Enterprise JavaBeans, Fourth Edition

Chapter 11. Session Beans

Entity beans provide an object-oriented model that makes it easier for developers to create, modify, and delete data from the database. They allow developers to be more productive by encouraging reuse, thus reducing development costs. For example, once a bean has been defined to represent a concept like a Ship, that bean can be reused throughout a business system without redefining, recoding, or retesting the business logic and data access.

However, entity beans are not the entire story. We have seen another kind of enterprise bean: the session bean. Session beans fill the gaps left by entity beans. They are useful for describing interactions between other beans (taskflow) and for implementing particular tasks. Unlike entity beans, session beans do not represent shared data in the database, but they can access shared data. This means that we can use session beans to read, update, and insert data. For example, we might use a session bean to provide lists of information, such as a list of all available cabins. Sometimes we might generate the list by interacting with entity beans, like the cabin list we developed in the TravelAgent EJB in Chapter 4. More frequently, session beans will generate lists by accessing the database directly.

When do you use an entity bean and when do you use a session bean? As a rule of thumb, an entity bean should provide a safe and consistent interface to a set of shared data that defines a concept. This data may be updated frequently. ...