/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package javax.servlet;

import java.io.IOException;
import java.util.Enumeration;

/**
 * Defines a generic, protocol-independent
 * servlet. To write an HTTP servlet for use on the
 * Web, extend {@link javax.servlet.http.HttpServlet} instead.
 * <p/>
 * <p><code>GenericServlet</code> implements the <code>Servlet</code>
 * and <code>ServletConfig</code> interfaces. <code>GenericServlet</code>
 * may be directly extended by a servlet, although it's more common to extend
 * a protocol-specific subclass such as <code>HttpServlet</code>.
 * <p/>
 * <p><code>GenericServlet</code> makes writing servlets
 * easier. It provides simple versions of the lifecycle methods
 * <code>init</code> and <code>destroy</code> and of the methods
 * in the <code>ServletConfig</code> interface. <code>GenericServlet</code>
 * also implements the <code>log</code> method, declared in the
 * <code>ServletContext</code> interface.
 * <p/>
 * <p>To write a generic servlet, you need only
 * override the abstract <code>service</code> method.
 *
 * @version $Rev$ $Date$
 */


public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable {

    private transient ServletConfig config;

    /**
     * Does nothing. All of the servlet initialization
     * is done by one of the <code>init</code> methods.
     */
    public GenericServlet() {
    }

    /**
     * Called by the servlet container to indicate to a servlet that the
     * servlet is being taken out of service.  See {@link Servlet#destroy}.
     */
    public void destroy() {
    }

    /**
     * Returns a <code>String</code> containing the value of the named
     * initialization parameter, or <code>null</code> if the parameter does
     * not exist.  See {@link ServletConfig#getInitParameter}.
     * <p/>
     * <p>This method is supplied for convenience. It gets the
     * value of the named parameter from the servlet's
     * <code>ServletConfig</code> object.
     *
     * @param name a <code>String</code> specifying the name
     *             of the initialization parameter
     * @return String                 a <code>String</code> containing the value
     *         of the initialization parameter
     */
    public String getInitParameter(String name) {
        return getServletConfig().getInitParameter(name);
    }

    /**
     * Returns the names of the servlet's initialization parameters
     * as an <code>Enumeration</code> of <code>String</code> objects,
     * or an empty <code>Enumeration</code> if the servlet has no
     * initialization parameters.  See {@link
     * ServletConfig#getInitParameterNames}.
     * <p/>
     * <p>This method is supplied for convenience. It gets the
     * parameter names from the servlet's <code>ServletConfig</code> object.
     *
     * @return Enumeration         an enumeration of <code>String</code>
     *         objects containing the names of
     *         the servlet's initialization parameters
     */
    public Enumeration<String> getInitParameterNames() {
        return getServletConfig().getInitParameterNames();
    }

    /**
     * Returns this servlet's {@link ServletConfig} object.
     *
     * @return ServletConfig         the <code>ServletConfig</code> object
     *         that initialized this servlet
     */
    public ServletConfig getServletConfig() {
        return config;
    }

    /**
     * Returns a reference to the {@link ServletContext} in which this servlet
     * is running.  See {@link ServletConfig#getServletContext}.
     * <p/>
     * <p>This method is supplied for convenience. It gets the
     * context from the servlet's <code>ServletConfig</code> object.
     *
     * @return ServletContext         the <code>ServletContext</code> object
     *         passed to this servlet by the <code>init</code>
     *         method
     */
    public ServletContext getServletContext() {
        return getServletConfig().getServletContext();
    }

    /**
     * Returns information about the servlet, such as
     * author, version, and copyright.
     * By default, this method returns an empty string.  Override this method
     * to have it return a meaningful value.  See {@link
     * Servlet#getServletInfo}.
     *
     * @return String                 information about this servlet, by default an
     *         empty string
     */
    public String getServletInfo() {
        return "";
    }

    /**
     * Called by the servlet container to indicate to a servlet that the
     * servlet is being placed into service.  See {@link Servlet#init}.
     * <p/>
     * <p>This implementation stores the {@link ServletConfig}
     * object it receives from the servlet container for later use.
     * When overriding this form of the method, call
     * <code>super.init(config)</code>.
     *
     * @param config the <code>ServletConfig</code> object
     *               that contains configutation
     *               information for this servlet
     * @throws ServletException if an exception occurs that
     *                          interrupts the servlet's normal
     *                          operation
     * @see UnavailableException
     */
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        init();
    }

    /**
     * A convenience method which can be overridden so that there's no need
     * to call <code>super.init(config)</code>.
     * <p/>
     * <p>Instead of overriding {@link #init(ServletConfig)}, simply override
     * this method and it will be called by
     * <code>GenericServlet.init(ServletConfig config)</code>.
     * The <code>ServletConfig</code> object can still be retrieved via {@link
     * #getServletConfig}.
     *
     * @throws ServletException if an exception occurs that
     *                          interrupts the servlet's
     *                          normal operation
     */
    public void init() throws ServletException {
    }

    /**
     * Writes the specified message to a servlet log file, prepended by the
     * servlet's name.  See {@link ServletContext#log(String)}.
     *
     * @param msg a <code>String</code> specifying
     *            the message to be written to the log file
     */
    public void log(String msg) {
        getServletContext().log(getServletName() + ": " + msg);
    }

    /**
     * Writes an explanatory message and a stack trace
     * for a given <code>Throwable</code> exception
     * to the servlet log file, prepended by the servlet's name.
     * See {@link ServletContext#log(String, Throwable)}.
     *
     * @param message a <code>String</code> that describes
     *                the error or exception
     * @param t       the <code>java.lang.Throwable</code> error
     *                or exception
     */
    public void log(String message, Throwable t) {
        getServletContext().log(getServletName() + ": " + message, t);
    }

    /**
     * Called by the servlet container to allow the servlet to respond to
     * a request.  See {@link Servlet#service}.
     * <p/>
     * <p>This method is declared abstract so subclasses, such as
     * <code>HttpServlet</code>, must override it.
     *
     * @param req the <code>ServletRequest</code> object
     *            that contains the client's request
     * @param res the <code>ServletResponse</code> object
     *            that will contain the servlet's response
     * @throws ServletException if an exception occurs that
     *                          interferes with the servlet's
     *                          normal operation occurred
     * @throws IOException      if an input or output
     *                          exception occurs
     */
    public abstract void service(ServletRequest req, ServletResponse res) throws ServletException, IOException;

    /**
     * Returns the name of this servlet instance.
     * See {@link ServletConfig#getServletName}.
     *
     * @return the name of this servlet instance
     */
    public String getServletName() {
        return config.getServletName();
    }
    
}