The abstract class
javax.servlet.Servlet
is designed for those who wish to
structure an entire web server around the servlet notion. For
example, in Sun’s Java Web Server, there is a servlet subclass
for handling plain HTML pages, another for processing CGI programs,
and so on. Unless you are writing your own web server, you will probably not
extend from this class, but rather its subclass
HttpServlet
, in the package
javax.servlet.http
. This class has a method:
public void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException;
The service method is passed two arguments,
request and response. The
request contains all the information about the request from the
browser, including its input stream should you need to read data. The
response argument contains information to get the response back to
the browser, including the output stream to write your response back
to the user.
But the web has several HTTP methods for passing data into a web page. Unimportant for plain HTML pages, this distinction becomes of interest when processing forms, i.e., web pages with fill-in-the-blank or choice items. Briefly, the GET method of HTTP is used to pass all the form data appended to the URL. GET URLs look like this, for example:
http://www.acmewidgets.com/cgi-bin/ordercgi?productId=123456
They have the advantage that the user can bookmark them, avoiding
having to fill in the form multiple times. But there is a limit of
about 1KB on the overall length of the URL. Since this must be a
single string, there is an encoding that allows spaces, tabs, colons,
and other characters to be presented as two hexadecimal digits:
%20 is the character hexadecimal 20, or the ASCII
space character. The POST method, by contrast, passes any parameters
as input on the socket connection, after the HTTP headers.
The default implementation of the service( )
method in the HttpServlet class figures out which
method was used to invoke the servlet. It dispatches to the correct
method: doGet( )
if a GET request, doPost( ) if a POST request, etc., passing along the
request and response arguments.
So while you can, in theory, override the service( ) method, it’s more common (and officially
recommended) to override either doGet( ),
doPost( ), or both.
The
simplest
HttpServlet is something like Example 18-1.
Example 18-1. HelloServlet.java
import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Simple Hello World Servlet
*/
public class HelloServlet extends HttpServlet{
public void doGet(HttpServletRequest request, HttpServletResponse response) throws
IOException {
PrintWriter out = response.getWriter( );
response.setContentType("text/html");
out.println("<H1>Hello from a Servlet</H1>");
out.println("<P>This servlet ran at ");
out.println(new Date().toString( ));
out.println("<P>Courtesy of HelloServlet.java 1.2 ");
}
}The program will give output resembling Figure 18-1.
You can do much more with servlets. Suppose you wanted to print a
dictionary -- a list of terms and their meanings -- from within
a servlet. The code would be pretty much as it was in Figure 18-1, except that you’d need a
doGet( )
method instead of a
doPost( ) method. Example 18-2 is
the code for
TermsServlet
.
Example 18-2. TermsServlet.java
/** A Servlet to list the dictionary terms.
*/
public class TermsServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
PrintWriter out = resp.getWriter( );
out.println("<HTML>");
out.println("<TITLE>Ian Darwin's Computer Terms and Acronyms</TITLE>");
out.println("<BODY>");
out.println("<H1>Ian Darwin's Computer Terms and Acronyms</H1>");
out.println("<TABLE BORDER=2>");
out.println("<TR><TH>Term<TH>Meaning</TR>");
// This part of the Servlet generates a list of lines like
// <TR> <TD>JSP <TD>Java Server Pages, a neat tool for ...
TermsAccessor tax = new TermsAccessor("terms.txt");
Iterator e = tax.iterator( );
while (e.hasNext( )) {
Term t = (Term)e.next( );
out.print("<TR><TD>");
out.print(t.term);
out.print("<TD>");
out.print(t.definition);
out.println("</TR>");
}
out.println("</TABLE>");
out.println("<HR></HR>");
out.println("<A HREF="servlet/TermsServletPDF
">Printer-friendly (Acrobat PDF) version</A>");
out.println("<HR></HR>");
out.println("<A HREF="mailto:[email protected]/subject=Question
">Ask about another term</A>");
out.println("<HR></HR>");
out.println("<A HREF="index.html">Back to HS</A> <A HREF="../
">Back to DarwinSys</A>");
out.println("<HR></HR>");
out.println("<H6>Produced by $Id: TermsServlet.java,v 1.1 2000/04/06
ian Exp $");
out.print(" using ");
out.print(tax.ident);
out.println("</H6>");
}
}