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Managing Persistent Data in the Business Tier

Learn about features, components, and usage scenarios of entity beans in Java EE development. Explore creating, finding, managing, and selecting entity beans in the business tier.

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Managing Persistent Data in the Business Tier

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  1. Managing Persistent Data in the Business Tier Entity EJBs

  2. Objectives • After completing this lesson, you should be able to do the following: • Identify the features of an entity bean • Distinguish between session beans and entity beans • Decide when to use an entity bean • Describe the various components of an entity bean • Differentiate bean-managed persistent beans from container-managed persistent beans

  3. Entity Beans • Are objects that can be stored in permanent storage • Represent persistent data in the database • Can be uniquely identified • Do not contain complex business logic • Do not model business processes but manage data for such processes • Can serve multiple clients simultaneously

  4. int x String s float f int y double d Representing Data in Entity Beans Entity bean object Table

  5. When to Use Entity Beans • You can use an entity bean in the following situations: • To represent a business entity • When the state of the bean must be persistent • When you need to represent a relationship between entities

  6. Callback Methods to Load and Store Data • ejbLoad()loads the data from the persistent storage to the bean. • ejbStore()saves the data from the bean instance to the persistent storage.

  7. Session Beans Versus Entity Beans • Entity Beans: • Are represented by nouns because they model business data • Are long-lived and do not depend on a client’s session • Contain callback methods for managing data (create, retrieve, persist, and so on) in the persistent storage • Session Beans: • Are represented by verbs because they model workflow • Are short-lived and have a lifetime of a client session • Manage their state, but do not contain callback methods to manage data because they do not represent data in the persistent storage

  8. Types of Entity Beans • Container-managed persistent (CMP) beans: The container provides the logic to search and manipulate the persistent data. • Bean-managed persistent (BMP) beans: The bean provider codes the logic to search and manipulate the persistent data in the callback methods.

  9. BMP Beans Versus CMP Beans • BMP beans: • Contain code for managing data persistence • Provide flexibility for bean developers to manage state • Are complicated to program • CMP beans: • Do not contain code for managing persistence because the container manages the persistence • Use abstract persistence schema and define the data retrieval/manipulation logic in the deployment descriptor • Are easier to program and contain lesser code

  10. Components of an Entity Bean • Home interface: Is used by clients to create, find, and destroy EJB objects • Component interface: Contains the declaration of all business methods in the bean class that can be invoked by the client • Bean class: Represents persistent data and contains methods to access or manipulate that data • Primary key class: Is used to uniquely identify an entity bean instance • Deployment descriptor: Contains information that is used by the container

  11. Creating, Removing, Finding, and Selecting Entity Beans • ejbCreate(): Initializes the entity bean in memory. This method may also insert the corresponding data into the persistent storage. • ejbRemove(): Removes the database data but does not remove the in-memory entity bean instance. The instance may return to the pool and release all resources in the ejbPassivate() method. • Finder methods: Used to find an entity bean. • Selector methods: Used to select entity beans and values of CMP fields.

  12. Home Interface of an Entity Bean • Remote home interface: • Extends the javax.ejb.EJBHome interface • Can be accessed by remote clients by using Java Naming and Directory Interface (JNDI) • Allows client to create handles for later reference • Local home interface: • Extends the javax.ejb.EJBLocalHome interface • Can be accessed by local clients • Remote/local home interface: • Contains methods to create, find, or remove entity objects • Contains business methods that are not applicable to a specific instance (home methods)

  13. Creating a Bean Instance • An entity bean can have one or more create() methods, or none defined in its home interface. • The create() method: • Enables a client to create a row in the database table that corresponds to the entity bean • Contains parameters that initialize the state of the created entity object • Returns the remote/local interface reference of the entity beans • Throws CreateException and any user-defined exceptions • Throws RemoteException if it is part of a remote home interface

  14. Finding an Entity Bean Instance • An entity bean: • Can have one or more finder methods in home interface • Must have a findByPrimaryKey(primarykey) method • Has finder methods that: • Find row or rows in the database table • Contain parameters that locate the requested entity object • Return entity bean’s component interface reference or collection of objects of component interface type • Must throw FinderException • Must throw RemoteException in a remote home interface • Have find as a prefix in their names

  15. Removing an Entity Bean • An entity bean with remote home interface: • Can have one or more remove() methods: remove(Handle handle) and remove(Object primarykey) • Throws RemoteException and RemoveException from remove() methods • An entity bean with local home interface: • Can have one remove() method: remove(Object primarykey) • Throws RemoveException from the remove() method • remove() methods remove the entity object and the row from the underlying database.

  16. Home Methods of Entity Beans • Home methods are provided by a bean provider. • Home methods contain business logic that is not specific to any bean instance. • An entity bean can have one or more home methods, or none. • The arguments and return types of remote home methods should be of RMI-IIOP type.

  17. Component Interfaces of an Entity Bean • Component interfaces define: • Business methods that are accessible by clients • Accessor methods for the bean attributes • Remote interfaces: • Are referenced by remote clients • Extend the javax.ejb.EJBObject interface • Local interfaces: • Are referenced by local clients • Extend the javax.ejb.EJBLocalObject interface

  18. Primary Key Class of an Entity Bean • Primary key: • Uniquely identifies each bean instance • Is used to find or remove an entity bean • Can be of any legal value type in RMI-IIOP • The primary key class: • Should implement java.io.Serializable • Can have a single primary key (a single field) to identify the entity bean • Can have composite keys (multiple fields) to identify the entity bean

  19. Bean Class of an Entity Bean • Initializes the bean instance through the ejbCreate() and ejbPostCreate() methods • Implements: • Finder methods through ejbFindxxx() methods • Home methods through ejbHomexxx() methods • Callback methods from the EntityBean interface • Business and private methods

  20. Bean Class of an Entity Bean • Contains ejbCreate() and ejbPostCreate() methods for each create() method. • The ejbCreate() method: • Is invoked when a client invokes the create() method to initialize persistent fields • Has primary key as return type • The ejbPostCreate() method: • Is invoked after the entity bean is created and before any other request from a client is processed • Has void as return type • Initializes any relationship fields for an entity bean

  21. javax.ejb.EntityBean Interface • All entity bean classes should implement the javax.ejb.EntityBean interface. public interface javax.ejb.EntityBean implements javax.ejb.EnterpriseBean { public void ejbActivate(); public void ejbLoad(); public void ejbPassivate(); public void ejbRemove(); public void ejbStore(); public void setEntityContext(EntityContext ctx); public void unSetEntityContext(); }

  22. Pooled Life Cycle of an Entity Bean Does not exist newInstance() setEntityContext() unsetEntityContext() ejbCreate()ejbPostCreate() ejbRemove() ejbPassivate() ejbActivate() ejbLoad() Ready ejbStore() Clients invoke business methods

  23. Deployment Descriptor <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE …> <ejb-jar> <enterprise-beans> <entity> <ejb-name>...</ejb-name> <home>...</home> <remote>...</remote> <ejb-class>...</ejb-class> <persistence-type>...</persistence-type> <prim-key-class>...</prim-key-class> <reentrant>False</reentrant> <abstract-schema-name>...</abstract-schema-name> <cmp-field> <field-name>...</field-name> </cmp-field> <primkey-field>...</primkey-field> </entity> </enterprise-beans> ...

  24. Deployment Descriptor ... <assembly-descriptor> <security-role> <description>Public</description> <role-name>PUBLIC</role-name> </security-role> <method-permission> <description>Public methods</description> <role-name>PUBLIC</role-name> <method> <ejb-name>...</ejb-name> <method-name>*</method-name> </method> </method-permission> </assembly-descriptor> </ejb-jar>

  25. Summary • In this lesson, you should have learned how to: • Identify features of an entity bean • Use entity beans • Distinguish between a session bean and an entity bean • Develop components of an entity bean • Differentiate BMP beans from CMP beans • Describe the life cycle of an entity bean

  26. Practice 13-1: Overview • This practice reviews entity bean concepts using paper-based questions.

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