NOTE For instructions on using Tomcat to run a servlet see Using Tomcat for Servlet in Java

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Following is a skeletal servlet It overrides the three life cycle methods, and it returns a response when its service( ) method is called This response is the same for all requests: it simply returns a message that shows what life cycle methods have been called and the name of the server However, your own servlet will provide implementations appropriate for your application
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NOTE For instructions on using Tomcat to run a servlet see Using Tomcat for Servlet
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// A simple servlet import javaio*; import javaxservlet*; public class ServletSkel extends GenericServlet { String msg = "";
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// Called once, at startup public void init(ServletConfig sc) { msg = "Servlet initialized"; } // Called once, when the servlet is removed public void destroy() { msg += " This won't be seen"; } // Called repeatedly to handle requests public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException { // Set the content type responsesetContentType("text/html"); // Get a response stream PrintWriter pw = responsegetWriter(); // Show that the service() method has been called // and display the server name msg += "<br>Inside service() Server: " + requestgetServerName(); pwprintln(msg); pwclose(); } }
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The output displayed in the browser is shown here:
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Notice that the server name is localhost This is because the servlet was run on the same computer as the browser Also note that the message added by destroy( ) is not shown As the comments inside destroy( ) indicate, you will not see the string added by destroy( ) because it is the last method called when the servlet is being removed Thus, service( ) is not called after destroy( ) is called
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Options and Alternatives
As explained, when extending GenericServlet, there is no need to override init( ) or destroy( ) if they are not needed Default implementations are provided To create a servlet that handles various HTTP requests, such as GET or POST, it is often more convenient to extend HttpServlet rather than GenericServlet The following recipe shows how
6:
Applets and Servlets
Handle HTTP Requests in a Servlet
Key Ingredients
Classes and Interfaces javaxservlethttp HttpServlet javaxservlethttp HttpServletRequest javaxservlethttp HttpServletResponse Methods void doGet(HttpServletRequest hsreq, HttpServletResponse hsrep) String getParameter(String paramName) PrintWriter getWriter( )
If you are creating a servlet that will be responding to HTTP requests, such as GET or POST, then you will usually want your servlet to extend HttpServlet rather than GenericServlet The reason is that HttpServlet provides methods that handle the various HTTP requests HttpServlet extends GenericServlet and is packaged in javaxservlethttp
Step-by-Step
To create a servlet based on HttpServlet, follow these steps: 1 Create a class that extends HttpServlet 2 If necessary, override the init( ) and destroy( ) life cycle methods However, do not override service( ) because it is implemented by HttpServlet and it automatically routes HTTP requests to the handlers defined by HttpServlet 3 Override the handler or handlers needed by your servlet The handler used in this recipe is doGet( ) It handles GET requests Information about the request is available through an HttpServletRequest object The response is returned via an HttpServletResponse object The example responds to requests by writing to the output stream linked to the response parameter The output stream is obtained by calling getWriter( ) Before responding, you can set the content type by calling setContentType( ) 4 You can obtain the value of a parameter associated with a request by calling getParameter( ) on the HttpServletRequest object
Discussion
HttpServlet defines several do methods that handle various HTTP requests The one used by the recipe is doGet( ) It handles GET requests and is shown here: void doGet(HttpServletRequest hsreq, HttpServletResponse hsrep) throws IOException, ServletException
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It is called when a GET request is received Information about the request is available through hsreq To respond, use hsrep An IOException is thrown if an I/O error occurs while handling the request A ServletException is thrown if the request fails HttpServletRequest extends ServletRequest and adds support for HTTP requests HttpServletResponse extends ServletResponse and adds support for HTTP responses To obtain the value of a parameter associated with the request, call getParameter( ) This method is inherited from ServletRequest and is shown here: String getParameter(String paramName) The name of the parameter is passed in paramName The value (in string form) is returned If the parameter is not found, null is returned For a description of setContentType( ) and getWriter( ), see Create a Simple Servlet Using GenericServlet
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