Ejbe slides

Module

EJBE
Entity EJBs
At the end of this module you will be able to:
• understand how entity EJBs stay persistent
• write entity EJBs from scratch
• connect an entity EJB to a data source

Developing Enterprise Applications with BEA WebLogic Server EJBE-1
Copyright © 2000, BEA Systems Inc.

Section

1
Entity EJB Basics
At the end of this section you will be able to:
• design an entity EJB that is mapped to a datasource
• describe the differences between CMP and BMP
• write a container managed entity EJB


What is an Entity EJB?
§ Entity EJBs have the following behavior:
Ø they are a representation of persistent data
Ø they can survive a crash
Ø multiple clients can be using EJBs that
represent the same data
Ø the EJB instance contains a “copy” of the data
in the persistent store


Persistence Management
§ The attributes of the object must be stored
using a persistent mechanism.
§ Examples of persistence management
include:
Ø serializationto a file
Ø relational database mappings through JDBC

Object in memory Persistent Storage
Client
-clientID database,
-corporateName files,
-address etc...
-town
-postcode


Shared Entity EJBs
§ When multiple clients share an entity EJB
they:
Ø receive their own instance
Ø share the underlying data
Ø do not have to handle synchronization

Client
Context Persistent Storage
Client
Entity EJB
Client
Server
Client
Container


Primary Keys
§ Every entity EJB has a set of attributes that
are unique when aggregated together.
§ The aggregated attributes are called the
primary key of an entity EJB.


Object-Relational Mapping
§ An O-R mapping stores each object in a
single row of a table.
§ A database column is associated with each
attribute of the class. Client
•Client Id
Primary •Corporatename
key •Address
•Town
•Postcode

clientId Corporate Name Address Town Postcode

411MEB MEB 1, Lichfield Road B ’ham B7 4TU
411PIT PITCH 34, London Road Chester C60 7YJ
411WC World Company 1, The Strand London W1 8PL


Identical Enitity EJBs
§ Two entity EJBs are identical if the
contents of their primary keys are
identical.

Primary Key
Client
Client
Entity EJB
Client
Server
Client
Container


Types of Persistence
§ There are two types of persistent EJBs:
Ø Container-Managed Persistence (CMP)
Ø Bean-Managed Persistence (BMP)

The Logic is
CMP here!
Client
Client

Entity EJB
BMP
Server
Client
Container


Entity vs. Session EJB Authoring

Differences Between Writing Entity EJBs and Session EJBs:
Differences Between Writing Entity EJBs and Session EJBs:
1. You must write aaprimary key class.
1. You must write primary key class.
2. The home interface must have aafindByPrimaryKey(<PrimaryKey>)
2. The home interface must have findByPrimaryKey(<PrimaryKey>)
method.
method.
3. The home interface can have other find(…) methods.
3. The home interface can have other find(…) methods.
4. The bean class must implement the javax.ejb.EntityBean interface.
4. The bean class must implement the javax.ejb.EntityBean interface.
5. The bean class must provide an ejbCreate(…) AND an
5. The bean class must provide an ejbCreate(…) AND an
ejbPostCreate(…) for each create(…) listed in the home interface.
ejbPostCreate(…) for each create(…) listed in the home interface.


Primary Key Classes
§ A primary key object is the unique
identifier of an EJB.
§ A primary key class must:
Ø bedeclared public
Ø implement java.io.Serializable
Ø have a no-argument constructor
Ø have all of its attributes be declared public

Sample Primary Key Class:
Sample Primary Key Class:
public class AlarmClockPK implements java.io.Serializable {
public int serialNumber;
public String currentStation;
}}

Primary Key Attributes
§ The EJB class must contain the same
parameters listed in the primary key.

AlarmClockPK AlarmClockBean
•int serialNum;
•int serialNum; •String currentStation;
•String currentStation; •long currentVolume;
•Date wakeUpTime;
•boolean isFm;


CMP EJB Creation
Container-Provided Classes
Entity Synchronization
: Client EJBHome EJBObject Context instance database

create(args)
ejbCreate(args)

extract container managed fields

create entity representation in DB

new

ejbPostCreate(args)
Return
remote stub
containing Create
primary key primary key
instance! instance with
values
extracted!

Home Interfaces
§ Entity EJB home interfaces must include
one or more find(…) methods.
Syntax for findByPrimaryKey(…) Method:
Syntax for findByPrimaryKey(…) Method:
public <RemoteIF> findByPrimaryKey(<PrimaryKey> pk)
throws FinderException, RemoteException;

Syntax for Other find(…) Methods:
Syntax for Other find(…) Methods:
public <RemoteIF> findWhatever(…)

public Enumeration findWhatever(…)


Attribute and find(…) Method Mapping


Finder Expressions
Syntax for Expressions:
Syntax for Expressions:
operator operand1 operand2

Available Operators:
Available Operators:
(), =, <, >, <=, >=, !, &, |, like

Available Operands:
Available Operands:
1. Another Expression
2. An EJB Attribute
3. A find(…) Method Parameter (Must precede with $)

Finder Expression Examples:
Finder Expression Examples:
(> balance $amount)

(& (> bal $amount) (! (= accountType ‘checking’)))

(= 1 1)

(like lastName M%)


Writing the Bean Class
§ The bean class must implement the
EntityBean interface.
The EntityBean Interface:
The EntityBean Interface:
public interface EntityBean extends EnterpriseBean {
public interface EntityBean extends EnterpriseBean {
public void ejbActivate();
public void ejbActivate();
public void ejbPassivate();
public void ejbPassivate();
public void ejbLoad();
public void ejbLoad();
public void ejbStore();
public void ejbStore();
public void setEntityContext(EntityContext ctx);
public void setEntityContext(EntityContext ctx);
public void unsetEntityContext()
public void unsetEntityContext()
public void ejbRemove();
public void ejbRemove();
}}

Some Other Guidelines:
Some Other Guidelines:
1. Your ejbCreate(…) methods should return void for CMP.
1. Your ejbCreate(…) methods should return void for CMP.
2. You do not have to implement any ejbFind(…) methods for CMP.
2. You do not have to implement any ejbFind(…) methods for CMP.


Entity EJB Life Cycle
instance throws
does not system exception from
exist any method

newInstance() unsetEntityContext()
setEntityContext(ec)
ejbFind<METHOD>()
ejbCreate(args)
ejbPostCreate(args) pooled
ejbRemove()

ejbActivate() ejbPassivate()

ready
ejbStore()
ejbLoad()

business method

Passivation
Entity Synchro-
: Client EJBHome EJBObject Container Context instance database
nization

business method
business method

ejbStore()
extract container managed fields
updated entity state in DB
ejbPassivate()


Activation
Entity Synchro-
: Client EJBHome EJBObject Container Context instance database
nization

business method
ejbActivate()

read entity state in DB

set container-managed fields

ejbLoad()

business method


Standard Entity EJB Exceptions
§ The EJB specification defines five
standard EJB application exceptions:
Ø javax.ejb.CreateException
Ø javax.ejb.DuplicateException
Ø javax.ejb.FinderException
Ø javax.ejb.ObjectNotFoundException
Ø javax.ejb.RemoveException


Finally, An Example!
§ Let’s write an entity EJB that tracks
different alarm clock settings.
§ The EJB will track:
Ø the time the alarm is supposed to go off
Ø the current radio station selected
Ø AM/FM radio selection


The Remote Interface

The AlarmClock Remote Interface:
The AlarmClock Remote Interface:

public interface AlarmClock extends Remote, EJBObject {
public interface AlarmClock extends Remote, EJBObject {
public Date getAlarmTime() throws RemoteException;
public Date getAlarmTime() throws RemoteException;
public String getRadioStation() throws RemoteException;
public String getRadioStation() throws RemoteException;
public boolean isFmSet() throws RemoteException;
public boolean isFmSet() throws RemoteException;
public void isFmSet(boolean fm) throws RemoteException;
public void isFmSet(boolean fm) throws RemoteException;
public void setAlarmTime(Date time) throws RemoteException;
public void setAlarmTime(Date time) throws RemoteException;
public void setRadioStation(String station)
public void setRadioStation(String station)
throws RemoteException;
throws RemoteException;
}}


The Home Interface and Primary Key
AlarmClockPK Primary Key Class:
AlarmClockPK Primary Key Class:
}}

The AlarmClockHome Home Interface:
The AlarmClockHome Home Interface:
public interface AlarmClockHome extends EJBHome {
public interface AlarmClockHome extends EJBHome {
AlarmClock create() throws CreateException, RemoteException;
AlarmClock create() throws CreateException, RemoteException;
AlarmClock create(Date time, String station, boolean fm)
AlarmClock create(Date time, String station, boolean fm)
throws CreateException, RemoteException;
throws CreateException, RemoteException;
AlarmClock findByPrimaryKey(AlarmClockPK id)
AlarmClock findByPrimaryKey(AlarmClockPK id)
Enumeration findAllFMSettings()
Enumeration findAllFMSettings()
}}


The Bean Class Finally!

The AlarmClockBean EJB Class:
The AlarmClockBean EJB Class:
public class AlarmClockBean implements EntityBean {
public class AlarmClockBean implements EntityBean {
public int id;
public int id;

transient protected EntityContext context;
transient protected EntityContext context;

// Container managed fields
// Container managed fields
public String station;
public String station;
public String wakeUpTime;
public String wakeUpTime;
public boolean isFm;
public boolean isFm;


The AlarmClockBean EJB Class Continued:
public void ejbActivate() {}
public void ejbActivate() {}
public void ejbLoad() {}
public void ejbLoad() {}
public void ejbPassivate() {}
public void ejbPassivate() {}
public void ejbRemove() {}
public void ejbRemove() {}
public void ejbStore() {}
public void ejbStore() {}

public void ejbCreate() {
public void ejbCreate() {
// Generate a unique ID for this EJB.
// Most EJBs pass receive this value as a parameter.
// Most EJBs pass receive this value as a parameter.
this.id = (int)(Math.random()*100000);

// Get the station and time from the environment
// Get the station and time from the environment
// properties.
// properties.
Properties props = context.getEnvironment();
Properties props = context.getEnvironment();
this.station = props.getProperty("Station");
this.station = props.getProperty("Station");
this.isFm = props.getProperty("isFM");
this.isFm = props.getProperty("isFM");
this.wakeUpTime = props.getProperty("Time");
this.wakeUpTime = props.getProperty("Time");
}}


EJBE-1, 2
public void ejbCreate(String time, String station, boolean isFm){
public void ejbCreate(String time, String station, boolean isFm){
// Most EJBs pass this value in.
// Most EJBs pass this value in.

this.wakeUpTime = time;
this.wakeUpTime = time;
this.station = station;
this.station = station;
this.isFm = new Boolean(isFm).toString();
this.isFm = new Boolean(isFm).toString();
}}

public void ejbPostCreate() {}
public void ejbPostCreate() {}
public void ejbPostCreate(String time, String station,
public void ejbPostCreate(String time, String station,
boolean isFm) {}
boolean isFm) {}


BMP Considerations
Things you Have to do For BMP Entity EJBs:
Things you Have to do For BMP Entity EJBs:
1. You must code the persistence management for the ejbCreate(…),
1. You must code the persistence management for the ejbCreate(…),
ejbRemove(), ejbLoad(), and ejbStore() methods.
ejbRemove(), ejbLoad(), and ejbStore() methods.
2. The ejbCreate(…) methods must return aaprimary key instance.
2. The ejbCreate(…) methods must return primary key instance.
3. The ejbFind(…) methods must be implemented.
3. The ejbFind(…) methods must be implemented.


BMP EJB Creation
Entity Synchronization
: Client EJBHome EJBObject Context instance database

create(args)
ejbCreate(args)
create representation
in DB
Return primary key instance
with unique values inserted
new

ejbPostCreate(args)

Return
remote stub
containing
primary key
instance!


The Alarm With BMP


EJBE-3
The Alarm With BMP


Transactions
§ Transactions can group multiple
statements into an atomic unit to ensure
integrity.
§ EJBs can have their transaction
demarcation level set:
Ø forthe entire EJB
Ø per method


EJB Transaction Attributes
§ The following entities can demarcate
transactions:
Ø an EJB client
Ø the container (automatic demarcation)
Ø the EJB component itself
§ The EJB specification defines 6 transaction
attributes:
Ø TX_NOT_SUPPORTED
Ø TX_SUPPORTS
Ø TX_MANDATORY
Ø TX-REQUIRED
Ø TX_REQUIRES_NEW
Ø TX_BEAN_MANAGED

TX_NOT_SUPPORTED
§ TX_NOT_SUPPORTED transactions cause
client transactions to be suspended
during the EJB’s execution.
Scenario 1:
1-begin
2-method Container
Container method EJB
3-commit
Client
Active Transaction
Scenario 2: No Transaction Active

Container method
method EJB
Client

TX_SUPPORTS
§ TX_SUPPORTS implies that the EJB will be
included in any client-started transactions.

Scenario 1:
1-begin
2-method Container
3-commit
Client
Active Transaction

Container method
method EJB
Client

TX_MANDATORY
§ TX_MANDATORY transactions require the
client to pass a global transaction to the
EJB.
Scenario 1:
1-begin
2-method Container
3-commit
Client
Active Transaction

Container TransactionRequired
method Exception EJB
Client

TX_REQUIRED
§ TX_REQUIRED transactions force the EJB
to be executed within a transaction started
either by the client or the container.
Scenario 1:
1-begin
2-method Container
3-commit
Client
Active Transaction
1-begin
Container 2-method EJB
method
Client 3-commit


TX_REQUIRES_NEW
§ TX_REQUIRES_NEW transactions force the
EJB to execute within a new transaction
that can only be started by the container.
Scenario 1 (try to avoid):
1-begin 1-begin
2-method Container
3-commit 3-commit
Client Active Transaction
No Transaction Active
Scenario 2:
Localized Transaction
1-begin
method
Client 3-commit


TX_BEAN_MANAGED
§ TX_BEAN_MANAGED transactions are
demarcated within the EJB only.
Scenario 1 (try to avoid):
1-begin 1-begin
2-method Container
Container
method EJB 2-method
3-commit 3-commit
Client
Active Transaction
No Transaction Active
Scenario 2: Localized Transaction

Container 1-begin
method method EJB 2-method
Client 3-commit


The UserTransaction Interface
§ Use the javax.jts.UserTransaction
object to demarcate transactions within a
client or EJB.
<<Interface>>
UserTransaction

begin()
commit()
rollback()
setRollbackOnly()
getStatus()
setTransactionTimeout()

To Obtain a New javax.jts.UserTransaction Instance From JNDI:
To Obtain a New javax.jts.UserTransaction Instance From JNDI:
UserTransaction trans =
(UserTransaction)ctx.lookup(“javax.jts.UserTransaction”);


Other Transaction Issues
§ An EJB’s conversational state is NOT
encompassed in a transaction.
§ A rollback() on a transaction that
encompasses an EJB will not revert the
EJB’s attributes!
§ Container and client managed transaction
objects can be accessed by the context:
To Obtain Change the Status of the Current Transaction Within an EJB:
To Obtain Change the Status of the Current Transaction Within an EJB:
contextObject.setRollbackOnly();
boolean setForRollback = contextObject.getRollbackOnly();


Reverting Conversational State
§ You can have your EJB implement the
SessionSynchronization interface.
SessionSynchronization Interface Methods:
SessionSynchronization Interface Methods:
// Called by the application server right after the start of aanew transaction. If you
// Called by the application server right after the start of new transaction. If you
// store attributes that may need to be reverted, do it here.
// store attributes that may need to be reverted, do it here.
public void afterBegin();
public void afterBegin();

// Called immediately before the completion of the transaction. It is NOT known if
// Called immediately before the completion of the transaction. It is NOT known if
// is committed or rolled back at this point.
// is committed or rolled back at this point.
public void beforeCompletion();
public void beforeCompletion();

// Called immediately after the completion of aatransaction. If the committed
// Called immediately after the completion of transaction. If the committed
// variable is true, then the transaction was successful. If the committed value
// variable is true, then the transaction was successful. If the committed value
// is false, the transaction was rolled back. You can recover EJB state from
// is false, the transaction was rolled back. You can recover EJB state from
// within this method if you need to.
// within this method if you need to.
public void afterCompletion(boolean committed);
public void afterCompletion(boolean committed);
}}

Review
§ In this section we talked about :
Ø container-managed and bean-managed
persistence
Ø the differences between session and entity
EJBs
Ø primary key classes
Ø transactions and their demarcation levels


Review
§ In this module we talked about:
Ø container and bean managed persistence
Ø primary key classes
Ø differences between session and entity EJBs
Ø writing ejbPostCreate(…) and
ejbFind(…) methods
Ø writing ejbLoad() and ejbStore()
methods
Ø transactions


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