Enterprise Java Beans

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Enterprise Java Beans

• Introduction
• Application Server
• Java 2 Enterprise Edition
• What is an Enterprise Bean ?
• EJB Properties
• EJB Overview
• Deployment Phase
• Type of beans
• Client access with interfaces
• Remote access
• Local Access

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Enterprise Java Beans

• Contents of an Enterprise Bean
• EJB Example
• EJB vs MTS
• A few EJB implementations
• Whats new in EJB 2.0
• Bibliography

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Introduction

• Enterprise Java Beans ( EJB ) is
• a middleware component model for Java and CORBA
• a specification for creating server-side,
  scalable, transactional, multi-user and secure
  enterprise-level applications
• Presented by Sun in the 1999, they are easier
  than other technologies as RMI or Corba

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Introduction

• This is the three level structure for Application
  Server

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Applicaton Server

• Presentation
• HTML Application
• Java Application
• Business Logic
• Data Access

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Presentation

• HTML
• Generated server-side HTML
• Runs on any Web browser
• Less client-side power

• Java
• Required Java virtual Machine
• More client side power
• Runned on a page
• Launched from a browser or a standalone
  application

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Business Logic

• Implements the logic of the application defining
  all the function that may be used from a client
• Change Business Rules Easily
• Re-use components
• Make complex applications manageable

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Data Access

• Utility to access external datas such as Database
  or other Web component

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J2EE Application Server

• Java 2 Enterprise Edition standardizes interfaces
  for Application Server components

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What is an Enterprise Bean ?

• Is a server side component written in Java
  Language
• Industry standard distribuited component model
• Incorporates the business logic of an application
  ( the code that implements the purpose of the
  application)

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EJB Properties

• Bean writers need not write
• Remote access Protocols
• Transactional Behaviour
• Threads
• Security
• State Management
• Object life cycle
• Resource pooling
• Persistence

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EJB Overview
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Deployment Phase
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Deployment Phase
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When to use Enterprise bean

• The application must be scalable.It will run on
  different machine and their location will remain
  transparent to the client
• Transaction requirement
• The application will have lot of different type
  of clients

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Type of beans

• Session Bean
• Entity Bean
• Message Driven Bean

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Session Bean

• Represents a single client inside the server
• The client calls the session bean to invoke
  methods of an application on the server
• Perform works for its client, hiding the
  complexity of interaction with other objects in
  the server
• Is not shared
• Is not persistent
• When the client stops the session,the bean can be
  assigned to another client from the server

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Session Bean

• Stateful session bean
• Stateless session bean

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Stateful Session Bean

• Contains the state of a single client session
• Information on the client
• On method called
• Return values
• This state is called conversational state and is
  not retained when the session ends, also if the
  client not removes the bean

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Stateless Session Bean

• Not maintain a conversational state for a
  particular client
• Contains values only for the duration of the
  single invocation
• Except during method invocation, all instances of
  stateless session bean are equivalent

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Entity Bean

• Represents a business object in a persistent
  storage mechanism such as a relational database
• Usually is a table in the database and each
  instance of that entity bean is a row in that
  table
• Properties
• Persistent
• Allow shared access
• Have primary key
• Have relationship with other entity beans.

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Entity Bean persistent

• Bean managed persistence
• Container managed persistence

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Bean managed persistence

• Who write the beans code must access the
  database and save his own data

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Container managed persistence

• The container save the data
• There is no code in the bean for access the
  database
• The container handles all database access
  required for the bean
• Links between beans are created using a structure
  called abstract schema

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Entity beans shared access

• Entity beans can be used by different clients
• Its important that they work whithin
  transactions
• The EJB container provides transaction management
  
• The transactions attribute are specified in the
  beans deployment description

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Entity beans primary key

• Each entity bean has a unique object identifier
  like a key in a database table

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Entity beans relationship

• Container managed persistent
• The container performs all the operation to
  create relationship
• Bean managed persistent
• The code to perform relations must be written in
  the bean

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Message Driven bean

• Allows applications to process messages
  asynchronously
• The messages may be sent by
• An application client
• Another enterprise bean
• A Web component

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Message Driven bean

• Retain no data or conversational state for a
  specific client
• All instances are equivalent, allowing the EJB
  container to assign a message to any
  message-driven bean instance. The container can
  pool these instances to allow streams of messages
  to be processed concurrently
• Can process messages from multiple clients

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Message Driven bean

• A client cant access directly to a message
  driven bean
• When a message arrive, the container gives it to
  a message driven bean
• The bean process the message

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Client access with interfaces

• A client may access a session or an entity bean
  only through the methods defined in the bean's
  interfaces
• They define the client's view of a bean
• Types of access
• Remote access
• Local access

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Remote access

• A remote client of an enterprise bean has the
  following traits
• It may run on a different machine and a different
  Java virtual machine than the enterprise bean it
  accesses (It is not required to run on a
  different JVM )
• It can be a Web component
• It can be another enterprise bean

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Remote access

• To create an enterprise bean with remote access,
  you must
• Code a remote interface
• Business methods
• Code a home interface
• Finder methods
• Home methods

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Remote access example
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Local access

• A local client has these characteristics
• It must run in the same JVM as the enterprise
  bean it accesses
• It may be a Web component or another enterprise
  bean
• To the local client, the location of the
  enterprise bean it accesses is not transparent
• It is often an entity bean that has a
  container-managed relationship with another
  entity bean

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Local access

• To create an enterprise bean with local access,
  you must
• Code the local interface
• Bean's business methods
• Code the local home interface
• Life cycle
• Finder methods

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Local interfaces

• If an entity bean is the target of a container
  managed relationship it MUST have local
  interfaces

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Contents of an Enterprise Bean

• Deployment descriptor
• Persistence type
• Transaction attribute
• Enterprise bean class
• Interfaces
• Helper classes
• Exception
• Utility classes

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EJB Example

• The OnLine Bank
• We will take a not completed system to give an
  idea to how choose if a component is an entity,
  session or message driven bean.

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EJB Example
 
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EJB Example

• The example has three component
• Services what the client can do in the system
  such as see the foreign currency , listed shares
  or make operations on his hown account.
• Accounts a database containing the accounts of
  all the clients of the bank with information
  about credit,debit,access etc..
• Security is a subsystem that receives all the
  alarm caused from wrong access and performs
  action about the situation
• ( calls police and stops operation of that
  client keeping information about him )

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EJB Example

• In this example is easy to create an EJB
  structure.
• Client will have a web page at client side to
  insert values and connect the system.This will be
  done using JSP ( Java Servlet Pages )
• Services will be a Statefull Session Bean and it
  will be different for each client connecting the
  system mantaining data about the client
  connected.
• Accounts will be formed by an Entity Bean for
  each account in the system with a code-account as
  primary key.
• Security will be a Message driven bean and will
  be called only from container if some operation
  are abnormal for result or the autentification
  for the same client fails too much times.

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EJB vs MTS

• Microsoft Transaction Server is based on the
  Component Object Model (COM) which is the
  middleware component model for Windows NT
• MTS can be defined as a component-based
  programming model

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EJB vs MTSAnalogies

• Implement business logic for Application Server
  components
• Have a Server and a container ( for MTS called
  MTS Executive )
• Similar architecture in both models
• A client invokes wrapped method

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EJB vs MTSDifference

• EJB
• Component instance are pooled
• Dont support heterogeneous transactions
• Portability accross multiple platforms using Java
  platform

• MTS
• Component is not created until the call from a
  client reaches the container
• Support heterogeneous transactions
• Portability only on Windows NT

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EJB vs MTSDifference

• EJB
• Invoked by clients using RMI
• Has both persistent and non-persistent components

• MTS
• Invoked by clients using DCOM or through local
  COM calls
• Components are not persistent, even though they
  may contain information

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A few EJB implementations

• WebLogic
• Bluestone
• Novera
• Persistence
• Oracle AS
• Oracle8i

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Whats new in EJB 2.0

• Released On April 26, 2001
• Integration with JavaTM Message Service (JMS) --
  Asynchronous Capabilities Streamline Systems
• Send asynchronous messages via the JMS API
• Container-Managed Persistence (CMP) --
  Simplifying and Expediting Application
  Development
• Used to isolate the application developer from
  the physical database schema
• Introduces for the first time a portable query
  language, based on the abstract schema

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Whats new in EJB 2.0

• Local Interfaces -- Streamlining Calls Between
  Local Beans
• The local interface may be defined for a bean
  during development, to allow streamlined calls to
  the bean if a caller is in the same container
• Inter-Server Interoperability -- Enabling
  Heterogeneous Environments
• Takes the benefit of cross-server application
  portability
• Able to deploy the EJB technology-based
  application across a heterogeneous environment
  mixing application servers from different vendors

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Bibliography

• The J2EE Tutorial
• http//java.sun.com/j2ee/tutorial/1_3-fcs/doc/EJBC
  oncepts.html
• Meeting about Application servers
• http//www.phxjug.org/meetings/silverstream/sld001
  .htm
• Developing Enterprise components
• http//spectral.mscs.mu.edu/EJB20001113/index.html
  
• A detailed Comparison of EJB MTS models
• http//members.tripod.com/gsraj/misc/ejbmts/ejbmts
  comp.html

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Bibliography

• What's new in the Enterprise JavaBeansTM 2.0
  Specification?
• http//java.sun.com/products/ejb/2.0.html
• Introduction to Enterprise JavaBeans
• http//cosmos.inesc.pt/a01/javacourses/ejb/
• Programming WebLogic Enterprise JavaBeans
• http//e-docs.bea.com/wls/docs61/ejb/index.html

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The MTS architecture

• Is made up of
• The MTS Executive (mtxex.dll)
• The Factory Wrappers and Context Wrappers for
  each component
• The MTS Server Component
• MTS clients
• Auxiliary systems like
• COM runtime services,
• Service Control Manager (SCM)
• The Microsoft Distributed Transaction Coordinator
  (MS-DTC)
• The Microsoft Message Queue (MSMQ)
• The COM-Transaction Integrator (COM-TI)

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The MTS architecture
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The EJB architecture

• Consists of
• An EJB server
• EJB containers that run within the server
• Home objects
• Remote EJBObjects
• Enterprise Beans
• EJB clients
• Auxiliary systems like
• Java Naming and Directory Interface (JNDI)
• Java Transaction Service (JTS)
• Security services

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The EJB architecture
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Stateful session beans life cycle

• The client invoke the create method
• The EJB container
• Instantiates the bean
• Invokes the setSessionContext
• Invokes ejbCreate
• The bean is ready

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Stateful session beans life cycle

• While in the ready state
• EJB container may passivate the bean moving it
  from memory to secondary storage
• A client may invoke a business method
• EJB container may activate a bean,moving it back
  to the ready stage, and then calls the bean's
  ejbActivate method
• A client may invoke the remove method and the
  container calls the bean's ejbRemove method

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Stateful session beans life cycle
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Stateless session beans life cycle

• The client invoke the create method
• The EJB container
• Instantiates the bean
• Invokes the setSessionContext
• Invokes ejbCreate
• The bean is ready

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Stateless session beans life cycle

• While in the ready state
• A client may invoke a business method
• A client may invoke the remove method and the
  container calls the bean's ejbRemove method
• Its never passivate

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Stateless session beans life cycle
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Entity beans life cycle

• The EJB container
• Creates the instance
• Calls the setEntityContext
• The entity bean moves to a pool of available
  instances

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Entity beans life cycle

• While in the pool
• Instance is not associated with any particular
  object identity
• All instances in the pool are identical
• EJB container may assign an identity to an
  instance when moving it to the ready stage
  invoking the ejbActivate method
• A client may invoke the create method
• EJB container calls ejbCreate and ejbPostCreate
• EJB container may remove the instance invoking
  unsetEntityContext

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Entity beans life cycle

• While in the ready state
• A client may invoke entity bean's business
  methods
• A client may invoke the remove method
• EJB container calls the ejbRemove method
• EJB container may invoke the ejbPassivate method

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Entity beans life cycle
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Message driven beans life cycle

• EJB container creates a pool of message-driven
  bean instances
• For each instance, the EJB container instantiates
  the bean
• It calls the setMessageDrivenContext
• It calls the instance's ejbCreate
• Like a stateless session bean,its never
  passivated, It has only two states
• Nonexistent
• Ready to receive messages.

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Message driven beans life cycle

• While in the ready state
• EJB container may call onMessage
• EJB container may call the ejbRemove

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Message driven beans life cycle
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Abstract schema

• Part of an entity bean's deployment descriptor
• Defines the bean's persistent fields and
  relationships.The term abstract distinguishes
  this schema from the physical schema of the
  underlying data store
• You specify the name of an abstract schema in the
  deployment descriptor

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Example of Abstract schema
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Abstract schema

• Persistent fields
• Are stored in the underlying data store
• Constitute the state of the bean. At runtime, the
  EJB container automatically synchronizes this
  state with the database
• During deployment, the container
• Maps the entity bean to a database table
• Maps the persistent fields to the table's columns

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Abstract schema

• Relationship fields
• Its like a foreign key in a database table.It
  identifies a related bean
• Like a persistent field, a relationship field is
  virtual and is defined in the enterprise bean
  class with access methods
• Unlike a persistent field, a relationship field
  does not represent the bean's state

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Abstract schema

• Multiplicity in Container-Managed Relationships
• One-to-one Each entity bean instance is related
  to a single instance of another entity bean
• One-to-many An entity bean instance may be
  related to multiple instances of the other entity
  bean
• Many-to-one Multiple instances of an entity bean
  may be related to a single instance of the other
  entity bean
• Many-to-many The entity bean instances may be
  related to multiple instances of each other

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Abstract schema

• Direction in Container-Managed Relationships
• Bidirectional relationship each entity bean has
  a relationship field that refers to the other
  bean. Through the relationship field, an entity
  bean's code can access its related object
• Unidirectional relationship only one entity bean
  has a relationship field that refers to the other

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EJB 3.0

• This section based on
• Mastering EJB 3.0 4th Ed

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EJB 3.0

• Key issues for distributed systems
• Remote Method Invocation
• Load Balancing
• Transparent failover
• Back-end integration to legacy systems
• Transactions
• Clustering (state replication across servers)
• Dynamic redeployment (upgrade while running)

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EJB 3.0

• More key issues
• Clean shutdown
• Logging auditing
• Systems management
• Threading
• Message-oriented middleware
• Component lifecycle
• Resource pooling

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EJB 3.0

• Yet more issues
• Security
• Caching
• Etc., etc.

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EJB 3.0

• All of these can be thought of as middleware
  services
• Developer of distributed app shouldnt have to
  worry about them
• Should concentrate on business logic
• Much too expensive to build from scratch
• Distributed app should be portable across
  middleware service providers

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EJB 3.0

• EJB spec defines standard interfaces between
  distributed components their containers

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EJB 3.0

• Advantages
• Industry standard
• Vendors design containers to EJB specs
• Portability possible across containers
• RAD capable because services provided by
  container

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EJB 3.0

• Whats an EJB?
• Server-side component encapsulating application
  functionality
• Business logic
• DB access
• Integrate with other legacy systems
• Serve multiple clients
• Browser
• Desktop apps
• Other EJBs
• Web app components (e.g., Servlets)
• Web Service clients

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EJB 3.0

• How do they work?
• EJB can be located anywhere, usually on another
  machine
• Use RMI-IIOP for remote invocation of methods (P.
  15)
• Location transparency
• Client thinks remot EJB is local
• Run inside EJB container
• Application servers

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EJB 3.0

• Who makes containers?
• Most of the usual enterprise suspects
• BEA, IBM, Red Hat (JBOSS), Sun, Oracle
• Closely related to web services
• P. 29

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EJB 3.0

• EJB 2.1 was NASTY!!
• HelloWorld EJB 2.1 (pp. 40-49)
• Many disappointed enterprise developers
• Vastly simplified in EJB 3.0
• HelloWorld EJB 3.0 (pp. 83-85)
• Many once burned twice shy developers
• Suns dilemma
• If we simplify it, will they resume coding EJBs?

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EJB 3.0

• EJB best practices
• For appropriate use only
• Remoting is required
• Distributed transactions required
• Component security required
• Persistence required
• Legacy system integration
• Scalability required

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EJB 3.0

• If your application components dont need to be
  distributed across servers, avoid using
  distributed components like EJBs

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EJB 3.0

• This section based on
• J2EE Development Without EJB

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EJB 3.0

• Written by a once burned twice shy EJB
  developer
• Observations apply to EJB 2.1 (and earlier)
  experiences
• EJB 3.0 spec in early stages when book written

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EJB 3.0

• Basic thesis is EJBs.
• Rarely necessary for web-based applications
• Commonly overused
• Symptom of over-engineering
• Can obtain most of needed EJB services via other
  programming techniques using lightweight
  containers (e.g., Spring)
• Java EE Java SE
• Aspect-oriented programming
• Dynamic proxies

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EJB 3.0

• Hes not alone
• Many other enterprise architects and developers
  have critized EJBs since early 2000s
• Martin Fowlers Law of Distributed Objects
• Dont do it!!

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EJB 3.0

• Some criticisms
• Too complex
• Specifications getting more complex
• EJB 3.0 tries to address this
• Unit testing hard to do
• Have to isolate app from container
• If something broken, is it container related or
  app related?

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EJB 3.0

• Other technologies (AOP) get the job done more
  easily
• Entity beans are a flop
• EJB 3.0 doesnt upgrade entity beans
• Left to wither on the Java EE vine
• Persistence API tries to address this

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EJB 3.0

• Metadata (a la .NET) are better than descriptor
  files
• Annotations designed to address this
• Only truly unique service provided by EJB is
  remote access to a component
• Java EE provides other services already
• JNDI, JTA, JCA, resource pooling, etc.

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EJB 3.0

• EJBs remain best choice for apps that genuinely
  need object distribution or use IIOP protocol
• EJBs also pretty good for apps based around
  asynchronous messaging
• Message-drive beans pretty simple

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EJB 3.0

• Financial services middleware likely good place
  for EJB-driven apps
• Argues that EJBs are declining technology will
  be legacy within 3 years
• Doesnt think EJB 3.0 will make much of a
  difference

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EJB 3.0

• Advocates use of lightweight containers like
  Spring.
• Oh btw, author is co-founder of Spring framework!
• Bias is always an issue!

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EJB 3.0

• Whats an enterprise developer to do?
• Distributed systems a holy grail of system
  architecture deployment since early 90s
• CORBA initiative began in 91
• Web Services are current frontier
• IT profession still learning when how to do
  distributed systems

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EJB 3.0

• References
• Master Enterprise JavaBeans 3.0, 4th Ed,
  Sriganesh et. Al., 2006
• J2EE Development Without EJB, Johnson, 2004