Entity Bean Chuyên đề Lập trình Java & J2EE Chương 15 Biên soạn: Th.S Nguyễn văn Lành Concepts  An entity bean represents persistent data stored in one row of a database table   When an entity bean is created, the data is written to the appropriate database table row, and if the data in an entity bean is updated, the data in the appropriate database table row is also updated Entity beans differ from session beans in several ways     Entity beans are persistent: entity bean's state exists beyond the lifetime of the application allow shared access: may be shared by multiple clients have primary keys: unique object identifier may participate in relationships with other entity beans When to Use Entity Beans  The bean represents a business entity, not a procedure  For example, CreditCardEJB would be an entity bean  CreditCardVerifierEJB would be a session bean  The bean's state must be persistent Types of persistent management  Bean-Managed Persistence   Container-Managed Persistence 1.1   the bean developer takes responsibility for persistent storage in attributes persistent attributes are defined as attributes of the bean class and given in the deployment descriptor Container-Managed Persistence 2.0/2.1   The bean developer works with an abstract bean class defines abstract access methods for reading and writing of attributes (During deployment the EJB container generates a derived class that implements all the abstract methods ) Constitution of a entity bean  Remote interface:   Home interface:   makes it possible to identify all entity bean identities uniquely Deployment Descriptor:   provides the implementation for the business methods, finder methods, home methods Primary key class   defines the methods that allow a client to create and find an entity bean Enterprise Bean class:   defines the business methods of the bean This configuration document defines the bean's runtime attributes, such as its transaction or security attributes Helper classes Remote Interface     Each method in the remote interface must match a method in the enterprise bean class The signatures of the methods in the remote interface must be identical to the signatures of the corresponding methods in the enterprise bean class The arguments and return values must be valid RMI types The throws clause must include java.rmi.RemoteException Home Interface   defines the methods that allow a client to create and find an entity bean create Method     has the same number and types of arguments as its matching ejbCreate method in the enterprise bean class returns the remote interface type of the enterprise bean The throws clause includes the exceptions specified by the throws clause of the corresponding ejbCreate and ejbPostCreate methods The throws clause includes the javax.ejb.CreateException & java.rmi.RemoteException Entity bean creation Home Interface  finder Method       Every finder method in the home interface corresponds to a finder method in the entity bean class The name of a finder method in the home interface begins with find, whereas the corresponding name in the entity bean class begins with ejbFind The number and types of arguments must match those of the corresponding method in the entity bean class The return type is the entity bean's remote interface type, or a collection of those types The exceptions in the throws clause include those of the corresponding method in the entity bean class The throws clause contains the javax.ejb.FinderException & java.rmi.RemoteException Home Interface  finder Method in bean class     All finder methods must begin with ejbFind You must have at least one finder method, called ejbFindByPrimaryKey You can have many different finder methods, each with different names and different parameters A finder method must return either the primary key for the entity bean it finds or a collection of primary keys if it finds more than one 10 BMP guidelines 13 BMP guidelines 14 BMP guidelines 15 16 Bean-DBMS synchronization 17 CMP Entity Beans  CMP Entity Beans Are Subclassed database-independent  The container generates the JDBC code by subclassing your entity bean class CMP Entity Beans Have No Declared Fields CMP Entity Beans Have a Query Language CMP Entity Beans Can Have ejbSelect() Methods     18 Bean instance Pooling 19 CMP Guidelines 20 CMP Guidelines 21 CMP Guidelines 22 23 24 25 26 27 ... Bean- DBMS synchronization 17 CMP Entity Beans  CMP Entity Beans Are Subclassed database-independent  The container generates the JDBC code by subclassing your entity bean class CMP Entity Beans... an entity bean is updated, the data in the appropriate database table row is also updated Entity beans differ from session beans in several ways     Entity beans are persistent: entity bean'' s... relationships with other entity beans When to Use Entity Beans  The bean represents a business entity, not a procedure  For example, CreditCardEJB would be an entity bean  CreditCardVerifierEJB