EJB Technology Presentation

EJB Technology

Pre-assessment Questions
1. Which statement defines a presentation tier?
a) Provides transparent access to the persistent store in order to
retrieve and store data.
b) Provides various business services required by an application client
in an enterprise application.
c) Contains the logic that is required for presenting the information to
an end user.
d) Communicates with the external resources, such as data stores
and applications.

©NIIT Architecting J2EE Solutions Lesson 2B / Slide 1 of 53

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1. Select the design pattern that allows you to implement synchronous
messaging in EJB components.
a) Service activator
b) Value list handler
c) Transfer object assembler
d) Transfer object

3. Which option identifies the pattern of integration tier?
a) Business delegate
b) Service locator
c) Session facade
d) Data access object


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1. Consider the statements:
Statement A: The Value list handler pattern accesses the persistent data
store.
Statement B: The Transfer object assembler pattern is used to create
complex transfer objects from various data sources.
Identify the correct option.
a) Statement A is True and statement B is False.
b) Statement A is False and statement B is True.
c) Both, statements A and B, are True.
d) Both, statements A and B, are false.


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1. Which pattern is used to carry multiple data elements in a single structure
across tiers?
a) Transfer object assembler
b) Transfer object
c) Data access object
d) Service activator


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Solutions to Pre-assessment Questions

1. c) Contains the logic that is required for presenting the information to an end
user
2. b) Value list handler
3. d) Data access object
4. b) Statement A is false and statement B is true
5. b) Transfer object


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Objectives

In this lesson you will learn to:

• Identify features of EJB container
• Identify the various states during the lifecycle of various types of EJBs
• Compare stateful and stateless session beans
• Compare session and entity beans
• Use DAO with entity beans
• Identify container managed transactions and bean managed transactions


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EJB Container Model

• EJB container
• Is a part of the application server in which an enterprise bean is created
and destroyed.
• Provides runtime environment for an enterprise bean and pools resources
such as socket connections and database connections in order to reduce
memory consumption and processing time.
• Is also responsible for managing the lifecycle of an enterprise bean.


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EJB Container Model (Contd.)
• Features of EJB Container (Contd.)
• Passivation
• Pooling stateless session bean and message driven bean instances is
quite easy, as they do not hold conversational state.
• A stateful session bean’s processing spans to multiple client requests,
therefore, you cannot assign a stateful bean instance to another
client request that is already servicing a client request.
• EJB container saves the conversational state of the bean instance into
a file.
• EJB container then sends the bean instance into the pool and assigns
the bean instance to another client request.
• The process of saving the conversational state is called passivation.
• When the original client request is encountered, the saved
conversational state read into a bean instance.
• Passivation of bean instances releases the resources acquired by the
bean.

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• Security
• Is important to distributed applications such as railway reservation
system, online shopping, and banking applications where an
unauthorized access can result in heavy loss of money.
• Ensures that the clients are authenticated before they are allowed to
perform certain operations.
• Features are defined in the deployment descriptor, which are read by
container


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• Distributed Remote Access
• Since your enterprise does not know how to receive calls from a
client, a client cannot directly invoke methods on the enterprise
bean.
• EJB container intercepts the calls and delegates them to the
enterprise bean.
• This adds great value to EJB applications, as you do not have to care
about networking issues.
• EJB Lifecycle Management
• EJB container manages the lifecycle of an enterprise bean.
• An enterprise beans passes through various stages in its lifecycle,
such as does not exist, pooled, and ready.
• EJB container is responsible for switching the state of the enterprise
bean between the states.


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• Lifecycle of EJBs
• All the beans has three states i.e. does not exist, pooled, and running.
• An EJB container is responsible for switching a bean between these states.
• However, the switching of a bean, between these states is entirely bean
specific.
• Lifecycle of stateless session beans
• The lifecycle of a stateless session bean consists of two stages:
• Ready Stage
• Does Not Exist Stage
• When a client uses an EJBHome to obtain a remote interface to a bean,
the container responds by creating an EJB Object, this is the ready state.
• Every method invocation on stateless session bean operates
independently, since it does not maintain a state.


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• Lifecycle of EJBs (Contd.)
• Lifecycle stages of stateless session beans


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• Lifecycle stages of a stateful session bean


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• In the lifecycle of a stateful session bean:
• The client initiates the lifecycle by invoking the create() method.
• The EJB container instantiates the bean and then invokes the
setSessionContext() and ejbCreate() methods in the session
bean.
• While in the ready state, EJB container may decide to passivate,
the bean by moving it from memory to secondary storage.
• If a client invokes a business method on the bean while it is in the
passive stage, the EJB container activates the bean, moving it
back to the ready stage.
• At the end of the lifecycle, the EJB container calls the bean’s
ejbRemove() method. The bean instance is ready for garbage
collection.


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• Lifecycle of entity beans
• The three stages in the lifecycle of an entity bean are:
• Pooled: In this stage, the EJB container creates the instances
of an entity bean.
• Ready: In this stage, an entity bean is activated.
• Does Not Exist: In this stage, an entity bean is
uninstantiated.


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• Lifecycle of an entity bean


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• In the lifecycle of entity bean:
• EJB container creates the instance, and it calls the
setEntityContext() method of the entity bean class to set the
context of the bean.
• After instantiation, the entity bean moves to a pool of available
instances.
• There are two ways to move from pooled stage to ready stage:
• Client invokes the create() method, causing EJB container to
call the ejbCreate() method.
• EJB container may call the ejbActivate() method.


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• Business methods are invoked in the ready stage.
• EJB container invokes ejbPassivate() method in order to move
the bean instance to Pooled state.
• At the end of the lifecycle, EJB container removes the instance
from the pool and invokes the unsetEntityContext() method.


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• Lifecycle of message driven beans
• Message-driven beans have a shorter lifespan as compared to the
session and entity beans because they remain active only while
receiving messages.
• The lifecycle of a message-driven bean contains two stages, Ready
and Does Not Exist.
• The message-driven beans are similar to stateless session beans in
that they do not have a passive stage.


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• Lifecycle of a message-driven bean


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• In the lifecycle of message-driven bean:
• At the start of its lifecycle, a message-driven bean is in the
Ready stage.
• In this stage, the message-driven bean instance remains in
pool to service the messages sent by clients.
• A single instance of a message-driven bean can only process
one message at a time.


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• To add a new message-driven bean instance to the pool, EJB
container performs the following steps:
• Call the setMessageDrivenContext() method to pass
the context object to a message-driven bean instance.
• Call the ejbCreate() method of the instance to initialize
the message-driven bean.
• In the message-driven bean lifecycle, the onMessage() method
is called whenever a message arrives from the client.
• The EJB container invokes the ejbRemove() method to remove
a message-driven bean instance from the pool.


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More on EJBs
• Stateful Session Beans Vs Stateless Session Beans
• Session beans can be used as a façade of entity beans.
• The following list shows the comparison between stateless and stateful
session beans:
• Client state: Stateless session beans cannot retain client state
between various client requests. Whereas, a stateful session bean
can retain client state between various client requests. Both,
stateless and stateful session beans remain in use as long as the
client session continues.
• Pooling: Since stateless session beans do not retain client state, an
EJB container takes advantage of this fact and pre-creates the
instances of a stateless session beans. A single stateless session
bean can process multiple client requests. Stateful session beans
retain client state therefore, it is difficult to pool a stateful session
bean. The EJB container saves the conversational state in a file
and later on retrieves the information from the file.


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More on EJBs (Contd.)

• Stateful Session Beans Vs Stateless Session Beans (Contd.)
• Lifecycle: Stateless session beans have only two states; does not exist
and ready. Whereas, a stateful session bean has three states does not
exist, pooled and ready. An EJB container is responsible for switching a
bean between various states.
• Activation and Passivation: An EJB container does not invoke
ejbActivate() and ejbPassivate() methods on a stateless session
bean. On the other hand, ejbActivate() and ejbPassivate() methods
are called by EJB container on a stateful session bean, for saving and
retrieving conversational state respectively from a file.


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• Stateful Session Beans Vs Stateless Session Beans (Contd.)
• ejbCreate(): There is only one ejbCreate() method in stateless
session bean that does not take any parameter. The reason is, if you
pass parameters to ejbCreate() method, the parameter values would
not be available to other methods. Therefore, it is of no use to pass
parameters to stateless session beans ejbCreate() method. On the
other hand, in stateful session bean you can declare as many
ejbCreate() methods as you want. You can pass any number of
parameters in ejbCreate() method, because the parameter values are
also available to other methods depending on the scope of the
parameters.
• Application: A stateless session can be used where a client session
spans up to a single method call. For, example verifying a customers
credit card. You can use a stateful session bean, where a client session
spans more than a single method call. For example, verifying customers
bank account number and process other transactions.


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• Session Beans Vs Entity Beans
• Session beans are processes that handle client request, whereas entity
beans contain data information.
• You can use a session beans to process the compression of a video file.
Whereas, an entity bean can be used to represent the bank account
information.
• The following list shows the comparison between session beans and
entity beans:
• Session beans represent a process that performs some business
logic for the client. Whereas, an entity bean represents core
business data.
• Entity beans are persistent objects that are stored in a permanent
storage such as a database. Whereas, session beans lifetime spans
as long as the client’s session is in continuation.


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• Session Beans Vs Entity Beans (Contd.)
• A session bean is coupled with a particular client while multiple clients
can share entity beans.
• Session beans do not survive system failure whereas an entity bean can
survive system failure.
• A session bean is created when a client request is encountered and
destroyed when the client request ends. Whereas entity beans are
persisted across multiple systems.
• An entity bean can be found if it already exists, whereas a session
cannot be found.
• You can modify an entity bean by directly accessing and updating the
underlying database. However, you cannot do the same with session
beans.


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• Applicability, Maintaining Business Process, and States of EJB Components
• The various EJB components are session beans, entity beans, and
message driven beans.
• Session Beans
• Session Beans are used whenever you want to write business logic
in an enterprise bean.
• Session beans always hide their identity and remain anonymous
unlike entity beans, which expose their identity by using primary
keys.
• A session bean can be used as a client of an entity bean.
• A session bean shields the complexity of an entity bean and
increases the response time of the application by locally accessing
the entity bean.
• In a session-entity façade scenario, the session bean can execute
create, read, update, and delete operations on behalf of remote
clients.


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(Contd.)
• Entity Beans
• Are used when you want to represent a database record in an
enterprise bean object.
• It is always a better approach to wrap a database record in an
entity bean object and perform database operations on the object,
instead of directly working on the database records.
• Are deployed in an application server, which provides transaction
services to the entity beans, in order to execute database
operations in a reliable manner.


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(Contd.)
• Message Driven Beans
• Message driven bean are message listeners.
• A message driven bean asynchronously receives messages from a
Messaging server.
• The supplier of the message can be a JMS application, an
enterprise bean, or a standalone application.
• You can use message driven beans to automatically deliver
messages in order to avoid polling of messages.
• A message driven bean can be used as a trigger, as you can create
message selectors in a message driven bean to handle only
specific messages.


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(Contd.)
• Maintaining Client Session State
• Maintaining client session state refers to the saving of client state
between various client requests.
• In EJB, a stateful session bean is used to maintain client session
state.
• A session bean uses serialization the client’s state in a file.
• Whenever another client request is encountered, the session bean
instance retrieves the state from the file and processes the client
request.


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(Contd.)
• Maintaining Presentation Process
• A presentation process is a method of handling the data retrieved
from a business process and transforming that data in a form that
is suitable for rendering at the client end.
• In J2EE, the presentation logic is handled by servlets and JSPs.
• In situations, where there is simple presentation logic and Web
interface is absent, a session bean can be used for handling
presentation process.


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(Contd.)
• Maintaining Business Process
• A business process consists of computational business logics. In
J2EE, session beans are used to handle business processes.
• Servlets, JSPs, and entity beans are not suited for performing
business processes.
• Both, stateless and stateful session beans, are able to handle
business processes.


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(Contd.)
• Performing Asynchronous Communication
• There are situations, when a client does not require a quick action
on its request.
• In such applications, you can use asynchronous messaging to
invoke methods on a remote object.
• Asynchronous messages do not block network traffic by waiting for
a response.


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• Using DAO for Entity Beans
• Data Access Objects (DAO) contain the logic to access a data source,
such as a database.
• They are used to separate the business logic from data access logic.
• The applications that use DAO objects are flexible, as they do not
directly access the data source for performing regular database
operations.
• Advantages of Using DAO with BMP Entity Beans are:
• Encapsulates the functionality of accessing, inserting, and updating
a database.
• Stores data retrieved by DAO in a value object, which is used to
hold values.


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• Using DAO for Entity Beans (Contd.)
• The various advantages of using DAO with an enterprise component
are:
• Making your BMP entity beans independent of database, as all the
database access logics reside in the DAO. In future, if there is a
change in database, you do not have to rewrite your entity bean
code to accommodate the changes.
• Separating business logic from data logic, thereby increasing the
readability and portability of the application.
• Centralizing the data access logic in a DAO so that it can be used
with different enterprise components.
• Providing easier migration from Bean Managed Persistence to
Container Managed Persistence, as data access logic is handled by
DAO.


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• EJB Local and Remote Models
• With EJB 2.0 specification, Sun has introduced local interfaces that can
be used instead of remote interfaces.
• The reason behind introducing local interfaces is that sometimes there
is no requirement for remotely invoking an enterprise component using
RMI/RMI-IIOP calls.
• Applicability of Both Models
• In EJB, both remote and local models can be used simultaneously.
• The remote model is used, where enterprise components are
placed in different application servers.
• The remote model enables you to remotely call methods on an
enterprise component.
• The local model can be used in situations where both the
enterprise component and their clients are to be placed in the
same address space.


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• EJB Local and Remote Models (Contd.)
• Comparison of Both Models
• The various advantages of local model are:
• Reduces the requirement of marshalling and unmarshalling of
data
• Reduces the requirement of stub and skeleton objects.
• Increases the response time of the application.
• The various disadvantages of local model are:
• They do not allow parameters to be passed by value.
• You cannot remotely invoke the methods of an enterprise
component that is using the local model.


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• EJB Local and Remote Models (Contd.)
• The various advantages of remote model are:
• Enabling an enterprise component to communicate with remote
clients.
• Enabling you to pass parameters by value.
• Enabling you to develop clients without the need to know where
the bean is located.
• The various disadvantages of remote model are:
• It is very slow to create beans through home interface, and calling
beans through interface is very slow.
• It is also necessary to generate stub and skeleton objects.
• Invoking remote object through stub is comparatively slow and it
increases the response time of the application.


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EJB Transaction Management
• A transaction is a single unit of work that consists of a sequence of
operations.
• There are two approaches of controlling transactions in EJB
• Distributed transactions: Spans over a multi-tier deployment, which
involves various participants. In a distributed transaction, if any
operation fails even on a single machine, the whole transaction is
aborted.
• Local transactions: Are performed within an application server.
• A transaction management system is responsible for managing transactions.
• The various components of the transaction management system are the
business logic components, transaction manager, and application server.


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EJB Transaction Management (Contd.)
• Container Managed Transactions
• EJB container is responsible for handling all method invocations in a
transaction.
• EJB container starts a transaction as soon as a bean method starts
executing. The EJB container commits or aborts the transaction just
before bean method completes execution.
• A transaction attribute in a container-managed transaction defines,
which methods are part of a transaction.
• Advantage of using a container-managed transaction is that you are not
required to write the complex transaction management code.
• You only need to specify transaction attributes in the deployment
descriptor file, in order to start, commit, or rollback a transaction.
• You can change the way transactions are handled by the container,
without accessing the source code.


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• Container Managed Transactions (Contd.)
• Transaction Attributes
• With container-managed transactions, we specify transaction
attributes for our enterprise bean in a file.
• EJB container uses transaction attributes to control transaction
processing.
• Transaction attributes specify, which methods can be part of a
transaction and how to control their execution.


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• The various transaction attributes are:
• Required: Joins an exiting transaction, if any, otherwise creates a
new transaction.
• RequiresNew: Always creates a new transaction for an enterprise
bean.
• Mandatory: Mandates that a transaction must be running before an
enterprise bean’s method is invoked.


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• Supports: Joins an already running transaction. If no transaction is
running, the container does not start a new transaction before running
the method.
• NotSupported: Does not allow a bean to run under a transaction.
Suspends an already running transaction and resumes it only after
enterprise bean’s method completes execution.
• Never: Does not allow a bean to run under a transaction. Throws
exception, if a transaction is already running.


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• Rolling Back a Transaction Using CMT
• Rolling back a transaction refers to aborting a transaction before
completion and undoing all the changes made by successful
operations in the transaction.
• A transaction can be rolled back in two ways.
• First, by explicitly calling setRollbackOnly()method.
• Second, container will roll back a transaction, whenever an
exception is thrown.
• However, if a bean throws an application exception then the
container does not rollback the transaction.
• In such situation you need to explicitly call the setRollbackOnly()
method to rollback the transaction.


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• The javax.ejb.SessionSynchronization Interface
• You can implement the javax.ejb.SessionSynchronization
interface only for stateful session beans, which use declarative
transactions for demarcating transactional boundaries.
• Stateful session beans use this interface to rollback a transaction,
when the transaction fails.
• The javax.ejb.SessionSynchronization interface has following
methods:
• afterBegin(): Indicates that a new transaction has started.
• beforeCompletion(): Is invoked before the transaction is
committed and after the operations in the current transaction
has completed.
• afterCompletion(): Indicates that the current transaction
has been completed.


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• Bean Managed Transactions
• With bean managed transactions you have to explicitly code transaction
for your bean. For that you will have to use Java Transaction API (JTA)
and JDBC.
• You can use bean-managed transactions in a session bean or message
driven bean only.
• Transactions in a message-driven bean should end before the
onMessage() method finishes execution.
• Entity beans do not have choice to use bean-managed transactions.
• In a stateful session bean, a bean-managed transaction can maintain
the transactional state across several method invocations.
• A bean-managed transaction can be used to support compensating
transactions.
• A compensating transaction is a transaction, which can be rolled back
even after being committed.


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• Bean Managed Transactions (Contd.)
• JDBC Transactions
• JDBC enables you to implement a bean-managed transaction.
• The JDBC transactions defined in an enterprise bean are controlled
by DBMS transaction manager.
• It implicitly starts a new transaction for the first SQL statement
that follows the commit, rollback, or connect statement.
• The transaction methods used in JDBC transactions are defined in
the javax.sql.Connection interface.


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• A bean method invokes the commit() or rollback() method of
the javax.sql.Connection interface to commit or rollback a JDBC
transaction.
• In JDBC, all the transactions are auto-committed. If at any stage,
you want to rollback a transaction, then you have to set the auto-
commit property of a transaction to false.
• You can do that by invoking setAutoCommit(boolean
autoCommit) of javax.sql.Connection interface and passing
false to the method as a parameter.


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• JTA Transactions
• J2EE transaction manager enables you to use JTA in an enterprise
bean or client code, for controlling transaction boundaries
programmatically.
• JTA enables you to start a transaction in a bean code, make other
beans part of the transaction, and commit or abort the transaction.
• JTA contains the javax.transaction.UserTransaction interface
that is used to programmatically control transactions.
• The getUserTransaction() method returns a reference of the
javax.transaction.UserTransaction interface. The various
methods in the javax.transaction.UserTransaction interface are:
• begin(): Starts a new transaction.
• commit(): Saves the changes made by a transaction.
• rollback(): Undoes the changes made by a transaction.
• setRollbackOnly(): Marks the current transaction for roll back.

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• You need to adhere to the following rules in implementing the
javax.transaction.UserTransaction interface:
• Transactions in an entity bean do not use the
javax.transaction.UserTransaction interface.
• Transactions in a stateless session bean method should end before
the method finishes execution. Failure in ending a transaction
leads to transaction rollback and the exceptions, RemoteException
or EJBException, are thrown.


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Summary
In this lesson, you learned:
• Services provided by an EJB container are bean pooling, security,
passivation, and bean lifecycle management in order to maximize the
use of resources.
• The container calls various methods of an enterprise bean in order to
service client requests and switch a bean among various states of bean
lifecycle, such as does not exists, pooled, and running.
• Stateless session beans do not hold conversational state on behalf of a
client. Whereas, stateful session bean retain client state between
various client requests.
• Session beans are business workflow applications whereas entity beans
are data objects.


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Summary (Contd.)

• Data Access Objects encapsulates the logic to insert, delete, or update the
database. The DAO provides a clear separation between data logic and
business logic. Using DAO you can make your BMP entity bean database
independent.
• Container Managed Transactions are used to declaratively, handle the
transactional boundaries of a transaction. You declare a bean’s transactional
requirements in the deployment descriptor file. The container reads the file
and provides the required transaction services.
• Bean Managed Transactions are used to programmatically, handle the
transactional boundaries of a transaction. You use JDBC and JTA to
programmatically handle the transactions of a bean.


EJB Technology Presentation

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