/** * Defines methods that all servlets must implement. * * <p>A servlet is a small Java program that runs within a Web server. * Servlets receive and respond to requests from Web clients, * usually across HTTP, the HyperText Transfer Protocol.  * * <p>To implement this interface, you can write a generic servlet * that extends * <code>javax.servlet.GenericServlet</code> or an HTTP servlet that * extends <code>javax.servlet.http.HttpServlet</code>. * * <p>This interface defines methods to initialize a servlet, * to service requests, and to remove a servlet from the server. * These are known as life-cycle methods and are called in the * following sequence: * <ol> * <li>The servlet is constructed, then initialized with the <code>init</code> method. * <li>Any calls from clients to the <code>service</code> method are handled. * <li>The servlet is taken out of service, then destroyed with the  * <code>destroy</code> method, then garbage collected and finalized. * </ol> * * <p>In addition to the life-cycle methods, this interface * provides the <code>getServletConfig</code> method, which the servlet  * can use to get any startup information, and the <code>getServletInfo</code> * method, which allows the servlet to return basic information about itself, * such as author, version, and copyright. * * @author     Various * * @see     GenericServlet * @see     javax.servlet.http.HttpServlet * */public interface Servlet {

    /**     * Called by the servlet container to indicate to a servlet that the      * servlet is being placed into service.     *     * <p>The servlet container calls the <code>init</code>     * method exactly once after instantiating the servlet.     * The <code>init</code> method must complete successfully     * before the servlet can receive any requests.     *     * <p>The servlet container cannot place the servlet into service     * if the <code>init</code> method     * <ol>     * <li>Throws a <code>ServletException</code>     * <li>Does not return within a time period defined by the Web server     * </ol>     *     *     * @param config            a <code>ServletConfig</code> object      *                    containing the servlet‘s     *                     configuration and initialization parameters     *     * @exception ServletException     if an exception has occurred that     *                    interferes with the servlet‘s normal     *                    operation     *     * @see                 UnavailableException     * @see                 #getServletConfig     *     */public void init(ServletConfig config) throws ServletException;

     /**     *     * Returns a {@link ServletConfig} object, which contains     * initialization and startup parameters for this servlet.     * The <code>ServletConfig</code> object returned is the one      * passed to the <code>init</code> method.      *     * <p>Implementations of this interface are responsible for storing the      * <code>ServletConfig</code> object so that this      * method can return it. The {@link GenericServlet}     * class, which implements this interface, already does this.     *     * @return        the <code>ServletConfig</code> object     *            that initializes this servlet     *     * @see         #init     *     */public ServletConfig getServletConfig();

     /**     * Called by the servlet container to allow the servlet to respond to      * a request.     *     * <p>This method is only called after the servlet‘s <code>init()</code>     * method has completed successfully.     *      * <p>  The status code of the response always should be set for a servlet      * that throws or sends an error.     *     *      * <p>Servlets typically run inside multithreaded servlet containers     * that can handle multiple requests concurrently. Developers must      * be aware to synchronize access to any shared resources such as files,     * network connections, and as well as the servlet‘s class and instance      * variables.      * More information on multithreaded programming in Java is available in      * <a href="http://www.mamicode.com/http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">     * the Java tutorial on multi-threaded programming</a>.     *     *     * @param req     the <code>ServletRequest</code> object that contains     *            the client‘s request     *     * @param res     the <code>ServletResponse</code> object that contains     *            the servlet‘s response     *     * @exception ServletException     if an exception occurs that interferes     *                    with the servlet‘s normal operation      *     * @exception IOException         if an input or output exception occurs     *     */public void service(ServletRequest req, ServletResponse res)    throws ServletException, IOException;

 /**     * Returns information about the servlet, such     * as author, version, and copyright.     *      * <p>The string that this method returns should     * be plain text and not markup of any kind (such as HTML, XML,     * etc.).     *     * @return         a <code>String</code> containing servlet information     *     */public String getServletInfo();

 /**     *     * Called by the servlet container to indicate to a servlet that the     * servlet is being taken out of service.  This method is     * only called once all threads within the servlet‘s     * <code>service</code> method have exited or after a timeout     * period has passed. After the servlet container calls this      * method, it will not call the <code>service</code> method again     * on this servlet.     *     * <p>This method gives the servlet an opportunity      * to clean up any resources that are being held (for example, memory,     * file handles, threads) and make sure that any persistent state is     * synchronized with the servlet‘s current state in memory.     *     */    public void destroy();}