java/jakarta/servlet/GenericServlet.java

/*
 * 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 jakarta.servlet;

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

/**
 * Defines a generic, protocol-independent servlet. To write an HTTP servlet for use on the Web, extend
 * {@link jakarta.servlet.http.HttpServlet} instead.
 * <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>
 * <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>
 * To write a generic servlet, you need only override the abstract <code>service</code> method.
 */
public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable {

    @Serial
    private static final long serialVersionUID = 1L;

    private transient ServletConfig config;

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

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

    /**
     * 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>
     * 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
     */
    @Override
    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>
     * 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
     */
    @Override
    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
     */
    @Override
    public ServletConfig getServletConfig() {
        return config;
    }

    /**
     * Returns a reference to the {@link ServletContext} in which this servlet is running. See
     * {@link ServletConfig#getServletContext}.
     * <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
     */
    @Override
    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
     */
    @Override
    public String getServletInfo() {
        return "";
    }

    /**
     * {@inheritDoc}
     * <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>.
     */
    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

    /**
     * A convenience method which can be overridden so that there's no need to call <code>super.init(config)</code>.
     * <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}.
     *
     * @exception ServletException if an exception occurs that interrupts the servlet's normal operation
     */
    public void init() throws ServletException {
        // NOOP by default
    }

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

    /**
     * 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>
     * 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
     *
     * @exception ServletException if an exception occurs that interferes with the servlet's normal operation occurred
     * @exception IOException      if an input or output exception occurs
     */
    @Override
    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
     */
    @Override
    public String getServletName() {
        return config.getServletName();
    }
}