001/*
002 *  Licensed to the Apache Software Foundation (ASF) under one or more
003 *  contributor license agreements.  See the NOTICE file distributed with
004 *  this work for additional information regarding copyright ownership.
005 *  The ASF licenses this file to You under the Apache License, Version 2.0
006 *  (the "License"); you may not use this file except in compliance with
007 *  the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 *  Unless required by applicable law or agreed to in writing, software
012 *  distributed under the License is distributed on an "AS IS" BASIS,
013 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 *  See the License for the specific language governing permissions and
015 *  limitations under the License.
016 */
017
018package org.apache.commons.daemon;
019
020/**
021 * Provides support for native daemon invocation. Using
022 * a platform dependant helper program, classes that implement the
023 * {@code Daemon} interface can be initialized, started and
024 * stopped according to the conventions of the underlying operating
025 * system.
026 * <p>
027 * Implementors of this interface must also provide a public constructor
028 * with no arguments so that instances can be created in an automated
029 * fashion.
030 * </p>
031 */
032public interface Daemon
033{
034
035    /**
036     * Initializes this {@code Daemon} instance.
037     * <p>
038     *   This method gets called once the JVM process is created and the
039     *   {@code Daemon} instance is created thru its empty public
040     *   constructor.
041     * </p>
042     * <p>
043     *   Under certain operating systems (typically Unix based operating
044     *   systems) and if the native invocation framework is configured to do
045     *   so, this method might be called with <i>super-user</i> privileges.
046     * </p>
047     * <p>
048     *   For example, it might be wise to create {@code ServerSocket}
049     *   instances within the scope of this method, and perform all operations
050     *   requiring <i>super-user</i> privileges in the underlying operating
051     *   system.
052     * </p>
053     * <p>
054     *   Apart from set up and allocation of native resources, this method
055     *   must not start the actual operation of the {@code Daemon} (such
056     *   as starting threads calling the {@code ServerSocket.accept()}
057     *   method) as this would impose some serious security hazards. The
058     *   start of operation must be performed in the {@code start()}
059     *   method.
060     * </p>
061     *
062     * @param context A {@code DaemonContext} object used to
063     * communicate with the container.
064     * @throws DaemonInitException An exception that prevented
065     * initialization where you want to display a nice message to the user,
066     * rather than a stack trace.
067     * @throws Exception Any exception preventing a successful
068     *                      initialization.
069     */
070    void init(DaemonContext context)
071        throws DaemonInitException, Exception;
072
073    /**
074     * Starts the operation of this {@code Daemon} instance. This
075     * method is to be invoked by the environment after the init()
076     * method has been successfully invoked and possibly the security
077     * level of the JVM has been dropped. Implementors of this
078     * method are free to start any number of threads, but need to
079     * return control after having done that to enable invocation of
080     * the stop()-method.
081     *
082     * @throws Exception If the start was not successful
083     */
084    void start()
085        throws Exception;
086
087    /**
088     * Stops the operation of this {@code Daemon} instance. Note
089     * that the proper place to free any allocated resources such as
090     * sockets or file descriptors is in the destroy method, as the
091     * container may restart the Daemon by calling start() after
092     * stop().
093     *
094     * @throws Exception If the stop was not successful
095     */
096    void stop()
097        throws Exception;
098
099    /**
100     * Frees any resources allocated by this daemon such as file
101     * descriptors or sockets. This method gets called by the container
102     * after stop() has been called, before the JVM exits. The Daemon
103     * can not be restarted after this method has been called without a
104     * new call to the init() method.
105     */
106    void destroy();
107}
108