Web programming Units WEB1P and WEB2P

Web programming in Java 2: Java Server Pages

Java Server Pages (JSP)

Read chapters 7 and 8 of Basham.

JSP is an extension of servlet technology. In fact, given a JSP, a servlet container simply translates it into a servlet that is then executed like any other.

A servlet might be described as a Java program with HTML in it; a JSP might be described as an HTML page with Java in it.

The great advantage of a JSP over a servlet is the same as that of a PHP page over a Perl script - the program looks like an HTML page rather than a computer program. This means that web page editors (e.g. Dreamweaver) can be used to get the look and feel of the page right. Dreamweaver also provides some support for the inclusion of Java code in the page.

JSPs can be located anywhere in your web application (except in the WEB-INF directory).

To make a JSP dynamic, you embed Java code in amongst the HTML. The scripting components are:

<%

String name = request.getParameter("name");
%>

<%= name %>

<%! int counter = 0; %>

<jsp:include page="header.jsp"/>

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

<%--
This is a JSP comment
--%>

<--
This is an HTML comment
-->

Consider this simple example of a JSP (PDF format) which does almost exactly the same as the servlet considered above.

• On the first page there are just two lines containing JSP code.
  • Line 1 stipulates (via a page directive) that the output will be HTML
  • Line 6 prints out the value of the "name" parameter
• Everything else is either implicit (the declaration of the servlet and all its necessary environment) or is assumed to be HTML output.
• Pages 2 and 3 show the servlet code that this page translated into. (I used Tomcat v4 - other servlet containers might translate it into different but equivalent code.) It is not necessary for you to understand everything here, but note the following:
  • The servlet is called "hello$jsp" (line 9). This is constructed from the JSP filename and is useful to know for debugging purposes.
  • The main method of the servlet is _jspService (lines 22-78).
  • After setting things up, the actual code to output the page begins at line 52. Note how the original source from the JSP is included as a comment.
  • Line 61 is the one that implements line 6 of the JSP.

It is generally agreed that using scriplets, declarations and expressions is a bad thing. Their use leads to cluttered unreadable code, where Java and HTML get hopelessly mixed up, but you need to know about them in case you see a page that includes them.

Most modern JSP pages use only directives and an occasional action, and do the rest of their task using custom tags (see below).

Java tag libraries

A further refinement of JSP is the use of tag libraries. You can either use the Java Standard Tag Libraries (JSTL) or define your own custom tag libraries.

Tag libraries try to obviate the need to include any Java code in the page, thereby making the page XHTML compatible and therefore more likely to be processable with XML tools.

JSP 2.0 also introduced the Expression Language, a means of accessing Java Beans and other objects directly in the HTML page. This reduces the number of tags that need to appear.

<c:if test="${status.totalVisits == 1000000}">
You are the millionth visitor to our site!
Congratulations!
</c:if>

<jim:display value="${record.lastUpdated}" format="large"/>

Hello ${me.name}