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Using JDBC from a Stateful Session Bean
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Many web interactions can t live with the limitations imposed by stateless session beans. Consider a more complex web-based form that spans four pages. As the user fills out each page and sends it to the web site, the session bean must accumulate the information and retain it across the four page clicks until all of the data is ready to be captured into a database. The need to retain information across method invocations calls for a stateful session bean. Another example in which a stateful session bean is appropriate is a commercial web site where a user shops online and accumulates a list of items to be purchased in
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Database calls from a stateless session bean
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an online shopping cart. After 40 or 50 clicks through the web site, the user may have accumulated five or six items in the shopping cart. If the user then clicks a button requesting display of the current shopping cart contents, those contents are probably most easily maintained as session bean state. In both of these examples, the session bean requires continuity of database access to effectively accomplish its tasks. Figure 22-5 shows the pattern, and the contrast to the pattern of interactions in Figure 22-4. Even if the bean can be implemented without instance variables (for example, by storing all of its state information in a back-end database), it needs one continuous database session to carry out its database access. The client-side API for the DBMS maintains this session, and the API itself will need to maintain session-state information across session bean method invocations.
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Database calls from a stateful session bean
Entity Bean Database Access
It s possible to implement complete, sophisticated web site applications using session beans deployed on a J2EE application server. However, programming an application using session beans tends to produce more procedural, and less object-oriented code. The object-oriented philosophy is to have object classes (in this case, EJB classes) represent real-world entities, such as customers or offices, and object instances represent individual customers or offices. But session beans don t represent any of those entities; they represent currently active user sessions. When database interaction is handled directly by session beans, the representation of real-world entities is basically left in the database; it doesn t have an object counterpart.
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Entity beans provide the object counterpart for real-world entities and the rows in a database that represent them. Entity bean classes embody customers and offices; individual entity bean instances represent individual customers and individual offices. Other objects (such as session beans) within the application server can interact with customers and offices using object-oriented techniques, by invoking the methods of the entity beans that represent them. To maintain this object-oriented model, there must be very close cooperation between the entity-bean representations of entities and their database representations. If a session bean invokes a customer entity bean method that changes a customer s credit limit, that change must be reflected in the database, so that an order-processing application using the database will use the new limit. Similarly, if an inventory management application adds to the quantity on hand for a particular product in the database, that product s entity bean in the application server must be updated. Just as an application server will passivate and reactivate session beans as necessary, it will passivate and reactivate entity beans repeatedly in response to a heavy workload. Before the application server passivates an entity bean, the bean s state must be saved in a persistent way, by updating the database. Similarly, when the application server reactivates an entity bean, its instance variables must be set to their values just before it was passivated, by reloading those values from the database. The entity bean class defines callback methods that an entity bean must provide to implement this synchronization. There is close correspondence between actions carried out on entity beans and database actions, as shown in Table 22-1. The J2EE specification provides two alternative ways to manage this coordination: I Bean-managed persistence. The entity bean itself is responsible for maintaining synchronization with the database. The application programmer who develops the entity bean and codes its implementation methods must use JDBC to read and write data in the database when necessary. The application server container notifies the bean when it takes actions that require database interaction. I Container-managed persistence. The EJB container provided by the application server is responsible for maintaining synchronization with the database. The container monitors interaction with the entity bean, and automatically uses JDBC to read and write data in the database and update the instance variables within the bean when needed. The application programmer who develops the entity bean and codes its implementation methods can focus on the business logic in the bean, and assumes that its instance variables will accurately represent the state of the data in the database. Note that entity beans are always stateful the distinction between these two bean types is not the difference between stateless and stateful beans, but rather, the difference between who is responsible for maintaining proper state. The next two sections discuss the practical issues associated with each type of entity bean, and the trade-offs between them.
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