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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 example is a simple guest book. Users log in and add comments.

<deftable>
Role Implementation
Model A `GuestBook` of `Guests`.
View login.jsp for new users
add.jsp for logged-in users.
Controller `GuestJsp`, a servlet to manage the state.
</deftable>

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.

When the example is compiled, browse

<def>http://localhost:8080/servlet/jsp.GuestJsp</def>

And you should see a page like:

<results> Hello, world </results>

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`.

<%@ page language=javascript %> <head> <title>&lt%= request.attribute.message %></title> </head> <body bgcolor='white'> <h1>&lt%= request.attribute.message %></h1> </body>

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 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.

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`

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:

<example> for (var guest in guestBook) { ... }

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.

// 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> <results> <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> </results>

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.

GuestJsp stores login information in the session variable.

<deftable>
Form Variable Meaning
action'login' to login or 'add' to add a comment
name user name
password user password
comment comment for the guest book
</deftable> ... // 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); ...

login.jsp and add.jsp just append different forms to the display code in the previous section.

<%@ 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.

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