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
017package com.google.common.util.concurrent;
018
019import com.google.common.annotations.Beta;
020
021import java.util.concurrent.ExecutionException;
022import java.util.concurrent.Executor;
023
024/**
025 * An object with an operational state, plus asynchronous {@link #start()} and {@link #stop()}
026 * lifecycle methods to transition between states. Example services include webservers, RPC servers
027 * and timers.
028 *
029 * <p>The normal lifecycle of a service is:
030 * <ul>
031 *   <li>{@linkplain State#NEW NEW} -&gt;
032 *   <li>{@linkplain State#STARTING STARTING} -&gt;
033 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
034 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
035 *   <li>{@linkplain State#TERMINATED TERMINATED}
036 * </ul>
037 *
038 * <p>There are deviations from this if there are failures or if {@link Service#stop} is called
039 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
040 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
041 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
042 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
043 * states it cannot ever leave them.
044 *
045 * <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes
046 * in this package which implement this interface and make the threading and state management
047 * easier.
048 *
049 * @author Jesse Wilson
050 * @author Luke Sandberg
051 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
052 */
053@Beta
054public interface Service {
055  /**
056   * If the service state is {@link State#NEW}, this initiates service startup and returns
057   * immediately. If the service has already been started, this method returns immediately without
058   * taking action. A stopped service may not be restarted.
059   *
060   * @return a future for the startup result, regardless of whether this call initiated startup.
061   *         Calling {@link ListenableFuture#get} will block until the service has finished
062   *         starting, and returns one of {@link State#RUNNING}, {@link State#STOPPING} or
063   *         {@link State#TERMINATED}. If the service fails to start, {@link ListenableFuture#get}
064   *         will throw an {@link ExecutionException}, and the service's state will be
065   *         {@link State#FAILED}. If it has already finished starting, {@link ListenableFuture#get}
066   *         returns immediately. Cancelling this future has no effect on the service.
067   */
068  ListenableFuture<State> start();
069
070  /**
071   * Initiates service startup (if necessary), returning once the service has finished starting.
072   * Unlike calling {@code start().get()}, this method throws no checked exceptions, and it cannot
073   * be {@linkplain Thread#interrupt interrupted}.
074   *
075   * @throws UncheckedExecutionException if startup failed
076   * @return the state of the service when startup finished.
077   */
078  State startAndWait();
079
080  /**
081   * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
082   */
083  boolean isRunning();
084
085  /**
086   * Returns the lifecycle state of the service.
087   */
088  State state();
089
090  /**
091   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
092   * this initiates service shutdown and returns immediately. If the service is
093   * {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been
094   * started nor stopped. If the service has already been stopped, this method returns immediately
095   * without taking action.
096   *
097   * @return a future for the shutdown result, regardless of whether this call initiated shutdown.
098   *         Calling {@link ListenableFuture#get} will block until the service has finished shutting
099   *         down, and either returns {@link State#TERMINATED} or throws an
100   *         {@link ExecutionException}. If it has already finished stopping,
101   *         {@link ListenableFuture#get} returns immediately. Cancelling this future has no effect
102   *         on the service.
103   */
104  ListenableFuture<State> stop();
105
106  /**
107   * Initiates service shutdown (if necessary), returning once the service has finished stopping. If
108   * this is {@link State#STARTING}, startup will be cancelled. If this is {@link State#NEW}, it is
109   * {@link State#TERMINATED terminated} without having been started nor stopped. Unlike calling
110   * {@code stop().get()}, this method throws no checked exceptions.
111   *
112   * @throws UncheckedExecutionException if the service has failed or fails during shutdown
113   * @return the state of the service when shutdown finished.
114   */
115  State stopAndWait();
116
117  /**
118   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
119   * executor.  The listener will have the corresponding transition method called whenever the
120   * service changes state. The listener will not have previous state changes replayed, so it is
121   * suggested that listeners are added before the service starts.
122   *
123   * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
124   * this method is guaranteed to be called whenever there is a state change.
125   *
126   * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
127   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
128   * thrown by {@linkplain MoreExecutors#sameThreadExecutor inline execution}) will be caught and
129   * logged.
130   *
131   * @param listener the listener to run when the service changes state is complete
132   * @param executor the executor in which the the listeners callback methods will be run. For fast,
133   *     lightweight listeners that would be safe to execute in any thread, consider
134   *     {@link MoreExecutors#sameThreadExecutor}.
135   * @since 13.0
136   */
137  void addListener(Listener listener, Executor executor);
138
139  /**
140   * The lifecycle states of a service.
141   *
142   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
143   */
144  @Beta // should come out of Beta when Service does
145  enum State {
146    /**
147     * A service in this state is inactive. It does minimal work and consumes
148     * minimal resources.
149     */
150    NEW,
151
152    /**
153     * A service in this state is transitioning to {@link #RUNNING}.
154     */
155    STARTING,
156
157    /**
158     * A service in this state is operational.
159     */
160    RUNNING,
161
162    /**
163     * A service in this state is transitioning to {@link #TERMINATED}.
164     */
165    STOPPING,
166
167    /**
168     * A service in this state has completed execution normally. It does minimal work and consumes
169     * minimal resources.
170     */
171    TERMINATED,
172
173    /**
174     * A service in this state has encountered a problem and may not be operational. It cannot be
175     * started nor stopped.
176     */
177    FAILED
178  }
179
180  /**
181   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
182   *
183   * @author Luke Sandberg
184   * @since 13.0
185   */
186  @Beta // should come out of Beta when Service does
187  interface Listener {
188    /**
189     * Called when the service transitions from {@linkplain State#NEW NEW} to
190     * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#start} or
191     * {@link Service#startAndWait} is called the first time.
192     */
193    void starting();
194
195    /**
196     * Called when the service transitions from {@linkplain State#STARTING STARTING} to
197     * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
198     */
199    void running();
200
201    /**
202     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
203     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or
204     * {@linkplain State#RUNNING RUNNING}.  This occurs when {@link Service#stop} is called.
205     *
206     * @param from The previous state that is being transitioned from.
207     */
208    void stopping(State from);
209
210    /**
211     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
212     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
213     * diagram.  Therefore, if this method is called, no other methods will be called on the
214     * {@link Listener}.
215     *
216     * @param from The previous state that is being transitioned from.  The only valid values for
217     *     this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or
218     *     {@linkplain State#STOPPING STOPPING}.
219     */
220    void terminated(State from);
221
222    /**
223     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
224     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
225     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
226     *
227     * @param from The previous state that is being transitioned from.  Failure can occur in any
228     *     state with the exception of {@linkplain State#NEW NEW} or
229     *     {@linkplain State#TERMINATED TERMINATED}.
230     * @param failure The exception that caused the failure.
231     */
232    void failed(State from, Throwable failure);
233  }
234}