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murach’s
Java
servlets and
JSP
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Copyright © 2003 Mike Murach & Associates. All rights reserved.
Chapter 4 How to develop JavaServer Pages
99
Section 2
The essence of servlet and
JSP programming
The best way to learn how to develop web applications in Java is to start
developing them. That’s why the chapters in this section take a hands-on
approach to developing web applications. In chapter 4, you’ll learn how to
code JavaServer Pages (JSPs), which is one way to develop web applications.
In chapter 5, you’ll learn how to code servlets, which is another way to
develop web applications. And in chapter 6, you’ll learn how to structure your
web applications so you combine the best features of JSPs and servlets.
With that as background, you’ll be ready to learn the other web
programming essentials. In chapter 7, you’ll learn how to work with sessions
and cookies so your application can keep track of its users. In chapter 8, you’ll
learn how to build better applications by using JavaBeans. And in chapter 9,
you’ll learn how to simplify the code in your JSPs by using custom tags. When
you complete this section, you’ll have the essential skills that you need for
designing, coding, and testing web applications that use JSPs and servlets.
Chapter 4 How to develop JavaServer Pages
101
4
How to develop
JavaServer Pages
In this chapter, you’ll learn how to develop a web application that consists of
HTML pages and JavaServer Pages (JSPs). As you will see, JSPs work fine as
long as the amount of processing that’s required for each page is limited. When
you complete this chapter, you should be able to use JSPs to develop simple
web applications of your own.
The Email List application ................................................. 102
The user interface for the application .......................................................... 102
The code for the HTML page that calls the JSP ......................................... 104
The code for the JSP .................................................................................... 106
How to create a JSP ........................................................... 108
How to code scriptlets and expressions ....................................................... 108
How to use the methods of the request object ............................................. 110
Where and how to save a JSP ...................................................................... 112
How to request a JSP .................................................................................... 114
When to use the Get and Post methods ....................................................... 116
How to use regular Java classes with JSPs .................... 118
The code for the User and UserIO classes .................................................. 118
Where and how to save and compile regular Java classes .......................... 120
A JSP that uses the User and UserIO classes .............................................. 122
How to use three more types of JSP tags ........................ 124
How to import classes .................................................................................. 124
How to code comments in a JSP .................................................................. 126
How to declare instance variables and methods .......................................... 128
A JSP that imports classes and declares instance variables ........................ 130
How to work with JSP errors ............................................. 132
How to debug JSP errors .............................................................................. 132
How to use a custom error page ................................................................... 134
When and how to view the servlet that’s generated for a JSP .................... 136
Perspective ......................................................................... 138
102
Section 2 The essence of servlet and JSP programming
The Email List application
This topic introduces you to a simple web application that consists of one
HTML page and one JavaServer Page, or JSP. Once you get the general idea of
how this application works, you’ll be ready to learn the specific skills that you
need for developing JSPs.
The user interface for the application
Figure 4-1 shows the two pages that make up the user interface for the
Email List application. The first page is an HTML page that asks the user to
enter a first name, last name, and email address. Then, when the user clicks on
the Submit button, the HTML page calls the JSP and passes the three user
entries to that page.
When the JSP receives the three entries, it could process them by checking
them for validity, writing them to a file or database, and so on. In this simple
application, though, the JSP just passes the three entries back to the browser so
it can display the second page of this application. From this page, the user can
return to the first page by clicking the Back button in the web browser or by
clicking the Return button that’s displayed on this page.
As simple as this application is, you’re going to learn a lot from it. In this
chapter, you’ll learn how enhance this application so it uses regular Java classes
to save the user entries in a text file. Then, in later chapters, you’ll learn how to
modify this application to illustrate other essential skills that apply to servlet
and JSP programming.
Chapter 4 How to develop JavaServer Pages
103
The HTML page
The JSP
Figure 4-1
The user interface for the application
104
Section 2 The essence of servlet and JSP programming
The code for the HTML page that calls the JSP
Figure 4-2 presents the code for the HTML page that calls the JSP. If you’ve
read chapter 3, you shouldn’t have any trouble following it. Here, the Action
attribute of the Form tag calls a JSP named show_email_entry.jsp that’s stored
in the same directory as the HTML page, and the Method attribute specifies that
the HTTP Get method should be used with this action. Then, when the user
clicks on the Submit button, the browser will send a request for the JSP.
You should also notice the Name attributes of the three text boxes that are
used in the table within this HTML page. These are the names of the param-
eters that are passed to the JSP when the user clicks on the Submit button. In
figure 4-1, the parameter names are firstName, lastName, and emailAddress and
the parameter values are John, Smith, and jsmith@hotmail.com.
Chapter 4 How to develop JavaServer Pages
105
The code for the HTML page
<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Chapter 4 - Email List application</title>
</head>
<body>
<h1>Join our email list</h1>
<p>To join our email list, enter your name and
email address below. <br>
Then, click on the Submit button.</p>
<form action="show_email_entry.jsp" method="get">
<table cellspacing="5" border="0">
<tr>
<td align="right">First name:</td>
<td><input type="text" name="firstName"></td>
</tr>
<tr>
<td align="right">Last name:</td>
<td><input type="text" name="lastName"></td>
</tr>
<tr>
<td align="right">Email address:</td>
<td><input type="text" name="emailAddress"></td>
</tr>
<tr>
<td></td>
<td><br><input type="submit" value="Submit"></td>
</tr>
</table>
</form>
</body>
</html>
Description
•
The Action and Method attributes for the Form tag set up a request for a JSP that will be
executed when the user clicks on the Submit button.
•
The three text boxes represent parameters that will be passed to the JSP when the user
clicks the Submit button.
•
The parameter names are firstName, lastName, and emailAddress, and the parameter
values are the strings that the user enters into the text boxes.
Figure 4-2
The code for the HTML page that calls the JSP
106
Section 2 The essence of servlet and JSP programming
The code for the JSP
Figure 4-3 presents the code for the JSP. As you can see, much of the JSP
code is HTML. In addition, though, Java code is embedded within the HTML
code in the form of JSP scriptlets and expressions. Typically, a scriptlet is used
to execute one or more Java statements while a JSP expression is used to
display text. To identify scriptlets and expressions, JSPs use tags. To distinguish
them from HTML tags, you can refer to them as JSP tags.
When you code a JSP, you can use the methods of the request object in your
scriptlets or expressions. Since you don’t have to explicitly create this object
when you code JSPs, this object is sometimes referred to as the implicit request
object. The scriptlet in this figure contains three statements that use the
getParameter method of the request object. Each of these statements returns the
value of the parameter that is passed to the JSP from the HTML page. Here, the
argument for each getParameter method is the name of the textbox on the
HTML page.
Once the scriptlet is executed, the values for the three parameters are
available as variables to the rest of the page. Then, the three expressions can
display these variables. Since these expressions are coded within the HTML
tags for a table, the browser will display these expressions within a table.
After the table, the JSP contains some HTML that defines a form. This form
contains only one control, a submit button named Return. When it is clicked, it
takes the user back to the first page of the application. If you have any trouble
visualizing how this button or the rest of the page will look when displayed by a
browser, please refer back to figure 4-1.
As you read this book, remember that it assumes that you already know the
basics of Java programming. If you have any trouble understanding the Java
code in this chapter, you may need a refresher course on Java coding. To
quickly review the basics of Java coding, we recommend that you use Murach’s
Beginning Java 2 because it contains all the Java skills that you’ll need for
working with this book.
Chapter 4 How to develop JavaServer Pages
107
The code for the JSP
<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Chapter 4 - Email List application</title>
</head>
<body>
<%
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
JSP scriptlet
String emailAddress = request.getParameter("emailAddress");
%>
<h1>Thanks for joining our email list</h1>
<p>Here is the information that you entered:</p>
<table cellspacing="5" cellpadding="5" border="1">
<tr>
<td align="right">First name:</td>
<td><%= firstName %></td>
</tr>
<tr>
JSP expression
<td align="right">Last name:</td>
<td><%= lastName %></td>
</tr>
<tr>
<td align="right">Email address:</td>
<td><%= emailAddress %></td>
</tr>
</table>
<p>To enter another email address, click on the Back <br>
button in your browser or the Return button shown <br>
below.</p>
<form action="join_email_list.html" method="post">
<input type="submit" value="Return">
</form>
</body>
</html>
Description
•
Although a JSP looks much like an HTML page, a JSP contains embedded Java code.
•
To code a scriptlet that contains one or more Java statements, you use the <% and %>
tags. To display any expression that can be converted to a string, you use the <%= and
%> tags.
•
When you code a JSP, you can use the implicit request object. This object is named
request. You can use the getParameter method of the request object to get the values of
the parameters that are passed to the JSP.
Figure 4-3
The code for the JSP
108
Section 2 The essence of servlet and JSP programming
How to create a JSP
Now that you have a general idea of how JSPs are coded, you’re ready to
learn some specific skills for creating a JSP. To start, you need to know more
about coding scriptlets and expressions.
How to code scriptlets and expressions
Figure 4-4 summarizes the information you need for coding scriptlets and
expressions within a JSP. To code a scriptlet, for example, you code Java
statements that end with semicolons within the JSP scriptlet tags. To code an
expression, you code any Java expression that evaluates to a string. Since
primitive data types like integers or doubles are automatically converted to
strings, you can also use expressions that evaluate to these data types.
When you’re coding a scriptlet or an expression, you can use any of the
methods of the implicit request object. In this figure, only the getParameter
method is used, but you’ll learn about two more methods of the request object
in the next figure.
In this figure, the first two examples show different ways that you can
display the value of a parameter. The first example uses a scriptlet to return the
value of the firstName parameter and store it in a String object. Then, this
example uses an expression to display the value. In contrast, the second ex-
ample uses an expression to display the value of the firstName parameter
without creating the firstName object.
The last example in this figure shows how two scriptlets and an expression
can be used to display an HTML line five times while a Java variable within the
HTML line counts from 1 to 5. Here, the first JSP scriptlet contains the code
that begins a while loop. Then, a line of HTML code uses a JSP expression to
display the current value of the counter for the loop. And finally, the second
scriptlet contains the code that ends the loop.
Chapter 4 How to develop JavaServer Pages
109
The syntax for a JSP scriptlet
<% Java statements %>
The syntax for a JSP expression
<%= any Java expression that can be converted to a string %>
The syntax for getting a parameter from the implicit request object
request.getParameter(parameterName);
Examples that use scriptlets and expressions
A scriptlet and expression that display the value of the firstName parameter
<%
String firstName = request.getParameter("firstName");
%>
The first name is <%= firstName %>.
An expression that displays the value of the firstName parameter
The first name is <%= request.getParameter("firstName") %>.
Two scriptlets and an expression that display an HTML line 5 times
<%
int numOfTimes = 1;
while (numOfTimes <= 5){
%>
<h1> This line is shown <%= numOfTimes %> of 5 times in a JSP.</h1>
<%
numOfTimes++;
}
%>
Description
•
Within a scriptlet, you can code one or more complete Java statements. Because these
statements are Java statements, you must end each one with a semicolon.
•
Within a JSP expression, you can code any Java expression that evaluates to a string.
This includes Java expressions that evaluate to any of the primitive types, and it includes
any object that has a toString method. Because a JSP expression is an expression, not a
statement, you don’t end it with a semicolon.
Figure 4-4
How to code scriptlets and expressions
110
Section 2 The essence of servlet and JSP programming
How to use the methods of the request object
In the last figure, you learned how to use the getParameter method to return
the value that the user entered into a textbox. Now, figure 4-5 summarizes that
method and illustrates it in a new context. This figure also summarizes and
illustrates two more methods of the implicit request object.
In most cases, the getParameter method returns the value of the parameter.
For a textbox, that’s usually the value entered by the user. But for a group of
radio buttons or a combo box, that’s the value of the button or item selected by
the user.
For checkboxes or independent radio buttons that have a Value attribute, the
getParameter method returns that value if the checkbox or button is selected and
a null value if it isn’t. For checkboxes or independent radio buttons that don’t
have a Value attribute, though, the getParameter method returns an “on” value if
the checkbox or button is selected and a null value if it isn’t. This is illustrated
by the first example in this figure.
To retrieve multiple values for one parameter name, you can use the
getParameterValues method as illustrated by the second example. This method
is useful for controls like list boxes that allow multiple selections. After you use
the getParameterValues method to return an array of String objects, you can use
a loop to get the values from the array.
To get the names of all the parameters sent with a request, you can use the
getParameterNames method to return an Enumeration object that contains the
names. Then, you can search through the Enumeration object to get the param-
eter names, and you can use the getParameter method to return the value for
each parameter name. This is illustrated by the third example.
If you’re not familiar with the Enumeration class, you can learn more about
it though the API. For most purposes, though, you only need to know that an
Enumeration object is a collection that can be searched element by element. To
determine if more elements exist in the collection, you can use the
hasMoreElements method, which returns a boolean value. And to get the next
element in the collection, you can use the nextElement method.
Chapter 4 How to develop JavaServer Pages
111
Three methods of the request object
Method
Description
getParameter(String param)
Returns the value of the specified parameter as a string if it
exists or null if it doesn’t. Often, this is the value defined in the
Value attribute of the control in the HTML page or JSP.
getParameterValues(String param)
Returns an array of String objects containing all of the
values that the given request parameter has or null if the parameter
doesn’t have any values.
getParameterNames()
Returns an Enumeration object that contains the names of
all the parameters contained in the request. If the request has no
parameters, the method returns an empty Enumeration object.
A scriptlet that determines if a checkbox is checked
<%
String rockCheckBox = request.getParameter("Rock");
// returns the value or "on" if checked, null otherwise.
if (rockCheckBox != null){
%>
You checked Rock music!
<% }
%>
A scriptlet that reads and displays multiple values from a list box
<%
String[] selectedCountries = request.getParameterValues("country");
// returns the values of items selected in list box.
for (int i = 0; i < selectedCountries.length; i++){
%>
<%= selectedCountries[i] %> <br>
<%
}
%>
A scriptlet that reads and displays all request parameters and values
<%
Enumeration parameterNames = request.getParameterNames();
while (parameterNames.hasMoreElements()){
String parameterName = (String) parameterNames.nextElement();
String parameterValue = request.getParameter(parameterName);
%>
<%= parameterName %> has value <%= parameterValue %>. <br>
<%
}
%>
Description
•
You can use the getParameter method to return the value of the selected radio button in a
group or the selected item in a combo box. You can also use it to return the value of a
selected check box or independent radio button, but that value is null if it isn’t selected.
•
If an independent radio button or a checkbox doesn’t have a Value attribute, this method
returns “on” if the control is selected or null if it isn’t.
Figure 4-5
How to use the methods of the request object
112
Section 2 The essence of servlet and JSP programming
Where and how to save a JSP
As figure 4-6 shows, you normally save the JSPs of an application in the
same directory that you use for the HTML pages of the application. The differ-
ence is that the name for a JSP requires a jsp extension. So if you’re using a text
editor that’s not designed for working with JSPs, you may have to place the
filename in quotation marks to make sure the file is saved with the jsp exten-
sion.
Like HTML pages, JSPs must be saved in a directory that’s available to the
web server. For Tomcat 4.0, you can use any directory under the webapps
directory. The root directory for the web applications that are presented in the
first 16 chapters of this book is the webapps\murach directory, and the
subdirectory for the Email List application that’s presented in this chapter is
email4. Note, however, that you can also use the webapps\ROOT directory that
Tomcat sets up as the default root directory. Or, you can create your own
subdirectories under the webapps directory.
Chapter 4 How to develop JavaServer Pages
113
Where the show_email_entry.jsp page is saved
c:\tomcat\webapps\murach\email4
Other places you can save your JSPs
c:\tomcat\webapps\yourDocumentRoot
c:\tomcat\webapps\yourDocumentRoot\yourSubdirectory
c:\tomcat\webapps\ROOT
c:\tomcat\webapps\ROOT\yourSubdirectory
A standard dialog box for saving a JSP
Description
•
JSPs are normally saved in the same directory as the HTML pages. This directory should
be a subdirectory of the web applications directory for your server. If you’re running
Tomcat on your PC, that directory is usually c:\tomcat\webapps or c:\jakarta-
tomcat\webapps.
•
For the first 16 chapters of this book, the document root directory for all applications is
the murach directory. As a result, the HTML and JSP files for each application are stored
in this directory or one of its subdirectories.
•
If you’re using Tomcat on your local system, you can also use webapps\ROOT as the
root directory for your applications. The ROOT directory is automatically set up when
you install Tomcat, and it is the default document root directory.
•
To make sure that the filename for a JSP is saved with the jsp extension when you’re
using an HTML or text editor, you can enter the filename within quotes.
Figure 4-6
Where and how to save a JSP
114
Section 2 The essence of servlet and JSP programming
How to request a JSP
After you create a JSP, you need to test it. One way to do that is to click on
a link or a button on an HTML page that requests the JSP. Another way is to
enter a URL into a web browser that requests the JSP. Figure 4-7 shows how to
request a JSP either way.
To request a JSP from an HTML form, you use the Action attribute of the
form to provide a path and filename that point to the JSP. This is illustrated by
the first example in this figure. Here, the assumption is that the HTML page and
the JSP are in the same directory. If they weren’t, you would have to supply a
relative or absolute path for the JSP file.
To request a JSP by entering its URL into a browser, you enter an absolute
URL as shown by the next two examples in this figure. The first example shows
the URL for the JSP when it’s stored on a local web server in the email4 direc-
tory of the murach directory. The second example shows the URL for the JSP if
the JSP was deployed on the Internet server for www.murach.com.
When you test a JSP by entering a URL, you will often want to pass param-
eters to it. To do that, you can add the parameters to the end of the URL as
shown by the last examples in this figure. Here, the question mark after the jsp
extension indicates that one or more parameters will follow. Then, you code the
parameter name, the equals sign, and the parameter value for each parameter
that is passed, and you separate multiple parameters with ampersands (&). If
you omit a parameter that’s required by the JSP, the getParameter method will
return a null value for that parameter.
When you use a Get method to request a JSP from another page, any
parameters that are passed to the JSP will be displayed in the browser’s URL
address. In this figure, for example, you can see the first two parameters that
have been attached to the URL. However, in the next figure, you’ll learn that the
Post method works differently.
Chapter 4 How to develop JavaServer Pages
115
A URL that includes parameters
How Tomcat maps directories to HTTP calls
Tomcat directory
URL
c:\tomcat\webapps\murach
http://localhost:8080/murach/
c:\tomcat\webapps\murach\email4
http://localhost:8080/murach/email4
c:\tomcat\webapps\ROOT
http://localhost:8080/
c:\tomcat\webapps\ROOT\email4
http://localhost:8080/email4
A Form tag that requests a JSP
<form action="show_email_entry.jsp" method="get">
Two URLs that request a JSP
http://localhost:8080/murach/email4/show_email_entry.jsp
http://www.murach.com/email4/show_email_entry.jsp
How to include parameters
show_email_entry.jsp?firstName=John
show_email_entry.jsp?firstName=John&lastName=Smith
Description
•
When you use the Get method to request a JSP from an HTML form, the parameters are
automatically appended to the URL.
•
When you code or enter a URL that requests a JSP, you can add a parameter list to it
starting with a question mark and with no intervening spaces. Then, each parameter
consists of its name, an equals sign, and its value. To code multiple parameters, use
ampersands (&) to separate the parameters.
Figure 4-7
How to request a JSP
116
Section 2 The essence of servlet and JSP programming
When to use the Get and Post methods
When you code a Form tag that requests a JSP, you can code a Method
attribute that specifies the HTTP method that’s used for the request. The Get
method is the default HTTP method, but the Post method is also commonly
used.
Figure 4-8 presents the pros and cons of using the Get and Post methods.
With either method, you can still test the page by appending the parameters to
the URL string. So the question really comes down to selecting the appropriate
method for the finished web application.
There are two primary reasons for using the Post method. First, since the
Post method doesn’t append parameters to the end of the URL, it is more
appropriate for working with sensitive data. If, for example, you’re passing a
parameter for a password or a credit card number, the Post method prevents
these parameters from being displayed in the browser. In addition, it prevents
the web browser from including these parameters in a bookmark for a page.
Second, you need to use the Post method if your parameters contain more than 4
KB of data.
For all other uses, the Get method is preferred. It runs slightly faster than
the Post method, and it lets the user bookmark the page along with the param-
eters that were sent to the page.
Chapter 4 How to develop JavaServer Pages
117
An HTML form tag that uses the Post method
<form action="show_email_entry.jsp" method="post">
A JSP that’s requested through the Post method
When to use the Get method
•
If you want to transfer data as fast as possible.
•
If the HTML form only needs to transfer 4 KB of data or less.
•
If it’s okay for the parameters to be displayed in the URL.
•
If you want users to be able to include parameters when they bookmark a page.
When to use the Post method
•
If you’re transferring over 4 KB of data.
•
If it’s not okay for the parameters to be appended to the URL.
Description
•
The visible difference between the Get and Post methods is the URL that’s displayed in
the browser. For Get requests from an HTML page, the parameters are appended to the
URL. For Post requests, the parameters are still sent, but they’re not displayed in the
browser.
•
You can test a JSP that uses either method by appending the parameters to the URL.
Figure 4-8
When to use the Get and Post methods
118
Section 2 The essence of servlet and JSP programming
How to use regular Java classes with
JSPs
In this topic, you’ll learn how to use regular Java classes to do the process-
ing that a JSP requires. In particular, you’ll learn how to use two classes named
User and UserIO to do the processing for the JSP of the Email List application.
The code for the User and UserIO classes
Figure 4-9 presents the code for a business class named User and an I/O
class named UserIO. The package statement at the start of each class indicates
where each class is stored. Here, the User class is stored in the business direc-
tory because it defines a business object while the UserIO class is stored in the
data directory because it provides the data access for the application.
The User class defines a user of the application. This class contains three
instance variables: firstName, lastName, and emailAddress. It includes a
constructor that accepts three values for these instance variables. And it includes
get and set methods for each instance variable.
In contrast, the UserIO class contains one static method named addRecord
that writes the values stored in a User object to a text file. This method accepts
two parameters: a User object and a string that provides the path for the file. If
this file exists, the method will add the user data to the end of it. If the file
doesn’t exist, the method will create it and add the data at the beginning of the
file.
If you’ve read the first six chapters of Murach’s Beginning Java 2, you
should understand the code for the User class. And if you’ve read chapters 16
and 17, you should understand the code in the UserIO class. The one exception
is the use of the synchronized keyword in the addRecord method declaration.
But this keyword just prevents two users from writing to the file at the same
time, which could lead to an error.
Chapter 4 How to develop JavaServer Pages
119
The code for the User class
package business;
public class User{
private String firstName;
private String lastName;
private String emailAddress;
public User(){}
public User(String first, String last, String email){
firstName = first;
lastName = last;
emailAddress = email;
}
public void setFirstName(String f){
firstName = f;
}
public String getFirstName(){ return firstName; }
public void setLastName(String l){
lastName = l;
}
public String getLastName(){ return lastName; }
public void setEmailAddress(String e){
emailAddress = e;
}
public String getEmailAddress(){ return emailAddress; }
}
The code for the UserIO class
package data;
import java.io.*;
import business.User;
public class UserIO{
public synchronized static void addRecord(User user, String filename)
throws IOException{
PrintWriter out = new PrintWriter(
new FileWriter(filename, true));
out.println(user.getEmailAddress()+ "|"
+ user.getFirstName() + "|"
+ user.getLastName());
out.close();
}
}
Note
•
The synchronized keyword in the declaration for the addRecord method of the UserIO
class prevents two users of the JSP from using that method at the same time.
Figure 4-9
The code for the User and UserIO classes
120
Section 2 The essence of servlet and JSP programming
Where and how to save and compile regular Java
classes
If you’re using Tomcat 4.0, figure 4-10 shows where and how to save your
compiled Java classes (the .class files) so Tomcat can access them. Usually,
you’ll save your source code (the .java files) in the same directory, but that’s not
required.
The two paths shown at the top of this figure show where the User and
UserIO classes that come with this book are saved. After that, the figure pre-
sents the syntax for other paths that can be used to store Java classes. If you
review these paths, you’ll see that each one places the Java classes in a
subdirectory of the WEB-INF\classes directory.
Since the User class contains a package statement that corresponds to the
business directory, it must be located in the WEB-INF\classes\business direc-
tory. In contrast, if the package statement specified “murach.email”, the com-
piled classes would have to be located in the WEB-INF\classes\murach\email
directory.
Since TextPad is designed for working with Java, you can use it to compile
regular Java classes. However, you may need to configure your system as
described in appendix A before it will work properly. In particular, you may
need to add the appropriate WEB-INF\classes directory to your classpath.
If you use the DOS prompt window to compile your classes, you can do that
as shown in this figure. Here, a DOS prompt is used to compile the User class.
To start, the cd command changes the current directory to the \WEB-
INF\classes directory. Then, the javac command is used with
“business\User.java” as the filename. This compiles the User class and stores it
in the business package, which is what you want.
Chapter 4 How to develop JavaServer Pages
121
Where the User class is saved
c:\tomcat\webapps\murach\WEB-INF\classes\business
Where the UserIO class is saved
c:\tomcat\webapps\murach\WEB-INF\classes\data
Other places to save your Java classes
c:\tomcat\webapps\yourDocumentRoot\WEB-INF\classes
c:\tomcat\webapps\yourDocumentRoot\WEB-INF\classes\packageName
c:\tomcat\webapps\ROOT\WEB-INF\classes
c:\tomcat\webapps\ROOT\WEB-INF\classes\packageName
The DOS prompt window for compiling the User class
Description
•
Although you can save the source code (the .java files) in any directory, you must save
the class files (the .class files) in the WEB-INF\classes directory or one of its
subdirectories. This can be subordinate to the ROOT directory or your own document
root directory.
•
To compile a class, you can use TextPad’s Compile Java command, your IDE’s compile
command, or the javac command from the DOS prompt window.
•
If you have trouble compiling a class, make sure your system is configured correctly as
described in appendix A.
Figure 4-10
Where and how to save and compile regular Java classes
122
Section 2 The essence of servlet and JSP programming
A JSP that uses the User and UserIO classes
Figure 4-11 shows the code for the JSP in the Email List application after it
has been enhanced so it uses the User and UserIO classes to process the param-
eters that have been passed to it. In the first statement of the body, a special type
of JSP tag is used to import the business and data packages that contain the User
and UserIO classes. You’ll learn how to code this type of tag in the next figure.
For now, though, you should focus on the other shaded lines in this JSP.
In the scriptlet of the JSP, the getParameter method is used to get the values
of the three parameters that are passed to it, and these values are stored in String
objects. Then, the next statement uses these strings as arguments for the con-
structor of the User class. This creates a User object that contains the three
values. Last, this scriptlet uses the addRecord method of the UserIO class to add
the three values of the User object to a file named UserEmail.txt that’s stored in
the WEB-INF\etc directory. Since the WEB-INF\etc directory isn’t web-
accessible, this prevents users of the application from accessing this file.
After the scriptlet, the code in the JSP defines the layout of the page. Within
the HTML table definitions, the JSP expressions use the get methods of the
User object to display the first name, last name, and email address values.
Although these JSP expressions could use the String objects instead, the code in
this figure is intended to show how the get methods can be used.
Chapter 4 How to develop JavaServer Pages
123
The code for a JSP that uses the User and UserIO classes
<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Chapter 4 - Email List application</title>
</head>
<body>
<%@ page import="business.*, data.*" %>
<%
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress = request.getParameter("emailAddress");
User user = new User(firstName, lastName, emailAddress);
UserIO.addRecord(user, "../webapps/murach/WEB-INF/etc/UserEmail.txt");
%>
<h1>Thanks for joining our email list</h1>
<p>Here is the information that you entered:</p>
<table cellspacing="5" cellpadding="5" border="1">
<tr>
<td align="right">First name:</td>
<td><%= user.getFirstName() %></td>
</tr>
<tr>
<td align="right">Last name:</td>
<td><%= user.getLastName() %></td>
</tr>
<tr>
<td align="right">Email address:</td>
<td><%= user.getEmailAddress() %></td>
</tr>
</table>
<p>To enter another email address, click on the Back <br>
button in your browser or the Return button shown <br>
below.</p>
<form action="join_email_list.html" method="post">
<input type="submit" value="Return">
</form>
</body>
</html>
Description
•
This JSP uses a scriptlet to create a User object and add it to a file, and it uses JSP
expressions to display the values of the User object’s instance variables.
•
Since the User and UserIO classes are stored in the business and data packages, the JSP
must import these packages as shown in figure 4-12.
Figure 4-11
A JSP that uses the User and UserIO classes
124
Section 2 The essence of servlet and JSP programming
How to use three more types of JSP
tags
So far, you’ve learned how to use the JSP tags for scriptlets and expressions.
Now, you’ll learn how to use three more types of JSP tags. All five types of JSP
tags are summarized at the top of figure 4-12.
How to import classes
Figure 4-12 also shows how to use a JSP directive to import classes in a
JSP. The type of directive that you use for doing that is called a page directive,
and the shaded statement at the start of the JSP shows how to code one.
To code a page directive for importing classes, you code the starting tag and
the word page followed by the Import attribute. Within the quotation marks
after the equals sign for this attribute, you code the names of the Java classes
that you want to import just as you do in a Java import statement. In the ex-
ample in this figure, all the classes of the business and data packages are
imported, plus the Date class in the java.util package.
Once the page directive imports the packages, the JSP can access the User
and UserIO classes and the Date class. The scriptlet that follows creates a User
object from the User class and uses the addRecord method of the UserIO class
to write the data for the User object to a text file. The last line in this example
uses the default constructor of the Date class as an expression in an HTML line.
This works because the JSP will automatically convert the Date object that’s
created by the constructor into a string.
Chapter 4 How to develop JavaServer Pages
125
The JSP tags presented in this chapter
Tag
Name
Purpose
<% %>
JSP scriptlet
To insert a block of Java statements.
<%= %>
JSP expression
To display the string value of an expression.
<%@ %>
JSP directive
To set conditions that apply to the entire JSP.
<%-- --%>
JSP comment
To tell the JSP engine to ignore code.
<%! %>
JSP declaration
To declare instance variables and methods for a JSP.
JSP code that imports Java classes
<%@ page import="business.*, data.*, java.util.Date" %>
<%
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress = request.getParameter("emailAddress");
User user = new User(firstName, lastName, emailAddress);
UserIO.addRecord(user, "../webapps/murach/WEB-INF/etc/UserEmail.txt");
%>
Today's date is <%= new Date() %>.
Description
•
To define the conditions that the JSP engine should follow when converting a JSP into a
servlet, you can use a JSP directive.
•
To import classes in a JSP, you use the import attribute of the page directive. This makes
the imported classes available to the entire page.
•
You can also use the page directive to define other conditions like error handling and
content type conditions. You’ll learn more about this directive throughout this book.
Figure 4-12
How to import classes
126
Section 2 The essence of servlet and JSP programming
How to code comments in a JSP
Figure 4-13 shows how to code JSP comments, and it shows how you can
code Java comments and HTML comments in a JSP. As a result, you need to
understand how the three types of comments are processed.
Like Java comments, any code within a JSP comment isn’t compiled or
executed. In contrast, any code within an HTML comment is compiled and
executed, but the browser doesn’t display it. This is illustrated by the HTML
comment in this figure. Here, the HTML line within the comment isn’t dis-
played by the browser, but the integer variable in the comment is incremented
each time the page is accessed. To prevent this integer variable from being
incremented, you have to use a JSP comment instead of an HTML comment.
Chapter 4 How to develop JavaServer Pages
127
A JSP comment
<%--
Today's date is <%= new java.util.Date() %>.
--%>
Java comments in a JSP scriptlet
<%
//These statements retrieve the request parameter values
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress = request.getParameter("emailAddress");
/*
User user = new User(firstName, lastName, emailAddress);
UserIO.addRecord(user, "../webapps/murach/WEB-INF/etc/UserEmail.txt");
*/
%>
An HTML comment in a JSP
<!--
<i>This page has been accessed <%= ++i %> times.</i>
-->
The value of the variable i is <%= i %>.
Description
•
When you code JSP comments, the comments aren’t compiled or executed.
•
When you code Java comments within a scriptlet, the comments aren’t compiled or
executed.
•
When you code HTML comments, the comments are compiled and executed, but the
browser doesn’t display them.
Figure 4-13
How to code comments in a JSP
128
Section 2 The essence of servlet and JSP programming
How to declare instance variables and methods
When a JSP is requested for the first time, one instance of the JSP is created
and loaded into memory, and a thread is started that executes the Java code in
the JSP. For each subsequent request for the JSP, another thread is started that
can access the one instance of the JSP. When you code variables in scriptlets,
each thread gets its own copy of each variable, which is usually what you want.
In some cases, though, you may want to declare instance variables and
methods that can be shared between all of the threads that are accessing a JSP.
To do that, you can code JSP declarations as shown in figure 4-14. Then, the
instance variables and methods are initialized when the JSP is first requested.
After that, each thread can access those instance variables and methods.
This is illustrated by the accessCount variable that’s declared as an instance
variable in this figure. This variable is incremented by one each time the JSP is
requested. Once it’s incremented, the current value is stored in a local variable.
Later, when the variable is displayed, it represents the total number of times that
the page has been accessed.
In this figure, the synchronized keyword prevents two threads from using
the method or modifying the instance variable at the same time. This results in a
thread-safe JSP. Without the synchronized keyword for the method, for ex-
ample, two or more threads could write to the text file at the same time, which
could lead to an I/O error if one thread tries to open the file before the previous
thread closes it. Without the synchronized and this keywords for the block of
code, two or more threads could update the accessCount variable at the same
time. If, for example, user 4 updates the accessCount before it is stored in the
localCount variable, the access count that’s displayed for user 3 will be 4.
Although that’s highly unlikely and insignificant in this case, this illustrates the
type of problem that can occur.
In most cases, though, it’s okay for multiple threads to access a method or
instance variable at the same time because no harm can come from it. That’s
why you only need to code a thread-safe JSP when multiple threads could cause
inaccurate or faulty results. The potential for this often arises when an instance
variable is modified by a thread or when a method accesses a data store. Then,
you can decide whether you need to provide for this by coding a thread-safe
JSP.
If you find your JSP becoming cluttered with declarations, you should
consider restructuring your program. In some cases, you may want to use
regular Java classes like the ones shown in this chapter. In other cases, you may
want to use a servlet as described in the next chapter. Better yet, you may want
to use a combination of servlets, JSPs, and other Java classes as described in
chapter 6.
Chapter 4 How to develop JavaServer Pages
129
JSP code that declares an instance variable and a method
<%@ page import="business.*, data.*, java.util.Date, java.io.*" %>
<%! int accessCount = 0; %>
<%!
public synchronized void addRecord(User user, String filename)
throws IOException{
PrintWriter out = new PrintWriter(
new FileWriter(filename, true));
out.println(user.getEmailAddress()+ "|"
+ user.getFirstName() + "|"
+ user.getLastName());
out.close();
}
%>
<%
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress = request.getParameter("emailAddress");
User user = new User(firstName, lastName, emailAddress);
addRecord(user, "../webapps/murach/WEB-INF/etc/UserEmail.txt");
int localCount = 0;
synchronized (this) {
accessCount++;
localCount = accessCount;
}
%>
.
.
.
<p><i>This page has been accessed <%= localCount %> times.</i></p>
Description
•
You can use JSP declarations to declare instance variables and methods for a JSP.
Unlike the variables defined in scriptlets, one set of instance variables and methods are
used by all the users of the JSP.
•
To code a thread-safe JSP, you must synchronize access to instance variables and
methods that should only be called by one thread at a time. Otherwise, two threads may
conflict when they try to modify the same instance variable at the same time or when
they try to execute the same method that accesses a data store at the same time.
•
To synchronize access to a method, you can use the synchronized keyword in the method
declaration. To synchronize access to a block of code, you can use the synchronized
keyword and the this keyword before the block.
Figure 4-14
How to declare instance variables and methods
130
Section 2 The essence of servlet and JSP programming
A JSP that imports classes and declares instance
variables
To illustrate how these JSP tags work together, figure 4-15 presents an
enhanced version of the Email List application. Here, the page that’s displayed
is the same as the page shown in figure 4-1 except that it displays two lines
below the Return button. The first line displays the date and time that the user
accessed the page. The second displays the number of times the page has been
accessed since it was initialized.
In the code for the JSP, the page directive imports the required classes
including the User and UserIO classes from the business and data packages.
This directive is followed by two declarations. The first declares an instance
variable for the number of users that access the JSP. The second declares an
instance variable for the path of the text file that will store the user data. The
scriptlet that follows is one that you’ve already seen so you should understand
how it works.
Since you’ve already seen the HTML tags that display the data for this JSP,
this figure doesn’t show these tags. They are the same HTML tags that are
shown in figure 4-11.
The first shaded line at the end of this JSP uses the constructor for the Date
class to return the current date and time. Since this constructor is coded within
an expression, each user of the JSP will get its own copy of the date and time.
As a result, the HTML line will display a different time for each user. In con-
trast, as explained earlier, the second shaded line displays the value of an
instance variable that is incremented by one for each thread.
Chapter 4 How to develop JavaServer Pages
131
A JSP that displays the date and an instance variable
The code for the JSP
<!doctype html public "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Chapter 4 - Email List application</title>
</head>
<body>
<%@ page import= "business.User, data.UserIO, java.util.Date, java.io.*" %>
<%! int accessCount = 0; %>
<%! String file = "../webapps/murach/WEB-INF/etc/UserEmail.txt"; %>
<% String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
String emailAddress = request.getParameter("emailAddress");
User user = new User(firstName, lastName, emailAddress);
UserIO.addRecord(user, file);
int localCount = 0;
synchronized (this) {
accessCount++;
localCount = accessCount;
}
%>
<!-- missing code that's the same as figure 4-11 -->
Today's date is <%= new Date() %>. <br>
<i>This page has been accessed <%= localCount %> times.</i>
</body>
</html>
Figure 4-15
A JSP that imports classes and declares instance variables
132
Section 2 The essence of servlet and JSP programming
How to work with JSP errors
When you develop JSPs, you will inevitably encounter errors. That’s why
the last three figures in this chapter show you how to work with JSP errors.
How to debug JSP errors
Figure 4-16 presents the two most common JSP errors. HTTP Status 404
means that Tomcat received the HTTP request but couldn’t find the requested
resource. You’ve already seen this in chapter 2. To fix this type of problem,
make sure that you’ve entered the correct path and filename for the request and
that the requested file is in the right location.
In contrast, HTTP Status 500 means that the server received the request and
found the resource but couldn’t fill the request. This usually means that the JSP
engine wasn’t able to compile the JSP due to a coding error in the JSP. To fix
this type of problem, you can review the other information provided by the error
page. In the error page in this figure, for example, you can see that a semicolon
is missing at the end of one of the statements in the JSP scriptlet.
To correct this type of error, you should fix the JSP so it will compile
correctly. To do this, you can open the JSP, add the semicolon, save it, and
refresh the screen. Then, the JSP engine will recognize that the JSP has been
modified and it will automatically attempt to compile and load the JSP.
This figure also gives some tips for debugging JSP errors. The first three
tips will help you solve most status 404 errors. The fourth one will help you find
the cause of most status 500 errors. And when all else fails, you can look at the
servlet that’s generated from the JSP as explained in figure 4-18.
Chapter 4 How to develop JavaServer Pages
133
An error page for a common JSP error
Common JSP errors
•
HTTP Status 404 – File Not Found Error
•
HTTP Status 500 – Internal Server Error
Tips for debugging JSP errors
•
Make sure the Tomcat server is running.
•
Make sure that the URL is valid and that it points to the right location for the requested
page.
•
Make sure all of the HTML, JSP, and Java class files are in the correct locations.
•
Read the error page carefully to get all available information about the error.
•
As a last resort, look at the servlet that’s generated for the JSP as shown in figure 4-18.
Figure 4-16
How to debug JSP errors
134
Section 2 The essence of servlet and JSP programming
How to use a custom error page
As you have just seen, the server displays its error page when an uncaught
exception is thrown at run time. Although that’s okay while you’re developing a
web application, you may not want error pages like that displayed once the
application is put into production. In that case, you can design your own error
pages and use them as shown in figure 4-17. That makes the user interface
easier to understand.
To designate the error page that should be used when any uncaught excep-
tion is thrown by a JSP page, you use the errorPage attribute of a page directive.
This attribute designates the JSP or HTML page that you want to use as the
error page. This is illustrated by the first example in this figure.
Then, if you want the error page to have access to the implicit exception
object, you set the isErrorPage attribute of its page directive to true as illustrated
by the second example in this figure. This makes an object named exception
available to the page. In the second example, you can see how this object is used
in a JSP expression to display the exception type and description. (When you
code an object name in an expression, its toString method is invoked.)
Chapter 4 How to develop JavaServer Pages
135
A custom error page in a browser
Two more attributes of a page directive
Attribute
Use
errorPage
To designate an error page for any uncaught exceptions that are thrown by the page.
isErrorPage
To identify a page as an error page so it has access to the implicit exception object.
A page directive that designates a custom error page
<%@ page errorPage="show_error_message.jsp" %>
A JSP that is designated as a custom error page
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Chapter 4 - Email List application</title>
</head>
<body>
<%@ page isErrorPage="true" %>
<h1>Error</h1>
<p>There was a problem adding your email to our list.
Please try back later.</p>
<p><i><%= exception %></i></p>
</body>
</html>
Description
•
You can use the errorPage attribute of a JSP page directive to designate an error page that
is requested if an uncaught exception is thrown at runtime. That error page can be either
a JSP or an HTML page.
•
If you want the error page to have access to the JSP implicit exception object, the error
page must be a JSP that includes a page directive with the isErrorPage attribute set to
true. Since the implicit exception object is an object of the java.lang.Throwable class,
you can call any methods in this class from the exception object.
Figure 4-17
How to use a custom error page
136
Section 2 The essence of servlet and JSP programming
When and how to view the servlet that’s
generated for a JSP
As you work with JSPs, you should keep in mind that JSPs are translated
into servlets and compiled before they are run. As a result, when a Status 500
error occurs, it’s usually because the JSP engine can’t compile the generated
servlet, even though that problem is caused by bad code in the related JSP.
As you have seen in figure 4-16, you can usually find the cause of a Status
500 error by reading the error page and proceeding from there. As a last resort,
though, it sometimes makes sense to view the servlet that has been generated
from the JSP. For instance, figure 4-18 shows the servlet that’s generated for the
JSP in figure 4-15.
Before you take a closer look at that servlet, though, you need to understand
how a servlet is generated, compiled, and instantiated. In particular, you need to
keep these facts in mind. First, each JSP is translated into a servlet class.
Second, the servlet class is compiled when the first user requests the JSP. Third,
one instance of the servlet class is instantiated when the first user requests the
JSP. Fourth, each user is treated as a thread by the servlet, and each thread gets
its own copy of the local variables of the servlet methods. With this conceptual
background, the generated servlet should make more sense.
At the start of this figure, you can see the path and filename for the servlet
that’s generated from the JSP in figure 4-15. Although all generated servlets are
stored in the work\localhost directory for Tomcat 4.0, the rest of the path is
dependent on the location of the actual JSP. If, for example, the JSP is saved in
the email4 directory of the murach application directory, then the generated
servlet is saved in the work\localhost\murach\email4 directory. The filename of
this servlet is generated from the JSP filename.
If you find the servlet file and open it with a text editor, you will probably
find the code overwhelming at first. But the partial code in this figure should
give you an idea of what’s going on. Somewhere near the start of the code,
you’ll find the instance variables of the JSP declarations declared as variables of
the class.
After that, you’ll find a JSP service method that contains all of the code
from the JSP scriptlet. Because this method is called for each user, each user (or
thread) will get a copy of the local variables used by this method. Then, after the
code that’s generated for the scriptlets, you’ll find out.write statements that
return HTML to the browser and out.print statements that return Java expres-
sions to the browser.
After you read the next chapter, which shows you how to develop servlets,
the code for this servlet should make more sense. But if you compare the code
for the JSP in figure 4-15 with its servlet code, you should get a good idea of
what’s going on. Remember, though, that you usually don’t need to view the
generated servlets for debugging purposes.
Chapter 4 How to develop JavaServer Pages
137
The location of the servlet class for the JSP in figure 4-15
C:\tomcat\work\localhost\murach\email4\show_0005femail_0005fentry$jsp.java
Part of the servlet class that’s generated from the JSP in figure 4-15
public class show_0005femail_0005fentry$jsp extends HttpJspBase {
Instance variables
int accessCount = 0;
from JSP declarations
String file = "../webapps/murach/WEB-INF/etc/UserEmail.txt";
...
}
public void _jspService(HttpServletRequest request, HttpServletResponse
response) throws java.io.IOException, ServletException {
...
String firstName = request.getParameter("firstName");
Local data from a
String lastName = request.getParameter("lastName");
JSP scriptlet
String emailAddress = request.getParameter("emailAddress");
User user = new User(firstName, lastName, emailAddress);
addRecord(user, file);
int localCount = 0;
synchronized (this) {
accessCount++;
localCount = accessCount;
}
...
A write statement
out.write("... <td align=\"right\">First name:</td> ...");
for HTML tags
out.print( firstName );
...
}
A print statement
}
from a JSP expression
How a servlet is generated, compiled, and instantiated from a JSP
•
When a JSP is first requested, the JSP engine translates the page into a servlet class and
compiles the class. When it does, it places all JSP scriptlets into a service method. As a
result, all variables defined by the scriptlets are local variables to the service method.
•
After the servlet class is compiled, one instance of the class is instantiated and a new thread
is created. Then, the service method is called.
•
For each subsequent user that requests the JSP, a new thread is created from the instance of
the servlet class and the service method is called. This method delivers a copy of the local
variables for each thread, but all threads share the instance variables.
When and how to view the servlet that’s generated for a JSP
•
Although you need to know that your JSPs are translated into servlets before they are
executed, you normally don’t need to view them. For advanced debugging problems,
though, it is sometimes useful to view the servlet for a JSP.
•
If you’re using Tomcat 4.0 or later, you can find the code for the Java servlet class in the
subdirectory of the tomcat\work\localhost directory. Starting with that directory, you’ll find
a directory structure that corresponds to the one for your JSPs.
•
If you’re using another JSP engine, you should find the servlet class in a similar directory
structure.
Figure 4-18
When and how to view the servlet that’s generated for a JSP
138
Section 2 The essence of servlet and JSP programming
Perspective
The goal of this chapter has been to provide you with the basics of coding,
saving, and viewing JSPs. At this point, you should be able to code simple, but
practical, JSPs of your own. In addition, you should understand how HTML pages
communicate with JSPs, how JSPs communicate with regular Java classes, and
how to fix some of the most common JSP errors.
In the next chapter, you’ll learn how to use the same types of skills with
servlets. In fact, the next chapter uses the same application that’s presented in this
chapter, but it uses a servlet instead of a JSP.
Summary
•
A JavaServer Page, or JSP, consists of HTML code plus Java code that’s embed-
ded in scriptlets and expressions. Within the scriptlets and expressions, you can
use the methods of the implicit request object to get the parameters that are passed
to the JSP.
•
The JSPs for an application are stored in the document root directory or one of its
subdirectories. Then, you can view a JSP by entering a URL that corresponds to its
directory location.
•
When you use the Get method to pass parameters to a JSP, the parameters are
displayed in the URL. When you use the Post method, they aren’t. Although the
Get method transfers data faster than the Post method, you can’t use the Get
method to transfer more than 4K of data.
•
You can use business and data classes from within scriptlets and expressions just
as you use them from Java classes. However, the business and data classes must be
stored in the WEB-INF\classes directory or one of its subdirectories.
•
You use a JSP directive known as a page directive to import classes for use in a
JSP.
•
When you use JSP comments, the comments aren’t compiled or executed. In
contrast, HTML comments are compiled and executed, but the browser doesn’t
display them.
•
You use JSP declarations to declare instance variables and methods for a JSP. To
code a thread-safe JSP, you must synchronize access to these instance variables
and methods.
•
You can use a page directive to designate an error page for an uncaught exception.
You can code this directive so it has access to the JSP implicit exception object and
you can use this object’s methods.
Chapter 4 How to develop JavaServer Pages
139
•
When a JSP is first requested, the JSP engine translates the page into a servlet and
compiles it. Then, one instance of the class is instantiated. For each subsequent
request, a new thread is created from this single instance of the servlet class. This
means that each requestor gets its own copy of the local variables, but all requestors
share the same instance variables.
Terms
JavaServer Page (JSP)
implicit request object
JSP declaration
parameter
JSP directive
thread-safe
scriptlet
page directive
error page
JSP expression
JSP comment
implicit exception
JSP tag
instance
object
request object
thread
Objectives
•
Code and test JavaServer Pages that require any of the features presented in this
chapter including scriptlets, expressions, page directives, JSP comments, JSP
declarations, and custom error pages.
•
Describe the directory structure that should be used for JSPs, business, and data
classes.
•
List two differences between the Get and Post methods that can be used to pass
parameters to a JSP.
•
Describe the difference between JSP comments and HTML comments.
•
Explain what is meant by a thread-safe JSP and describe what you have to do to
develop one.
•
Describe the process of running a JSP and its effect on local and instance vari-
ables.
Exercise 4-1
Create a custom error page
In this exercise, you’ll create a custom error page for the Email List application.
1. Run the HTML document named join_email_list.html that’s in the
webapps\murach\email4 directory so it accesses and runs the JSP named
show_email_entry.jsp.
2. Create a custom error page named show_error_page.jsp that displays the type
of error, the current date, and the number of times the error page has been
accessed. Save this JSP in the webapps\murach\email4 directory.
3. Edit the JSP file named show_email_entry.jsp that’s in the email4 directory so
it uses the show_error_page.jsp.
140
Section 2 The essence of servlet and JSP programming
4.
Test the custom error page. To do that, you can cause an exception by changing
the properties of the file so it’s read-only. In Windows, you can do this by right
clicking the file icon in Explorer, selecting Properties, and selecting Read-only.
Exercise 4-2
Create a new JSP
In this exercise, you’ll modify the HTML document for the Email List
application, and you’ll create a new JSP that responds to the HTML document.
1.
Modify the HTML document named join_email_list.html that’s in the email4
directory so it has this line of text after the Email address box: “I’m interested
in these types of music.” Then, follow this line with a list box that has options
for Rock, Country, Bluegrass, and Folk music. This list box should be followed
by the Submit button, and the Submit button should link to a new JSP named
show_music_choices.jsp
2.
Create a new JSP named show_music_choices.jsp that responds to the changed
HTML document. This JSP should start with an H1 line that reads like this:
Thanks for joining our email list, John Smith.
And this line should be followed by text that looks like this:
We’ll use email to notify you whenever we have new releases for
these types of music:
Country
Bluegrass
In other words, you list the types of music that correspond to the items that are
selected in the list box. And the entire web page consists of just the heading
and text lines that I’ve just described.
3.
Test the HTML document and the new JSP by running them. Note the
parameter list that is passed to the JSP by the HTML document. Then, test the
new JSP by using a URL that includes a parameter list.