Definition of an Entity Bean

An entity bean is a remote object that manages persistent data, performs complex business logic, potentially uses several dependent Java objects, and can be uniquely identified by a primary key. Entity beans are normally coarse-grained persistent objects, in that they utilize persistent data stored within several fine-grained persistent Java objects.

Note: Fine-grained persistent Java objects typically manage persistent data that has a one-to-one mapping between the data and a table column. Coarse-grained persistent Java objects use or manage persistent data stored in several fine-grained persistent objects.

Managing Persistent Data

An entity bean manages its data persistentcy through callback methods, which are defined in the javax.ejb.EntityBean interface. When you implement the EntityBean interface in your bean class, you develop each of the callback functions as designated by the type of persistence that you choose: bean-managed persistence or container-managed persistence. The container invokes the callback functions at designated times. That is, the contract between the container and the entity bean designates the order that the callback methods are invoked and who manages the bean's persistent data.

Uniquely Identified by a Primary Key

Each entity bean has a persistent identity associated with it. That is, the entity bean contains a unique identity that can be retrieved if you have the primary key. Given the primary key, a client can retrieve the entity bean. If the bean is not available, the container instantiates the bean and repopulates the persistent data for you.

The type for the unique key is defined by the bean provider.

Performing Complex Logic Involving Dependent Objects

When designing your EJB application, you need to keep in mind the aspects of each type of object.

• Session beans are typically used for performing simple tasks for a remote client.

• Entity beans are typically used for performing complex tasks that involve coarse-grained persistence for remote clients.

• Java objects, persistent or otherwise, are used for simple tasks for local clients.

Enterprise JavaBeans are remote objects and are used for interacting with clients over a network. Remote objects have a higher overhead for verifying security and transaction information. Thus, when you design your application, you may have an entity or session bean interacting with the client, but also have the Enterprise JavaBean invoke other dependent Java objects to perform tasks or manage persistent data.

Entity beans are normally used to manage complex, coarse-grained persistent data for a remote client. Be careful to separate the difference between an entity bean and a persistent object. Your entity bean should be more than just a persistent object; it should manage and return complex data to justify using a remote object for managing data.

You can have an entity bean that calls one or more dependent objects within the application. The entity bean is a remote object and thus its primary function is interacting with the client over the network. You should not have an entity bean invoking another entity bean within the same node on the network. If you need to design multiple objects within your application, design your application so that the entity bean facilitates the communication and data management between the client and other Java objects.

Figure 4-1 demonstrates how the client interacts with either a session or an entity bean, which then manages the application for the client with other Java objects within the application. The Java objects that make up the backend of the application can be persistent objects. The figure also shows how both the entity bean and a persistent Java object can be persistent and store data within the database.

Figure 4-1 Relationship of Enterprise JavaBeans to Java objects

For example, if you are managing a shopping cart for an online bookstore, you would have the following requirements:

• identify the customer for the shopping cart

• add items to the customer's shopping cart

• calculate the price for all items taking into account any discounts, sale items, and shipping costs

• ship items to the customer

In this scenario, the entity bean could do the following:

• retrieve the customer information from a dependent persistent object

• retrieve item information from an item dependent persistent object

• record the number of items for each item in the order

• retrieve discount and shipping costs

• coordinate the shipment to the designated customer address

Thus, this entity bean not only retrieves persistent information from other objects, but would also maintain its own persistent data and perform complex calculations.

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