Transcript EJB 6

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Knowledge Byte
In this section, you will learn about:
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Declaring relationships in BMP entity beans
Object-to-relational mapping in EJB
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Lesson 2C / Slide 1 of 22
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Declaring Relationships in BMP Entity
Beans
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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
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Lesson 2C / Slide 2 of 22
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Declaring Relationships in BMP Entity
Beans (Contd.)
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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.
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Lesson 2C / Slide 3 of 22
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Declaring Relationships in BMP Entity
Beans (Contd.)
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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
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Lesson 2C / Slide 4 of 22
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Declaring Relationships in BMP Entity
Beans (Contd.)
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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.
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Lesson 2C / Slide 5 of 22
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Declaring Relationships in BMP Entity
Beans (Contd.)
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Directionality:
• It refers to the direction in which you can navigate a relationship.
• Relationships can be:
• Unidirectional
• Bi-directional
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Lesson 2C / Slide 6 of 22
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Declaring Relationships in BMP Entity
Beans (Contd.)
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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 bidirectional 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 bidirectional relationships consists of the get and set methods.
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Lesson 2C / Slide 7 of 22
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Object-to-Relational Mapping in EJB
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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.
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Lesson 2C / Slide 8 of 22
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From the Expert’s Desk
In this section, you will learn:
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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
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Lesson 2C / Slide 9 of 22
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Best Practices
Choosing Between HTTP Session Object and
Stateful Session Bean to Maintain Client State
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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
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Lesson 2C / Slide 10 of 22
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Best Practices
Choosing Between HTTP Session Object and
Stateful Session Bean to Maintain Client State
(Contd.)
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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.
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Lesson 2C / Slide 11 of 22
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Best Practices
Choosing Between HTTP Session Object and
Stateful Session Bean to Maintain Client State
(Contd.)
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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.
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Lesson 2C / Slide 12 of 22
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Best Practices
Choosing Between HTTP Session Object and
Stateful Session Bean to Maintain Client State
(Contd.)
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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.
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Lesson 2C / Slide 13 of 22
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Best Practices
Choosing Between HTTP Session Object and
Stateful Session Bean to Maintain Client State
(Contd.)
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Stateful Session Beans have the following advantages:
• Enable the Web-based and non-Web-based clients to store clientspecific 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.
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Lesson 2C / Slide 14 of 22
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Tips and Tricks
Optimizing the Performance of Stateful Session
Beans
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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
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Lesson 2C / Slide 15 of 22
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Tips and Tricks
Optimizing the Use of JDBC in BMP Entity Beans
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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.
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Lesson 2C / Slide 16 of 22
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Tips and Tricks
Optimizing the Use of JDBC in BMP Entity Beans
(Contd.)
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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.
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Lesson 2C / Slide 17 of 22
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Tips and Tricks
Optimizing the Use of JDBC in BMP Entity Beans
(Contd.)
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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 threetiered.
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Lesson 2C / Slide 18 of 22
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FAQs
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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.
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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.
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Lesson 2C / Slide 19 of 22
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FAQs (Contd.)
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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.
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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.
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Lesson 2C / Slide 20 of 22
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Challenge
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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.
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Lesson 2C / Slide 21 of 22
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Solutions to Challenge
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secondary storage
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entity
bean-managed persistence entity beans and container-managed persistence
entity beans
ejbPostCreate()
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environment
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Lesson 2C / Slide 22 of 22