Download import javax.ejb.

Session
Beans
-) stateless
-) stateful
Session Beans
A session bean represents a single client
inside the J2EE server.
To access an application that is deployed on the server, the
client invokes the session bean's methods. The session bean
performs work for its client, shielding the client from
complexity by executing business tasks inside the server.
At any given time, only one client has access
to the bean instance.
The state (i.e. the values of the instance
variables) of the bean is not persistent,
existing only for a short period of time.
A session bean can be stateful or stateless.
Stateful session Beans
In a stateful session bean, the instance
variables represent the state of a unique
client-bean session. This state is often
called the conversational state.
The state is retained for the duration of the
client-bean session. If the client removes
the bean or terminates, the session ends and
the state disappears.
Stateless session Beans
A stateless session bean does not maintain a
conversational state for a particular client.
When a client invokes the method of a
stateless bean, the bean's instance variables
may contain a state, but only for the
duration of the invocation. When the method
is finished, the state is no longer retained.
Stateless vs. stateful session Beans
All instances of a stateless bean are
equivalent, allowing the EJB container to
assign an instance to any client.
=> Stateless session beans can support
multiple clients, and offer better
scalability for applications that require
large numbers of clients.
Typically, an application requires fewer stateless session
beans than stateful session beans to support the same number
of clients.
Stateless vs. stateful session Beans
The EJB container can write a stateful
session bean to secondary storage.
However, stateless session beans are never
written to secondary storage.
Therefore, stateless beans may offer better
performance than stateful beans.
Stateful session Beans – examples of use
•The bean needs to hold information about the
client across method invocations.
•The bean mediates between the client and the
other components of the application,
presenting a simplified view to the client.
•Behind the scenes, the bean manages the work
flow of several enterprise beans.
Stateless session Beans – examples of use
•In a single method invocation, the bean
performs a generic task for all clients. For
example, you might use a stateless session
bean to send an e-mail that confirms an
online order.
•The bean fetches from a database a set of
read-only data that is often used by clients.
Stateless Example
Stateless Example:
Euro Converter
Stateless example – The Component (Remote) Interface
package statelessDemo;
import java.rmi.*;
import javax.ejb.*;
public interface Converter extends EJBObject {
public double convert(int lire);
}
Stateless example – The Home Interface
package statelessDemo;
import java.rmi.*;
import javax.ejb.*;
public interface ConverterHome extends EJBHome {
public Converter create() throws RemoteException,
CreateException;
}
Stateless example – The bean
package statelessDemo;
import java.rmi.*;
import javax.ejb.*;
public class ConverterBean implements SessionBean {
private SessionContext sessionContext;
public void ejbCreate() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sessionContext)
{ this.sessionContext = sessionContext; }
private static double ratio=1936.27;
public double convert(int lire) {
return lire/ratio;
}
}
Stateless example – The descriptor
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ejb-jar PUBLIC "-//Sun Microsystems, Inc.//DTD Enterprise
JavaBeans 1.1//EN" "http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd">
<ejb-jar>
<description>A simple demo of a stateless Bean</description>
<display-name>An Euro Converter Bean</display-name>
<enterprise-beans>
<session>
<description>A converter from Lire to Euro</description>
<ejb-name>EuroConverter</ejb-name>
<home>statelessDemo.ConverterHome</home>
<remote>statelessDemo.Converter</remote>
<ejb-class>statelessDemo.ConverterBean</ejb-class>
<session-type>Stateless</session-type>
<transaction-type>Container</transaction-type>
</session>
</enterprise-beans>
Stateless example – The descriptor
<assembly-descriptor>
<container-transaction>
<method>
<ejb-name>EuroConverter</ejb-name>
<method-name>*</method-name>
</method>
<trans-attribute>Required</trans-attribute>
</container-transaction>
</assembly-descriptor>
<ejb-client-jar />
</ejb-jar>
Stateless example – The client
import javax.ejb.*;
import javax.naming.InitialContext;
public class ConverterClient {
public ConverterClient() {
}
Get naming context
public static void main(String[] args) {
and object reference
try {
InitialContext ctx=new InitialContext();
JNDI name
Object objref=ctx.lookup("L2EConverter");
statelessDemo.ConverterHome home =
Cast to correct type
(statelessDemo.ConverterHome)
javax.rmi.PortableRemoteObject.narrow(
objref,statelessDemo.ConverterHome.class);
statelessDemo.Converter bean=home.create();
Get a bean instance from container
Stateless example – The client
int lire=100000;
Do your business
System.out.println(lire+" Lire = "+
bean.convert(lire)+" Euro");
} catch (javax.naming.NamingException ex) {
System.out.println("NamingException: "+ex);
} catch (ClassCastException cc) {
System.out.println(" ClassCastException : "+cc);}
catch (javax.ejb.CreateException ce) {
System.out.println("CreateException: "+ce);
} catch (java.rmi.RemoteException re) {
System.out.println("RemoteException: "+re);
}
}
}
Stateless example – execution
RUN
100000 Lire = 51.64568990894865 Euro
Stateful Example
Stateful Example:
Dollar Converter
Stateful example – The Component (Remote) Interface
import java.rmi.*;
import javax.ejb.*;
public interface DollarConverter extends EJBObject {
public double convertInEuro(double dollar) throws
java.rmi.RemoteException;
public double convertInDollar(double euro) throws
java.rmi.RemoteException;
public void setRate(double euro_dollar_ratio) throws
java.rmi.RemoteException;
}
Stateful example – The Home Interface
import java.rmi.*;
import javax.ejb.*;
public interface DollarConverterHome extends EJBHome {
public DollarConverter create() throws RemoteException,
CreateException;
}
Stateful example – The bean
import java.rmi.*;
import javax.ejb.*;
public class DollarConverterBean implements SessionBean {
private SessionContext sessionContext;
public void ejbCreate() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sessionContext)
{ this.sessionContext = sessionContext; }
Stateful example – The bean
private double euro_dollar_ratio=1;
public double convertInEuro(double dollar) throws
java.rmi.RemoteException{
return dollar/euro_dollar_ratio;
}
public double convertInDollar(double euro) throws
java.rmi.RemoteException{
return euro*euro_dollar_ratio;
}
public void setRate(double euro_dollar_ratio) throws
java.rmi.RemoteException{
this.euro_dollar_ratio=euro_dollar_ratio;
}
}
Stateful example – The descriptor
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE ejb-jar PUBLIC "-//Sun Microsystems, Inc.//DTD Enterprise
JavaBeans 1.1//EN" "http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd">
<ejb-jar>
<description>A simple demo of a stateful Bean</description>
<display-name>An Euro to Dollar Converter Bean</display-name>
<enterprise-beans>
<session>
<description>A converter from Euro to Dollar</description>
<ejb-name>DollarConverter</ejb-name>
<home>DollarConverterHome</home>
<remote>DollarConverter</remote>
<ejb-class>DollarConverterBean</ejb-class>
<session-type>Stateful</session-type>
<transaction-type>Container</transaction-type>
</session>
</enterprise-beans>
Stateful example – The descriptor
<assembly-descriptor>
<container-transaction>
<method>
<ejb-name>DollarConverter</ejb-name>
<method-name>*</method-name>
</method>
<trans-attribute>Required</trans-attribute>
</container-transaction>
</assembly-descriptor>
<ejb-client-jar />
</ejb-jar>
Stateful example – The client
import javax.ejb.*;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
public class ConverterClient {
public ConverterClient() {}
public static void main(String[] args) {
Object objref=null;
DollarConverterHome home=null;
Get naming context
DollarConverter bean=null;
and object reference
try {
InitialContext ctx=new InitialContext();
objref=ctx.lookup("EDC");
Cast to correct type
JNDI name
home =(DollarConverterHome)PortableRemoteObject.narrow(
objref,DollarConverterHome.class);
bean=home.create();
Get a bean instance from container
Stateful example – The client
bean.setRate(0.978);
Do your business
double euro=1000;
System.out.println(euro+" Euro = "
+bean.convertInDollar(euro)+" Dollar");
double dollar=1000;
System.out.println(dollar+" Dollar = "
+bean.convertInEuro(dollar)+" Euro");
} catch (Exception e) {
e.printStackTrace();
}
}
}
Stateful example – execution
RUN
1000.0 Euro = 978.0 Dollar
1000.0 Dollar = 1022.4948875255624 Euro
Session Beans Lifecycle: client’s view
Start
Does not exist
Is not referenced
Crash,
Timeout
Exists
Is not referenced
Client
releases
reference
home.create()
Client
obtains
handle
Client
releases
reference
Client invokes method
(NoSuchObject Exception)
Does not exist
Is referenced
object.remove(),
home.remove(),
Crash, timeout
Exists
Is referenced
Client
invokes
method
Stateless session Beans Lifecycle
Client invokes method
(NoSuchObject Exception)
Does not exist
Client invokes create()
Container executes:
1) newInstance()
2) setSessionContext(sc)
3) ejbCreate()
Client invokes remove()
Container executes:
ejbRemoved()
Exists
In the pool
Client invokes method
Stateful session Beans Lifecycle
Client invokes create()
Container executes:
1) newInstance()
2) setSessionContext(sc)
3) ejbCreate()
Does not exist
Client invokes remove(),
or timeout is reached
Container executes:
ejbRemoved()
ejbPassivate()
Client invokes
non TX method
Exists,
method ready
in the pool
Client invokes
TX method
AfterBegin()
Client invokes commit
beforeCompletion()
afterCompletion(true)
Client invokes
TX method
Passive
ejbActivate()
Exists,
method ready
in TX
Client invokes rollback
afterCompletion(false)