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 static com.google.common.util.concurrent.Internal.toNanosSaturated;
018
019import com.google.common.annotations.GwtIncompatible;
020import com.google.common.annotations.J2ktIncompatible;
021import com.google.errorprone.annotations.CanIgnoreReturnValue;
022import com.google.errorprone.annotations.DoNotMock;
023import java.time.Duration;
024import java.util.concurrent.Executor;
025import java.util.concurrent.TimeUnit;
026import java.util.concurrent.TimeoutException;
027
028/**
029 * An object with an operational state, plus asynchronous {@link #startAsync()} and {@link
030 * #stopAsync()} lifecycle methods to transition between states. Example services include
031 * webservers, RPC servers and timers.
032 *
033 * <p>The normal lifecycle of a service is:
034 *
035 * <ul>
036 *   <li>{@linkplain State#NEW NEW} -&gt;
037 *   <li>{@linkplain State#STARTING STARTING} -&gt;
038 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
039 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
040 *   <li>{@linkplain State#TERMINATED TERMINATED}
041 * </ul>
042 *
043 * <p>There are deviations from this if there are failures or if {@link Service#stopAsync} is called
044 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
045 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
046 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
047 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
048 * states it cannot ever leave them.
049 *
050 * <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes
051 * in this package which implement this interface and make the threading and state management
052 * easier.
053 *
054 * @author Jesse Wilson
055 * @author Luke Sandberg
056 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
057 */
058@DoNotMock("Create an AbstractIdleService")
059@J2ktIncompatible
060@GwtIncompatible
061@ElementTypesAreNonnullByDefault
062public interface Service {
063  /**
064   * If the service state is {@link State#NEW}, this initiates service startup and returns
065   * immediately. A stopped service may not be restarted.
066   *
067   * @return this
068   * @throws IllegalStateException if the service is not {@link State#NEW}
069   * @since 15.0
070   */
071  @CanIgnoreReturnValue
072  Service startAsync();
073
074  /** Returns {@code true} if this service is {@linkplain State#RUNNING running}. */
075  boolean isRunning();
076
077  /** Returns the lifecycle state of the service. */
078  State state();
079
080  /**
081   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
082   * this initiates service shutdown and returns immediately. If the service is {@linkplain
083   * State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been started nor
084   * stopped. If the service has already been stopped, this method returns immediately without
085   * taking action.
086   *
087   * @return this
088   * @since 15.0
089   */
090  @CanIgnoreReturnValue
091  Service stopAsync();
092
093  /**
094   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
095   *
096   * @throws IllegalStateException if the service reaches a state from which it is not possible to
097   *     enter the {@link State#RUNNING} state. e.g. if the {@code state} is {@code
098   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
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   * @throws TimeoutException if the service has not reached the given state within the deadline
109   * @throws IllegalStateException if the service reaches a state from which it is not possible to
110   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is {@code
111   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
112   * @since 28.0
113   */
114  default void awaitRunning(Duration timeout) throws TimeoutException {
115    awaitRunning(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
116  }
117
118  /**
119   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
120   * than the given time.
121   *
122   * @param timeout the maximum time to wait
123   * @param unit the time unit of the timeout argument
124   * @throws TimeoutException if the service has not reached the given state within the deadline
125   * @throws IllegalStateException if the service reaches a state from which it is not possible to
126   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is {@code
127   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
128   * @since 15.0
129   */
130  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
131  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
132
133  /**
134   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
135   *
136   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
137   * @since 15.0
138   */
139  void awaitTerminated();
140
141  /**
142   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
143   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
144   *
145   * @param timeout the maximum time to wait
146   * @throws TimeoutException if the service has not reached the given state within the deadline
147   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
148   * @since 28.0
149   */
150  default void awaitTerminated(Duration timeout) throws TimeoutException {
151    awaitTerminated(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
152  }
153
154  /**
155   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
156   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
157   *
158   * @param timeout the maximum time to wait
159   * @param unit the time unit of the timeout argument
160   * @throws TimeoutException if the service has not reached the given state within the deadline
161   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
162   * @since 15.0
163   */
164  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
165  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
166
167  /**
168   * Returns the {@link Throwable} that caused this service to fail.
169   *
170   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
171   * @since 14.0
172   */
173  Throwable failureCause();
174
175  /**
176   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
177   * executor. The listener will have the corresponding transition method called whenever the
178   * service changes state. The listener will not have previous state changes replayed, so it is
179   * suggested that listeners are added before the service starts.
180   *
181   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
182   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
183   * invoked in the same order as the underlying service enters those states. Additionally, at most
184   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
185   * may execute concurrently, and listeners may execute in an order different from the one in which
186   * they were registered.
187   *
188   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
189   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
190   * logged.
191   *
192   * @param listener the listener to run when the service changes state is complete
193   * @param executor the executor in which the listeners callback methods will be run. For fast,
194   *     lightweight listeners that would be safe to execute in any thread, consider {@link
195   *     MoreExecutors#directExecutor}.
196   * @since 13.0
197   */
198  void addListener(Listener listener, Executor executor);
199
200  /**
201   * The lifecycle states of a service.
202   *
203   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
204   * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
205   * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
206   * {@code A -> B}.
207   *
208   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
209   */
210  enum State {
211    /** A service in this state is inactive. It does minimal work and consumes minimal resources. */
212    NEW,
213
214    /** A service in this state is transitioning to {@link #RUNNING}. */
215    STARTING,
216
217    /** A service in this state is operational. */
218    RUNNING,
219
220    /** A service in this state is transitioning to {@link #TERMINATED}. */
221    STOPPING,
222
223    /**
224     * A service in this state has completed execution normally. It does minimal work and consumes
225     * minimal resources.
226     */
227    TERMINATED,
228
229    /**
230     * A service in this state has encountered a problem and may not be operational. It cannot be
231     * started nor stopped.
232     */
233    FAILED,
234  }
235
236  /**
237   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
238   *
239   * <p>All methods are no-ops by default, implementors should override the ones they care about.
240   *
241   * @author Luke Sandberg
242   * @since 15.0 (present as an interface in 13.0)
243   */
244  abstract class Listener {
245    /** Constructor for use by subclasses. */
246    public Listener() {}
247
248    /**
249     * Called when the service transitions from {@linkplain State#NEW NEW} to {@linkplain
250     * State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called the first
251     * time.
252     */
253    public void starting() {}
254
255    /**
256     * Called when the service transitions from {@linkplain State#STARTING STARTING} to {@linkplain
257     * State#RUNNING RUNNING}. This occurs when a service has successfully started.
258     */
259    public void running() {}
260
261    /**
262     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
263     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or {@linkplain
264     * State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
265     *
266     * @param from The previous state that is being transitioned from.
267     */
268    public void stopping(State from) {}
269
270    /**
271     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
272     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
273     * diagram. Therefore, if this method is called, no other methods will be called on the {@link
274     * Listener}.
275     *
276     * @param from The previous state that is being transitioned from. Failure can occur in any
277     *     state with the exception of {@linkplain State#FAILED FAILED} and {@linkplain
278     *     State#TERMINATED TERMINATED}.
279     */
280    public void terminated(State from) {}
281
282    /**
283     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
284     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
285     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
286     *
287     * @param from The previous state that is being transitioned from. Failure can occur in any
288     *     state with the exception of {@linkplain State#NEW NEW} or {@linkplain State#TERMINATED
289     *     TERMINATED}.
290     * @param failure The exception that caused the failure.
291     */
292    public void failed(State from, Throwable failure) {}
293  }
294}