Developing Web Components

Target Audience and Content

The target audience for this guide is the Web Component provider, i.e. the person in charge of developing the web components on the server side.

The content of this guide is the following:

  1. Target Audience and Content
  2. Introduction
  3. The JSPs
  4. The Servlets
  5. Accessing an EJB from a Servlet or JSP page

Introduction

A Web Component is a generic term which denotes both JSP pages and Servlets. Web components are packaged in a .war file and can be deployed in a JOnAS server via the web service. Web components can be integrated in a J2EE application by packing the .war file in an .ear file (refer to the J2EE Application Programmer's Guide).

The JOnAS distribution includes a Web application example: The EarSample example.

The directory structure of this application is the following:

etc/xml contains the web.xml file describing the web application
etc/resources/web contains html pages and images; JSP pages can also be placed here.
src/org/objectweb/earsample/servlets servlet sources
src/org/objectweb/earsample/beans beans sources

The bean directory is not needed if beans coming from another application will be used.

The JSP pages

Java Server Pages (JSP) is a technology that allows regular, static HTML, to be mixed with dynamically-generated HTML written in Java programming language for encapsulating the logic that generates the content for the page. Refer to the Java Server PagesTM and the Quickstart guide for more details.

Example:

The following example shows a sample JSP page that lists the content of a cart.

    <!-- Get the session -->
    <%@ page session="true" %>

    <!-- The import to use -->
    <%@ page import="java.util.Enumeration" %>
    <%@ page import="java.util.Vector"      %>

    <html>
    <body bgcolor="white">
      <h1>Content of your cart</h1><br>
      <table>
        <!-- The header of the table -->
        <tr bgcolor="black">
          <td><font color="lightgreen">Product Reference</font></td>
          <td><font color="lightgreen">Product Name</font></td>
          <td><font color="lightgreen">Product Price</font></td>
        </tr>

        <!-- Each iteration of the loop display a line of the table -->
        <%
          Cart cart = (Cart) session.getAttribute("cart");
          Vector products = cart.getProducts();
          Enumeration enum = products.elements();
          // loop through the enumeration
          while (enum.hasMoreElements()) {
              Product prod = (Product) enum.nextElement();
        %>
        <tr>
          <td><%=prod.getReference()%></td>
          <td><%=prod.getName()%></td>
          <td><%=prod.getPrice()%></td>
        </tr>
        <%
        } // end loop
        %>
      </table>
    </body>
    </html>
    

It is a good idea to hide all the mechanisms for accessing EJBs from JSP pages by using a proxy java bean, referenced in the JSP page by the usebean special tag. This technique is shown in the alarm example, where the .jsp files communicate with the EJB via a proxy java bean ViewProxy.java.

The Servlets

Servlets are modules of Java code that run in an application server for answering client requests. Servlets are not tied to a specific client-server protocol. However, they are most commonly used with HTTP, and the word "Servlet" is often used as referring to an "HTTP Servlet."

Servlets make use of the Java standard extension classes in the packages javax.servlet (the basic Servlet framework) and javax.servlet.http (extensions of the Servlet framework for Servlets that answer HTTP requests).

Typical uses for HTTP Servlets include:

For more details refer to the JavaTM Servlet Technology and the Servlets tutorial.

Example:

The following example is a sample of a Servlet that lists the content of a cart.
This example is the servlet version of the previous JSP page example.

    import java.util.Enumeration;
    import java.util.Vector;
    import java.io.PrintWriter;
    import java.io.IOException;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import javax.servlet.http.HttpSession;

    public class GetCartServlet extends HttpServlet {

        protected void doGet(HttpServletRequest req, HttpServletResponse res)
                             throws ServletException, IOException {

            res.setContentType("text/html");
            PrintWriter out = res.getWriter();

            out.println("<html><head><title>Your cart</title></head>");
            out.println("<body>");
            out.println("<h1>Content of your cart</h1><br>");
            out.println("<table>");

            // The header of the table
            out.println("<tr>");
            out.println("<td><font color="lightgreen">Product Reference</font></td>");
            out.println("<td><font color="lightgreen">Product Name</font></td>");
            out.println("<td><font color="lightgreen">Product Price</font></td>");
            out.println("</tr>");

            // Each iteration of the loop display a line of the table
            HttpSession session = req.getSession(true);
            Cart cart = (Cart) session.getAttribute("cart");
            Vector products = cart.getProducts();
            Enumeration enum = products.elements();
            while (enum.hasMoreElements()) {
                Product prod = (Product) enum.nextElement();
                int prodId = prod.getReference();
                String prodName = prod.getName();
                float prodPrice = prod.getPrice();
                out.println("<tr>");
                out.println("<td>" + prodId + </td>);
                out.println("<td>" + prodName + </td>);
                out.println("<td>" + prodPrice + </td>);
                out.println("</tr>");
            }

            out.println("</table>");
            out.println("</body>");
            out.println("</html>");
            out.close();
        }
    }
    

Accessing an EJB from a Servlet or JSP page

Starting with JOnAS 2.6 with its web container service, it is possible to access an enterprise java bean and its environment in a J2EE-compliant way.

The following sections describe:

  1. How to access the Remote Home interface of a bean.
  2. How to access the Local Home interface of a bean.
  3. How to access the environment of a bean.
  4. How to start transactions in servlets.
Note that all the following code examples are taken from the The EarSample example provided in the JOnAS distribution.

Accessing the Remote Home interface of a bean:

In this example the servlet gets the Remote Home interface OpHome registered in JNDI using an EJB reference, then creates a new instance of the session bean:
import javax.naming.Context;
import javax.naming.InitialContext;

//remote interface
import org.objectweb.earsample.beans.secusb.Op;
import org.objectweb.earsample.beans.secusb.OpHome;

        Context initialContext = null;
        try {
            initialContext = new InitialContext();
        } catch (Exception e) {
            out.print("<li>Cannot get initial context for JNDI: ");
            out.println(e + "</li>");
            return;
        }
      // Connecting to OpHome thru JNDI
        OpHome opHome = null;
        try {
            opHome = (OpHome) PortableRemoteObject.narrow(initialContext.lookup("java:comp/env/ejb/Op"), OpHome.class);
        } catch (Exception e) {
            out.println("<li>Cannot lookup java:comp/env/ejb/Op: " + e + "</li>");
            return;
        }
        // OpBean creation
        Op op = null;
        try {       
            op = opHome.create("User1"); 
        } catch (Exception e) {
            out.println("<li>Cannot create OpBean: " + e + "</li>");
            return;
        }
Note that the following elements must be set in the web.xml file tied to this web application:
  <ejb-ref>
    <ejb-ref-name>ejb/Op</ejb-ref-name>
    <ejb-ref-type>Session</ejb-ref-type>
    <home>org.objectweb.earsample.beans.secusb.OpHome</home>
    <remote>org.objectweb.earsample.beans.secusb.Op</remote>
    <ejb-link>secusb.jar#Op</ejb-link>
  </ejb-ref>

Accessing the Local Home of a bean:

The following example shows how to obtain a local home interface OpLocalHome using an EJB local reference:
//local interfaces
import org.objectweb.earsample.beans.secusb.OpLocal;
import org.objectweb.earsample.beans.secusb.OpLocalHome;


      // Connecting to OpLocalHome thru JNDI
        OpLocalHome opLocalHome = null;
        try {
            opLocalHome = (OpLocalHome) 
                initialContext.lookup("java:comp/env/ejb/OpLocal");
        } catch (Exception e) {
            out.println("<li>Cannot lookup java:comp/env/ejb/OpLocal: " + e + "</li>");
            return;
        }
This is found in the web.xml file:
  <ejb-local-ref>
    <ejb-ref-name>ejb/OpLocal</ejb-ref-name>
    <ejb-ref-type>Session</ejb-ref-type>
    <local-home>org.objectweb.earsample.beans.secusb.OpLocalHome</local-home>
    <local>org.objectweb.earsample.beans.secusb.OpLocal</local>
    <ejb-link>secusb.jar#Op</ejb-link>
  </ejb-local-ref>

Accessing the environment of the component:

In this example, the servlet seeks to access the component's environment:
       String envEntry = null;
        try {
            envEntry = (String) initialContext.lookup("java:comp/env/envEntryString");
        } catch (Exception e) {
            out.println("<li>Cannot get env-entry on JNDI " + e + "</li>");
            return;
        }
This is the corresponding part of the web.xml file:
  <env-entry>
    <env-entry-name>envEntryString</env-entry-name>
    <env-entry-value>This is a string from the env-entry</env-entry-value>
    <env-entry-type>java.lang.String</env-entry-type>
  </env-entry>

Starting transactions in servlets:

The servlet wants to start transactions via the UserTransaction:
import javax.transaction.UserTransaction;

       // We want to start transactions from client: get UserTransaction
        UserTransaction utx = null;
        try {
            utx = (UserTransaction) initialContext.lookup("java:comp/UserTransaction");
        } catch (Exception e) {
            out.println("<li>Cannot lookup java:comp/UserTransaction: " + e + "</li>");
            return;
        }

        try { 
            utx.begin();
            opLocal.buy(10);
            opLocal.buy(20); 
            utx.commit();

        } catch (Exception e) {
            out.println("<li>exception during 1st Tx: " + e + "</li>");
            return;
        }