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A servlet is an extension to a server that enhances
the server's functionality. The most common use for
a servlet is to extend a web server by providing dynamic
web content. Web servers display documents written in
HyperText Markup Language (HTML) and respond to user
requests using the HyperText Transfer Protocol (HTTP).
HTTP is the protocol for moving hypertext files across
the internet. HTML documents contain text that has been
marked up for interpretation by an HTML browser such as
Netscape.
Servlets are easy to write. All you need is the
Java® 2 Platform software, and JavaServerTM
Web Development Kit (JWSDK).
You can download a free copy of the
JWSDK.
This lesson shows you how to create a very simple form that invokes
a basic servlet to process end user data entered on the form.
About the Example
A browser accepts end user input through an HTML form. The
simple form used in this lesson has one text input field
for the end user to enter text and a Submit button.
When the end user clicks the Submit button, the simple servlet is
invoked to process the end user input.
In this example, the simple servlet returns an HTML page that displays
the text entered by the end user.
HTML Form
The HTML form is embedded in this
HTML file.
The diagram shows how the HTML page looks when it is
opened in a browser.
The HTML file and form are similar to the simple
application and applet examples in
Lesson 4 so you can
compare the code and learn how servlets, applets, and
applications handle end user inputs.
When the user clicks the Click Me button, the
servlet gets the entered text, and returns an HTML page with
the text.
The HTML page returned to the browser by the
ExampServlet.java
servlet is shown below. The servlet code to retrieve the
user's input and generate the HTML page follows with a discussion.
Note:
To run the example, you have to put the servlet and
HTML files in the correct directories for
the Web server you are using. For example, with Java WebServer 1.1.1,
you place the servlet in the ~/JavaWebServer1.1.1/servlets
and the HTML file in the
~/JavaWebServer1.1.1/public_html directory.
Servlet Backend
ExampServlet.java builds an HTML page
to return to the end user. This means the servlet code does not use any
Project Swing or
Abstract Window Toolkit (AWT) components or have event handling code.
For this simple servlet, you only need to import these packages:
-
java.io for system input and output. The
HttpServlet class uses the IOException
class in this package to signal that an input or output
exception of some kind has occurred.
-
javax.servlet, which contains generic
(protocol-independent) servlet classes. The
HttpServlet class uses the ServletException
class in this package to indicate a servlet problem.
-
javax.servlet.http, which contains HTTP
servlet classes. The HttpServlet class is in this
package.
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ExampServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<title>Example</title>" +
"<body bgcolor=FFFFFF>");
out.println("<h2>Button Clicked</h2>");
String DATA = request.getParameter("DATA");
if(DATA != null){
out.println(DATA);
} else {
out.println("No text entered.");
}
out.println("<P>Return to
<A HREF="../simpleHTML.html">Form</A>");
out.close();
}
}
Class and Method Declarations
All servlet classes extend the HttpServlet abstract class.
HttpServlet simplifies writing HTTP servlets by providing a
framework for handling the HTTP protocol. Because HttpServlet
is abstract, your servlet class must extend it and override at
least one of its methods. An abstract class is a class
that contains unimplemented methods and cannot be instantiated itself.
public class ExampServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
The ExampServlet class is declared public
so the web server that runs the servlet, which is not
local to the servlet, can access it.
The ExampServlet class defines a doPost
method with the same name, return type, and parameter list as the
doPost method in the HttpServlet class.
By doing this, the ExampServlet class overrides and
implements the doPost method in the
HttpServlet class.
The doPost method performs the HTTP POST
operation, which is the type of operation specified in
the HTML form used for this example. The other possibility is the
HTTP GET operation, in which case you would implement
the doGet method instead.
In short, POST requests are for sending any amount of
data directly over the connection without changing the URL, and
GET requests are for getting limited amounts of
information appended to the URL. POST requests
cannot be bookmarked or emailed and do not change the Uniform
Resource Locators (URL) of the response. GET
requests can be bookmarked and emailed and add information to the URL
of the response.
The parameter list for the doPost method takes a
request and a response object. The browser
sends a request to the servlet and the servlet sends a response back
to the browser.
The doPost method implementation accesses information
in the request object to find out who made the
request, what form the request data is in, and which HTTP headers
were sent, and uses the response object to create
an HTML page in response to the browser's request.
The doPost method throws an IOException
if there is an input or output problem when it handles the request,
and a ServletException if the request could not be
handled. These exceptions are handled in the HttpServlet
class.
Method Implementation
The first part of the doPost method uses
the response object to create an HTML page.
It first sets the response content type to be
text/html,
then gets a PrintWriter object for formatted text output.
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<title>Example</title>" +
"<body bgcolor=#FFFFFF>");
out.println("<h2>Button Clicked</h2>");
The next line uses the request object to
get the data from the text field on the form and store it
in the DATA variable. The getparameter
method gets the named parameter, returns null if the
parameter was not set, and an empty string if the parameter
was sent without a value.
String DATA = request.getParameter("DATA");
The next part of the doPost method gets the data out of the
DATA
parameter and passes it to the response object to add to the
HTML response page.
if(DATA != null){
out.println(DATA);
} else {
out.println("No text entered.");
}
The last part of the doPost method creates a link to take the
end user from the HTML response page back to the original form, and
closes the response.
out.println("<P>Return to
<A HREF="../simpleHTML.html">Form</A>");
out.close();
}
Note:
To learn how to use the other methods available in the
HttpServlet, HttpServletRequest,
and HttpServletResponse classes, see
The
Java Tutorial trail on
Servlets.
More Information
You can find more information on servlets in the
Servlets
trail in
The Java Tutorial.
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