Basic Servlet Structure

Here's the outline of a basic servlet that handles GET requests. GET requests, for those unfamiliar with HTTP, are requests made by browsers when the user types in a URL on the address line, follows a link from a Web page, or makes an HTML form that does not specify a METHOD. Servlets can also very easily handle POST requests, which are generated when someone creates an HTML form that specifies METHOD="POST". We'll discuss that in later sections.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class SomeServlet extends HttpServlet {
 public void doGet(HttpServletRequest request,
                   HttpServletResponse response)
     throws ServletException, IOException {
    
   // Use "request" to read incoming HTTP headers (e.g. cookies)
   // and HTML form data (e.g. data the user entered and submitted)
  
   // Use "response" to specify the HTTP response line and headers
   // (e.g. specifying the content type, setting cookies).
  
   PrintWriter out = response.getWriter();
   // Use "out" to send content to browser
 }
}

(Download template source code -- click with the right mouse on the link or hold down SHIFT while clicking on the link.)

To be a servlet, a class should extend HttpServlet and override doGet or doPost (or both), depending on whether the data is being sent by GET or by POST. These methods take two arguments: an HttpServletRequest and an HttpServletResponse. The HttpServletRequest has methods that let you find out about incoming information such as FORM data, HTTP request headers, and the like. The HttpServletResponse has methods that lets you specify the HTTP response line (200, 404, etc.), response headers (Content-Type, Set-Cookie, etc.), and, most importantly, lets you obtain a PrintWriter used to send output back to the client. For simple servlets, most of the effort is spent in println statements that generate the desired page. Note that doGet and doPost throw two exceptions, so you are required to include them in the declaration. Also note that you have to import classes in java.io (for PrintWriter, etc.), javax.servlet (for HttpServlet, etc.), and javax.servlet.http (for HttpServletRequest and HttpServletResponse). Finally, note that doGet and doPost are called by the service method, and sometimes you may want to override service directly, e.g. for a servlet that handles both GET and POST request.