriptors

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Home Interface

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Primary Key Class

Local Interfaces

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Entity Bean class

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Maps

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Models

Persistent Data

Java Class

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Database Schema

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Entity Definition

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For example, an entity bean class can map to a relational table definition. An entity bean instance of that class will then map to a row in that table.

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The Entity Bean’s Remote Interface

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Client

Entity Bean

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Remote Invokes Interface Business Method signatures

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The Home Interface

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create

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Entity Bean Object

Home Interface

Uses

find

Client

Entity Bean Object

Methods

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destroy

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Entity Bean Object

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The home interface is used by the clients to create, find and destroy entity bean objects.

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Local Interface

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Local Clients

Entity Beans

Exposes

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Entity Beans

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Methods

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Container

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Local Interface allows the beans to expose its methods to other beans that reside within the same container (local clients). ACCP2005/EJB 2.0/ Session 5 / 14 of 33

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The Primary Key Class

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 Primary keys make each entity bean look different.  A primary key is an object itself which contains other objects and data that is necessary to identify an entity bean data instance.

 The primary key class has to be serializable, and has

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to follow the rules of Java object serialization.

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 The unique identifier, or primary key, enables client to

locate the particular entity bean.

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The Deployment Descriptors

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Contacts the deployment descriptor for bean deployment

Container

Deployment descriptors

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List of properties

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Informs the container about the bean and classes

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Pooling of Entity Bean Instances

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Represents

Instantiates

Storage

EJB

Entity Entity Bean 1 Bean 1

Data 1

Container

Data 2

Entity Bean 2

Data n

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Entity Bean n

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Pooled and Recycled

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When the bean instance is used ,it is assigned to handle different client request.

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Ways to Persist Entity Beans

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Bean-managed Persistence

Entity Bean

Entity Beans map themselves in the data store

Data Store

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Deployment Descriptor

Container- Managed Persistence

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Persistent Beans

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Working with Entity Beans

 The entity bean instance and the underlying database can be

considered as one and the same.

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 When the beanmanaged persistent bean calls the ejbCreate()

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method, it creates the database data.

 In case of containermanaged persistence, the container contains required data access logic, leaving the bean’s methods empty of data access logic.

 The finder methods are used to find the existing entity bean in

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storage. The finder methods do not create any new database data, but they load old entity bean data.

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Modifying Entity Beans Directly though the Database

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EJB Container/ Server

Entity Beans Existing Application

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O/R Mapping

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Direct Database Modifications

Bean data

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Database

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Developing and Using Entity Beans

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 In order to write an entity bean class, the javax.ejb.EntityBean interface has to be implemented. It is this interface that defines the call back method used by the container which the bean must implement.

* setEntityContext(javax.ejb.EntityContext)

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* unsetEntityContext()

* ejbRemove()

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* ejbActivate()

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* ejbPassivate()

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* ejbLoad()

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* ejbStore()

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ejbCreate( )

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 Used to initialize the fields of a bean instance which can be used for a particular client to create the underlying database data.  The parameters vary with respect to the ejbCreate() method. Therefore, there are multiple ways to initialize an entity bean instance.

 The ejbCreate() methods in the home interface have to be

duplicated in the bean class.

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ejbFind ( )

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EJB Container/ Server

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Client Code

1. Call create() 2. Call ejbCreate() Home object

Entity bean instance

6. Returns object to the client

5. Create EJB object 4. Returns primary key

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EJB object

3. Create database data

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Creating a BMP EJB and an EJB object

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ejbFind ( ) - rules

 All finder methods have to begin with “ejbFind”.  There has to be at least one finder method by the name

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ejbFindByPrimaryKey(). The method finds a unique entity bean instance based on the unique primary key.

 There can be various finder methods, with different names and

different parameters.

 The finder method has to return the primary key for the entity

bean it finds. Other than this, it can also give an enumeration of primary keys.

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 The client will never call the finder methods on the bean instance

itself

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ejbRemove( ) - I

 Called to remove data from the database  The instance of the bean can be recycled to handle data from a

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different database

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 Does not take any parameters

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ejbRemove( )- II

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EJB Container/ Server

Entity bean instance

2. Call ejbRemove() 1. Call remove() Home object Client Code

2. Call ejbRemove()

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EJB object 1. Call remove()

3. Remove database data

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Destroying an entity bean’s data representation

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Entity Contexts

 Enterprise beans have a context object which identifies the bean’s

environment.

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 Context objects are accessible to the beans in order to retrieve information, such as transaction and security information.

 The interface to be used for entity beans is the

javax.ejb.EntityContext.

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 Two new methods have been added to entity beans: * getEJBObject() * getPrimaryKey()

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Message-Driven Beans

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 Allows JMS applications to receive messages asynchronously (the sender is independent of the receiver receiving and processing the messages).

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 Includes business logic, which may include operations such as: * Performing computation on received data * Initiating a step or condition in a workflow * Storing data * Sending a message

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Uses of Message-Driven Beans

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Asynchronous messaging. Integrating two applications in a loosely coupled but

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reliable manner.

Mere message delivery or message content should drive

other events in the system.

Create message selectors which are designed to consume

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only specific messages and thus act as triggers.

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Implementation of Message Beans

 It mainly implements two interfaces

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javax.ejb.MessageDrivenBean interface and the javax.jms.MessageListener interface.

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 The developer needs to implement the main businesslogic in the onMessage() method. The bean implements three other methods viz. ejbCreate() ejbRemove() and setMessageDrivenContext() which the container uses for managing the lifecycle of the messagedriven bean.