CSM15: Component-Based Software development

Lecture 8Java Persistence - Entity Classes

What is an entity bean/class?

JEE 5 simplifies the handling of “Persistence” in Java

But what do we mean by persistence, and what is an entity?

Entities are things

ID LastName FirstName888 Krause Paul999 Schneider Steve777 Brush Basil007 Bond James

Customer Table

Entities are things

ID LastName FirstName888 Krause Paul999 Schneider Steve777 Brush Basil007 Bond James

Customer Table

ID: 888LastName:

KrauseFirstName:

Paul

ID: 999LastName: Schneider

FirstName: Steve

ID: 777LastName: Brush

FirstName: BasilID: 007

LastName: Bond

FirstName: James

Customer Beans

persistence and entities

An Entity is an object-oriented representation of an underlying “persistent store”

The Persistent Store may be a:

relational database

object database

clumsy storing serialized objects to files...

Nouns and verbs

Entity beans are data objects - things - Nouns -

customer, booking, animal, order, ...

Session beans are processes - verbs -

membership registration, checkout, order submission, ...

Entity beans (data) will interact with Session Beans (processes)

Managing persistence

In JEE 5, we no longer need to use deployment descriptors to manage persistence. Instead, we:

Create a persistence unit (see next slide)

specify the data source

specify the entity manager

The Container does the rest

What’s a Persistence unit?

A persistence unit defines the entity classes that are managed by the entity manager

Persistence units are defined by the persistence.xml configuration file.

Each persistence unit must be identified with a name that is unique to the persistence unit's scope.

Start a new project

Choose File > New Project (Ctrl-Shift-N).

Select Web Application from the Web category.

Name the project CustomerBook

Set the server to Sun Java System Application Server

Set the Java EE Version to Java EE 5, click Finish

create a persistence unit

Using New/File, create a new Persistence Unit using the wizard

You can use the default name for the persistence unit

Use the default, Top Link, for the Persistence Provider

the entity manager is in TopLink Essential.jar

Select jdbc/sample (this will connect to the Java DB)

What do you see?

The Entity manager

The Entity Manager:

Creates and removes persistent entity instances

Finds entities by the entity’s primary key

Allows queries to be run on entities

Will manage the storage of an entity’s data to its associated database

entities

An entity is a lightweight persistence domain object. Typically an entity represents a table in a relational database, and each entity instance corresponds to a row in that table. The primary programming artifact of an entity is the entity class, although entities can use helper classes.

The persistent state of an entity is represented either through persistent fields or persistent properties. These fields or properties use object/relational mapping annotations to map the entities and entity relationships to the relational data in the underlying data store.

Sun Java EE 5 Tutorial

requirements for entity classes

The class must be annotated with the javax.persistence.Entity annotation.

The class must have a public or protected, no-argument constructor. The class may have other constructors.

The class must not be declared final. No methods or persistent instance variables must be declared final.

Persistent instance variables must be declared private, protected, or package-private, and can only be accessed directly by the entity class's methods. Clients should access the entity's state through accessor or business methods.

entity classes from a DB

Tools > Java DB Database > Start Java DB Server

New/File & select Entity Class from the Persistence category

Select the CUSTOMER table and click Add

Note DISCOUNT_CODE is also added

Click Next, then specify ejb as the package and Finish

Customer.java

Customer.java

Customer.java

Customer.java

Customer.java

Follow up

Code the Servet by following:

http://testwww.netbeans.org/kb/55/customer-book.html