001/*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import com.google.common.annotations.Beta;
018import com.google.common.annotations.GwtIncompatible;
019import com.google.errorprone.annotations.CanIgnoreReturnValue;
020import java.util.concurrent.Executor;
021import java.util.concurrent.TimeUnit;
022import java.util.concurrent.TimeoutException;
023
024/**
025 * An object with an operational state, plus asynchronous {@link #startAsync()} and
026 * {@link #stopAsync()} lifecycle methods to transition between states. Example services include
027 * webservers, RPC servers 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#stopAsync} 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
054@GwtIncompatible
055public interface Service {
056  /**
057   * If the service state is {@link State#NEW}, this initiates service startup and returns
058   * immediately. A stopped service may not be restarted.
059   *
060   * @return this
061   * @throws IllegalStateException if the service is not {@link State#NEW}
062   *
063   * @since 15.0
064   */
065  @CanIgnoreReturnValue
066  Service startAsync();
067
068  /**
069   * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
070   */
071  boolean isRunning();
072
073  /**
074   * Returns the lifecycle state of the service.
075   */
076  State state();
077
078  /**
079   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
080   * this initiates service shutdown and returns immediately. If the service is
081   * {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been
082   * started nor stopped. If the service has already been stopped, this method returns immediately
083   * without taking action.
084   *
085   * @return this
086   * @since 15.0
087   */
088  @CanIgnoreReturnValue
089  Service stopAsync();
090
091  /**
092   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
093   *
094   * @throws IllegalStateException if the service reaches a state from which it is not possible to
095   *     enter the {@link State#RUNNING} state. e.g. if the {@code state} is
096   *     {@code State#TERMINATED} when this method is called then this will throw an
097   *     IllegalStateException.
098   *
099   * @since 15.0
100   */
101  void awaitRunning();
102
103  /**
104   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
105   * than the given time.
106   *
107   * @param timeout the maximum time to wait
108   * @param unit the time unit of the timeout argument
109   * @throws TimeoutException if the service has not reached the given state within the deadline
110   * @throws IllegalStateException if the service reaches a state from which it is not possible to
111   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is
112   *     {@code State#TERMINATED} when this method is called then this will throw an
113   *     IllegalStateException.
114   *
115   * @since 15.0
116   */
117  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
118
119  /**
120   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
121   *
122   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
123   *
124   * @since 15.0
125   */
126  void awaitTerminated();
127
128  /**
129   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
130   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
131   *
132   * @param timeout the maximum time to wait
133   * @param unit the time unit of the timeout argument
134   * @throws TimeoutException if the service has not reached the given state within the deadline
135   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
136   * @since 15.0
137   */
138  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
139
140  /**
141   * Returns the {@link Throwable} that caused this service to fail.
142   *
143   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
144   *
145   * @since 14.0
146   */
147  Throwable failureCause();
148
149  /**
150   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
151   * executor. The listener will have the corresponding transition method called whenever the
152   * service changes state. The listener will not have previous state changes replayed, so it is
153   * suggested that listeners are added before the service starts.
154   *
155   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
156   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
157   * invoked in the same order as the underlying service enters those states. Additionally, at most
158   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
159   * may execute concurrently, and listeners may execute in an order different from the one in which
160   * they were registered.
161   *
162   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
163   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
164   * logged.
165   *
166   * @param listener the listener to run when the service changes state is complete
167   * @param executor the executor in which the listeners callback methods will be run. For fast,
168   *     lightweight listeners that would be safe to execute in any thread, consider
169   *     {@link MoreExecutors#directExecutor}.
170   * @since 13.0
171   */
172  void addListener(Listener listener, Executor executor);
173
174  /**
175   * The lifecycle states of a service.
176   *
177   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
178   * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
179   * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
180   * {@code A -> B}.
181   *
182   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
183   */
184  @Beta // should come out of Beta when Service does
185  enum State {
186    /**
187     * A service in this state is inactive. It does minimal work and consumes minimal resources.
188     */
189    NEW {
190      @Override
191      boolean isTerminal() {
192        return false;
193      }
194    },
195
196    /**
197     * A service in this state is transitioning to {@link #RUNNING}.
198     */
199    STARTING {
200      @Override
201      boolean isTerminal() {
202        return false;
203      }
204    },
205
206    /**
207     * A service in this state is operational.
208     */
209    RUNNING {
210      @Override
211      boolean isTerminal() {
212        return false;
213      }
214    },
215
216    /**
217     * A service in this state is transitioning to {@link #TERMINATED}.
218     */
219    STOPPING {
220      @Override
221      boolean isTerminal() {
222        return false;
223      }
224    },
225
226    /**
227     * A service in this state has completed execution normally. It does minimal work and consumes
228     * minimal resources.
229     */
230    TERMINATED {
231      @Override
232      boolean isTerminal() {
233        return true;
234      }
235    },
236
237    /**
238     * A service in this state has encountered a problem and may not be operational. It cannot be
239     * started nor stopped.
240     */
241    FAILED {
242      @Override
243      boolean isTerminal() {
244        return true;
245      }
246    };
247
248    /** Returns true if this state is terminal. */
249    abstract boolean isTerminal();
250  }
251
252  /**
253   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
254   *
255   * <p>All methods are no-ops by default, implementors should override the ones they care about.
256   *
257   * @author Luke Sandberg
258   * @since 15.0 (present as an interface in 13.0)
259   */
260  @Beta // should come out of Beta when Service does
261  abstract class Listener {
262    /**
263     * Called when the service transitions from {@linkplain State#NEW NEW} to
264     * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called
265     * the first time.
266     */
267    public void starting() {}
268
269    /**
270     * Called when the service transitions from {@linkplain State#STARTING STARTING} to
271     * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
272     */
273    public void running() {}
274
275    /**
276     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
277     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or
278     * {@linkplain State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
279     *
280     * @param from The previous state that is being transitioned from.
281     */
282    public void stopping(State from) {}
283
284    /**
285     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
286     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
287     * diagram. Therefore, if this method is called, no other methods will be called on the
288     * {@link Listener}.
289     *
290     * @param from The previous state that is being transitioned from. The only valid values for
291     *     this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or
292     *     {@linkplain State#STOPPING STOPPING}.
293     */
294    public void terminated(State from) {}
295
296    /**
297     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
298     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
299     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
300     *
301     * @param from The previous state that is being transitioned from. Failure can occur in any
302     *     state with the exception of {@linkplain State#NEW NEW} or {@linkplain State#TERMINATED
303     *     TERMINATED}.
304     * @param failure The exception that caused the failure.
305     */
306    public void failed(State from, Throwable failure) {}
307  }
308}