Overriding Service, Get, and Post

When a request is made, the Sun Java System Web Server hands the incoming data to the servlet engine to process the request. The request includes form data, cookies, session information, and URL name-value pairs, all in a type HttpServletRequest object called the request object. Client metadata is encapsulated as a type HttpServletResponse object called the response object. The servlet engine passes both objects as the servlet's service() method parameters.

The default service() method in an HTTP servlet routes the request to another method based on the HTTP transfer method (POST, GET, and so on). For example, HTTP POST requests are routed to the doPost() method, HTTP GET requests are routed to the doGet() method, and so on. This enables the servlet to perform different request data processing depending on the transfer method. Since the routing takes place in service(), there is no need to generally override service() in an HTTP servlet. Instead, override doGet(), doPost(), and so on, depending on the expected request type.

The automatic routing in an HTTP servlet is based simply on a call to request.getMethod(), which provides the HTTP transfer method. In a Sun Java System Web Server, request data is already preprocessed into a name-value list by the time the servlet sees the data, so simply overriding the service() method in an HTTP servlet does not lose any functionality. However, this does make the servlet less portable, since it is now dependent on preprocessed request data.

Override the service() method (for generic servlets) or the doGet() or doPost() methods (for HTTP servlets) to perform tasks needed to answer the request. Very often, this means collating the needed information (in the request object or in a JDBC result set object), and then passing the newly generated content to a JSP for formatting and delivery back to the client.

Most operations that involve forms use either a GET or a POST operation, so for most servlets you override either doGet() or doPost(). Implement both methods to provide for both input types or simply pass the request object to a central processing method, as shown in the following example:

public void doGet (HttpServletRequest request,
                  HttpServletResponse response)
         throws ServletException, IOException {
   doPost(request, response);
}

All request-by-request traffic in an HTTP servlet is handled in the appropriate doOperation() method, including session management, user authentication, JSPs, and accessing Sun Java System Web Server features.

If a servlet intends to call the RequestDispatcher method include() or forward(), be aware the request information is no longer sent as HTTP POST, GET, and so on. In other words, if a servlet overrides doPost(), it may not process anything if another servlet calls it, if the calling servlet happens to receive its data through HTTP GET. For this reason, be sure to implement routines for all possible input types, as explained above. RequestDispatcher methods always call service().

For more information, see Calling a Servlet Programmatically.

Arbitrary binary data, such as uploaded files or images, can be problematic, because the web connector translates incoming data into name-value pairs by default. You can program the web connector to properly handle these kinds of data and package them correctly in the request object.