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Session Beans
Objectives
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Introduction to Session Beans
Local and Remote Session Beans
Stateless and Stateful Session Beans
Session Bean Lifecycle
Accessing Session Beans
Introduction to Session Beans
• Session beans represent a client's interaction with an
enterprise application.
• Session beans can be accessed by Servlets, other Enterprise
JavaBeans (EJB), or even desktop applications.
• Session beans encapsulate business methods and provide an
interface for client code.
• A session bean typically represents a single client's interaction
with the application and plays the role of the Controller in the
MVC design pattern.
• To accommodate the different ways clients interact with
applications, session beans come in two varieties: stateful and
stateless.
EJB Container Overview
• All EJB instances are running within the EJB container. The
container is a runtime environment (set of classes generated
by deployment) that controls an EJB component instance and
provides all necessary management services
• Transaction management: ensuring transaction properties of
multiple distributed transaction executions.
• Persistence management: ensuring a persistent state of an
entity bean that is backed up by database.
• Life cycle management: ensuring the EJB component state
transitions in its life cycle.
• Security management: authentication and authorization
services, integrity, and encryption management.
Session Beans In EJB Architecture
Local and Remote Interfaces
• Local beans are meant to be accessed from within
the same container. Remote session beans, as the
name implies, may be accessed from remote sources.
• Remote session beans can be accessed through the
Java Naming and Directory Interface (JNDI), a
directory service provided by the EJB container.
Remote beans can also be accessed locally.
• Remote or local access can be defined by the session
bean's interface using the @Remote or @Local
annotations.
Stateless Session Beans
• A stateless session bean is a session bean that
does not maintain state information across
multiple calls.
• Stateless session beans can be pooled by the
EJB container.
Stateful Session Beans
• A stateful session bean keeps its internal state
between invocations from the client.
• The stateful session beans will be less efficient
because the EJB container cannot simply grab the
next available session bean and hand it to the client.
• The client must be matched with the same bean
instance that serviced the last request from the
client.
• The state that the session bean maintains is the
internal state of the object.
(cont.)
app server
site2
Entity
Bean
db
servlet
session
bean
HTML
JSP
web server
client-tier
web-tier
app server
site1
business-tier
J2EE EJB architecture
Entity
Bean
app server
site3
db
data-tier
EJB Components
• An enterprise bean is a distributed server component that
lives in an EJB container and is accessed by remote clients
over network via its remote interface or is accessed by other
local enterprise beans on the same server via its local
interface.
• The EJB component is a remotely executable component
deployed on its server and it is a self-descriptive component
specified by its Deployment Descriptor (DD) in a XML format.
• Each EJB component has a business logic interface (remote or
local) that clients can run the business logic operations via this
interface without knowing the detail implementation behind
the interface.
EJB Components (cont.)
• Each EJB component has a home interface. An
instance of an EJB component is created and
managed by its factory named home interface
on the EJB container.
• Every enterprise bean must have a home
interface and a remote (local) interface.
• The EJB classes behind home and remote (or
local) interfaces are the implementations of
these two interfaces.
EJB Components (cont.)
• An EJB component is a black-box component. A
client of an EJB component only knows what the
component does but not how it does.
• A client makes a request to an EJB component with
its deployed name by looking up at JNDI to get an
Object Reference (OR) of this EJB component by
which client can create an instance of this EJB server
component. Finally, the client invokes the business
methods of this EJB instance.
• The EJB implementation class may also locate and
access other EJB beans at remote sites by using EJB
context information.
1.Lookup(“EJB”)
JNDI
Registers with
Client
2,Create()
EJB Home
Interface
New()
3. Invoke method
EJB
Class
EJB Object
Interface
EJB Context
EJB container
(security, transaction, life cycle, persistence)
EJB
The Client access to EJB on server
Server
Entity Bean
• Bean Managed Persistence (BMP) entity beans,
where persistent storage management (JDBC SQL) is
coded by bean developers. .
• Container Managed Persistence (CMP) entity beans,
where the persistent storage management is
specified by the deployment tool and managed by
the container.
• An entity bean is backed up by a relational database.
• The EJB implementation class implements either sessionBean
or entityBean interface, both of that implement
EnterpriseBean interface
Java.ejb.EnterpriseBean
Javax.ejb.EntityBean
Javax.ejb.SessionBean
EntityBean
SessionBean
EJB implementation class hierarchy
Session Beans
• As its name implies, a session bean is an interactive bean and
its lifetime is during the session with a specific client. It is nonpersistent.
• When a client terminates the session, the bean is not longer
associated with the client and is terminated as well.
• A server site session bean represents a particular client. It
responses on behalf of a client and terminates when the
client session is over.
• Session beans are often designed for major and complex
business logic and flow control in front of entity beans.
• A session bean may control the dialogues with entity bean
business objects. They may also make requests to another
session bean or to other Web components such as JSP, Servlet,
or HTML pages.
• stateless session beans and stateful session beans.
Stateless Session Bean
• The stateless session bean simply defines a set
of independent operations that can be
performed on behalf of clients.
• A stateless session bean plays a role of
controller and perform some procedural
operation on behalf of client during its session.
Life Cycle of a Stateless Session
Bean
• The life cycle of a stateless session bean is very
simple since it does not need to keep any state and
lives only during the session. Its life cycle has only
two stages: not-exist and method ready for the
invocation of business methods.
• The not-exist stage basically is where the bean
interface and class files are located. The method
stage is where the instantiated bean instance is
loaded into memory.
(cont.)
• The EJB container manages a bean instance pool to
reduce the number of component instantiations so
that expenses on the creations and removals of bean
instances can be significatelly reduced.
• There are two type methods in a enterprise bean:
the business methods and the bean life cycle
methods.
• The business methods are called by clients and life
cycle methods ( callback) methods are called back by
the EJB container when the EJB container thinks it is
necessary.
• The EJB callback methods are underlined in the
diagram and others are notated in the boxes.
(cont.)
• A client requests a new session bean instance by
create() method of bean home interface, and the
container calls the class’s newInstance() method to
create a new bean object ; the container then calls
the setSessionContext() method to pass in the
context environment object; it calls back the
ejbCreate() method to initialize the instance.
• programmers can define EJB container callback
method ejbCreate() which is only called once during
any stateless session bean life cycle.
• When the remove() method is called the ejbRemove()
is then called next; the bean may be pulled out from
the ready stage and is back to not-exist stage.
Not
Exist
create()
remove()
Class.newInstance()
ejbRomove()
setSessionContext()
ejbCreate()
(only once)
business method
invocation
Method
Ready
Life Cycle of a Stateless Session Bean
•
Stateless session beans have the advantage of
being able to be pooled. Since no state is saved with
the session, there is no need to match a specific
instance of the bean to a particular client.
• If subsequent calls are serviced by different
instances, the client application does not know (or
care).
• As a result, the total number of session bean
instances may be smaller than the total number of
clients accessing the application without impacting
performance.
Your first Stateless Session Bean(2.X)
This stateless session bean performs a temperature conversion
from a Fahrenheit temparature to its Ceilcius temparature. First,
two interfaces ( Home interface and Remote interface) are
specified in F2CHome.java and F2C.java files perspectively.
//F2C.java specifies remote interface for this converter session bean
//It exposes the business method fToC()
package f2c;
import javax.ejb.EJBObject;
import java.rmi.RemoteException;
import java.math.*;
public interface F2C extends EJBObject {
public double fToC(double f) throws RemoteException;
}
(cont.)
//The file F2CHome.java specifies the home interface for this EJB
package f2c;
import java.io.Serializable;
import java.rmi.RemoteException;
import javax.ejb.CreateException;
import javax.ejb.EJBHome;
public interface F2CHome extends EJBHome {
Converter create() throws RemoteException, CreateException;
}
(cont.)
• We define the implementation of this stateless
session bean in the F2CBean.java file.
• The fToC() method implementation is specified in
this file; the declaration of this method is listed in its
remote interface.
• Notice that this bean class does not have its own
state property. It simply takes client inputs and
performs the conversion operations, and then
returns the results. It specifies the implementations
of the EJB interfaces listed above.
• After it completes its service it will not remember
what happened in the past.
Bean Class
//The file F2CBean.java specifies the EJB implementation class .
package f2c;
import java.rmi.RemoteException;
import javax.ejb.SessionBean;
import javax.ejb.SessionContext;
import java.math.*;
public class F2CBean implements SessionBean {
public double fToC(double f) {
double temp=(f-32)*5./9;
return temp;
}
// It must have a default constructor; All EJB container
//call back methods are also listed
public F2CBean() {}
public void ejbCreate() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sc) {}
}
Client of Stateless Session Bean
Web JSP client for this stateless session bean EJB component
//index.jsp file.
<%-- Web Client for the EJB: index.jsp --%>
<%@ page
import="f2c.TempConv,f2c.TempConvHome,javax.ejb.*,
java.rmi.RemoteException, javax.naming.*,javax.rmi.*,
java.text.DecimalFormat" %>
<%!
private TempConv conv = null;
public void jspInit() {
try {
InitialContext ic = new InitialContext();
Object objRef = ic.lookup("java:comp/env/ejb/myBean");
(cont.)
TempConvHome home =
(TempConvHome)PortableRemoteObject.narrow(objRef,
TempConvHome.class);
conv = home.create();
} catch (RemoteException ex) {
System.out.println("Couldn't create bean."+
ex.getMessage());
} catch (CreateException ex) {
System.out.println("Couldn't create bean."+
ex.getMessage());
} catch (NamingException ex) {
System.out.println("Unable to lookup home: "+ "myBean "+
ex.getMessage());
}
}
(cont.)
public void jspDestroy() {
conv = null;
}
%>
<html>
<head>
<title>Temperature Converter</title>
</head>
<body bgcolor="white" ><center>
<h4><b>Temperature Converter</b></h4>
<p>Enter a temperature in Fahrenheit degree:</p>
<form method="get">
<input type="text" name="degree" size="25">
<br>
<p>
<input type="submit" name="fToC" value="Fahrenheit to Celsius">
(cont.)
</form>
<%
DecimalFormat twoDigits = new DecimalFormat ("0.00");
String degree = request.getParameter("degree");
if ( degree != null && degree.length() > 0 ) {
double d = Double.parseDouble(degree);
%>
<%
if (request.getParameter("fToC") != null ) {
%>
<p>
<%= degree %> in Fahrenheit degree is equivalent to
<%= twoDigits.format(conv.fToC(d)) %> in Celsius degree.
<%
}
%>
<%
}
%>
</center></body>
</html>
(cont.)
• Web clients of this application locate the home
object of this session bean by the Java Naming
and Directory Interface (JNDI). The InitialContext
class is the context for performing JNDI naming
operations. The lookup() method takes the bean's
JNDI name “myBean” (deployed name) as the
argument:
Context initialContext = new InitialContext();
F2CHome home =
(F2CHome)PortableRemoteObject.narrow(initialContext.look
up(“ java:comp/env/ejb/myBean"),F2CHome.class);
(cont.)
• The PortableRemoteObject.narrow() method must be
used in order to access a remote bean object via JNDI
lookup.
• This method converts the RMI-IIOP compatible remote
home stub into a Java object.
• For a local clients, the client and EJB bean are in the
same server, the return value of the
InitialContext.lookup() method is not a stub and you
can directly cast it to the local home interface just like
the following statement.
LocalF2CHome home =
(LocalF2CHome)initialContext.lookup("java:comp/env/ejb
/myBean");
(cont.)
• The detail procedures of the compilation,
configuration, deployment of this session
bean and its Web client can be found in the
section 6.7 Examples and Lab Practice.
Client gets the input from clients via
index.jsp and locates this session EJB; it
then gets the required services from the
bean and display the converted
temperature on the page.
• This is a simplest Web application of a
stateless Java enterprise session bean.
Stateless Session Bean(EJB3.X)
Session Bean Interface:
package com.datavikings.sessionbeans;
import javax.ejb.Remote;
@Remote
public interface HelloSessionRemote {
String hiThere(String name);
}
Sample Stateless Session Bean Implementation
(cont.)
package com.datavikings.sessionbeans;
import javax.ejb.Stateless;
@Stateless
public class HelloSessionBean implements
HelloSessionRemote {
public String hiThere(String name) {
return "Hi there, " + name + "!";
}
}
package com.datavikings.servlet;
import com.datavikings.sessionbeans.HelloSessionRemote;
import java.io.IOException;
import java.io.PrintWriter;
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class HelloServlet extends HttpServlet {
@EJB
HelloSessionRemote greeter;
protected void processRequest(HttpServletRequest
request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
if(request.getParameter("name") != null) {
out.println(greeter.hiThere(request.getParameter("name
")) + "<br />");
}
out.println("<formmethod=\"post\"
action=\"HelloServlet\">");
out.println("Your name:<input type=\"text\" name=\"name\"
/>");
out.println("<input type=\"submit\" value=\"Say Hi\" />");
out.println("</form>");
out.close();
}
(cont.)
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
(cont.)
• As the examples show, a session bean is very similar
to a POJO (Plain Old Java Object). The only
differences are the annotations, without which the
example could easily be a plain Servlet application
with a helper class. The @Remote annotation in the
interface tells the EJB container that the session bean
can be accessed remotely with this interface. We
could have used @Local, but (as stated previously)
the remote interface gives us more flexibility should
the application need to scale in the future.
• The @Stateless annotation in the session bean
implementation tells the EJB container that we
are not interested in keeping the state of the
session bean and that it should not worry about
trying to match it with a particular client. The
@EJB annotation is a signal that the interface we
used is actually an EJB and as such will be injected
by the EJB container. This is the simplest way to
access a session bean, by injecting it into a data
member of the client object.
The Stateful Session Bean
• A stateful session bean keeps its internal state between
invocations from the client. It seems pretty obvious, but there
are some consequences that must be understood. First, the
stateful session beans will be less efficient because the EJB
container cannot simply grab the next available session bean
and hand it to the client. The client must be matched with the
same bean instance that serviced the last request from the
client. If no such bean exists, a new one must be created to
handle the client (and only that client). As a result, there will
be as many stateful session beans as there are clients.
Contrast this with the stateless bean, which can be pooled,
and reduces the total number of instantiated objects and thus
conserves memory.
The Stateful Session Bean
• The state that the session bean maintains is the
internal state of the object. This should not be
confused with the Java Persistence API, which makes
data persistent by writing it to a database.
• If the application is shut down or the server running
the EJB container loses power, the session bean
ceases to exist. Rather, it is a way to keep track of the
conversational state between the application and a
client. It is sort of like remembering someone's name
for the duration of a phone call rather than writing
their name down in your address book to keep
permanently.
The Stateful Session Bean
• A stateful session bean maintains a specific
internal state that corresponds to a particular
client session; they cannot be pooled. An
analogy:When you get ready to leave, you expect
the valet to bring you the car that belongs to you,
not just any car in the lot
• stateless session beans can. An analogy of this
would be eating dinner at a fancy restaurant.
If you ask the waiter for a fork he will bring
you one from a “pool” of forks, because they
are all pretty much the same.
The Stateful Session Bean
• Because of this, the stateful session bean must exist
for the duration of the client session in case the
client needs it again. EJB container decides
to“passivate,” or serialize a stateful session bean and
place it in a more long-term storage area in order to
free some memory.
• Before a passivated session bean can service the
client the EJB container must locate it and unserialize
it to place it back in memory.
The Life Cycle of a Stateful Session
Bean
Life Cycle of a Stateful Session Bean
Your First Stateful Session Bean(2.x)
• simple on-line shopping cart with stateful session
bean class with its Home interface and Remote
interface.
• This stateful session been has a vector data state
which is a cart holding all the items customer put in
during the shopping session.
• The customer can also remove any items from this
cart and review the cart during that session.
– Home interface (CartHome)
– Remote interface (Cart)
– Session bean class (CartBean)
Your First Stateful Session Bean
• The home interface is like an EJB factory that defines
the create() methods and clients may invoke it to
create a new instance of the bean. For example,
clients call this create() method:
• Cart myCart = home.create(“Allen”);
• Every create() method in the home interface has its
corresponding ejbCreate() callback method in the
bean class. The signatures of the ejbCreate()
methods in the CartBean class is as follows.
• public void ejbCreate(String name) throws
CreateException
Your First Stateful Session Bean
(cont.)
• The CartHome.java is a Home interface file for this
statefull session bean. This home interface extends
the javax.ejb.EJBHome interface.
package shoppingCart;
import java.rmi.RemoteException;
import javax.ejb.*;
public interface CartHome extends EJBHome {
Cart create(String name) throws RemoteException,
CreateException;}
Your First Stateful Session Bean
(cont.)
• The Cart.java is a Remote interface file which declares all business
methods that the CartBean implements. This Remote interface
extends javax.ejb.EJBObject, and defines the business methods that
a remote client may invoke.
package shoppingCart;
import java.util.*;
import javax.ejb.EJBObject;
import java.rmi.RemoteException;
public interface Cart extends EJBObject {
public void addItem(String item) throws RemoteException; public
void removeItem(String item) throws RemoteException; public
Vector getCart() throws RemoteException;
}
Your First Stateful Session Bean (cont.)
//The CartBean.java is a stafull session bean class file which implement all
//bean interface and overrides the container callback methods.
package shoppingCart;
import java.util.*;
import javax.ejb.*;
import java.rmi.*;
public class CartBean implements SessionBean {
String name;
Vector cart;
SessionContext sessionContext;
//ejbCreate() is called back by EJB container after clients invoke
//create() method. It creates a vector to hold all shopped items
public void ejbCreate(String name) throws CreateException {
if (name == null) {
throw new CreateException("creation failed."); }
else { this.name = name; }
cart = new Vector();
}
Your First Stateful Session Bean (cont.)
//Add an new item to the cart
public void addItem(String item) { cart.add(item); }
//Remove an existing item from the cart
public void removeItem(String item) throws RemoteException {
boolean result = cart.remove(item);
if (result == false) { throw new RemoteException(“Can’t find it”); }
}
//Return this cart
public Vector getCart() {
return cart; }
public CartBean() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sc) {sessionContext=sc ;}
// sessionContext is useful when this session needs to use
//other resources in the session context}
}
Stateful Session Bean EJB3.x Example
package com.datavikings.sessionbeans;
import javax.ejb.Remote;
@Remote
public interface HelloSessionRemote {
String hiThere(String name); }
Stateful Session Bean Implementation:
package com.datavikings.sessionbeans;
import javax.ejb.Stateful;
@Stateful(mappedName=”HelloSessionBean”)
public class HelloSessionBean implements
HelloSessionRemote { String name = null;
(cont.)
public String hiThere(String n) {
if(name == null) {
name = n;
return "Hi there, " + name + ". It's nice to meet you";}
else {
return "Hey, I remember you! Your name is " + name + "!";
}
}
}
Sample Servlet Client:
package com.datavikings.servlet;
import com.datavikings.sessionbeans.HelloSessionRemote;
import java.io.IOException;
import java.io.PrintWriter;
import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.servlet.ServletException;
import javax.servlet.http.*;
public class HelloServlet extends HttpServlet {
protected void processRequest(HttpServletRequest
request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
HelloSessionRemote greeter = null;
try {InitialContext ctx = new InitialContext();
if(request.getSession().getAttribute("greeter") == null) {
greeter = (HelloSessionRemote)
ctx.lookup("HelloSessionBean");
request.getSession().setAttribute("greeter", greeter);}
else { // Otherwise, get reference from session
greeter = (HelloSessionRemote)
request.getSession().getAttribute("greeter");}
}
catch(NamingException e) {e.printStackTrace();}
if(request.getParameter("name") != null) {
out.println(greeter.hiThere(request.getParameter("name")) + "<br
/>");
}
out.println("<form method=\"post\" action=\"HelloServlet\">");
out.println("Your name:<input type=\"text\" name=\"name\" />");
out.println("<input type=\"submit\" value=\"Say Hi\" />");
out.println("</form>");
out.close();
}
(cont.)
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}