    001    /*
    002     * Copyright (C) 2009 The Guava Authors
    003     *
    004     * Licensed under the Apache License, Version 2.0 (the "License");
    005     * you may not use this file except in compliance with the License.
    006     * You may obtain a copy of the License at
    007     *
    008     * http://www.apache.org/licenses/LICENSE-2.0
    009     *
    010     * Unless required by applicable law or agreed to in writing, software
    011     * distributed under the License is distributed on an "AS IS" BASIS,
    012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    013     * See the License for the specific language governing permissions and
    014     * limitations under the License.
    015     */
    016
    017    package com.google.common.util.concurrent;
    018
    019    import com.google.common.annotations.Beta;
    020
    021    import java.util.concurrent.ExecutionException;
    022
    023    /**
    024     * An object with an operational state, plus asynchronous {@link #start()} and
    025     * {@link #stop()} lifecycle methods to transfer into and out of this state.
    026     * Example services include webservers, RPC servers and timers. The normal
    027     * lifecycle of a service is:
    028     * <ul>
    029     *   <li>{@link State#NEW} -&gt;</li>
    030     *   <li>{@link State#STARTING} -&gt;</li>
    031     *   <li>{@link State#RUNNING} -&gt;</li>
    032     *   <li>{@link State#STOPPING} -&gt;</li>
    033     *   <li>{@link State#TERMINATED}</li>
    034     * </ul>
    035     *
    036     * If the service fails while starting, running or stopping, its state will be
    037     * {@link State#FAILED}, and its behavior is undefined. Such a service cannot be
    038     * started nor stopped.
    039     *
    040     * <p>Implementors of this interface are strongly encouraged to extend {@link
    041     * AbstractService}, {@link AbstractExecutionThreadService}, or {@link
    042     * AbstractIdleService}, which make the threading and state management easier.
    043     *
    044     * @author Jesse Wilson
    045     * @since 9.0 (in 1.0 as
    046     *     {@code com.google.common.base.Service})
    047     */
    048    @Beta // TODO(kevinb): make abstract class?
    049    public interface Service {
    050      /**
    051       * If the service state is {@link State#NEW}, this initiates service startup
    052       * and returns immediately. If the service has already been started, this
    053       * method returns immediately without taking action. A stopped service may not
    054       * be restarted.
    055       *
    056       * @return a future for the startup result, regardless of whether this call
    057       *     initiated startup. Calling {@link ListenableFuture#get} will block
    058       *     until the service has finished starting, and returns one of {@link
    059       *     State#RUNNING}, {@link State#STOPPING} or {@link State#TERMINATED}. If
    060       *     the service fails to start, {@link ListenableFuture#get} will throw an
    061       *     {@link ExecutionException}, and the service's state will be {@link
    062       *     State#FAILED}. If it has already finished starting, {@link
    063       *     ListenableFuture#get} returns immediately. Cancelling the returned
    064       *     future is unsupported and always returns {@code false}.
    065       */
    066      ListenableFuture<State> start();
    067
    068      /**
    069       * Initiates service startup (if necessary), returning once the service has
    070       * finished starting. Unlike calling {@code start().get()}, this method throws
    071       * no checked exceptions, and it cannot be {@linkplain Thread#interrupt
    072       * interrupted}.
    073       *
    074       * @throws UncheckedExecutionException if startup failed
    075       * @return the state of the service when startup finished.
    076       */
    077      State startAndWait();
    078
    079      /**
    080       * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
    081       */
    082      boolean isRunning();
    083
    084      /**
    085       * Returns the lifecycle state of the service.
    086       */
    087      State state();
    088
    089      /**
    090       * If the service is {@linkplain State#STARTING starting} or {@linkplain
    091       * State#RUNNING running}, this initiates service shutdown and returns
    092       * immediately. If the service is {@linkplain State#NEW new}, it is
    093       * {@linkplain State#TERMINATED terminated} without having been started nor
    094       * stopped.  If the service has already been stopped, this method returns
    095       * immediately without taking action.
    096       *
    097       * @return a future for the shutdown result, regardless of whether this call
    098       *     initiated shutdown. Calling {@link ListenableFuture#get} will block
    099       *     until the service has finished shutting down, and either returns
    100       *     {@link State#TERMINATED} or throws an {@link ExecutionException}. If
    101       *     it has already finished stopping, {@link ListenableFuture#get} returns
    102       *     immediately.  Cancelling this future is unsupported and always returns
    103       *     {@code false}.
    104       */
    105      ListenableFuture<State> stop();
    106
    107      /**
    108       * Initiates service shutdown (if necessary), returning once the service has
    109       * finished stopping. If this is {@link State#STARTING}, startup will be
    110       * cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED
    111       * terminated} without having been started nor stopped. Unlike calling {@code
    112       * stop().get()}, this method throws no checked exceptions.
    113       *
    114       * @throws UncheckedExecutionException if shutdown failed
    115       * @return the state of the service when shutdown finished.
    116       */
    117      State stopAndWait();
    118
    119      /**
    120       * The lifecycle states of a service.
    121       *
    122       * @since 9.0 (in 1.0 as
    123       *     {@code com.google.common.base.Service.State})
    124       */
    125      @Beta // should come out of Beta when Service does
    126      enum State {
    127        /**
    128         * A service in this state is inactive. It does minimal work and consumes
    129         * minimal resources.
    130         */
    131        NEW,
    132
    133        /**
    134         * A service in this state is transitioning to {@link #RUNNING}.
    135         */
    136        STARTING,
    137
    138        /**
    139         * A service in this state is operational.
    140         */
    141        RUNNING,
    142
    143        /**
    144         * A service in this state is transitioning to {@link #TERMINATED}.
    145         */
    146        STOPPING,
    147
    148        /**
    149         * A service in this state has completed execution normally. It does minimal
    150         * work and consumes minimal resources.
    151         */
    152        TERMINATED,
    153
    154        /**
    155         * A service in this state has encountered a problem and may not be
    156         * operational. It cannot be started nor stopped.
    157         */
    158        FAILED
    159      }
    160    }