What Is an Entity Bean?
An entity bean represents a business object in a persistent storage mechanism. Some examples of business objects are customers, orders, and products. In the J2EE SDK, the persistent storage mechanism is a relational database. Typically, each entity bean has an underlying table in a relational database, and each instance of the bean corresponds to a row in that table. For code examples of entity beans, please refer to chapters 5 and 6.
What Makes Entity Beans Different from Session Beans?
Entity beans differ from session beans in several ways. Entity beans are persistent, allow shared access, have primary keys, and may participate in relationships with other entity beans.
Because the state of an entity bean is saved in a storage mechanism, it is persistent. Persistence means that the entity bean's state exists beyond the lifetime of the application or the J2EE server process. If you've worked with databases, you're familiar with persistent data. The data in a database is persistent because it still exists even after you shut down the database server or the applications it services.
There are two types of persistence for entity beans: bean-managed and container-managed. With bean-managed persistence, the entity bean code that you write contains the calls that access the database. If your bean has container-managed persistence, the EJB container automatically generates the necessary database access calls. The code that you write for the entity bean does not include these calls. For additional information, see the section Container-Managed Persistence (page 53).
Entity beans may be shared by multiple clients. Because the clients might want to change the same data, it's important that entity beans work within transactions. Typically, the EJB container provides transaction management. In this case, you specify the transaction attributes in the bean's deployment descriptor. You do not have to code the transaction boundaries in the beanthe container marks the boundaries for you. See Chapter 14 for more information.
Each entity bean has a unique object identifier. A customer entity bean, for example, might be identified by a customer number. The unique identifier, or primary key, enables the client to locate a particular entity bean. For more information see the section Primary Keys for Bean-Managed Persistence (page 113).
Like a table in a relational database, an entity bean may be related to other entity beans. For example, in a college enrollment application, StudentEJB and CourseEJB would be related because students enroll in classes.
You implement relationships differently for entity beans with bean-managed persistence and those with container-managed persistence. With bean-managed persistence, the code that you write implements the relationships. But with container-managed persistence, the EJB container takes care of the relationships for you. For this reason, relationships in entity beans with container-managed persistence are often referred to as container-managed relationships.
The term container-managed persistence means that the EJB container handles all database access required by the entity bean. The bean's code contains no database access (SQL) calls. As a result, the bean's code is not tied to a specific persistent storage mechanism (database). Because of this flexibility, even if you redeploy the same entity bean on different J2EE servers that use different databases, you won't need to modify or recompile the bean's code. In short, your entity beans are more portable.
In order to generate the data access calls, the container needs information that you provide in the entity bean's abstract schema.
Part of an entity bean's deployment descriptor, the abstract schema defines the bean's persistent fields and relationships. The term abstract distinguishes this schema from the physical schema of the underlying data store. In a relational database, for example, the physical schema is made up of structures such as tables and columns.
You specify the name of an abstract schema in the deployment descriptor. This name is referenced by queries written in the Enterprise JavaBeans Query Language ("EJB QL"). For an entity bean with container-managed persistence, you must define an EJB QL query for every finder method (except findByPrimaryKey). The EJB QL query determines the query that is executed by the EJB container when the finder method is invoked. To learn more about EJB QL, see Chapter 8.
You'll probably find it helpful to sketch the abstract schema before writing any code. Figure 31 represents a simple abstract schema that describes the relationships between three entity beans. These relationships are discussed further in the sections that follow.
Figure 31 A High-Level View of an Abstract Schema
The persistent fields of an entity bean are stored in the underlying data store. Collectively, these fields constitute the state of the bean. At runtime, the EJB container automatically synchronizes this state with the database. During deployment, the container typically maps the entity bean to a database table and maps the persistent fields to the table's columns.
A CustomerEJB entity bean, for example, might have persistent fields such as firstName, lastName, phone, and emailAddress. In container-managed persistence, these fields are virtual. You declare them in the abstract schema, but you do not code them as instance variables in the entity bean class. Instead, the persistent fields are identified in the code by access methods (getters and setters).
A relationship field is like a foreign key in a database tableit identifies a related bean. Like a persistent field, a relationship field is virtual and is defined in the enterprise bean class with access methods. But unlike a persistent field, a relationship field does not represent the bean's state. Relationship fields are discussed further in Direction in Container-Managed Relationships (page 55).
Multiplicity in Container-Managed Relationships
There are four types of multiplicities:
One-to-one: Each entity bean instance is related to a single instance of another entity bean. For example, to model a physical warehouse in which each storage bin contains a single widget, StorageBinEJB and WidgetEJB would have a one-to-one relationship.
One-to-many: An entity bean instance may be related to multiple instances of the other entity bean. A sales order, for example, can have multiple line items. In the order application, OrderEJB would have a one-to-many relationship with LineItemEJB.
Many-to-one: Multiple instances of an entity bean may be related to a single instance of the other entity bean. This multiplicity is the opposite of a one-to-many relationship. In the example mentioned in the previous item, from the perspective of LineItemEJB the relationship to OrderEJB is many-to-one.
Many-to-many: The entity bean instances may be related to multiple instances of each other. For example, in college each course has many students, and every student may take several courses. Therefore, in an enrollment application, CourseEJB and StudentEJB would have a many-to-many relationship.
Direction in Container-Managed Relationships
The direction of a relationship may be either bidirectional or unidirectional. In a bidirectional relationship, each entity bean has a relationship field that refers to the other bean. Through the relationship field, an entity bean's code can access its related object. If an entity bean has a relative field, then we often say that it "knows" about its related object. For example, if OrderEJB knows what LineItemEJB instances it has and if LineItemEJB knows what OrderEJB it belongs to, then they have a bidirectional relationship.
In a unidirectional relationship, only one entity bean has a relationship field that refers to the other. For example, LineItemEJB would have a relationship field that identifies ProductEJB, but ProductEJB would not have a relationship field for LineItemEJB. In other words, LineItemEJB knows about ProductEJB, but ProductEJB doesn't know which LineItemEJB instances refer to it.
EJB QL queries often navigate across relationships. The direction of a relationship determines whether a query can navigate from one bean to another. For example, a query can navigate from LineItemEJB to ProductEJB, but cannot navigate in the opposite direction. For OrderEJB and LineItemEJB, a query could navigate in both directions, since these two beans have a bidirectional relationship.
When to Use Entity Beans
You should probably use an entity bean under the following conditions:
The bean represents a business entity, not a procedure. For example, CreditCardEJB would be an entity bean, but CreditCardVerifierEJB would be a session bean.
The bean's state must be persistent. If the bean instance terminates or if the J2EE server is shut down, the bean's state still exists in persistent storage (a database).