JavaServer Pages

JavaServer Pages
By: Abdalla Mahmoud.

Contents
JavaServer Pages................................................................................ 1
Contents ........................................................................................... 1
1. Introduction ................................................................................... 3
2. First JSP ........................................................................................ 4
3. Syntax Elements............................................................................. 4
3.1. Comments ............................................................................... 4
3.2. Scriptlets ................................................................................. 5
3.3. Expressions .............................................................................. 5
3.4. Declarations ............................................................................. 6
3.5. Directives................................................................................. 6
4.1. Example................................................................................... 7
4.1.1. JSP .................................................................................... 7
4.1.2. Corresponding Servlet Class .................................................. 8
5. Implicit Variables ............................................................................ 9
6. Hoax Example - Revisited ................................................................ 9
6.1. Process Servlet ........................................................................ 10
6.2. View Servlet ............................................................................ 10

1

1. Introduction
JavaServer Pages are web components that generate dynamic web content. Like servlets,
JSPs process requests from agents and generate responses. However, JSPs are designed to
be more close the nature of web pages than language classes. JSPs are compared to servlets
as follows:

JSPs Servlets
They are not java classes. They are dynamic They are java classes.
web pages.
Include content with embedded java code. Include java code embedded with content.
They are more close to presentation logic. They are more close the business logic.
They are not compiled. They are compiled and described in the
deployment descriptor.
They are translated to servlets automatically They are not translated to something else
by the application server on deployment. and deployed by themselves as primitive
The translation process in transparent to the web components.
developer.

Here's a programmatic example demonstrating the practical differences between JSPs and
servlets:

file: index.jsp file: WEB-INF/classes/w/
IndexServlet.java
<html> package w ;

<head> import javax.servlet.* ;
<title>First JSP</title> import javax.servlet.http.* ;
</head> import java.io.* ;

<body> public class IndexServlet extends HttpServlet {
<%
String name = public void doGet(HttpServletRequest
request.getParameter("user_name") ; request, HttpServletResponse
out.print("Welcome " + name) ; response) throws ServletException,
%> IOException{
</body>
PrintWriter out =
</html> response.getWriter() ;
out.println("<html>") ;
out.println("<head>") ;
out.println(
"<title>First JSP</title>") ;
out.println("</head>") ;

3

out.println("<body>") ;

String name =
request.getParameter(
"user_name") ;
out.println("Welcome " + name) ;

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

}

}

2. First JSP
Here's an example of a JavaServer Page that generates dynamic output:

file: index.jsp
<html>

<head>
<title><% out.print("First JSP") ; %></title>
</head>

<body>
<%
String name = request.getParameter("user_name") ;
out.print("Welcome " + name) ;
%>
</body>

</html>

3. Syntax Elements
JSP primarily enclose page contents, or static parts of the page, with embedded JSP
elements of different types of declarations and actions. Here're some syntax elements of the
JSP technology.

3.1. Comments

Comments can be included in JSPs between <%-- and --%>. See the following example:

4

Example: JSP Comment
<html>

<head>
<title>First JSP</title>
</head>

<body>
<%-- Print a welcome message to the user %-->
<%
%>
</body>

</html>

3.2. Scriptlets

Sciptlets are snippets of java code that generate dynamic content. Scriptlets can be included
in JSPs between <% and %>. See the following example:

Example: Scriptlets
<html>

<head>
</head>

<body>
<%
%>
</body>

</html>

3.3. Expressions

Expressions are java language expressions that generate values inserted within content.
Expressions can be included in JSPs between <%= and %>. See the previous example
rewritten using expressions as the following:

Example: Expressions
<html>

5

<head>
</head>

<body>
Welcome <%= request.getParameter("user_name") %>
</body>

</html>

3.4. Declarations

Declarations are any class valid declaration that will be included in the generated servlet.
Declarations can be used in other scriptlets and expression elements. Declarations can be
included in the JSP between <%! and %>. See the following example:

Example: Declarations
<%!

int count ;

public int getVisitorNumber() {
return ++count ;
}

%>
<html>

<head>
<title>Example JSP</title>
</head>

<body>
Welcome, you are visitor number: <%= getVisitorNumber() %>
</body>

</html>

3.5. Directives

Directives provide information about the page to the JSP engine. Directives can be included
in JSP pages between <%@ and %>. JSP Directives include:

Directive Purpose
<%@ include file="header.html" %> include a file within the page.

6

<%@ page import="java.io.File" %> import a name used in a declaration.

See the following example:

Example: Directives
<%@ page import="java.io.File" %>
<html>

<%@ include file="header.html" %>

<body>
<%
File f = new File(request.getParameter("file")) ;
//display file content code
%>
</body>

</html>

file: header.html
<head>
</head>

4. JSP Translation Process
As mentioned earlier, JSPs are translated transparently to servlets. Although the
translation process is transparent to the developer, knowledge of the translation process
helps understanding JSP syntax. Translation process is simple and straightforward:

• a _jspService() method is generated to handle all HTTP request methods.
• Contents are printed to the output stream in order in _jspService() mehtod.
• Comments are ignored.
• Scriptlets are embedded within the _jspService() method in order.
• Expressions are printed to the output stream in order within _jspService().
• Declarations are written within the servlet's class body.

4.1. Example

Here is an example of how a JSP is translated into a servlet in runtime.

4.1.1. JSP

example.jsp
<%!

7

int count ;

return ++count ;
}

%>
<html>

<head>
<title>Example JSP</title>
</head>

<body>
Welcome, you are visitor number: <%= getVisitorNumber() %>
</body>

</html>

4.1.2. Corresponding Servlet Class

Runtime Class
package foo ;

import javax.servlet.* ;
import javax.servlet.http.* ;
import java.io.* ;

public class ExampleServlet extends HttpServlet {

int count ;

return ++count ;
}

public _jspService(HttpServletRequest request, HttpServletResponse
response) throws ServletException, IOException {

//bold are implicit variables generated automatically
PrintWriter out = request.getWriter() ;
HttpSession session = request.getSession() ;

out.println("<html>") ;
out.println("<head>") ;
out.println("<title>Example JSP</title>") ;
out.println("</head>") ;

8

out.println("<body>") ;
out.print("Welcome, you are visitor number: ") ;
out.print(getVisitorNumber()) ;
out.println() ;
out.println("</body>") ;
out.println("</html>") ;
}

public void goGet(HttpServletRequest request, HttpServletResponse
response)
throws ServletException, IOException {

_jspService(request, response) ;
}

public void goPost(HttpServletRequest request, HttpServletResponse
response) throws ServletException, IOException {

_jspService(request, response) ;

}

//same for rest HTTP methods methods

}

5. Implicit Variables
Implicit variables are variables defined by the application server on translating the JSP into
the corresponding servlet. Implicit variables can be used by other scriptlets and expression
elements. Every implicit variable represent some aspect to the web developer. Here is a list
of some implicit variables:

Variable Aspect
out Response stream.
request HTTP request.
response HTTP response.
session HTTP session object.

6. Hoax Example - Revisited
Our target is to develop a web application that allows a user to add two numbers. consist of
the following resources:

9

• A form page (page.html).
• A servlet that process and display the output (ProcessServlet).
• A JSP that display the output (viewResult.jsp).

The web application will work in the following scenario: the user requests page.html, fill
the form, post the data. The data is processed by ProvessServlet, which will forward the
request to the viewResult JSP to view the data. Although a trivial example, it's the main
concept in separating the controller from the view in web design patterns using JSPs.

6.1. Process Servlet

file: WEB-INF/classes/webapp/ProcessServlet.java
package webapp ;

import javax.servlet.* ;
import javax.servlet.http.* ;
import java.io.*;

public class ProcessServlet extends HttpServlet {

public void doPost(HttpServletRequest request, HttpServletResponse
response) throws IOException, ServletException {

float num1 = Float.parseFloat(request.getParameter("num1")) ;
float num2 = Float.parseFloat(request.getParameter("num2")) ;
float result = num1 + num2 ;

//set attribute to be shared with the delegates

request.setAttribute("result", reuslt) ;

//create a request dispatcher from the request object
RequestDispatcher rd =
request.getRequestDispatcher("/viewResult.jsp") ;
//forward the request passing request and response objects
rd.forward(request, response) ;

}

}

6.2. View Servlet

file: viewResult.jsp
<html>

10

<head>
<title>Result by Process Servlet</title>
</head>

<body>
Result = <%= request.getAttribute("result") %>
</body>

</html>

11

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