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Chapter 7
Java Server Pages
Objectives
• Explain how the separation of concerns principle
applies to JSP
• Describe the operation and life-cycle of a JSP
• List and describe the JSP element types
• Explain the operation of a JSP error page
• Explain the operation of include and forward
directives
• Explain the purpose of a Java Bean and how
one can be used from a JSP
• Explain the purpose of a taglib and how a tag
can be used from a JSP
Separation of Concerns
• A basic principle of software design:
Software modules should have distinct
responsibilities as much as possible
input
data
retrieval
calculation
output
Separation of Content
From Processing (1/2)
• A servlet generating an HTML document
intermixes Java code and HTML
public void doGet(HttpServletRequest req,
HttpServletResponse res) throws IOException() {
PrintWriter out = res.getWriter();
res.setContentType("text/html");
out.println("<html><head><title>"
+ "Example HTML" + "</title></head><body>";
...
out.println("</html>");
out.close();
}
Separation of Content
From Processing (2/2)
• Java Server Pages (JSP) allow static
HTML content to be separated from
dynamic content (created by Java code)
within a document
<%@ page %>
<html>
<head><title>Example Page</title></head>
<body>
<h1>Example Page</h1>
<%
String name = req.getParameter("name");
%>
<p>Hello, <%= name %></p>
...
</html>
JSP Elements
• JSP tags belong to one of five element
types
Declaration: declare variable or procedure
Expression: evaluate a Java expression
Scriptlet:
execute a Java code segment
Directive:
a JSP compiler command
Action Element:
interacts with another component
JSP Declaration
• A declaration can be used to initialize
variables or define procedures for use
later in the JSP
<%! Date today = new Date(); %>
<%! // return the difference between two integers
public int diff(int a, int b) {
return (a < b) ? (b - a) : (a – b);
}
%>
JSP Expression
• A JSP expression is evaluated and then
replaced by that value
<%! Date today = new Date(); %>
<p>Today is: <%= today.toString() %> </p>
Generated HTML:
<p>Today is: Mon Jun 15 08:05:00 EST 2009 </p>
JSP Scriptlet
• A JSP scriptlet is a short section of Java
code that is executed in-line
• Scriptlets have access to several
pseudovariables:
– out
– session
– request
– response
HTML output stream
current HttpSession object
HTTPRequest object
HTTPResponse object
JSP Scriptlet
• This scriptlet displays the current date and
time
<% // output the current date and time
Date today = new Date();
out.println("<p>Today is: " + date.toString()
+ " </p>";
%>
JSP Scriptlet
• The scriptlet checks to see if a Boolean
session attributed named "login" exists
<% // check to see if the user is logged in
boolean loggedIn = ((Boolean)
session.getAttribute("login")).booleanValue();
if (loggedIn) {
out.println("<p>Please select an option</p>";
}
else {
out.println("<p>Please login</p>";
}
%>
JSP Scriptlet
• The scriptlet maintains a "hit count"
<%
// get or create hit count
Integer hitCount =
(Integer) session.getAttribute("hits");
if (hitCount == null) {
hitCount = new Integer(0):
}
// increment and save hit count
hitCount = new Integer(hitCount.intValue() + 1);
session.setAttribute("hits", hitCount);
// display hit count
<p>Hit Count: <%= hitCount.intValue() %></p>
%>
JSP Life-Cycle
• JSP's are actually executed as servlets
• The first time a JSP is requested, it is
translated to a servlet and compiled
• Each subsequent request executes the
same servlet
JSP
translated
upon first
request
Servlet
JSP Life-Cycle
JSP Directive
• The page directive appears at the top of
each JSP and controls its execution
• The page directive also specifies imports
<%@ page import="java.util.Date, java.io.*" %>
JSP Errors
• Compilation errors and run-time
exceptions are logged and then returned
as an HTML response
???
JSP Error Page
• The proper way to handle errors is to
declare an error page
• The error page is invoked whenever a JSP
compilation or run-time error occurs
• The error page can display a standard
error message
JSP Error Page
• JSP that might throw an error:
<%@ page import="java.util.*" errorPage="stderror.jsp" %>
... possible errors ...
• stderror.jsp:
<%@ page IsErrorPage="true" %>
<html><head><title>Error Page</title></head>
<body>
<p>That request cannot be completed at this time.</p>
<p>Please contact your system administrator.</p>
</body></html>
JSP Action Element
• JSP action elements control interactions
with other JSP's, including
– include: add the contents of another JSP
– forward: send a request to another JSP for
handling
JSP Include
• A JSP include can be used to add
standard content to a JSP, such as
headers and footers
<%@ page %>
<jsp:include page="stdheader.jsp" />
...unique content of this JSP...
<jsp:include page="stdfooter.jsp" />
JSP Forward
• A JSP forward causes control to be
transferred to a different JSP
<%@ page %>
<% // verify that user is logged in
Boolean loggedIn =
(Boolean) session.getAttribute("login");
if (loggedIn == null
|| loggedIn.booleanValue() == false) {
%>
<jsp:forward page="login.jsp" />
<%
}
%>
the scriptlet is
continued
after the directive,
which is not a
Java statement
Java Beans
• A Java Bean is a Java class that can be
invoked from a JSP
• This allows complex processing required
by a JSP to be separated from the JSP
• A JSP can instantiate an instance of a
bean class and invoke its methods
Java Bean Example
• Java Bean
package bean;
public class AdderUtility {
public int add(int a, int b) {
return a + b;
}
}
• JSP
<%@ page %>
<jsp:useBean id="adder" class="bean.AdderUtility"
scope="session" />
<%
int a, b, sum;
…
int sum = adder.add(a, b);
%>
Java Bean Uses
• Java Beans are typically used to
encapsulate complex application logic,
including calculations, database access, or
network access
Application
JSP
Java Bean
Database
Network
JSP Taglibs
• A tag library allows programmers to create
custom tags to invoke application
functions with JSPs
• Each custom tag is associated with a Java
class that is executed when the tag is
evaluated
Taglib Example
• JSP code:
tag body content
<tags:boxin border="green">
This code appears in a green box
</tags:boxin>
This code appears in a green box
Taglib Example
• BoxInTag.java
public class BoxInTag extends BodyTagSupport {
private String border;
returns the tag body
public void setBorder(String border) {
content, i.e., the
this.border = border;
text between the
}
public int doStartTag() throws JspTagException {
tag opening and
return EVAL_BODY_BUFFERED;
closing
}
public int doEndTag() throws JspTagException {
JspWriter out = pageContext.getOut();
String content = bodyContent.getString();
try {
out.println("<div style=\"border-style: solid; border-color: "
+ border + "\">" + content + "</div>" );
}
catch (IOException ioe) {
ioe.printStackTrace();
}
return EVAL_PAGE;
}
}
Tag Library Definition File (TLD)
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE taglib PUBLIC
"-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.2//EN"
"http://java.sun.com/j2ee/dtd/web-jsptaglibrary_1_2.dtd">
<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance“
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
version="2.0">
<tlib-version>1.0</tlib-version>
<description>Show Standings Tag</description>
<tag>
<name>boxin</name>
<tag-class>tags.BoxInTag</tag-class>
<description> Draw a box around the content </description>
<body-content>tagdependent</body-content>
</tag>
</taglib>
Review
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Java Server Pages
JSP life-cycle
JSP Elements
JSP include and forward directives
Java Beans with JSP
JSP taglibs
