The Heaven For Techy People: enterprise bean

Although the J2EE application client and the Web client access the same enterprise bean, their code refers to the bean's home by different names. The J2EE application client refers to the bean's home asejb/SimpleConverter, but the Web client refers to it as ejb/TheConverter. These references are in the parameters of the lookup calls. In order for the lookup method to retrieve the home object, you must map the references in the code to the enterprise bean's JNDI name. Although this mapping adds a level of indirection, it decouples the clients from the beans, making it easier to assemble applications from J2EE components.

To map the enterprise bean references in the clients to the JNDI name of the bean, follow these steps.

In the tree, select ConverterApp.

Select the JNDI Names tab.

To specify a JNDI name for the bean, in the Application table locate the ConverterEJB component and enter MyConverter in the JNDI Name column.

To map the references, in the References table enter MyConverter in the JNDI Name for each row.

Figure 2-1 shows what the JNDI Names tab should look like after you've performed the preceding steps.

Figure 2-1 ConverterApp JNDI Names

The Web client is contained in the JSP page j2eetutorial/examples/src/ejb/converter/index.jsp. A JSP page is a text-based document that contains static template data, which can be expressed in any text-based format such as HTML, WML, and XML; and JSP elements, which construct dynamic content.

Coding the Web Client

The statements (in bold in the following code) for locating the home interface, creating an enterprise bean instance, and invoking a business method are nearly identical to those of the J2EE application client. The parameter of the lookup method is the only difference; the motivation for using a different name is discussed in Specifying the JNDI Names.

The classes needed by the client are declared with a JSP page directive (enclosed within the <%@ %> characters). Because locating the home interface and creating the enterprise bean are performed only once, this code appears in a JSP declaration (enclosed within the <%! %> characters) that contains the initialization method, jspInit, of the JSP page. The declaration is followed by standard HTML markup for creating a form with an input field. A scriptlet (enclosed within the <% %> characters) retrieves a parameter from the request and converts it to a BigDecimal object. Finally, JSP expressions (enclosed within <%= %> characters) invoke the enterprise bean's business methods and insert the result into the stream of data returned to the client.

<%@ page import="Converter,ConverterHome,javax.ejb.*,
javax.naming.*, javax.rmi.PortableRemoteObject,
java.rmi.RemoteException" %>
<%!
   private Converter converter = null;
   public void jspInit() {
      try {
         InitialContext ic = new InitialContext(); Object objRef = ic.lookup(" java:comp/env/ejb/TheConverter"); ConverterHome home = (ConverterHome)PortableRemoteObject.narrow( objRef, ConverterHome.class); converter = home.create();       } catch (RemoteException ex) {
         ...
      } 
   }
   ...
%>
<html>
<head>
    <title>Converter</title>
</head>

<body bgcolor="white">
<h1><center>Converter</center></h1>
<hr>
<p>Enter an amount to convert:</p>
<form method="get">
<input type="text" name="amount" size="25">
<br>
<p>
<input type="submit" value="Submit">
<input type="reset" value="Reset">
</form>
<%
   String amount = request.getParameter("amount");
   if ( amount != null && amount.length() > 0 ) {
      BigDecimal d = new BigDecimal (amount);
%>
   <p><%= amount %> dollars are  
      <%= converter.dollarToYen(d) %>  Yen.
   <p><%= amount %> Yen are 
      <%= converter.yenToEuro(d) %>  Euro.
<%
    }
%>
</body>
</html>

Compiling the Web Client

The J2EE server automatically compiles Web clients that are JSP pages. If the Web client were a servlet, you would have to compile it.

Packaging the Web Client

To package a Web client, you run the New Web Component wizard of the deploytool utility. During this process the wizard performs the following tasks.

Creates the Web application deployment descriptor

Adds the component files to a WAR file

Adds the WAR file to the application's ConverterApp.ear file

After the packaging process, you can view the deployment descriptor by selecting Tools

Descriptor Viewer.

To start the New Web Component wizard, select File

New

Web Component. The wizard displays the following dialog boxes.

Introduction dialog box
1. Read the explanatory text for an overview of the wizard's features.
2. Click Next.

WAR File dialog box
1. Select Create New WAR File In Application.
2. In the combo box, select ConverterApp.
3. In the WAR Display Name field, enter ConverterWAR.
4. Click Edit.
5. In the tree under Available Files, locate the j2eetutorial/examples/build/ejb/converter directory.
6. Select index.jsp and click Add.
7. Click OK.
8. Click Next.

Choose Component Type dialog box
1. Select the JSP radio button.
2. Click Next.

Component General Properties dialog box
1. In the JSP Filename combo box, select index.jsp.
2. Click Finish.

Specifying the Web Client's Enterprise Bean Reference

When it invokes the lookup method, the Web client refers to the home of an enterprise bean:

Object objRef = ic.lookup("java:comp/env/ejb/TheConverter");

You specify this reference as follows:

In the tree, select ConverterWAR.

Select the EJB Refs tab.

Click Add.

In the Coded Name column, enter ejb/TheConverter.

In the Type column, select Session.

In the Interfaces column, select Remote.

In the Home Interface column, enter ConverterHome.

In the Local/Remote Interface column, enter Converter.

The Heaven For Techy People

Friday, June 15, 2012

J2EE Tutorial-12(Specifying the JNDI Names)

J2EE Tutorial-11(Creating the Web Client)

Coding the Web Client

Compiling the Web Client

Packaging the Web Client

Specifying the Web Client's Enterprise Bean Reference