Types of EJBs

There are two types of EJBs: session beans and entity beans. An easy way to think of the difference is that a session bean implements one or more business tasks, while an entity bean is a complex business entity. A session bean might contain methods that query and update data in a relational table; an entity bean represents business data directly or indirectly through another persistent bean.

Session beans are often used to implement services. For example, an application developer might implement one or several session beans that retrieve and update inventory data in a database. You can use session beans to replace stored procedures in the database server, thereby achieving the scalability inherent in the Oracle8i Java server.

Entity beans are often used to facilitate business services that involve data and computations on that data. For example, an application developer might implement an entity bean to retrieve and perform computation on items within a purchase order. Your entity bean can manage multiple dependent persistent objects in performing its necessary tasks.

Persistence

Session beans are not inherently persistent. Persistence can refer either to a characteristic of the bean--entity beans are persistent, session beans are not inherently persistent--or it can refer to data that a bean might save, so that the data can be retrieved in a future instantiation. Persistent data is saved in the database.

Therefore, a session bean can save its state in an Oracle8i database, but it does not directly represent business data. Entity beans persist the business data either automatically--in a container-managed persistent entity bean--or by way of methods that use JDBC or SQLJ and are coded into the bean--in a bean-managed persistent entity bean.

Implementing the synchronization interface can make data storage and retrieval automatic for session beans.

Session Beans

Created by a client, a session bean is usually specific to that client. In Oracle8i more than one client can share a session bean.

Session beans are transient in that they do not survive a server crash or a network failure. If, after a crash, you instantiate a bean that had previously existed, the state of the previous instance is not restored. State can only be restored to entity beans.

Stateful Session Beans

A stateful session bean maintains its state between method calls. For example, a single instance of a session bean might open a JDBC database connection and use the connection to retrieve some initial data from the database. For example, a shopping-cart application bean could load a customer profile from the database as soon as it's activated, then that profile would be available for any method in the bean to use.

A typical stateful session EJB is a relatively coarse-grained object. A single bean almost always contains more than one method, and the methods provide a unified, logical service. For example, the session EJB that implements the server side of a shopping cart on-line application would have methods to return a list of objects that are available for purchase, put items in the customer's cart, place an order, change a customer's profile, and so on.

The state that a session bean maintains is called the "conversational state" of the bean, as the bean is maintaining a connection with a single client, similar to a telephone conversation.

Keep in mind that the state of a bean is still transient data, with respect to the bean itself. If the connection from the client to the bean is broken, the state can be lost. This depends on whether the client is unable to reconnect before timeout.

Entity Beans

Entity beans are persistent in that they do survive a server crash or a network failure. When an entity bean is re-instantiated, the state of previous instances is automatically restored. For more information on entity beans, see "Definition of an Entity Bean".

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