Post on 12-Jan-2016
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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() { } public static void main(String[] args) { try { InitialContext ctx=new InitialContext(); Object objref=ctx.lookup("L2EConverter");
statelessDemo.ConverterHome home = (statelessDemo.ConverterHome) javax.rmi.PortableRemoteObject.narrow( objref,statelessDemo.ConverterHome.class);
statelessDemo.Converter bean=home.create();
Get naming context and object reference
Cast to correct type
Get a bean instance from container
JNDI name
Stateless example – The client
int lire=100000; 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); } }}
Do your business
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; DollarConverter bean=null; try { InitialContext ctx=new InitialContext(); objref=ctx.lookup("EDC"); home =(DollarConverterHome)PortableRemoteObject.narrow(
objref,DollarConverterHome.class);
bean=home.create();
Get naming context and object reference
Cast to correct type
Get a bean instance from container
JNDI name
Stateful example – The client
bean.setRate(0.978);
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(); } }}
Do your business
Stateful example – execution
RUN
1000.0 Euro = 978.0 Dollar1000.0 Dollar = 1022.4948875255624 Euro
Session Beans Lifecycle: client’s view
Does not existIs not referenced
ExistsIs not referenced
ExistsIs referenced
Does not existIs referenced
Start
home.create()
Clientreleasesreference
Crash,Timeout
Clientreleasesreference
Clientobtainshandle
object.remove(),home.remove(),Crash, timeout
Clientinvokesmethod
Client invokes method (NoSuchObject Exception)
Stateless session Beans Lifecycle
ExistsIn the pool
Does not exist
ejbRemoved()
Client invokes method
Client invokes method (NoSuchObject Exception)
1) newInstance()2) setSessionContext(sc)3) ejbCreate()
Client invokes create()Container executes: Client invokes remove()
Container executes:
Stateful session Beans Lifecycle
Passive
ejbPassivate()
ejbActivate()
Exists, method ready
in the pool
Does not existClient invokes remove(), or timeout is reachedContainer executes:ejbRemoved()
Client invokes create()Container executes:1) newInstance()2) setSessionContext(sc)3) ejbCreate()
Client invokes non TX method
Exists, method ready
in TX
Client invokes TX method
Client invokes TX methodAfterBegin()
Client invokes commitbeforeCompletion()afterCompletion(true)
Client invokes rollbackafterCompletion(false)