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This chapter acts as a Servlet tutorial. You will learn how to use important
techniques for Servlet development by writing some typical Servlets, ranging
from very simple to rather complex. All examples in this chapter are fully
functional and complete Servlets which have been successfully compiled and
run.
This section shows how to
-
use the framework that makes up a simple Servlet
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write a Servlet that provides static content (i.e. it produces the same
output every time it is called by a client)
We start our venture into Servlet programming with the well-known "Hello
World" example, this time named more suitably "Hello Client":
1: import java.io.*;
2: import javax.servlet.*;
3: import javax.servlet.http.*;
4:
5: public class HelloClientServlet extends HttpServlet
6: {
7: protected void doGet(HttpServletRequest req,
8: HttpServletResponse res)
9: throws ServletException, IOException
10: {
11: res.setContentType("text/html");
12: PrintWriter out = res.getWriter();
13: out.println("<HTML><HEAD><TITLE>Hello Client!</TITLE>"+
14: "</HEAD><BODY>Hello Client!</BODY></HTML>");
15: out.close();
16: }
17:
18: public String getServletInfo()
19: {
20: return "HelloClientServlet 1.0 by Stefan Zeiger";
21: }
22: }
When you
compile
this Servlet and
run
it by requesting a URL which is assigned to it in a Web Browser it produces
the following output:
Let's have a look at how the Servlet works.
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Lines 1 to 3 import some packages which contain many classes which are
used by the Servlet (almost every Servlet needs classes from these
packages).
1: import java.io.*;
2: import javax.servlet.*;
3: import javax.servlet.http.*;
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The Servlet class is declared in line 5. Our Servlet extends javax.servlet.http.HttpServlet, the
standard base class for HTTP Servlets.
5: public class HelloClientServlet extends HttpServlet
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In lines 7 through 16 HttpServlet's doGet method is getting overridden.
7: protected void doGet(HttpServletRequest req,
8: HttpServletResponse res)
9: throws ServletException, IOException
10: {
...
16: }
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In line 11 we use a method of the HttpServletResponse object to set the content type of
the response that we are going to send. All response headers must be set
before a PrintWriter or ServletOutputStream is requested to write body data to the
response.
11: res.setContentType("text/html");
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In line 12 we request a PrintWriter object to write text to the response
message.
12: PrintWriter out = res.getWriter();
[API 2.0] ServletResponse.getWriter() is a
new feature of JSDK version 2.0. If your Servlet engine does not support
JSDK 2.0 you can replace the above line by " ServletOutputStream out = res.getOutputStream();". This change can be
made in most of the example Servlets in this tutorial. The advantages of
using ServletResponse.getWriter() are discussed in section
4.4.
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In lines 13 and 14 we use the PrintWriter to write the text of type
text/html (as specified through the content type).
13: out.println("<HTML><HEAD><TITLE>Hello Client!</TITLE>"+
14: "</HEAD><BODY>Hello Client!</BODY></HTML>");
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The PrintWriter gets closed in line 15 when we are finished writing to it.
15: out.close();
This line is included for completeness. It is not strictly necessary. The
Web Server closes the PrintWriter or ServletOutputStream automatically when a service call
returns. An explicit call to close() is useful when you want to do some
post-processing after the response to the client has been fully written.
Calling close() tells the Web Server that the response is finished and the
connection to the client may be closed as well.
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In lines 18 through 21 we override the getServletInfo() method which is supposed to
return information about the Servlet, e.g. the Servlet name, version,
author and copyright notice. This is not required for the function of the
HelloClientServlet but can provide valuable information to the user of a Servlet who
sees the returned text in the administration tool of the Web Server.
18: public String getServletInfo()
19: {
20: return "HelloClientServlet 1.0 by Stefan Zeiger";
21: }
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