Knowledge Byte In this section, you will learn about: Declaring relationships in BMP entity beans Object-to-relational

1. Knowledge Byte • In this section, you will learn about: • Declaring relationships in BMP entity beans • Object-to-relational mapping in EJB Collaborate

2. Declaring Relationships in BMP Entity Beans • The bean developer writes the code for defining and managing relationships between BMP entity beans. • Various types of the relationships that can exist between BMP entity beans are: • One-to-One Relationships • One-to-Many Relationships • Many-to-Many Relationships Collaborate

3. Declaring Relationships in BMP Entity Beans (Contd.) • One-to-One Relationships: • Specify that each instance of an entity bean is related to a single instance of another entity bean. • For example, in a banking application, each account may only have one particular loan associated with it. This means that the relationship between the Account and Loan entity beans is one-to-one. • One-to-Many Relationships: • Specify that each instance of an entity bean is related to several instances of another entity bean. • For example, in a banking application, the relationship between the Customer and Account entity beans is one-to-many. This means that a customer can have more than one account in a bank but one account can belong to only one customer. Collaborate

4. Declaring Relationships in BMP Entity Beans (Contd.) • Many-to-Many Relationships: • Specify that multiple instances of an entity bean are related to several instances of another entity bean. • For example, in a banking application, an account can be shared between multiple customers. In addition, one customer can have several accounts. As a result, the relationship between the Account and the Customer entity beans is many-to-many. • Can either be: • Fake many-to-many relationship • True many-to-many relationship Collaborate

5. Declaring Relationships in BMP Entity Beans (Contd.) • Fake many-to-many relationship: • Is implemented by using an association table. • For example, you can create the CustomerAccount table as an association table linking the Customer and Account tables. • True many-to-many relationship: • Is implemented using the ejbLoad() method, which invokes a finder method by using the home interface of the other entity bean in the relationship. The finder method uses the primary key of the other entity bean as an argument. Collaborate

6. Declaring Relationships in BMP Entity Beans (Contd.) • Directionality: • It refers to the direction in which you can navigate a relationship. • Relationships can be: • Unidirectional • Bi-directional Collaborate

7. Declaring Relationships in BMP Entity Beans (Contd.) • Unidirectional relationship: • Specifies that only one BMP entity bean consists of the get and set methods and a field that references the other BMP entity bean. For example, in a unidirectional relationship between two entity beans A and B, a client can only navigate from A to B and not from B to A. • Bi-directional relationship: • Specifies that each BMP entity bean consists of one field pointing to the other BMP entity bean in the relationship. For example, in a bi-directional relationship between two entity beans A and B, a client can navigate from A to B and from B to A. • Specifies that each entity bean that participates in the bi-directional relationships consists of the get and set methods. Collaborate

8. Object-to-Relational Mapping in EJB • Object-to-relational mapping: • Is the method of mapping an object to a relational database. • Enables you to store Java objects in a relational database, such as SQL Server or Oracle. • Enables you to issue queries for retrieving particular information. • Can be performed using the following two methods: • By providing the code in your application using a database access API, such as JDBC. • By using an object-relational mapper, such as Sun's JavaBlend. Collaborate

9. From the Expert’s Desk • In this section, you will learn: • Best Practice on: • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State • Tips and Tricks on: • Optimizing the Performance of Stateful Session Beans • Optimizing the Use of JDBC in BMP Entity Beans • FAQs on session beans Collaborate

10. Best Practices • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State • While interacting with an enterprise application, the operation performed by the client during a session may require storing some client-specific information. • The following methods are used to maintain the client state in an application: • HTTP Session Object • Stateful Session Beans Collaborate

11. Best Practices • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) • HTTP Session Object: • Is a simple object that can be used for storing client-specific information for a particular session. • Has the following advantages: • Requires less resources for maintaining the state in a client. • Requires less time in maintaining the client state. • Enables the clients using Servlet and JSP to maintain the client state easily. Collaborate

12. Best Practices • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) • The HTTP Session Object has the following disadvantages: • Client state is lost if a client restarts or logs-out from the Web browser. • Client is allowed to change its system while accessing an enterprise bean. Collaborate

13. Best Practices • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) • Stateful Session Beans: • Can be used to maintain the client state. • Store the conversational state of an individual client with the enterprise bean. • For example, in an online bookshop application, a stateful session bean is used for storing the names of books selected by a customer. The names stored are retained even if the client calls other methods of the bean. Collaborate

14. Best Practices • Choosing Between HTTP Session Object and Stateful Session Bean to Maintain Client State (Contd.) • Stateful Session Beans have the following advantages: • Enable the Web-based and non-Web-based clients to store client-specific information. • Improve scalability of an application by using a shared pool to store the bean instances. • Provide support for transactions, security services, and RMI. • Allow a client to perform multiple operations in a single HTTP client request. Collaborate

15. Tips and Tricks • Optimizing the Performance of Stateful Session Beans • The measures to optimize the performance of a stateful session bean are: • Define an optimal cache size • Define an optimal time out value for each bean instance • Remove a stateful session bean, explicitly • Declare transient variables Collaborate

16. Tips and Tricks • Optimizing the Use of JDBC in BMP Entity Beans • You use the JDBC packages, java.sql and javax.sql, to connect to a database. • JDBC provides a connection pool that: • Consists of several open database connections. You can specify the minimum and maximum number of open connections in the connection pool. • Helps in optimizing performance, as you do not require establishing a new connection with the database. Collaborate

17. Tips and Tricks • Optimizing the Use of JDBC in BMP Entity Beans (Contd.) • When you use JDBC in the BMP entity beans, the following criteria should be considered: • Use batch transaction in order to execute multiple statements on a connection. • Use the PreparedStatement object to execute a statement multiple times. • Retrieve data from a database iteratively, in small amounts, instead of retrieving the entire data at one go. • Store the read-only and read-mostly data from a table in a cache. • Use the batch update feature of the statement object when you need to issue several queries to a database simultaneously. • Specify the appropriate isolation level according to the requirements of the application. Collaborate

18. Tips and Tricks • Optimizing the Use of JDBC in BMP Entity Beans (Contd.) • You need to choose the right type of driver to optimize the use of JDBC driver in the BMP entity beans. • When selecting a driver, the following criteria should be considered: • Use the Native protocol - All - Java driver while communicating with a database using an applet. • Use the Native API - Partly - Java driver while using a Java client to communicate with a database. • Use the JDBC-ODBC driver if the database with which you establish a connection does not support any driver. • Use the JDBC - Net - All Java driver for establishing communication between a client and a proxy server when the application is three-tiered. Collaborate

19. FAQs • How does a stateful session bean maintain the client conversational state? • A stateful session bean stores the values of variables in a secondary storage while passivating the bean instance associated with a client. • What is the purpose of the Passive stage in a stateful session bean life cycle? • In the Passive stage, EJB Container enables an idle stateful session bean instance to service another client’s request. Collaborate

20. FAQs (Contd.) • Is it necessary to override the ejbStore() and ejbLoad() methods in a BMP entity bean application if there are no delete and update options? • The ejbStore() and ejbLoad() methods are used to synchronize the bean state with a database. For this reason, it is necessary to implement and override these methods. • What is the use of ejbPostCreate() method in a BMP entity bean? • The ejbPostCreate() method can be used for passing the reference of an EJB object to other entity beans. Collaborate

21. Challenge • Stateful session beans save client’s conversational state in____________. • Persistent objects in the J2EE enterprise bean applications are represented using _________beans. • Entity beans are of two types, _________ and __________. • The _________ method should have the same parameters as the ejbCreate() method in a BMP entity bean. • The context object stores the information about the ___________ of an EJB. Collaborate

22. Solutions to Challenge • secondary storage • entity • bean-managed persistence entity beans and container-managed persistence entity beans • ejbPostCreate() • environment Collaborate