JSP Templates

JSP templates encourage the clear and flexible model-view-controller architecture. It's an example of the old JSP spec's "model 2." This tutorial works through a simple guest book example using JSP templates.

A powerful advantage of JSP is the ability to separate an application's business logic from its presentation. Using Smalltalk object-oriented terminology, JSP encourages MVC (model-view-controller) web applications. JSP classes or beans are the model, JSP is the view, and a servlet is the controller.

The GuestJsp skeleton forwards a "Hello, World" string to a login.jsp page. The skeleton establishes the architecture for the guest book. The details will be filled in below.

JSP templates start with servlet processing and then forward the results to a JSP page for formatting.

Forwarding uses a Servlet 2.1 feature of the ServletContext, getRequestDispatcher(). The request dispatcher lets servlets forward and include any subrequests on the server. It's a more flexible replacements for SSI includes. The RequestDispatcher can include the results of any page, servlet, or JSP page in a servlet's page. GuestJsp will use dispatcher.forward() to pass control to the JSP page for formatting.

package jsp.GuestJsp;

import java.io.*;
import java.util.*;

import javax.servlet.*;
import javax.servlet.http.*;

/**
 * GuestJsp is a servlet controlling user
 * interaction with the guest book.
 */
public class GuestJsp extends HttpServlet {
  /**
   * doGet handles GET requests
   */
  public void doGet(HttpServletRequest req,
                    HttpServletResponse res)
    throws ServletException, IOException
  {
    // Save the message in the request for login.jsp
    req.setAttribute("message", "Hello, world");

    // get the application object
    ServletContext app = getServletContext();

    // select login.jsp as the template
    RequestDispatcher disp;
    disp = app.getRequestDispatcher("login.jsp");

    // forward the request to the template
    disp.forward(req, res);
  }
}

The servlet and the jsp page communicate with attributes in the HttpRequest object. The skeleton stores "Hello, World" in the "message" attribute. When login.jsp starts, it will grab the string and print it.

Since Resin's JavaScript understands extended Bean patterns, it translates the request.getAttribute("message") into the JavaScript equivalent request.attribute.message.

Servlet Review

For those coming to JSP from an ASP or CGI background, Servlets replace CGI scripts taking advantage of Java's strength in dynamic class loading. A servlet is just a Java class which extends Servlet or HttpServlet and placed in the proper directory. Resin will automatically load the servlet and execute it.

doc
- index.html
- login.jsp
- add.jsp
- WEB-INF
  - classes
    - jsp
      - GuestJsp.class
      - GuestBook.class
      - Guest.class

The url /servlet/classname forwards the request to the Servlet Invoker. The Invoker will dynamically load the Java class classname from doc/WEB-INF/classes and try to execute the Servlet's service method.

Resin checks the class file periodically to see if the class has changed. If so, it will replace the old servlet with the new servlet.

The next step, after getting the basic framework running, is to create the model.

The GuestBook model

The guest book is straightforward so I've just included the API here. It conforms to Bean patterns to simplify the JavaScript. The same API will work for HashMap, file-based, and database implementations.

JSP files only have access to public methods. So a JSP file cannot create a new GuestBook and it can't add a new guest. That's the responsibility of the GuestJsp servlet.

jsp.Guest.java API

package jsp;

public class Guest {
  Guest();
  public String getName();
  public String getComment();
}

Resin's JavaScript recognizes Bean patterns. So JSP pages using JavaScript can access getName() and getComment() as properties. For example, you can simply use guest.name and guest.comment

jsp.GuestBook.java API

package jsp;

public class GuestBook {
  GuestBook();
  void addGuest(String name, String comment);
  public Iterator iterator();
}

Resin's JavaScript also recognizes the iterator() call, so you can use a JavaScript for ... each to get the guests:

for (var guest in guestBook) {
  ...
}

GuestBook as application attribute

To keep the example simple, GuestJsp stores the GuestBook in the application (ServletContext). As an example, storing data in the application is acceptable but for full-fledged applications, it's better just to use the application to cache data stored elsewhere.

jsp.GuestJsp.java

// get the application object
ServletContext app = getServletContext();

GuestBook guestBook;

// The guestBook is stored in the application
synchronized (app) {
  guestBook = (GuestBook) app.getAttribute("guest_book");

  // If it doesn't exist, create it.
  if (guestBook == null) {
    guestBook = new GuestBook();
    guestBook.addGuest("Harry Potter", "Griffindor rules");
    guestBook.addGuest("Draco Malfoy", "Slytherin rules");
    app.setAttribute("guest_book", guestBook);
  }
}

RequestDispatcher disp;
disp = app.getRequestDispatcher("login.jsp");

// synchronize the Application so the JSP file 
// doesn't need to worry about threading
synchronized (app) {
  disp.forward(req, res);
}

The JSP file itself is simple. It grabs the guest book from the application and displays the contents in a table. Normally, application objects need to be synchronized because several clients may simultaneously browse the same page. GuestJsp has taken care of synchronization before the JSP file gets called.

<%@ page language=javascript %>

<head>
<title>Hogwarts Guest Book</title>
</head>

<body bgcolor='white'>

<h1>Hogwarts Guest Book</h1>
<table>
<tr><td width='25%'><em>Name</em><td><em>Comment</em>
<%
var guestBook = application.attribute.guest_book

for (var guest in guestBook) {
  out.writeln("<tr><td>" + guest.name + "<td>" + guest.comment);
}
%>
</table>

</body>

<h1>Hogwarts Guest Book</h1>

<table>
<tr><td><em>Name</em></td><td><em>Comment</em>
</td></tr><tr><td>Harry Potter</td><td>Griffindor Rules
</td></tr><tr><td>Draco Malfoy</td><td>Slytherin Rules
</td></tr></table>

The guest book logic is simple. If the user has not logged in, she sees comments and a form to log in. After login, she'll see the comments and a form to add a comment. login.jsp formats the login page and add.jsp formats the add comment page.

...

// name from the session
String sessionName = session.getValue("name");

// action from the forms
String action = request.getParameter("action");

// name from the login.jsp form
String userName = request.getParameter("name");

// password from the login.jsp form
String password = request.getParameter("password");

// comment from the add.jsp form
String comment = request.getParameter("comment");

// login stores the user in the session
if (action != null && action.equals("login") &&
    userName != null &&
    password != null && password.equals("quidditch")) {
  session.putValue("name", userName);
}

// adds a new guest
if (action != null && action.equals("add") &&
    sessionName != null &&
    comment != null) {
  guestBook.addGuest(sessionName, comment);
}

String template;
// if not logged in, use login.jsp
if (session.getValue("name") == null)
  template = "login.jsp";
// if logged in, use add.jsp
else
  template = "add.jsp";

RequestDispatcher disp;
disp = app.getRequestDispatcher(template);

...

<%@ page language=javascript %>
<head>
<title>Hogwarts Guest Book: Login</title>
</head>
<body bgcolor='white'>

<h1>Hogwarts Guest Book</h1>
<table>
<tr><td width='25%'><em>Name</em><td><em>Comment</em>
<%
var guestBook = application.attribute.guest_book

for (var guest in guestBook) {
  out.writeln("<tr><td>" + guest.name + "<td>" + guest.comment);
}
%>
</table>
<hr>

<form action='GuestJsp' method='post'>
<input type=hidden name='action' value='login'>
<table>
<tr><td>Name:<td><input name='Name'>
<tr><td>Password:<td><input name='Password' type='password'>
<tr><td><input type=submit value='Login'>
</table>
</form>
</body>

The Resin demo shows a few ways to extend the guest book, including adding some intelligence to the form processing.

jsp templates

Introduction

Template Skeleton: Hello, World

Servlet Review

Displaying the Guest Book

The GuestBook model

GuestBook as application attribute

Guest book logic

Conclusion