Download Session Beans

Session Beans
Contents
1.
Session bean concepts
2.
Stateless session beans
3.
Stateful session beans
2
1. Session Bean Concepts

Session bean interfaces and classes

Defining metadata

Packaging and deployment

Operation at run time
3
Session Bean Interfaces and Classes

To define a session bean:
•
•
Define a remote and/or local interface
Define the bean class
Client calls business
methods on bean,
via the EJB object
EJB object
EJB bean
Proxy for the
EJB bean object
Business functionality
or business data
You define remote and/or local interface
Your EJB tool generates the source code
for the EJB object
Delegates
You define the bean class
You write the methods specified in the
remote and/or local interface
4
Defining Metadata

You can provide metadata to accompany session beans
•

There are 3 ways to provide metadata
•
•
•

E.g. transactional requirements, security, etc.
Implicit defaults
Annotations
Deployment descriptors
Are annotations or deployment descriptors better?
•
Discuss 
5
Packaging and Deployment
Enterprise application (EAR file)
EJB module
(EJB-JAR file)
Remote
interface
Remote interface
Local interface Local i/f
Local interface
Bean
Bean class
Remote i/f
Bean class
Web module
(WAR file)
Remote
interface
Remote interface
Local interface
Servlets/JSPs
Local interface
Client code
Client code
Application client
Client code
6
Operation at Run Time

At start-up, the EJB server can create a pool of beans

When a client accesses a bean, it just gets an EJB object

When a client calls a method, a bean is acquired from pool

When a bean is unneeded, the bean is returned to pool
EJB objects
Bean pool
Client 1
Client 2
Client 3
7
2. Stateless Session Beans

Overview of stateless session beans

Defining the remote / local interfaces

Defining exceptions

Ensuring consistency

Defining the bean class

Stateless session bean life cycle

Using the bean

Using the bean in a Web application
8
Overview of Stateless Session Beans

Stateless session beans do not retain client-specific state
between method calls
•

Lightweight and scalable
If a client invokes a series of methods on a stateless
session bean…
•
•
The methods might be executed on different bean instances
This doesn't matter at all, because stateless session beans do not
contain any client-specific state
9
Defining the Remote / Local Interfaces

The remote and/or local interface specifies the 'business
methods' for your bean
•

Annotate an interface with @Remote or @Local
Example remote and local interfaces:
package demos.sessionbeans.ejb;
import javax.ejb.Remote;
TempConverterBeanRemote.java
in the DemosSessionBeansEJB project
@Remote
public interface TempConverterBeanRemote {
public double fToC(double fahr);
public double cToF(double celsius);
}
package demos.sessionbeans.ejb;
import javax.ejb.Local;
TempConverterBeanLocal.java
in the DemosSessionBeansEJB project
@Local
public interface TempConverterBeanLocal {
public double fToC(double fahr);
public double cToF(double celsius);
}
10
Ensuring Consistency

If you want to expose the same business interface
remotely and locally…
•

The remote and local interfaces must be the same
To ensure the remote and local interfaces are the same…
•
Define a base interface that the remote and local interfaces extend
public interface TempConverter {
public double fToC(double fahr);
public double cToF(double celsius);
}
@Remote
public interface TempConverterBeanRemote extends TempConverter
{}
@Local
public interface TempConverterBeanLocal extends TempConverter
{}
11
Defining the Bean Class
The next step is to write the bean class, which implements
the business methods for your bean

In EJB 3.x, the implementation class is a POJO

•
Just annotate the class with @Stateless or @Stateful, and
implement the remote and/or local interface
package demos.sessionbeans.ejb;
import javax.ejb.Stateless;
TempConverterBean.java
in the DemosSessionBeansEJB project
@Stateless
public class TempConverterBean
implements TempConverterBeanRemote, TempConverterBeanLocal
{
public TempConverterBean() {}
public double fToC(double fahr)
{ return (fahr - 32) * 5 / 9;
}
public double cToF(double celsius)
{ return (celsius * 9 / 5) + 32; }
}
12
Stateless Session Bean Life Cycle

(1 of 2)
The life cycle of a stateless session bean is very simple:
Does Not Exist
Class.newInstance()
@PreDestroy method
injections
@PostConstruct method
Ready Pool
business method
13
Stateless Session Bean Life Cycle
(2 of 2)
A bean class can define methods to be invoked during
bean start-up and shut-down

•
•
Annotate start-up method with @PostConstruct
Annotate shut-down method with @PreDestroy
@Stateless
TempConverterBean.java
public class TempConverterBean
implements TempConverterBeanRemote, TempConverterBeanLocal
{
…
@PostConstruct
public void myPostConstructMethod()
{
System.out.println("Bean created at: " + new Date());
}
@PreDestroy
public void myPreDestroyMethod()
{
System.out.println("Bean destroyed at: " + new Date());
}
}
14
Using the Bean (1 of 3)

Generally you can inject a bean reference where needed
•
•
Declare a variable of the remote interface type
Annotate the variable with @EJB
import javax.ejb.EJB;
import demos.sessionbeans.ejb.TempConverterBeanRemote;
…
@EJB
private TempConverterBeanRemote tempConverterBean;

You can explicitly tell the EJB container how to resolve the
dependency injection:
@EJB(mappedName="MyTempConverterBean")
private TempConverterBeanRemote tempConverterBean;

You can inject a reference to a bean on a different server:
@EJB(mappedName="corbaname:iiop:localhost:3700#MyTempConverterBean")
private TempConverterBeanRemote tempConverterBean;
15
Using the Bean (2 of 3)

You can also get a bean via a manual global JNDI lookup...
•
To look-up a bean by its remote interface type:
InitialContext context = new InitialContext();
bean = (TempConverterBeanRemote)context.lookup(
"demos.sessionbeans.ejb.TempConverterBeanRemote");
•
To look-up a bean by its mappedName:
InitialContext context = new InitialContext();
bean = (TempConverterRemote)context.lookup("MyTempConverterBean");
•
To look-up a bean on a specific server:
Hashtable env = new Hashtable();
env.put("org.omg.CORBA.ORBInitialHost", "localhost");
env.put("org.omg.CORBA.ORBInitialPort", "3700");
InitialContext context = new InitialContext(env);
bean = (TempConverterRemote)context.lookup("MyTempConverterBean");
16
Using the Bean (3 of 3)

After you have acquired access to a bean, you can use it
as follows:
Main.java
public class Main
{
in the DemosSessionBeansClient project
…
private static void testStatelessBean()
{
TempConverterBeanRemote tc = … ;
…
try
{
System.out.println("212F = " + tc.fToC(212) + "C");
System.out.println("100C = " + tc.cToF(100) + "F");
}
catch (InvalidTempException ex) {…}
}
}
17
Using the Bean in a Web App (1 of 2)

A Web app can access an EJB via its remote interface
public class MyServlet extends HttpServlet
{
@EJB
private TempConverterBeanRemote bean;
Using injection
protected void doGet(…) …
{
…
out.println("212F = " + bean.fToC(212) + "C");
out.println("100C = " + bean.cToF(100) + "F");
Using global JNDI lookup
public class MyServlet extends HttpServlet
{
protected void doGet(…) …
{
InitialContext context = new InitialContext();
TempConverterBeanRemote bean =
(TempConverterRemote)context.lookup("MyTempConverterBean");
…
out.println("212F = " + bean.fToC(212) + "C");
out.println("100C = " + bean.cToF(100) + "F");
18
Using the Bean in a Web App (2 of 2)

If a bean has a local interface, you can access it from Web
apps in the same EAR application file
•

Same as on previous slide, but using local interface(!)
But... you can't lookup the bean's local interface in JNDI
•
•
Most Java EE servers don't add bean local interfaces to JNDI!
Solution:


Manually add the bean's local interface to your local JNDI namespace
Lookup the bean's local interface in your local JNDI namespace
@EJB(name="ejb/TempConverter",
beanInterface=demos.sessionbeans.ejb.TempConverterBeanLocal.class)
public class MyServlet extends HttpServlet
{
protected void doGet(…) …
{
InitialContext context = new InitialContext();
TempConverterBeanLocal bean =
(TempConverterBeanLocal) context.lookup("java:comp/env/ejb/TempConverter");
…
19
3. Stateful Session Beans

Overview of stateful session beans

Defining the remote /local interface

Defining the bean class

Stateful session bean life cycle

Using the bean in a client application

Using the bean in a Web application
20
Overview of Stateful Session Beans

Stateful session beans hold conversations with a particular
client, which span multiple method calls
•
•
Stateful session beans must therefore retain state between method
calls
The state is specific to a particular client
21
Defining the Remote / Local Interface
The remote / local interfaces specify the 'business
methods' for your bean

•
The rules for defining remote / local interfaces are the same as for
stateless session beans
For example:

package demos.sessionbeans.ejb;
import javax.ejb.Remote;
import java.util.List;
ShoppingCartBeanRemote.java
in the DemosSessionBeansEJB project
@Remote
public interface ShoppingCartBeanRemote
{
public void addItem(String productName);
public void removeItem(String productName);
public List<String> getItems();
}
22
Defining the Bean Class (1 of 2)

The bean class must be annotated with @Stateful
•

Indicates it's a stateful session bean
The bean class must implement the business methods that
are specified in the remote / local interfaces
•
•
The bean class (and almost always does) have instance variables
The instance variables are preserved in a stateful session bean 
23
Defining the Bean Class (2 of 2)
package demos.sessionbeans.ejb;
ShoppingCartBean.java
import javax.ejb.Stateful;
import java.util.*;
@Stateful
public class ShoppingCartBean implements ShoppingCartBeanRemote
{
private List<String> items = new ArrayList<String>();
public void addItem(String productName)
{
items.add(productName);
}
public void removeItem(String productName)
{
items.remove(productName);
}
public List<String> getItems()
{
return items;
}
}
24
Stateful Session Bean Life Cycle (1 of 2)

The life cycle of a stateful session bean is as follows:
Does Not Exist
Class.newInstance()
@PreDestroy method
injections
@PostConstruct
@PrePassivate()
Ready
@PostActivate()
Passive
business method
25
Stateful Session Bean Life Cycle (2 of 2)
The following example shows how to define life cycle
methods for a stateful session bean

ShoppingCartBean.java
@Stateful
public class ShoppingCartBean
implements ShoppingCartBeanRemote, ShoppingCartBeanLocal
{
…
@PostConstruct
public void myPostConstructMethod() {…}
@PreDestroy
public void myPreDestroyMethod() {…}
@PrePassivate
public void myPrePassivateMethod() {…}
@PostActivate
public void myPostActivateMethod() {…}
}
26
Using the Bean in a Client App
package demos.sessionbeans.appclient;
import demos.sessionbeans.ejb.ShoppingCartBeanRemote;
import javax.ejb.EJB;
…
public static void testStatefulBean() {
try {
InitialContext context = new InitialContext();
ShoppingCartBeanRemote cart = (ShoppingCartBeanRemote)
context.lookup("demos.sessionbeans.ejb.ShoppingCartBeanRemote");
cart.addItem("Sky Sports");
cart.addItem("OLED 4KHD TV");
cart.addItem("Carving skis");
cart.removeItem("Sky Sports");
System.out.println("Items in your shopping cart:");
for (String item : cart.getItems()) {
System.out.printf("Item: %s\n", item);
}
} catch (Exception ex) {
System.out.println("Exception: " + ex.getMessage());
}
}
Main.java
in the DemosSessionBeansClient project
27
Using the Bean in a Web App (1 of 4)

You can access stateful session beans in a Web app
•

Do not use injection for stateful session beans
•
•
•

However, you must be very careful...
This would mean each servlet instance gets its own session bean
This probably isn't what you want!
You want each client to get its own session bean
Do use manual JNDI lookup
•
•
•
•
Look up the bean using JNDI
Insert the bean into HTTP session scope
Next time round, get the bean from HTTP session scope (not JNDI)
This way, each client has its own session bean
28
Using the Bean in a Web App (2 of 4)

Initial Web page, to enable user to enter an item to be
added to shopping cart:
<h2>Manage your Shopping Cart</h2>
ManageShoppingCart.html
<form action="ShoppingCartServlet" method="get">
<input type="text" name="txtItem" />
<input type="submit" value="Add to cart" />
</form>
ShoppingCartServlet
29
Using the Bean in a Web App (3 of 4)
Servlet (controller)

ShoppingCartServlet.java
@WebServlet(name="ShoppingCartServlet", urlPatterns={"/ShoppingCartServlet"})
@EJB(name="ejb/Cart", beanInterface=demos.sessionbeans.ejb.ShoppingCartBeanLocal.class)
public class ShoppingCartServlet extends HttpServlet {
protected void doGet(…)
{
// Get existing ShoppingCart bean from HTTP session, or lookup new one.
HttpSession sess = request.getSession();
ShoppingCartBeanLocal bean = (ShoppingCartBeanLocal)sess.getAttribute("Cart");
if (bean == null)
{
InitialContext context = new InitialContext();
bean = (ShoppingCartBeanLocal)context.lookup("java:comp/env/ejb/Cart");
sess.setAttribute("Cart", bean);
}
// Invoke business method on bean, to add item to shopping cart.
bean.addItem(request.getParameter("txtItem"));
// Redisplay HTML form and shopping cart JSP.
…
}
}
30
Using the Bean in a Web App (4 of 4)

JSP (view)
DisplayShoppingCart.jsp
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%-- Does session scope have a "Cart" attribute? --%>
<c:if test="${not empty sessionScope.Cart}">
<%-- Get items in ShoppingCart bean, and assign to local variable --%>
<c:set var="itemsInCart" value="${sessionScope.Cart.items}" />
<%-- Iterate through the items --%>
Items in shopping cart: <br/>
<ul>
<c:forEach var="item" items="${itemsInCart}">
<li>${item}</li>
</c:forEach>
</ul>
</c:if>
31
Summary

Introduction to EJBs
•

EJBs are transactional middle-tier components
Session beans provide functionality in the "middle tier"
•
•
Stateless session beans are conversation-less
Stateful session beans are conversational (per-client)
32
Any Questions?
33