jsp tutorial

JavaServer Pages(TM) Tutorial

Welcome to the JavaServer PagesTM (JSPTM) technology, the cross-platform method of
generating dynamic content for the Web.

If you have reached this learn-by-example tutorial, you are probably new to the
technology. You might be a Web developer or enterprise developer who wants to use
JavaServer Pages to develop dynamic Web applications. The sections in this tutorial
contain a series of topics and example applications that teach you how to use the
essential features of JavaServer Pages technology:
s “A First JSP Application” on page 4
s “Handling HTML Forms” on page 7
s “Using Scripting Elements” on page 18
s “Handling Exceptions” on page 26

1

Installing and Running the Example
Applications
The example applications described in this tutorial are packaged so that they can be
easily installed and run on the Tomcat JSP and server implementation. To run the
examples:

1. Download and install Tomcat

2. Download the example applications

The complete binary and source code for three of the examples are packaged in
the Web application archives helloworld.war, hellouser.war, and
email.war, contained in the zip archive examples.zip. To install the
applications in Tomcat, download examples.zip into the directory
TOMCAT_HOME/webapps and unzip the archive. The fourth example, number
guess, is already installed in Tomcat in the directory TOMCAT_HOME/webapps/
examples/jsp/num.

3. Configure Tomcat for the example applications

When an archived Web application is accessed, Tomcat 3.2 automatically unpacks
it into the directory TOMCAT_HOME/webapps/appName and adds the context for
each archive to the server startup file. If you are using an earlier version of
Tomcat you will need to:
s Unpack the Web application archive with the command jar xvf
appName.war.
s Add the following line to the file TOMCAT_HOME/conf/server.xml for each
application:
<Context path="/appName" docBase="webapps/appName" debug="0"
reloadable="true" />

When a Web application archive is unpacked, its contents are deposited into the
directories listed in the following table. This directory layout is required by the
Java Servlet specification and is one that you usually will use while developing an
application.

Directory Contents

appName JSP, HTML, and image files
appName/WEB-INF/classes classes accessed by JSP files

2 JavaServer Pages Tutorial

4. Open the URL of the first page of each example in a Web browser:
s http://localhost:8080/helloworld/helloworld.jsp
s http://localhost:8080/hellouser/hellouser.jsp
s http://localhost:8080/examples/jsp/num/numguess.jsp
s http://localhost:8080/email/email.jsp

Installing and Running the Example Applications 3

A First JSP Application
FIGURE 1-1 shows what is perhaps the simplest JSP application one could write. It
continues the illustrious computer science Hello, World tradition. CODE EXAMPLE 1-1
and CODE EXAMPLE 1-2 show how the example is put together.

FIGURE 0-1 Duke Says Hello

CODE EXAMPLE 0-1 The Duke Banner (dukebanner.html)
<table border="0" width="400" cellspacing="0" cellpadding="0">
<tr>
<td height="150" width="150">   </td>
<td width="250">   </td>
</tr>
<tr>
<td align="right" width="250">
<img src="duke.waving.gif">
</td>
</tr>

</table>
<br>


CODE EXAMPLE 0-2 The JSP Page (helloworld.jsp)
<%@ page info="a hello world example" %>

<html>
<head><title>Hello, World</title></head>

<body bgcolor="#ffffff" background="background.gif">
<%@ include file="dukebanner.html" %>
<table>
<tr>
<td width=150>   </td>
<td width=250 align=right>
<h1>Hello, World!</h1> </td>
</tr>
</table>

</body>
</html>

The Page Directive
The page directive is a JSP tag that you will use in almost every JSP source file you
write. In helloworld.jsp, it’s the line that looks like this:
<%@ page info="a hello world example" %>

The page directive gives instructions to the JSP container that apply to the entire JSP
source file. In this example, page specifies an informative comment that will become
part of the compiled JSP file. In other cases, page might specify the scripting
language used in the JSP source file, packages the source file would import, or the
error page called if an error or exception occurs.

You can use the page directive anywhere in the JSP file, but it’s good coding style to
place it at the top of the file. Because it’s a JSP tag, you can even place it before the
opening <html> tag.

The Include Directive
The include directive inserts the contents of another file in the main JSP file, where
the directive is located. It’s useful for including copyright information, scripting
language files, or anything you might want to reuse in other applications. In this
example, the included file is an HTML table that creates a graphic banner.

A First JSP Application 5

You can see the content of the included file by viewing the page source of the main
JSP file while you are running Hello, World. The included file does not contain
<html> or <body> tags, because these tags would conflict with the same tags in the
calling JSP file.

A Note About the JSP Tags
As you use the examples in this chapter, remember that the JSP tags are case
sensitive. If, for example, you type <%@ Page %>, instead of <%@ page %>, your
tag will not be recognized, and the JSP implementation will throw an exception.
Some of the attributes on the tags take class names, package names, pathnames or
other case-sensitive values as well.

If you have any doubts about the correct spelling or syntax of any JSP tag, see the
JavaServer Pages Syntax Card.

How To Run the Example
Install the example as described in “Installing and Running the Example
Applications” on page 2. Then, open a Web browser and go to:
http://localhost:8080/helloworld/helloworld.jsp


Handling HTML Forms
One of the most common parts of an electronic commerce application is an HTML
form in which a user enters some information. The information might be a
customer’s name and address, a word or phrase entered for a search engine, or a set
of preferences gathered as market research data.

What Happens to the Form Data
The information the user enters in the form is stored in the request object, which is
sent from the client to the JSP container. What happens next?

FIGURE 1-2 represents how data flows between the client and the server (at least
when you use Tomcat; other JSP containers may work a little differently).

FIGURE 0-2 How Data is Passed Between the Client and the Server

response
Client

JSP Container & response
request Web Server

response

JSP File Component
Component

request
request

The JSP container sends the request object to whatever server-side component
(JavaBeansTM component, servlet, or enterprise bean) the JSP file specifies. The
component handles the request, possibly retrieving data from a database or other
data store, and passes a response object back to the JSP container. The JSP
container passes the response object to the JSP page, where its data is formatted

Handling HTML Forms 7

according the page’s HTML design. The JSP container and Web server then send the
revised JSP page back to the client, where the user can view the results in the Web
browser. The communications protocol used between the client and server can be
HTTP, or it can be some other protocol.

The request and response objects are always implicitly available to you as you
author JSP source files. The request object is discussed in more detail later in this
tutorial.

How To Create a Form
You typically define an HTML form in a JSP source file, using JSP tags to pass data
between the form and some type of server-side object (usually a bean). In general,
you do the following things in your JSP application:

1. Start writing a JSP source file, creating an HTML form and giving each form
element a name.

2. Write the bean in a .java file, defining properties, get, and set methods that
correspond to the form element names (unless you want to set one property value
at a time explicitly).

3. Return to the JSP source file. Add a <jsp:useBean> tag to create or locate an
instance of the bean.

4. Add a <jsp:setProperty> tag to set properties in the bean from the HTML
form (the bean needs a matching set method).

5. Add a <jsp:getProperty> tag to retrieve the data from the bean (the bean
needs a matching get method).

6. If you need to do even more processing on the user data, use the request object
from within a scriptlet.

The Hello, User example will make these steps more clear.


A Dynamic Hello Application
The Hello, User JSP application shown in FIGURE 1-3 and FIGURE 1-4 expands on the
Hello, World application. The user has an opportunity to enter a name into a form
and the JSP page generates a new page that displays the name.

FIGURE 0-3 The User Enters a Name

FIGURE 0-4 Then Duke Says Hello


Example Code
CODE EXAMPLE 1-3, CODE EXAMPLE 1-4, CODE EXAMPLE 1-5, and CODE EXAMPLE 1-6
contain the code for the Duke banner, main JSP page, response JSP page, and
JavaBeans component that handles the name input.

CODE EXAMPLE 0-3 The Duke Banner (dukebanner.html)

<table border="0" width="400" cellspacing="0" cellpadding="0">
<tr>
<td height="150" width="150">   </td>
</tr>
<tr>
<td align="right" width="250"> <img src="duke.waving.gif">
</td>
</tr>
</table>
<br>

CODE EXAMPLE 0-4 The Main JSP File (hellouser.jsp)
<%@ page import="hello.NameHandler" %>

<jsp:useBean id="abean" scope="page" class="hello.NameHandler" />
<jsp:setProperty name="abean" property="*" />

<html>

<head><title>Hello, User</title></head>
<body bgcolor="#ffffff" background="background.gif">

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


<table border="0" width="700">
<tr>
<td width="550">
<h1>My name is Duke. What’s yours?</h1>
</td>
</tr>

<tr>
<td width="150"   </td>
<td width="550">
<form method="get">
<input type="text" name="username" size="25">
<br>
<input type="submit" value="Submit">
<input type="reset" value="Reset">
</td>
</tr>
</form>
</table>

<%
if ( request.getParameter("username") != null ) {
%>
<%@ include file="response.jsp" %>

<%
}
%>

</body>
</html>


CODE EXAMPLE 0-5 The Response File (response.jsp)

<table border="0" width="700">

<tr>
<td width="150">
 
</td>

<td width="550">
<h1>Hello, <jsp:getProperty name="abean" property="username" />!
</h1>

</td>
</tr>
</table>

CODE EXAMPLE 0-6 The Bean That Handles the Form Data (namehandler.java)
package hello;

public class NameHandler {

private String username;

public NameHandler() {
username = null;
}

public void setUsername( String name ) {
username = name;
}

public String getUsername() {
return username;
}
}


Constructing the HTML Form
An HTML form has three main parts: the opening and closing <form> tags, the
input elements, and the Submit button that sends the data to the server. In an
ordinary HTML page, the opening <form> tag usually looks something like this:
<form method=get action=someURL>

In a JSP application, the action attribute specifies the component or JSP file that
will receive the data the user enters in the form. You can omit the action attribute
if you want the data processed by the object specified in the <jsp:useBean> tag.
(This is similar to using action in other Web applications, where it specifies a CGI
script or other program that will process the form data.)

The rest of the form is constructed just like a standard HTML form, with input
elements, a Submit button, and perhaps a Reset button. Be sure to give each input
element a name, like this:
<input type="text" name="username">

Using the GET and POST Methods
The HTTP GET and POST methods send data to the server. In a JSP application, GET
and POST send data to the server. (The data, along with the rest of the JSP
application is compiled into a Java servlet that returns a response to the client Web
browser; for more information, see “How the JSP Page Is Compiled” on page 16.)

In theory, GET is for getting data from the server and POST is for sending data there.
However, GET appends the form data (called a query string) to an URL, in the form
of key/value pairs from the HTML form, for example, name=John. In the query
string, key/value pairs are separated by & characters, spaces are converted to +
characters, and special characters are converted to their hexadecimal equivalents.
Because the query string is in the URL, the page can be bookmarked or sent as email
with its query string. The query string is usually limited to a relatively small number
of characters.

The POST method, however, passes data of unlimited length as an HTTP request
body to the server. The user working in the client Web browser cannot see the data
that is being sent, so POST requests are ideal for sending confidential data (such as a
credit card number) or large amounts of data to the server.


Writing the Bean
If your JSP application uses a bean, you can write the bean according to the design
patterns outlined in the JavaBeans component architecture, remembering these general
points:
s If you use a <jsp:getProperty> tag in your JSP source file, you need a
corresponding get method in the bean.
s If you use a <jsp:setProperty> tag in your JSP source file, you need one or
more corresponding set methods in the bean.

Setting properties in and getting properties from a bean is explained a bit more in
the next section.

Getting Data From the Form to the Bean
Setting properties in a bean from an HTML form is a two-part task:
s Creating or locating the bean instance with <jsp:useBean>
s Setting property values in the bean with <jsp:setProperty>

The first step is to instantiate or locate a bean with a <jsp:useBean> tag before you
set property values in the bean. In a JSP source file, the <jsp:useBean> tag must
appear above the <jsp:setProperty> tag. The <jsp:useBean> tag first looks for
a bean instance with the name you specify, but if it doesn’t find the bean, it
instantiates one. This allows you to create a bean in one JSP file and use it in another,
as long as the bean has a large enough scope.

The second step is to set property values in the bean with a <jsp:setProperty>
tag. The easiest way to use <jsp:setProperty> is to define properties in the bean
with names that match the names of the form elements. You would also define
corresponding set methods for each property. For example, if the form element is
named username, you would define a property username property and methods
getUsername and setUsername in the bean.

If you use different names for the form element and the bean property, you can still
set the property value with <jsp:setProperty>, but you can only set one value at
a time. For more information on the syntax variations of <jsp:setProperty>, see
the JavaServer Pages Syntax Card.

Checking the Request Object
The data the user enters is stored in the request object, which usually implements
javax.servlet.HttpServletRequest (or if your implementation uses a different
protocol, another interface that is subclassed from
javax.servlet.ServletRequest).


You can access the request object directly within a scriptlet. Scriptlets are discussed
in more detail in the next section, but for now it’s enough to know that they are
fragments of code written in a scripting language and placed within <% and %>
characters. In JSP version 1.1, you must use the Java programming language as your
scripting language.

You may find some of these methods useful with the request object:

Method Defined In Job Performed

getRequest javax.servlet.jsp.PageContext Returns the current
request object
getParameterNames javax.servlet.ServletRequest Returns the names of
the parameters
request currently
contains
getParameterValues javax.servlet.ServletRequest Returns the values of
the parameters
request currently
contains
getParameter javax.servlet.ServletRequest Returns the value of a
parameter if you
provide the name

You’ll find other methods as well, those defined in ServletRequest,
HttpServletRequest, or any subclass of ServletRequest that your
implementation uses.

The JSP container always uses the request object behind the scenes, even if you do
not call it explicitly from a JSP file.

Returning Data to the JSP Page
Once the user’s data has been sent to the server, you may want to retrieve the data
and display it in the JSP page. To do this, use the <jsp:getProperty> tag, giving
it the bean name and property name:
<h1>Hello, <jsp:getProperty name="abean" property="username"/>!

The bean names you use on the <jsp:useBean>, <jsp:setProperty>, and
<jsp:getProperty> tags must match, for example:
hellouser.jsp:
<jsp:useBean id="abean" scope="session" class="hello.NameHandler" />
<jsp:setProperty name="abean" property="*" />


response.jsp:
<h1>Hello, <jsp:getProperty name="abean" property="username"/>!

In this example, the tags are in two files, but the bean names still must match. If they
don’t, Tomcat throws an error, possibly a fatal one.

The response the JSP container returns to the client is within the implicit response
object, which the JSP container creates.

How the JSP Page Is Compiled
It is very important to understand how a JSP page is compiled when your user loads
it into a Web browser. The compilation process is illustrated in FIGURE 1-5.

FIGURE 0-5 How a JSP Page is Compiled

JSP File JSP File Graphics

Component

.java ﬁle

Data Store
Web Browser
via HTTP or
other protocol
.class ﬁle

Client Server Data Store


First of all, a JSP application is usually a collection of JSP files, HTML files, graphics
and other resources. When the user loads the page for the first time, the files that
make up the application are all translated together, without any dynamic data, into
one Java source file (a .java file) with a name that your JSP implementation
defines. Then, the .java file is compiled to a .class file. In most implementations,
the .java file is a Java servlet that complies with the Java Servlet API. This entire
stage is known as translation time.

When the user makes a request of the JSP application (in this case, when the user
enters something in the form and clicks Submit), one or more of the application’s
components (a bean, enterprise bean, or servlet) handles data the user submits or
retrieves data dynamically from a data store and returns the data to the .java file
where it is recompiled in the .class file. The .class file, being a Java servlet,
returns the data to the client Web browser by its service method. When the user
makes a new request, the component obtains or handles the data again and returns
it to the .java file, which is compiled again into the .class file. This stage is
known as request time.

How to Run the Example
http://localhost:8080/hellouser/hellouser.jsp


Using Scripting Elements
At some point, you will probably want to add some good, old-fashioned program-
ming to your JSP files. The JSP tags are powerful and encapsulate tasks that would
be difficult or time-consuming to program. But even so, you will probably still want
to use scripting language fragments to supplement the JSP tags.

The scripting languages that are available to you depend on the JSP container you
are using. With Tomcat, you must use the JavaTM programming language for
scripting, but other vendors’ JSP containers may include support for other scripting
languages).

How To Add Scripting
First, you’ll need to know a few general rules about adding scripting elements to a
JSP source file:

1. Use a page directive to define the scripting language used in the JSP page (unless
you are using the Java language, which is a default value).

2. The declaration syntax <%! ... %> declares variables or methods.

3. The expression syntax <%= ... %> defines a scripting language expression and
casts the result as a String.

4. The scriptlet syntax <% ... %> can handle declarations, expressions, or any
other type of code fragment valid in the page scripting language.

5. When you write a scriptlet, end the scriptlet with %> before you switch to HTML,
text, or another JSP tag.

The Difference Between <%, <%=, and <%!
Declarations, expressions, and scriptlets have similar syntax and usage, but also
some important differences. Let’s explore the similarities and differences here, with
some examples.

Declarations (between <%! and %> tags) contain one or more variable or method
declarations that end or are separated by semicolons:
<%! int i = 0; %>
<%! int a, b; double c; %>
<%! Circle a = new Circle(2.0); %>


You must declare a variable or method in a JSP page before you use it in the page.
The scope of a declaration is usually a JSP file, but if the JSP file includes other files
with the include directive, the scope expands to cover the included files as well.

Expressions (between <%= and %> tags) can contain any language expression that is
valid in the page scripting language, but without a semicolon:
<%= Math.sqrt(2) %>
<%= items[i] %>
<%= a + b + c %>
<%= new java.util.Date() %>

The definition of a valid expression is up to the scripting language. When you use
the Java language for scripting, what’s between the expression tags can be any
expression defined in the Java Language Specification. The parts of the expression are
evaluated in left-to-right order. One key difference between expressions and
scriptlets (which are described next and appear between <% and %> tags) is that a
semicolon is not allowed within expression tags, even if the same expression
requires a semicolon when you use it within scriptlet tags.

Scriptlets (between <% and %> tags) allow you to write any number of valid
scripting language statements, like this:
<%
String name = null;
if (request.getParameter("name") == null) {
%>

Remember that in a scriptlet you must end a language statement with a semicolon if
the language requires it.

When you write a scriptlet, you can use any of the JSP implicit objects or classes
imported by the page directive, declared in a declaration, or named in a
<jsp:useBean> tag.

Using Scripting Elements 19

The Number Guess Game
The Number Guess game makes good use of scriptlets and expressions, as well as
using the knowledge of HTML forms you gained in the last example.

FIGURE 0-6 About to Guess a Number


Example Code

CODE EXAMPLE 0-7 Displaying the Number Guess Screen (numguess.jsp)


<%@ page import = "num.NumberGuessBean" %>

<jsp:useBean id="numguess" class="num.NumberGuessBean"
scope="session" />
<jsp:setProperty name="numguess" property="*" />

<html>
<head><title>Number Guess</title></head>
<body bgcolor="white">
<font size=4>

<% if (numguess.getSuccess() ) { %>

Congratulations! You got it.
And after just <%= numguess.getNumGuesses() %> tries.<p>

<% numguess.reset(); %>
Care to <a href="numguess.jsp">try again</a>?

<% } else if (numguess.getNumGuesses() == 0) { %>

Welcome to the Number Guess game.<p>
I’m thinking of a number between 1 and 100.<p>

<form method=get>
What’s your guess? <input type=text name=guess>
<input type=submit value="Submit">
</form>

<% } else { %>

Good guess, but nope. Try <b><%= numguess.getHint() %></b>.
You have made <%= numguess.getNumGuesses() %> guesses.<p>

I’m thinking of a number between 1 and 100.<p>

<form method=get>
What’s your guess? <input type=text name=guess>
<input type=submit value="Submit">
</form>

<% } %>

</font>
</body>
</html>

CODE EXAMPLE 0-8 Handling the Guess (NumberGuessBean.java)
// Number Guess Game
// Written by Jason Hunter, CTO, K&A Software
// jasonh@kasoftware.com, http://www.servlets.com
// Copyright 1999, K&A Software
// Distributed by Sun Microsystems with permission

package num;

import java.util.*;
public class NumberGuessBean {

int answer;
boolean success;
String hint;
int numGuesses;

public NumberGuessBean() {
reset();
}

public void setGuess(String guess) {
numGuesses++;

int g;


try {
g = Integer.parseInt(guess);
}
catch (NumberFormatException e) {
g = -1;
}
if (g == answer) {
success = true;
}
else if (g == -1) {
hint = "a number next time";
}
else if (g < answer) {
hint = "higher";
}
else if (g > answer) {
hint = "lower";
}
}
public boolean getSuccess() {
return success;
}
public String getHint() {
return "" + hint;
}

public int getNumGuesses() {
return numGuesses;
}

public void reset() {
answer = Math.abs(new Random().nextInt() % 100) + 1;
success = false;
numGuesses = 0;
}
}

Using Scripting Elements in a JSP File
The file numguess.jsp is an interesting example of the use of scripting elements,
because it is structured as you might structure a Java programming language source
file, with a large if ... else statement within scriptlet tags. The difference is that
the body of each statement clause is written in HTML and JSP tags, rather than in a
programming language.


You are not required to write scriptlets mingled with HTML and JSP tags, as shown
in numguess.jsp. Between the <% and %> tags, you can write as many lines of
scripting language code as you want. In general, doing less processing in scriptlets
and more in components like servlets or Beans makes your application code more
reusable and portable. Nonetheless, how you write your JSP application is your
choice, and Tomcat specifies no limit on the length of a scriptlet.

Mingling Scripting Elements with Tags
When you mingle scripting elements with HTML and JSP tags, you must always end
a scripting element before you start using tags and then reopen the scripting element
afterwards, like this:
<% } else { %> 

... tags follow ...

<% } %> 

At first, this may look a bit strange, but it ensures that the scripting elements are
transformed correctly when the JSP source file is compiled.

When Are the Scripting Elements Executed?
A JSP source file is processed in two stages—HTTP translation time and request
processing time.

At HTTP translation time, which occurs when a user first loads a JSP page, the JSP
source file is compiled to a Java class, usually a Java servlet. The HTML tags and as
many JSP tags as possible are processed at this stage, before the user makes a
request.

Request processing time occurs when your user clicks in the JSP page to make a
request. The request is sent from the client to the server by way of the request
object. The JSP container then executes the compiled JSP file, or servlet, using the
request values the user submitted.

When you use scripting elements in a JSP file, you should know when they are
evaluated. Declarations are processed at request processing time and are available to
other declarations, expressions, and scriptlets in the compiled JSP file. Expressions
are evaluated at request processing time. The value of each expression is converted

to a String and inserted in place in the compiled JSP file. Scriptlets also are
evaluated at request processing time, using the values of any declarations that are
made available to them.

http://localhost:8080/examples/jsp/num/numguess.jsp


Handling Exceptions
What was happening the last time you used a JSP application and you entered
something incorrectly? If the application was well written, it probably threw an
exception and displayed an error page. Exceptions that occur while a JSP application
is running are called runtime exceptions.

Just as in a Java application, an exception is an object that is an instance of
java.lang.Throwable or one of its subclasses. Throwable has two standard
subclasses—java.lang.Exception, which describes exceptions, and
java.lang.Error, which describes errors.

Errors are different from exceptions. Errors usually indicate linkage or virtual ma-
chine problems that your Web application probably won’t recover from, such as run-
ning out of memory. Exceptions, however, are conditions that can be caught and
recovered from. These exceptions might be, for example, a NullPointerExcep-
tion or a ClassCastException, which tell you that a null value or a value of the
wrong data type has been passed to your application while it is running.

Runtime exceptions are easy to handle in a JSP application, because they are stored
one at a time in the implicit object named exception. You can use the exception
object in a special type of JSP page called an error page, where you display the
exception’s name and class, its stack trace, and an informative message for your
user.

A runtime exception is thrown by the compiled JSP file, the Java class file that
contains the translated version of your JSP page. This means that your application
has already been compiled and translated correctly. (Exceptions that occur while a
file is being compiled or translated are not stored in the exception object and have
their messages displayed in the command window, rather than in error pages. These
are not the type of exception described in this tutorial.)

This tutorial describes how to create a simple JSP application with several display
pages, a JavaBeans component, and one error page that gives informative error
messages to the user. In this example, the bean tracks which JSP page the user was
working in when the exception was thrown, which gives you, the developer,
valuable information so that you can display an informative message. This is a
simple error tracking mechanism; we will describe more complex ones later in this
book.


How To Add Error Pages
Even though we call them error pages, the specialized JSP pages we describe here
actually display information about exceptions. To add error pages that display
exception information to a Web application, follow these steps:

1. Write your component so that it throws certain exceptions under certain
conditions.

2. In the JSP file, use a page directive with errorPage set to the name of a JSP file
that will display a message to the user when an exception occurs.

3. Write an error page file, using a page directive with isErrorPage="true". In
the error page file, use the exception object to get information about the
exception.

4. Use a simple tracking mechanism in your component to help you gather
information about what your user was doing when the exception was thrown.

5. Use messages, either in your error page file or included from other files, to give
your user information relevant to what he or she was doing when the exception
was thrown.

An Email Address Finder Example
This example, named email, stores names and email addresses in a map file based
on the java.util.TreeMap class defined in the JDK 1.2. The TreeMap class creates
a data structure called a red-black tree. In the tree, data is stored with a key and a
value. In this example, the name is the key and the email address is the value.

When you add an entry to the map file, you enter both a name (the key) and an
email address (the value). You can look up or delete an email address by entering
just a name. The name cannot be null because it is a key. If a user tries to enter a null
name, the application throws an exception and displays an error page.

Handling Exceptions 27

So What’s a Red-Black Tree?
For those of you who are curious about algorithms, a red-black tree is an extended
binary tree that looks something like this (conceptually, at least):

If you are viewing this document online, you will see that some nodes are red and
some are black. If you are viewing this document in print, the red nodes look a
shade or two lighter than the black.

The red-black tree has nodes that are either leaf nodes or branch nodes. Leaf nodes
are the small nodes at the end of a line, while branch nodes are the larger nodes that
connect two or more lines. Data is stored in a balanced structure in the tree, using
the following conditions:
s Every node has two children or is a leaf.
s Every node is colored red or black.
s Every leaf node is colored black.
s If a node is red, then both of its children are black.
s Every path from the root to a leaf contains the same number of black nodes.

If you want more detail on how a tree map works, you can find it in Introduction to
Algorithms by Corman, Leiserson, and Rivest. The advantage of a tree map is that
you can create a map file that stores data in ascending order (sorted by keys) and
that has fast search times.

How the Example Is Structured
The email example has three pages with HTML forms, two response files, one error
page, and one JavaBeans component. You can visualize the file structure as
something like this:


Map.java
email.jsp

lookup.jsp
lookupresponse.jsp

delete.jsp
deleteresponse.jsp

error.jsp

s Map.java is a JavaBeans component that creates the map file.
s email.jsp is a JSP page that displays a form where the user enters a name and
email address.
s lookup.jsp is a JSP page that lets a user search for an email address that
matches a name.
s lookupresponse.jsp is included in lookup.jsp and displays the entry the
user wants to look up.
s delete.jsp is a JSP page that lets the user delete an email address that matches
a name.
s deleteresponse.jsp is included in delete.jsp and displays the entry that
was deleted from the map file.
s error.jsp is an error page that displays information about handling exceptions
that occur while adding, looking up, or deleting entries in the map file.

The code for email is shown in CODE EXAMPLE 1-9 through CODE EXAMPLE 1-15, along
with miniature versions of its screens. You may want to install and run the example
while you look at the code. The instructions are in “How To Run the Example” on
page 45.

CODE EXAMPLE 0-9 Adding a Name and Email Address (email.jsp)
<%@ include file="copyright.html" %>

<%@ page isThreadSafe="false" import="java.util.*, email.Map"
errorPage="error.jsp" %>

<jsp:useBean id="mymap" scope="session" class="email.Map" />
<jsp:setProperty name="mymap" property="name" param="name" />
<jsp:setProperty name="mymap" property="email" param="email" />


<% mymap.setAction( "add" ); %>

<html>
<head><title>Email Finder</title></head>
<body bgcolor="#ffffff" background="background.gif" link="#000099">



<form method="get">
<table border="0" cellspacing="0" cellpadding="5">

<tr>
<td align="right">
<h1>Email Finder</h1> </td>
</tr>

<tr>
<td width="120" align="right"><b>Name</b></td>
<td align="left"><input type="text" name="name" size="35"></td>
</tr>

<tr>
<td width="120" align="right"><b>Email Address</b></td>
<td align="left"><input type="text" name="email" size="35"></td>
</tr>

<tr>
<td align="right">
Please enter a name and an email address.
</td>
</tr>

<tr>
<td align="right">
<input type="submit" value="Add">

</td>
</tr>



<%
String rname = request.getParameter( "name" );
String remail = request.getParameter( "email" );
if ( rname != null) {
mymap.put( rname, remail );
}
%>

<tr>
<td align="right">
The map file has <font color="blue"><%= mymap.size() %>
</font> entries.
</td>
</tr>

<tr>
<td align="right">
<a href="lookup.jsp">Lookup</a>  |  
<a href="delete.jsp">Delete</a>
</td>
</tr>

</table>
</form>

</body>
</html>

CODE EXAMPLE 0-10 Looking Up a Name in the Map File (lookup.jsp)



<% mymap.setAction( "lookup" ); %>

<html>

<head><title> Email Finder </title></head>

<form method="get">

<tr>
<td align="right">
</tr>

<tr>
<td width="120" align="right"> <b>Name</b></td>
<td align="left">
<input type="text" name="name" size="35"></td> </tr>
<tr>
<td align="right">
Please enter a name for which
<br>
you’d like an email address.
</td>
</tr>

<tr>
<td align="right">
The map file has <font color="blue"> <%= mymap.size() %></font>
entries.
</td>
</tr>

<tr>
<td align="right"> <input type="submit" value="Lookup"> </td>
</tr>


<% if ( request.getParameter( "name" ) != null ) { %>
<%@ include file="lookupresponse.jsp" %>
<% } %>

<tr>
<td align="right">
<a href="email.jsp">Add</a>   |  
<a href="delete.jsp">Delete</a>
</td>
</tr>

</table>
</form>

</body>
</html>

CODE EXAMPLE 0-11 Displaying the Lookup Response (lookupresponse.jsp)

<%@ page import="java.util.*, email.Map" %>

<tr>
<td align="right">
<b> Success! </b>
</td>
</tr>
<tr>
<td align="right">
<jsp:getProperty name="mymap" property="name" />
<br>
<jsp:getProperty name="mymap" property="email" />
</td>
</tr>


CODE EXAMPLE 0-12 Deleting an Email Address (delete.jsp)





<% mymap.setAction( "delete" ); %>

<html>
<head><title> Email Finder </title></head>

<form method="get">

<tr>
<td align="right">
</tr>
<tr>
<td width="120" align="right"><b>Name</b></td>
<td align="left"> <input type="text" name="name" size="35"> </td>
</tr>

<tr>
<td align="right">
Please enter a name you would like to delete.
</td>
</tr>

<tr>
<td align="right">
The map file has <font color="blue"> <%= mymap.size() %></font>
entries.
</td>
</tr>

<tr>
<td align="right"> <input type="submit" value="Delete"> </td>
</tr>



<%@ include file="deleteresponse.jsp" %>
<%
mymap.remove( request.getParameter("name") ) ;
}
%>

<tr>
<td align="right">
<a href="email.jsp">Add</a>   |  
<a href="lookup.jsp">Lookup</a>
</td>
</tr>

</table>
</form>

</body>
</html>

CODE EXAMPLE 0-13 Displaying the Delete Response (deleteresponse.jsp)

<%@ page import="java.util.*, email.Map" %>

<tr>
<td width="120">  
</td>
<td align="right"> <b>Success!</b>
</td>
</tr>
<tr>
<td align="right">
<br>
<br><p>
has been deleted from the map file.
</td>
</tr>

CODE EXAMPLE 0-14 Displaying Exception Messages (error.jsp)

<%@ page isErrorPage="true" import="java.util.*, email.Map" %>

<html>
<head><title>Email Finder</title></head>

<tr>
<td width="150" align="right">   </td>
<td align="right" valign="bottom"> <h1> Email Finder </h1> </td>
</tr>


<tr>
<td align="right"> <b>Oops! an exception occurred.</b> </td>
</tr>

<tr>
<td align="right"> The name of the exception is
<%= exception.toString() %>.
</td>
</tr>

<tr>
<td align="right">   </td>
</tr>

<% if (mymap.getAction() == "delete" ) { %>
<tr>
<td width=150 align=right>   </td>
<td align=right>
<b>This means that ...</b>
<p>The entry you were trying to
<font color="blue">delete</font> is not in the map file <br>
<b><i>or</i></b>
<br>
you did not enter a name to delete.
<p>
Want to try <a href="delete.jsp">again</a>?
</td>
</tr>

<% }

else if (mymap.getAction() == "lookup" ) { %>
<tr>


<td align="right">
<b><i>This means that ...</b></i>
<p>the entry you were trying to
<font color="blue">look up</font>
is not in the map file, <b><i>or</i></b>
<br>
you did not enter a name to look up.
<p>
Want to try <a href="lookup.jsp">again</a>?
</td>
</tr>

<% }

else if (mymap.getAction() == "add" ) { %>
<tr>
<td align="right">
<b><i>This means that ...</b></i>
<p>You were trying to <font color="blue">add</font>
an entry with a name of null.
<br>
The map file doesn’t allow this.
<p>
Want to try <a href="email.jsp">again</a>?
</td>
</tr>

<% } %>

</table>

CODE EXAMPLE 0-15 Creating the Map File (Map.java)
package email;
import java.util.*;

public class Map extends TreeMap {

// In this treemap, name is the key and email is the value

private String name, email, action;
private int count = 0;

public Map() { }

public void setName( String formName ) {
if ( formName != "" ) {


name = formName;
}
}

public String getName()
return name;
}

public void setEmail( String formEmail ) {
if ( formEmail != "" ) {
email = formEmail;
System.out.println( name );// for debugging only
System.out.println( email );// for debugging only
}
}

public String getEmail() {
email = get(name).toString();
return email;
}

public void setAction( String pageAction ) {
action = pageAction;
}

public String getAction() {
return action;
}

}

Handling Exceptions in the Bean
In this example, the code that throws exceptions is in the TreeMap class, which our
email.Map bean extends, so we won’t need to write code that throws exceptions in
the bean.

The methods that we use from TreeMap are shown below, with their exceptions:
s public Object get( Object key )
throws ClassCastException, NullPointerException
- retrieves an entry from the map file
s public Object put( Object key, Object value )
- adds an entry to the map file


s public Object remove( Object key )
- removes an entry from the map file
s int size()
- returns the number of entries in the map file

Of course, if you need more information about these methods, you can find it in the
Javadoc API reference for java.util.TreeMap.

The TreeMap class throws a ClassCastException when the user tries to enter
data of the wrong type in the map file, for example, an int where the map file is
expecting a String. Keep in mind that the TreeMap class is also used with Java
client applications. In our JSP application, this exception won’t occur, because the
user enters a name and an email address in an HTML form, which always passes
data as strings to the bean. Even if the user typed 6 as a name, the value is still sent
as a String.

However, the get, put, and remove methods throw a NullPointerException if
the user enters nothing and a null value is passed to the bean. This is the most
common exception that the email application needs to handle. This exception might
occur while your user is trying to add, look up, or remove an entry from the map
file. Remember that the key (in this case, the name) cannot be null.

When the User Tries to Add a Null Value
The first case, where the user attempts to add a null name or email address, is
handled by some simple code in the bean and in email.jsp. (Here null means the
user has entered nothing in the form text box. It does not handle the case where the
user enters one or more blank spaces, then presses Return.)

The code that handles adding null values is in the setName and setEmail methods
of Map.java and in a scriptlet in email.jsp (CODE EXAMPLE 1-16):

CODE EXAMPLE 0-16 Catching a Null Value on Add
Map.java:

public void setName( String formName ) {
if ( formName != "" ) {
name = formName;
}
}
public void setEmail( String formEmail ) {
if ( formEmail != "" ) {
email = formEmail;
System.out.println( name ); // for debugging only


System.out.println( email ); // for debugging only
}
}
email.jsp:

<%
String rname = request.getParameter( "name" );
String remail = request.getParameter( "email" );
if ( rname != null) {
mymap.put( rname, remail );
}
%>

Both setName and setEmail check whether the user has entered a null value in the
form before setting their respective properties. If the form value is null, the bean
does not set a property, the put method does not add a value to the map file, and no
exception is thrown.

When the User Tries to Look Up a Null Value
But if you go to the Lookup or Delete page of the example and try to look up or delete
an entry that isn’t in the map file at all, the email application throws a
NullPointerException and displays the error page. The code that handles
looking up null values is shown in CODE EXAMPLE 1-17.

CODE EXAMPLE 0-17 Catching a Null Value on Look Up
lookup.jsp:

<%@ include file="lookupresponse.jsp" %>
<% } %>
lookupresponse.jsp:
<tr>
<td align="right">
<font face="helvetica" size="-2">
<br>
</font>
</td>
</tr>

This example has two pieces of code that work together. The page lookup.jsp,
where you enter a name you want to look up in the map file, has a scriptlet that
checks whether or not the user has entered a name in the form. If the user doesn’t


enter a name, or enters a name that doesn’t exist in the map file, the bean throws a
NullPointerException and the application displays the error page—which is the
desired behavior! In this case, you can be happy that the error page is displayed.

You may have noticed that the lines from lookupresponse.jsp use the
<jsp:getProperty> tag to retrieve the name and email address from the bean.
You could also try to retrieve the email address using expressions, something like
this:
<%= request.getParameter( "name" ) %>
<br>
<%= mymap.get( request.getParameter( "name" ) ) %>

If you use these lines, the application would behave a little differently. Rather than
throwing a NullPointerException and displaying an error page, it would
display the name the user entered, with the word null below it in the JSP page. In
Tomcat, the <jsp:getProperty> tag intentionally handles null values differently
than scriptlets or expressions. The way null values are handled will vary according
to the JSP container you use.

When the User Tries to Delete a Null Value
Handling the case of a user trying to delete a null value is very similar to handling
the lookup of a null value. The code that handles null values that occur while you
are trying to delete an entry is shown in CODE EXAMPLE 1-18.

CODE EXAMPLE 0-18 Catching a Null Value on Delete
delete.jsp:

<%@ include file="deleteresponse.jsp" %>
<%
mymap.remove( request.getParameter("name") ) ;
}
%>
deleteresponse.jsp:

<tr>
<td align="right">
<font face="helvetica" size="-2">
<br>


<br><p>
has been deleted from the map file.
</font>
</td>
</tr>

Calling an Error Page From Another Page
To link the display pages to the error page, each display page in the email application
uses a page directive with the errorPage attribute, like this:

In the code examples, the files that use this directive are email.jsp, lookup.jsp,
and delete.jsp. You can only specify one error page for each JSP page.

This means that you can design a JSP application so that each JSP page calls a
different error page, or so that several JSP pages call one error page. In the email
application, several JSP pages call one error page, as it simplifies the number of files
you need to maintain for one application. In designing your applications, the choice
is up to you.

You should always use at least one error page in a JSP application. If you don’t
specify an error page, the exception message and stack trace are displayed in the
command window from which the JSP container was started, while the Web browser
displays a non-informative HTTP error message, for example, a 404 or 501 message.
This is definitely not a graceful way to handle exceptions.

Writing an Error Page
An error page is different from an ordinary JSP page. In an error page, you must
explicitly set the isErrorPage attribute of the page directive to true. You also
have access to the exception object, which gives you information about the
exception.

First, let’s look at an example of the page directive for an error page:
<%@ page isErrorPage="true" import="java.util.*, email.Map" %>

Once you have set isErrorPage to true, you can use the exception object.
exception is of type java.lang.Throwable, so you can use any of the methods
defined in Throwable with exception in a scriptlet or expression, for example:
s <%= exception.toString() %>
s <% exception.printStackTrace(); %>


The expression exception.toString() displays the exception’s class name, for
example, java.lang.NullPointerException, while
exception.printStackTrace() displays the exception’s stack trace. The class
name and stack trace are probably very helpful to you the developer, but probably
not very helpful to your user. To get around this, you may want to write some type
of tracking mechanism to provide information that helps you give an informative
message to your user.

Writing a Simple Tracking Mechanism
The email example uses a property named action in Map.java to track which page
the user was working in when the exception was thrown. That gives you valuable
information to help you write an informative error message for your user. The bean
has a variable named action, a getAction method, and a setAction method.
The variable and method declarations in the bean look like this:
private String action;

public void setAction( String pageAction ) {
action = pageAction;
}

public String getAction() {
return action;
}

Each of the pages email.jsp, lookup.jsp, and delete.jsp sets the value of
action with a line like this one (which comes from email.jsp):
<% mymap.setAction( "add" ); %>

If an exception occurs, error.jsp checks the value of action and includes an
appropriate message for each value, using lines like these:
<% if (mymap.getAction() == "delete" ) { %>
.. text message here ..
else if (mymap.getAction() == "lookup" ) { %>
else if (mymap.getAction() == "add" ) { %>
<% } %>

Of course, this is a very simple way to implement tracking. If you move into
developing J2EE applications, you can write applications that save state.


s http://localhost:8080/email/email.jsp


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