Java Server Page

Introduction

• Servlets handle HTTP requests by a Java program. They produce an HTTP response by writing HTTP/HTML to an output object
• If the processing done by a servlet is small and the amount of writing is large, then servlets become messy to write, hard to read and difficult to maintain
• JSP pages (like ASP and many other systems) consist mainly of HTML instructions with some embedded HTTP instructions and a mechanism to embed executable code (for JSP, this is Java code)
• These are good for the presentation-side of a Web application

MVC model

• Smalltalk invented the Model-View-Controller pattern to handle user interfaces
  • The model is the object which contains the state variables and methods of the "system" e.g. for an airconditioner, it would hold current temperature, temperature that has been set and methods to turn on or off, reset temperature etc
  • The view is e.g a GUI object that displays the state of the system and has buttons, sliders, etc to change values
  • The controller handles actions invoked by the view and calls methods on the model. It also takes changes in the model and tells the view to change its appearance
• The MVC has been used even when it is not quite the right description
• IBM came up with something they called MVC for Web applications, even though it isn't quite right:
  • The model is the backend logic module
  • The view is the Java Server Page that creates HTML for the browser to display
  • The controller is the servlet that handles input from the browser and calls the business logic and JSP as appropriate

Basic JSP page

• JSP pages are mainly HTML text
• Tags <% ... %> contains Java code
• A tag <%= value %> inserts the value of a Java expression into the HTML document

Deploying JSP pages

• Deployment of servlets and JSP pages seems to me to be far more complex than it needs to be
• Under the Tomcat home there is a directory webapps
• Create your own subdirectories
  • jan
  • jan/jsp
  • jan/WEB-INF
  • jan/WEB-INF/classes
• Compiled servlet files go into jan/WEB-INF/classes
• JSP pages go into jan/jsp
• A servlet such as HelloWWW is referenced by

  
        http://jan.newmarch.name:8080/jan/servlet/helloWWW
        

• A JSP page such as hello.jsp is referenced by

  
        http://jan.newmarch.name:8080/jan/jsp/hello.jsp
        

• That was the simple version - it can get much messier
• When you modify a JSP page or a servlet class, you may need to restart Tomcat to register the changes

Request parameter

It is customary in a servlet to call the HttpServletRequest request parameter request. In JSP, that is its name:

Application objects

• You can define your own objects and call them
• Java code:
• The Java class should be copied to the servlet location e.g. TOMCAT_HOME/webapps/jan/WEB-INF/classes
• A JSP calling this is
• Tomcat v1.5.5 seems to require the Java classes to be in a package - Tomcat 1.4 didn't

Forwarding control

• A servlet can forward control to a JSP page by e.g.

  
      RequestDispatcher dispatcher = request.
          getRequestDispatcher("/template.jsp");
      if (dispatcher != null)
          dispatcher.forward(request, response);
        

• One JSP page can forward control to another by

  
        <jsp:forward page="/main.jsp" />
        

Including another JSP page

• The syntax <%@ include file="relativeURL" %> includes a file at that point, which could be another JSP page

Declarations

• If you want to declare a Java variable that can be used throughout the JSP page, this can be done using the syntax <%! ... %> such as

  
        <%! int i = 0; %>
        

• Methods can also be defined e.g.

  
        <%! public String f(int i) {
                    if (i < 3) return("...");
                    ...
            }
        %>
        

Mixing HTML and Java

• Java code appears within <% ... >
• HTML code appears as normal text
• Java constructs can be spread around HTML e.g.

  
  <% if (Calendar.getInstance().get(Calendar.AM_PM) == Calendar.AM) { %>
       <p> Good Morning  </p>
  <% } else { %>
       <p> Good Afternoon  </p>
  <% } %>
        

• This can get messy very quickly

Custom tags

• JSP pages contain a mixture of Java and HTML, and can become messy
• One way of cotrolling this is define custom classes and hide most of the logic in methods
• This doesn't help if the methods have to contain lots of HTML! It looks like a servlet again
• An alternative is to define new XML tags which actually hide Java code inside themselves.
• Using a custom tag makes things look like lots of HTML/XML intead of Java/HTML
• There are many libraries of custom tags, and you can define your own

Java Standard Tag Library

• The JSTL is a set of custom tags - the spec etc is at JavaServer Pages Standard Tag Library
• They include many common activities
• There are libraries such as
  • Core
  • i18n
  • XML
  • SQL
• Each tag library is defined at a URL, and this must be used to define an XML namespace (similar to a Java package) e.g.

  
        <%@ taglib prefix="c" url="http://java.sun.com/jsp/jstl/core" %>
        

  defines the Core namespace "c".
  (There seems to be nothing at this URL, and all the documents/tutorials point to different URLs, none of which exist!)
• The standard libraries and customary tags are (from file:///usr/local/src/jakarta-taglibs-standard-1.1.2/doc/GettingStarted.html)

  Funtional Area	URI	Prefix	Example
  Core	http://java.sun.com/jsp/jstl/core	c	<c:tagname ...>
  XML processing	http://java.sun.com/jsp/jstl/xml	x	<x:tagname ...>
  I18N capable formatting	http://java.sun.com/jsp/jstl/fmt	fmt	<fmt:tagname ...>
  Database access (SQL)	http://java.sun.com/jsp/jstl/sql	sql	<sql:tagname ...>
  Functions	http://java.sun.com/jsp/jstl/functions	fn	fn:functionName(...)

Standard tags

From The JavaTM Web Services Tutorial - Using JSTL

Area	Function	Tags	TLD	Prefix
Core	Expression Language Support	catch out remove set	/jstl-c	c
	Flow Control	choose   when   otherwise forEach forTokens if
	URL Management	import   paramredirect   param url   param
XML	Core	out parse set	/jstl-x	x
	Flow Control	choose  when  otherwise forEach if
	Transformation	transform   param
I18n	Locale	setLocale	/jstl-fmt	fmt
	Message formatting	bundle message   param setBundle
	Number and dateformatting	formatNumber formatDate parseDate parseNumber setTimeZone timeZone
Database		setDataSource	/jstl-sql	sql
	SQL	query   dateParam   param transaction update  dateParam   param

Using JSTL tags

Expression language

• JSTL evaluates expressions in tags, both as attributes and as tag content
• Expressions are enclosed in ${ ... }
• The expression language is based on ECMAScript (JavaScript) and XPath

JSTL Implicit objects

The EL (expression language) has a number of inbuilt variables including

• pageContext
• pageContext.request
• param
• paramValues
• header
• headerValues

JSTL and i18n

• The i18n tag library is defined by the URL http://java.sun.com/jsp/jstl/fmt
• The customary namespaces is "fmt":

  
        <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
        

• Formatting tags are
  • fmt:setLocale
  • fmt:formatNumber
  • fmt:formatDate
  • fmt:timeZone
  • fmt:setTimeZone
  • fmt:parseDate
  • fmt:parseNumber
  • fmt:requestEncoding
  • fmt:bundle

Expression variables

• Variables in expressions can be implicit ones as above
• You can also put your own variables in the application context, eg. from a servlet
• Servlet
• JSP:

Summary

Java Server Pages act as the presentation end of a backend web application. They allow a mix of HTML and Java in various ways

• Include all the Java code in JSP scripts and expressions
• Define extra Java classes and just include a few calls to them in JSP scripts and expressions
• Use a tag and expression library such as JSTL
• Build and use a tag library

References

• Create internationalized JSP applications by Sing li
• Java Web Services Tutorial Sun MicroSystems
• ` JavaServer Pages Standard Tag Library Sun MicroSystems
• Apache Jakarta Tomcat Apache Software Foundation
• Jakarta Taglibs Apache Software Foundation
• Internationalize JSP-based Websites Govind Seshadri
• Common problems with i18n and servlets/jsp-s Anton Tagunov
• JSP Standard Tag Library eases Webpage development Steve Small
• JSP Standard Tag Libraries, Part 1 Sue Spielman
• JSP Standard Tag Libraries, Part 2 Sue Spielman
• JSTL 1.0: Standardizing JSP, Part 1 Hans Bergsten
• JSTL 1.0: What JSP Applications Need, Part 2 Hans Bergsten