Bài giảng Lập trình mạng Entity Bean - GV. Nguyễn Xuân Vinh

Files in an Entity bean Entity Bean Remote Interface Primary Key Class Entity Bean Class Deployment Descriptors Home Interface Local Interfaces... Persistent DataMaps Database Schema

EJB: Entity Bean

Presenter: Nguyễn Xuân VinhInformation Technology Faculty

 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

Three ways to make an object persistent

Object Relational Mapping

Object database Persistence

Database Storage

Object Relational Mapping

Relational Database

Manual Mapping

Use an Relational Mapping Product

Object-Object

Object Database Persistence

 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

 This language adds a layer of abstraction from the relational database queries

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

TRƯỜNG ĐẠI HỌC NÔNG LÂM TP.HCMComponents deployed in Multi-tier Deployment- KHOA CÔNG NGHỆ THÔNG TIN

Application logic components

Application logic components are components that provide methods which

perform common tasks

Computing the price

of a ticket

Billing of products

TRƯỜNG ĐẠI HỌC NÔNG LÂM TP.HCMComponents deployed in Multi-tier DeploymentKHOA CÔNG NGHỆ THÔNG TIN -

Persistent Data Components

Object Serialization of data Database

Examples are air ticket

information such as ticket

number and amount

Employee data such as salaries and place of work

Files in an Entity bean

Entity Bean

Remote Interface

Primary Key Class

Entity Bean Class

Deployment Descriptors

Home Interface Local

Interfaces

Persistent Data

Maps

Database Schema

Entity Definition

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

The Entity Bean’s Remote Interface

Client

Entity Bean

Invokes

Business Method signatures Remote Interface

Entity Bean Object

Entity Bean Object

Entity Bean Object

Entity Beans

Exposes

Local Interface allows the beans to expose its methods to other beans that

Container

The Primary Key Class

 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

to follow the rules of Java object serialization

 The unique identifier, or primary key, enables client

to locate the particular entity bean

The Deployment Descriptors

Container

Deployment descriptors

List of properties

Contacts the deployment descriptor for bean deployment

Informs the container about the bean and classes

Entity Bean 2

Entity Bean n

Instantiates

Storage Data 1 Data 2 Data n Represents

Entity Beans

Pooled and Recycled

Entity Bean 1

When the bean instance

is used ,it is assigned to handle different client

Ways to Persist Entity Beans

Data Store

Entity Bean

Entity Beans map themselves in the data store

Bean-managed Persistence

Managed Persistence

Container-Deployment Descriptor

Persistent Beans

Deployment Descriptor tells the

container about the persistent fields

and then the container handles the data logic

Working with Entity Beans

 The entity bean instance and the underlying database can be

considered as one and the same

 When the bean-managed persistent bean calls the ejbCreate()

method, it creates the database data

 In case of container-managed 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

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

Existing Application

Database

Modifications

Bean data

Developing and Using Entity Beans

 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

 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

ejbFind ( )

Home object

EJB object

Entity bean instance

EJB Container/ Server

1 Call create()

6 Returns object to the client

2 Call ejbCreate()

4 Returns primary key

Client Code

3 Create database data

5 Create EJB object

Creating a BMP EJB and an EJB object

 All finder methods have to begin with “ejbFind”

 There has to be at least one finder method by the name

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

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

 Called to remove data from the database

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

different database

 Does not take any parameters

ejbRemove( )- II

Home object

EJB object

Entity bean instance

EJB Container/ Server

Client Code

3 Remove database data

1 Call remove()

2 Call ejbRemove()

Destroying an entity bean’s data representation

 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

 Two new methods have been added to entity beans:

* getEJBObject()

* getPrimaryKey()

 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

Implementation of Message Beans

 It mainly implements two interfaces

javax.ejb.MessageDrivenBean interface and the

javax.jms.MessageListener interface

 The developer needs to implement the main business-logic 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

message-driven bean

 There are three main ways to make objects persistent:

 Java Object Serialization

 Object-Relational Mapping

 Object Database Management System

 Object serialization is a method by which the current state of Java objects can be captured and saved permanently

 There are two ways of mapping the objects to the relational data:

 Manual mapping: This is done using a database access API, which can be

JDBC or SQL/J.

 Use an object-relational mapping product It can be Sun’s JavaBlend or Object people’s TOP link.

 Application logic components

 Persistent data components

 The entity bean comprises the following files:

 The entity bean class

 The remote interface

 The Local Interface

 The home interface

 The primary key class

 The deployment descriptors

instance pool, the ejbActivate() callback method is used.

pool, this method is called

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

environment These context objects are accessible to the beans in order to retrieve transaction and security information