In a previous article, we tried to alleviate fears you may have had about building your first EJBs by walking you through the implementation details of a simple stateless session EJB. In this article, I will explore the important details of entity beans and how they are used to encapsulate data entities to be accessed by business objects.
Installing the EJB runtime and development environment
An EJB must be executed inside of an EJB container; therefore you must install a J2EE-compliant EJB container. To help you deploy your EJB, you also need a deployment tool. The J2EE reference implementation from Sun contains an EJB container and an EJB deployment tool. The J2EE reference implementation can be found in the J2EE development kit from Sun.
The J2EE development kit relies on the Java 2 SDK, Standard Edition (J2SE) development kit, so you will need to download it as well. You need the J2SE SDK to run the reference implementation J2EE server and to build and run EJB applications.
Once you have downloaded the development kits, install them, and select your desired location for each kit. Define an environment variable named JAVA_HOME and point it to the install directory for the J2SE development kit. Next define an environment variable named J2EE_HOME and point it to the install directory for the J2EE development kit. Now, you are ready to begin writing your entity bean.
Overview of entity beans
An entity bean is intended to represent the business logic for an entity existing in persistent storage. Entity beans share some of the same qualities that you would find in a relational database, for example:
- Entity beans are persistent—An entity bean's state exists beyond the lifetime of the application in which it is created, or for that matter, beyond the lifetime of the EJB container. This implies that the entity bean can be restored to its original state by the EJB container.
- Entity beans allow shared access—They may be shared by multiple clients and the concurrency is handled by the container.
- Entity beans have primary keys—Primary-key classes exist to identify an instance of an entity bean. The primary key contains all the information needed to find a persistent entity.
- Entity beans may participate in relationships—Local interfaces have been introduced to manage relationships between beans.
- Entity beans can participate in transactions—Since data can be accessed and changed by multiple clients, it is important for entity beans to be able to specify the transactional properties for their interaction. Transaction properties are specified declaratively in deployment descriptors and transaction boundaries are handled by the container.
The object-relational mapping implied by entity beans requires that an entity bean be responsible for inserting, updating, selecting, and removing data within the data source. This process of managing the communication between the component and the data source is called persistence. In other words, persistence is this process of writing the information to an external data source.
Enterprise Java Bean resources
- Advanced Programming with EJB Technology
- Design Considerations when Using Entity Beans with Container Managed Persistence
- Designing Entity Beans for Improved Performance
- J2EE SDK 1.4 Developer Release for Windows
Entity bean persistence mechanisms
There are two types of persistence for entity beans: bean-managed persistence (BMP) and container-managed persistence (CMP). With BMP, the programmer is responsible to write all of the code within the entity bean to access the data source. BMP allows more flexibility to the programmer because all access to the data source is controlled by the programmer.
With CMP the EJB container handles all database access required by the entity bean. As a result, the bean's data-access code is not coupled to a specific data source. This frees the programmer from writing any of the data-access code and allows the entity bean to be deployed in different containers and/or against different data-sources.
Entity bean relationships
Figure A illustrates the relationships that exist between a client, an EJB container, and entity beans:
|Entity Bean Interfaces and Classes|
An entity bean is composed of a minimum of three mandatory classes/interfaces and two optional interfaces (a local interface and a local home interface). For our examples, we will implement a very simple entity bean that sets or returns the username, password or name for a given user. We will assume that the entity bean is to be applied against a table named "user" containing three varchar columns named "username", "password", and "name".
The Component interface
The first mandatory class/interface is the Component interface. The Component interface defines the business methods visible to an EJB client and it must extend javax.ejb.EJBObject. The code for our Component interface is as follows:
* User is a generic user.
public interface User extends EJBObject
public String getUsername()
public void setUsername(String userName)
public void setPassword(String text)
public String getName()
public void setName(String name)
The Home interface
The second mandatory class/interface is the Home interface. The Home interface is used by a remote EJB client to obtain a reference to a bean's component interface and it must extend javax.ejb.EJBHome.
The Home interface defines the following methods:
- Create—Creates an entity bean instance
- Remove—Removes an entity bean instance
- Finder methods—These methods "find" one or more entity bean instances. Finder method names must start with "find". For a CMP entity bean, the finder method findByPrimaryKey must be defined.
- Home methods—These methods act like static methods for an entity bean. Home methods can be called from the home interface (remote or local) to execute entity bean methods.
The code for our Home interface is as follows:
* UserHome is the remote home interface for the User entity bean.
public interface UserHome extends EJBHome
* Create user.
public User create(String username, String password, String name)
* Find user by primary key.
public User findByPrimaryKey(String key)
* Find all users.
public Collection findAll()
* Generate username.
public String generateUsername()
The Local Home interface
The first optional interface is the Local Home interface. The Local Home interface is used by a local EJB client to obtain a reference to a bean's component interface and it must extend javax.ejb.EJBLocalHome.
Clients running in the same container as the entity bean, use the local home interface to create, remove, and find instances of the entity bean. The local home interface defines create, remove, finder methods, and home methods, just like the remote home interface (Listing A).
The Local interface
The second optional interface is the Local interface, which must extend javax.ejb.EJBLocalObject. Clients running in the same container as the entity bean interact with the entity bean via its Local interface, which defines the local business methods of the entity bean.
* UserLocal is the local interface for the User entity bean.
public interface UserLocal extends EJBLocalObject
/** Get username. */
public String getUsername();
/** Set username. */
public void setUsername(String userName);
/** Set password. */
public void setPassword(String text);
/** Get name. */
public String getName();
/** Set name. */
public void setName(String name);
The Enterprise Bean class
The third mandatory class/interface is the Enterprise Bean class. The Enterprise Bean class supplies the implementation for the Component interface and the Home interface. The Enterprise Bean class for a session EJB must implement the javax.ejb.EntityBean interface. The code for our Enterprise Bean class is in Listing B.
CMP configuration file
Finally, for your entity bean to be properly constructed by an EJB container, you must tell the container the specifics about the database, table, and field/column mappings to be generated and managed. The following snippet is a typical example of a CMP configuration to use to declare your User entity bean, a "user" table and the field/column mappings (Listing C).
In this article, I explored the details of entity beans and how they are used to encapsulate data entities to be accessed by business objects. The EJB architecture defines three distinct types of enterprise beans; session beans, entity beans and message-driven beans. Session beans and entity beans are invoked synchronously by an enterprise bean client. Message-driven beans (MDBs) are invoked by a message container, such as a publish/subscribe topic.
In the next article, I will look at message-driven beans and how they can be used to listen for Java Message Service (JMS) messages.