Programming Tutorials

Basic Servlet Structure

By: aathishankaran in JSP Tutorials on 2007-02-14  

Listing outlines of a basic servlet that handles GET requests. GET requests, for those unfamiliar with HTTP, are the usual type of browser requests for Web pages. A browser generates this request when the user types a URL on the address line, follows a link from a Web page, or submits an HTML form that does not specify a METHOD. Servlets can also very easily handle POST requests, which are generated when someone submits an HTML form that specifies

METHOD="POST". For details on using HTML forms. To be a servlet, a class should extend HttpServlet and override doGet or doPost, depending on whether the data is being sent by GET or by POST. If you want the same servlet to handle both GET and POST and to take the same action for each, you can simply have doGet call doPost, or vice versa.

Both of these methods take two arguments: an HttpServletRequest and an HttpServletResponse. The HttpServletRequest has methods by which you can find out about incoming information such as form data, HTTP request headers, and the client’s hostname. The HttpServletResponse lets you specify outgoing information such as HTTP status codes (200, 404, etc.), response headers (Content-Type, Set-Cookie, etc.), and, most importantly, lets you obtain a PrintWriter used to send the document content back to the client. For simple servlets, most of the effort is spent in println statements that generate the desired page. Form data, HTTP request headers, HTTP responses, and cookies will all be discussed in detail in the following chapters. Since doGet and doPost throw two exceptions, you are required to include them in the declaration. Finally, you have to import classes in java.io (for PrintWriter, etc.), javax.servlet (for HttpServlet, etc.), and javax.servlet.http (for HttpServletRequest and HttpServlet- Response).

Strictly speaking, HttpServlet is not the only starting point for servlets, since servlets could, in principle, extend mail, FTP, or other types of servers. Servlets for these environments would extend a custom class derived from Generic- Servlet, the parent class of HttpServlet. In practice, however, servlets are used almost exclusively for servers that communicate via HTTP (i.e., Web and application servers), and the discussion in this article will be limited to this usage.

ServletTemplate.java

import java.io.*;
import javax.servlet.*; 
import javax.servlet.http.*; 
public class ServletTemplate 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 status 
// code and headers (e.g. the content type, cookies).
PrintWriter out = response.getWriter();
// Use "out" to send content to browser
} 
}