EJB 3 - Tutorial

Enterprise Java bean is a server-side component that encapsulates the business logic of an application. EJB can be be used when you need a scalability, data/transaction integrity and if you need to add thin clients for your application.

EJB container provides system-level services to enterprise beans, the bean developer can concentrate on solving business problems. The EJB container, rather than the bean developer, is responsible for system-level services such as transaction management and security authorization.

There are two types of EJBs in EJB3

• Session Bean : Performs a task for a client; optionally may implement a web service
• Message-Driven Bean : Acts as a listener for a particular messaging type, such as the Java Message Service API

There three types of Session beans as well,

• Stateful Session Beans : The instance variables of this kind of bean represent the state of a unique client-bean session. A session bean is not shared; it can have only one client,


• Stateless Session Beans : When a client invokes the methods of a stateless bean, the bean’s instance variables may contain a state specific to that client, but only for the duration of the invocation. Can be shared across multiple clients.


• Singleton Session Beans : A singleton session bean is instantiated once per application, and exists for the life cycle of the application

A message-driven bean is an enterprise bean that allows Java EE applications to process messages asynchronously. It normally acts as a JMS message listener. Messages can be sent by any Java EE component or by a JMS application or system that does not use Java EE technology. Message-driven beans can process JMS messages or other kinds of messages. The most visible difference between message-driven beans and session beans is that clients do not access message-driven beans through interfaces.

Creating a stateless Enterprise Bean with NetBeans

We can create and deploy simple EJB with NetBeans and deploy it on glassfish server.

• First create web application with NetBeans (File --> New Project --> Java Web --> Web Application).


• Right click the project and add a Session Bean to it (New --> Session bean). This is a slateless session bean and you can add following code into it.
  

  package com.sun.tutorial.javaee.ejb;
  
  import java.math.BigDecimal;
  import javax.ejb.Stateless;
  
  @Stateless
  public class ConverterBean {
  
  private BigDecimal yenRate = new BigDecimal("115.3100");
  private BigDecimal euroRate = new BigDecimal("0.0071");
  
  public BigDecimal dollarToYen(BigDecimal dollars) {
    BigDecimal result = dollars.multiply(yenRate);
    return result.setScale(2, BigDecimal.ROUND_UP);
  }
  
  public BigDecimal yenToEuro(BigDecimal yen) {
    BigDecimal result = yen.multiply(euroRate);
    return result.setScale(2, BigDecimal.ROUND_UP);
  }
  
  }



• Add a Servelt to the project (New --> Servlet).We use this servlet to call our EJB and to display result in the web browser. Add following code into the servlet.
  

  package converter.web;
  
  import com.sun.tutorial.javaee.ejb.ConverterBean;
  import java.io.IOException;
  import java.io.PrintWriter;
  import java.math.BigDecimal;
  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 ConverterServlet extends HttpServlet {
  
   @EJB
   ConverterBean converterBean ;
  
   public void doGet(HttpServletRequest request, HttpServletResponse response)
           throws ServletException, IOException {
  
       BigDecimal euroAmount = performOperation(request);
  
       response.setContentType("text/html");
       PrintWriter pw = response.getWriter();
       pw.println(euroAmount);
   }
  
   public BigDecimal performOperation(HttpServletRequest request) {
       try {
           String amount = request.getParameter("amount");
           if (amount != null && amount.length() > 0) {
               BigDecimal d = new BigDecimal(amount);
               BigDecimal yenAmount = converterBean.dollarToYen(d);
  
               // call the ConverterBean.yenToEuro() method to get the amount in Euros
               BigDecimal euroAmount = converterBean.yenToEuro(yenAmount);
               return euroAmount;
           }
       } catch (Exception e){
           System.out.println(e.getMessage());
       }
       return new BigDecimal(0);
   }
  }
  

  Here we are referring ConverterBean with @EJB annotation and it uses dependency injection to obtain a reference to ConverterBean .


• Change the index.jsp to add text box and set form action to the Servlet. Modify Index.jsp as follows.
  

    <body>
        <form id="form1" action="ConverterServlet">
            < div >
                <input id="amount" name="amount" type="text" />
                <input id="Submit1" type="submit" value="submit"/>
            </div>
        </form>
    </body>

• Changing the web.xml to route appropriate request to servlet. Add following content to web.xml inside web-app tag.

  <servlet>
 <servlet-name>ConverterServlet</servlet-name>
 <servlet-class>
 SimpleServlet
 </servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>ConverterServlet</servlet-name>
 <url-pattern>/converterServlet</url-pattern>
 </servlet-mapping>

• Running the application: Run the application and go to the url 

http://localhost:8080/<proj_name>/converterServlet

References:

http://download-llnw.oracle.com/javaee/6/tutorial/doc/bnblr.html