Overview of EJB

1
Overview of EJB

• The core of transaction J2EE applications

2
What Well Cover

• What Is an Enterprise Bean?
• Benefits of Enterprise Beans
• When to Use Enterprise Beans
• Types of Enterprise Beans
• The Contents of an Enterprise Bean (OOO)
• Naming Conventions for Enterprise Beans (OOO)
• The Life Cycles of Enterprise Beans (OOO)

3
What Is an Enterprise Bean?

• Server side component that encapsulates some
  business logic
• Business logic code that fulfills the
  applications purpose
• Example EJBs implement inventory controls
• checkInventoryLevel
• orderProduct
• Remote clients can access these services

4
Benefits of Enterprise Beans

• Simplify the development of large distributed
  applications
• EJB container provides system level services
  (like JTA and Security)
• Beans contain business logic client developer
  makes thin (presentation only) clients
• Beans are portable assemblers can put together
  new applications from existing beans

5
When to Use Enterprise Beans

• If the application must be scalable (can be moved
  to multiple servers transparently)
• Transactions are required to insure data
  integrity
• Application requires a variety of clients

6
Types of Enterprise Beans

• Session - Performs a task for a client
  implements a Web service
• Entity - Represents a business entity object
  that exists in persistent storage
• Message-Driven - Acts as a listener for the Java
  Message Service API, processing messages
  asynchronously

7
The Contents of an Enterprise Bean

• The following files are required for EJBs
• Deployment descriptor An XML file that specifies
  information about the bean such as its
  persistence type and transaction attributes.
• Enterprise bean class Implements the methods
  defined in the following interfaces
• Interfaces remote/local/Web service
• Helper classes Other classes needed by the
  enterprise bean class, such as exception and
  utility classes

8
Naming Conventions for Enterprise Beans

• Enterprise bean name (DD) EJB AccountEJB
• EJB JAR display name (DD) JAR AccountJAR
• Enterprise bean class Bean AccountBean
• Home interface Home AccountHome
• Remote interface Account
• Local home interface LocalHome
  AccountLocalHome
• Local interface Local AccountLocal
• Abstract schema (DD) Account

9
The Life Cycles of Enterprise Beans

• Life cycles are managed by the J2EE server
• Life cycles vary between types of EJBs
• Most life cycle events are reflected by call back
  methods in the EJBs

10
What We Covered

• What Is an Enterprise Bean?
• Benefits of Enterprise Beans
• When to Use Enterprise Beans
• Types of Enterprise Beans
• The Contents of an Enterprise Bean (OOO)
• Naming Conventions for Enterprise Beans (OOO)
• The Life Cycles of Enterprise Beans (OOO)

11
Session Beans

• EJB Workhorse

12
What Well Cover

• What Is a Session Bean?
• State Management Modes
• When to Use Session Beans
• The Life Cycle of a Stateful Session Bean (OOO)
• The Life Cycle of a Stateless Session Bean (OOO)

13
What Is a Session Bean?

• Representative of a single client on the server
• Session bean performs work for its client
• Similar to an interactive session it is not
  shared
• It is not persistent

14
State Management Modes

• Stateful/Stateless
• Stateful instance variables (state) are
  maintained for the life of the session freed
  when terminated
• Stateless instance variables maintain state
  only for the life of a single invocation
• All instances equivalent any can go to any
  other client
• Only stateless session beans can implement Web
  Services
• May have better performance

15
When to Use Session Beans

• Use a session bean if
• one client at a time has access to instance.
• The state of the bean is not persistent.
• The bean implements a Web service.
• Use a Stateful bean if
• state represents the interaction between the bean
  and a specific client.
• needs to hold information about the client across
  method invocations.
• The bean mediates between the client and the
  other components, presenting a simplified view to
  the client.
• the bean manages the work flow of several
  enterprise beans.
• You can use stateless if
• The bean's state has no data for a specific
  client.
• the bean performs a generic task for all clients
  in a single invocation.
• The bean fetches from a database a set of
  read-only data that is often used by clients.

16
The Life Cycle of a Stateful Session Bean
17
The Life Cycle of a Stateful Session Bean
(narrative)

• client initiates the life cycle by invoking the
  create method
• The EJB container instantiates bean and invokes
  the setSessionContext and ejbCreate methods
• Bean is ready for business methods use (ready
  stage)
• While ready the EJB container may decide to
  passivate the bean by moving it from memory to
  secondary storage.
• ejbPassivate method invoked immediately before
  passivating
• ejbActivate method called before moved back to
  ready
• Client invokes remove method - container calls
  bean's ejbRemove
• Instance eligible for garbage collection
• Only create/remove called by client

18
The Life Cycle of a Stateless Session Bean

• life cycle has just two stages nonexistent and
  ready for the invocation of business methods

19
What We Covered

• What Is a Session Bean?
• State Management Modes
• When to Use Session Beans
• The Life Cycle of a Stateful Session Bean (OOO)
• The Life Cycle of a Stateless Session Bean (OOO)

20
Entity Beans

• Persistently stored business objects

21
What Well Cover

• What Is an Entity Bean?
• What Makes Entity Beans Different from Session
  Beans?
• Container-Managed Persistence
• When to Use Entity Beans
• The Life Cycle of an Entity Bean

22
What Is an Entity Bean?

• A business object in persistent storage
• Examples, customers, orders, products
• In J2EE usually stored in RDBMS
• Each bean may have an underlying table
• Each instance may correspond to a row

23
What Makes Entity Beans Different from Session
Beans?

• Entity beans are persistent
• Exists beyond lifetime of application or J2EE
  server
• Bean managed vs container managed
• Entity Beans may be shared by clients
• Since it may be changed, needs to work in
  transactions
• Each has a primary key Entity bean with the
  same key is the same bean

24
Container-Managed Persistence

• EJB container handles all database access
• abstract schema defines the bean's persistent
  fields and relationships (name specified in DD)
• persistent fields are stored in the underlying
  data store (the state of the bean)
• relationship field is like a foreign key (defines
  a related bean)
• Multiplicity one to one, one to many, many to
  one, many to many
• Direction bidirectional or unidirectional
• EJB QL often used to navigate across relationships

25
Abstract Schema (diagram)
26
CMR (Multiplicity)

• One-to-one Each entity bean instance is related
  to a single instance of another entity bean.
• One-to-many An entity bean instance related to
  multiple instances of the other entity bean.
• Many-to-one Multiple instances of an entity bean
  related to a single instance of the other entity
  bean.
• Many-to-many The entity bean instances related
  to multiple instances of each other

27
CMR - Direction

• Bidirectional each has a reference to the other
• Unidirectional one has a relationship field
  that refers to the other
• EJB QL often used to negotiate relationships
  directions dictate its ability to do so.

28
When to Use Entity Beans

• The bean represents a business entity, not a
  procedure (CreditCard, not CreditCardVerifier)
• The bean's state must be persistent

29
The Life Cycle of an Entity Bean
30
The Life Cycle of an Entity Bean (narrative)

• EJB container creates the instance
• Calls setEntityContext passing context to bean
• Moved to a pool of available instances
• Identity assigned to instance when it moves to
  the ready stage
• Two paths from pooled to ready
• Client calls create container calls ejbCreate
  and ejbPostCreate
• 2nd EJB container invokes the ejbActivate
• Two paths from ready to pooled
• Client calls remove container calls EJBRemove
• EJB container may invoke the ejbPassivate method
• End of life container calls unsetEntityContext
• With bean managed primary key not set
  automatically must be done in ejbCreate and
  ejbActivate

31
What We Covered

• What Is an Entity Bean?
• What Makes Entity Beans Different from Session
  Beans?
• Container-Managed Persistence
• When to Use Entity Beans
• The Life Cycle of an Entity Bean

32
Message Driven Beans

• Always listening

33
What Well Cover

• What Is a Message-Driven Bean?
• What Makes Message-Driven Beans Different from
  Session and Entity Beans?
• When to Use Message-Driven Beans
• The Life Cycle of a Message-Driven Bean

34
What Is a Message-Driven Bean?

• Allows J2EE applications to process messages
  asynchronously
• Acts as a JMS listener (similar to an event
  listener)
• Messages may be sent by Java client, J2EE server,
  other messaging system
• Can process JMS messages or other kinds of
  messages

35
What Makes Message-Driven Beans Different?

• Much like a stateless session bean
• instances retain no data or conversational state
• All instances are equivalent
• single bean can process messages from multiple
  clients
• Execute on receipt of client message in a queue
• Asynchronously invoked
• Short lived
• May be transaction aware
• When message arrives onMessage method called
• If part of a transaction, on rollback, message
  will be redelivered

36
When to Use Message-Driven Beans

• Session and entity beans only allow synchronous
  send/receive of messages (with MDB you shouldnt
  receive messages in them)
• Only asynchronous means is MDB

37
The Life Cycle of a Message-Driven Bean
38
What We Covered

• What Is a Message-Driven Bean?
• What Makes Message-Driven Beans Different from
  Session and Entity Beans?
• When to Use Message-Driven Beans
• The Life Cycle of a Message-Driven Bean

39
EJB Client Types

• Multiple uses for interfaces

40
What Well Cover

• Defining Client Access with Interfaces
• Remote Clients
• Local Clients
• Local Interfaces and Container-Managed
  Relationships
• Deciding on Remote or Local Access
• Web Service Clients
• Method Parameters and Access

41
Defining Client Access with Interfaces

• Does not apply to message beans
• Access thru interfaces define client access
• All else hidden (DD, implementations, etc.
• Well designed interfaces
• Shield clients from complexity
• Allow change of implementation
• Decide remote/local/web service interface

42
Remote Clients

• Traits of a remote client
• May run on different machine/JVM
• May be web component, application client or EJB
• Location of the EJB is transparent
• Must have remote/home interface
• Remote interface has business methods
• Home interface has life cycle methods create
  /remove
• Entity beans have finder methods

43
Remote Clients (diagram)
44
Local Clients

• Traits of local clients
• 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.
• Must have local interface and local home
  interface for EJBs

45
Local Interfaces and Container-Managed
Relationships

• Entity beans which are target of CMR must have a
  local interface
• Direction of relationship determines whether it
  is a target bidirectional, both must have local
  interfaces
• EJBs that are part of CMR must live in same EAR.

46
Deciding on Remote or Local Access

• CMR requires local access
• Tightly coupled beans candidate for local access
• Type of client app clients always require
  remote. Web/EJB depends on distribution
• Distributed scenarios allow remote access
• Performance local quicker than remote
• Flexibility
• Can allow both remote and local interfaces

47
Web Service Clients

• Two ways to access
• JAX-RPC Java xml remote procedure calls
• Invoke business methods on stateless session
  beans
• Correct protocols (SOAP, HTTP, WSDL) independent
  of client implementation
• Accessed through the Web service endpoint
  interface
• Not accompanied by a home interface

48
Method Parameters and Access

• Isolation
• Clients and bean operate on different copies
• Protects from client mod of data
• Even though local clients can modify same copy
  dont depend on it may need to make remote
• Granularity of data
• Remote calls should have coarser grained data

49
What We Covered

• Defining Client Access with Interfaces
• Remote Clients
• Local Clients
• Local Interfaces and Container-Managed
  Relationships
• Deciding on Remote or Local Access
• Web Service Clients
• Method Parameters and Access

50
EJB Query Language
51
What Well Cover

• Terminology
• Simplified Syntax
• Example Queries
• Full Syntax
• EJB QL Restrictions

52
Terminology

• Abstract schema definition of persistent fields
  and relationships
• Abstract schema name logical name of schema
• Abstract schema type all ejbql evaluates to a
  type- if a schema name then the local interface

53
Terminology (continued)

• Backus-Naur Form (BNF) notation for describing
  syntax
• Navigation traversing relationship .
• Path expression expression that navigates to a
  related entity bean
• Persistent field virtual field for entity bean
  stored in DB
• Relationship field virtual field defining a
  related entity bean

54
Simplified Syntax

• EJB QL query has four clauses SELECT, FROM,
  WHERE, and ORDER BY
• High level BNF - EJB QL select_clause
  from_clause where_clauseorderby_clause
• Select defines type of object
• From defines scope of query
• Where conditional that restricts objects selected
• Order By return in a particular order

55
Example Queries - Simple Finder Queries

• SELECT OBJECT(p) FROM Player p
• Data retrieved All players
• Finder method findall()
• The FROM clause declares an identification
  variable named p, omitting the optional keyword
  AS
• Player element is the abstract schema name of the
  PlayerEJB returns LocalPlayer from
  LocalPlayerHome

56
Example Queries - Simple Finder Queries
(continued)

• SELECT DISTINCT OBJECT(p) FROM Player p WHERE
  p.position ?1
• Data retrieved players with position specified
  by parameter
• Finder method findByPosition(String position)
• Description OBJECT keyword must precede a
  stand-alone variable p. DISTINCT keyword
  eliminates duplicate values. Where restricts
  player to position ?1 value.

57
Example Queries - Simple Finder Queries
(continued)

• SELECT DISTINCT OBJECT(p) FROM Player p WHERE
  p.position ?1 AND p.name ?2
• Data retrieved The players with the specified
  position and name.
• Finder method findByPositionAndName(String
  position, String name)
• Description position and name elements are
  persistent fields of the PlayerEJB entity bean

58
Example Queries - Queries That Navigate to
Related Beans

• SELECT DISTINCT OBJECT(p) FROM Player p, IN
  (p.teams) AS t WHERE t.city ?1
• Data retrieved The players whose teams belong to
  the specified city.
• Finder method findByCity(String city)
• Description p variable represents the PlayerEJB
  and t represents related TeamEJB
• P.teams navigation operator collection op and
  terminal
• t. used as single instance

59
Example Queries -Queries That Navigate to
Related Beans

• SELECT DISTINCT OBJECT(p) FROM Player p, IN
  (p.teams) AS t WHERE t.league ?1
• Data retrieved The players that belong to the
  specified league.
• Finder method findByLeague(LocalLeague league)
• Description as above, but parameter is object

60
Example Queries -Queries That Navigate to Related
Beans

• SELECT DISTINCT OBJECT(p) FROM Player p, IN
  (p.teams) AS t WHERE t.league.sport ?1
• Data retrieved The players who participate in
  the specified sport.
• Finder method findBySport(String sport)
• Description sport persistent field belongs to
  the LeagueEJB bean

61
Example Queries - Other Conditional Expressions

• SELECT OBJECT(p) FROM Player p WHERE p.teams IS
  EMPTY
• Data retrieved All players who do not belong to
  a team.
• Finder method findNotOnTeam()
• Description If a player does not belong to a
  team, then the teams collection is empty and the
  conditional expression is TRUE

62
Example Queries - Other Conditional Expressions

• SELECT DISTINCT OBJECT(p) FROM Player p WHERE
  p.salary BETWEEN ?1 AND ?2
• Data retrieved The players whose salaries fall
  within the range of the specified salaries.
• Finder method findBySalaryRange(double low,
  double high)
• Description BETWEEN expression has three
  arithmetic expressions a persistent field
  (p.salary) and the two input parameters (?1 and
  ?2).
• Equals p.salary ?1 AND p.salary

63
Example Queries - Other Conditional Expressions

• SELECT DISTINCT OBJECT(p1) FROM Player p1, Player
  p2 WHERE p1.salary p2.salary AND p2.name ?1
• Data retrieved All players whose salaries are
  higher than the salary of the player with the
  specified name.
• Finder method findByHigherSalary(String name)
• Description two identification variables (p1 and
  p2) of the same type (Player). Two identification
  variables are needed because the WHERE clause
  compares the salary of one player (p2) with that
  of the other players (p1).

64
What We Covered

• Terminology
• Simplified Syntax
• Example Queries
• Full Syntax
• EJB QL Restrictions