3.1 Basic Servlet Structure

Listing 3.1 outlines 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 enters a URL on the address line, follows a link from a Web page, or submits an HTML form that either does not specify a METHOD or specifies METHOD="GET" . Servlets can also easily handle POST requests, which are generated when someone submits an HTML form that specifies METHOD="POST" . For details on the use of HTML forms and the distinctions between GET and POST , see Chapter 19 (Creating and Processing HTML Forms).

Listing 3.1 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 query data from HTML forms.     // 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.   } } 

Servlets typically extend HttpServlet and override doGet or doPost , depending on whether the data is being sent by GET or by POST . If you want a servlet to take the same action for both GET and POST requests, simply have doGet call doPost , or vice versa.

Both doGet and doPost take two arguments: an HttpServletRequest and an HttpServletResponse . The HttpServletRequest lets you get at all of the incoming data; the class has methods by which you can find out about information such as form (query) data, HTTP request headers, and the client's hostname. The HttpServletResponse lets you specify outgoing information such as HTTP status codes (200, 404, etc.) and response headers ( Content-Type , Set-Cookie , etc.). Most importantly, HttpServletResponse lets you obtain a PrintWriter that you use to send 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 are all discussed in the following chapters.

Since doGet and doPost throw two exceptions ( ServletException and IOException ), you are required to include them in the method declaration. Finally, you must import classes in java.io (for PrintWriter , etc.), javax.servlet (for HttpServlet , etc.), and javax.servlet.http (for HttpServletRequest and HttpServletResponse ).

However, there is no need to memorize the method signature and import statements. Instead, simply download the preceding template from the source code archive at http://www.coreservlets.com/ and use it as a starting point for your servlets.