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Figure B.5 Stateful session bean life cycle diagram
Listing B.3
Stateless session bean implementation example
import java.util.*; import javax.ejb.*; /** * Stateless session bean implementation class example. * Note: Does not explicitly implement bean client interface. */ public class BookingAgentBean implements SessionBean { /** Session context instance. */ private SessionContext sessionContext; /** Called by container before invocation. */ public void setSessionContext(SessionContext sessionContext) { this.sessionContext = sessionContext; } /** Called by container after creation. */ public void ejbCreate() throws CreateException {} /** Possibly called by container before removal. */ public void ejbRemove() {} /** Not actually used in stateless session beans. */ public void ejbActivate() {} /** Not actually used in stateless session beans. */ public void ejbPassivate() {} public Collection getBookingDates(String userId) { // Query database for list of booking dates // and return a Collection of Date instances. // ...
Crafting enterprise beans
return null; } public void book(String userId, Date date, int attendees) throws BookingException { // Add a booking to the database. // Throw a BookingException if there is a // scheduling conflict. // ... } }
Implementing entity beans The purpose of the ejbCreate() and ejbRemove() methods in the entity bean implementation differs from that of the session bean counterparts. Entity bean ejbCreate() methods may accept parameters as well. The entity bean ejbCreate() method creates an object in the persistent data store and returns the object s primary key. The container calls the ejbRemove() method to remove the object from the persistent store. The bean implementation must implement one ejbCreate() method for each create() method in the client interface. Entity beans are not required to implement any create() methods. The bean implementation must also implement one ejbPostCreate() method for each ejbCreate() method; both methods must have the same parameters. The container invokes ejbPostCreate() after calling the ejbCreate() method and associating your instance with an EJBObject in other words, after your bean is client-accessible. While entity beans using BMP return a primary key object from the ejbCreate() method, entity beans using CMP should return null. The container will create the object in the persistent store and generate the primary key. Entity beans using BMP must implement two additional container callback methods, ejbLoad() and ejbStore(). These methods allow the container to synchronize an entity bean instance s data with a data store. The container uses ejbLoad() to tell the entity bean instance to load its data from the store, possibly for the first time, or to synchronize the beans data at the beginning of a transaction. The container uses ejbStore() to tell the bean instance to persist its data to the store. The container invokes each of these two callback methods at a time that depends on several factors, including your vendor s implementation, your bean s transaction configuration, and your caching settings, if any. The container uses the ejbActivate() and ejbPassivate() methods to prepare an in-memory entity bean instance for creation and removal. For a new
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object, the container will invoke the ejbActivate() method before calling ejbLoad() for the first time. This allows the bean implementation to set up any transient resources needed for the ejbLoad() implementaBusiness method tion. When a container removes an instance from memory (without removing the object from the persistent store), it will call ejbStore() to ensure that all data has Ready been persisted and then call ejbPassivate() to release resources (figure B.6). Create Remove The bean provider is responsible for implementing the finder methods for BMP entity beans. Though the finder methods declared in the home interface and the actual Pooled bean implementation have a one-to-one correspondence, the method names and return values are slightly different. Unset context Set context Specifically, finder method names in the bean implementation are prefixed with an ejb . In addition, while the Does Not finder method in the client interface returns a collection Exist of EJBObject instances or remote stubs, the finder method in your implementation class returns a Collection of primary keys. Using the minimally required Figure B.6 Entity bean life cycle findByPrimaryKey() method as an example, the corrediagram sponding finder method implementation would be called ejbFindByPrimaryKey(). Entity bean implementations using CMP depart significantly from other EJB bean implementations (listing B.4). First, the implementation class for a CMP entity bean is declared abstract. Each persistent field also has a set of abstract getters and setters. For example, for the user ID field in our booking entity, we would have an abstract getUserId() and setUserId() method pair. The container subclasses your bean s implementation class, implementing the field accessors as well as the finder methods. You declare finder method functionality for CMP beans separately from your business logic, using EJB QL. EJB QL is syntactically comparable to a limited version of SQL. However, EJB QL is not tied to relational databases. EJB 2.0 also introduced the notion of CMR. If you use local interfaces and CMP, you can configure relationships between your entity beans. For example, you could have a one-to-many relationship constraint between two entity bean types, much as you would between two tables in a relational database. This feature allows you to model complex data maps using entity beans and abstracts out this relationship management logic from the data store, further decoupling your application logic from the underlying persistence mechanism (listing B.4).
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