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.GwtIncompatible;
018import com.google.common.annotations.J2ktIncompatible;
019import com.google.errorprone.annotations.CanIgnoreReturnValue;
020import com.google.errorprone.annotations.DoNotMock;
021import java.util.concurrent.Executor;
022import java.util.concurrent.TimeUnit;
023import java.util.concurrent.TimeoutException;
024
025/**
026 * An object with an operational state, plus asynchronous {@link #startAsync()} and {@link
027 * #stopAsync()} lifecycle methods to transition between states. Example services include
028 * webservers, RPC servers and timers.
029 *
030 * <p>The normal lifecycle of a service is:
031 *
032 * <ul>
033 *   <li>{@linkplain State#NEW NEW} -&gt;
034 *   <li>{@linkplain State#STARTING STARTING} -&gt;
035 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
036 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
037 *   <li>{@linkplain State#TERMINATED TERMINATED}
038 * </ul>
039 *
040 * <p>There are deviations from this if there are failures or if {@link Service#stopAsync} is called
041 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
042 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
043 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
044 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
045 * states it cannot ever leave them.
046 *
047 * <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes
048 * in this package which implement this interface and make the threading and state management
049 * easier.
050 *
051 * @author Jesse Wilson
052 * @author Luke Sandberg
053 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
054 */
055@DoNotMock("Create an AbstractIdleService")
056@J2ktIncompatible
057@GwtIncompatible
058@ElementTypesAreNonnullByDefault
059public interface Service {
060  /**
061   * If the service state is {@link State#NEW}, this initiates service startup and returns
062   * immediately. A stopped service may not be restarted.
063   *
064   * @return this
065   * @throws IllegalStateException if the service is not {@link State#NEW}
066   * @since 15.0
067   */
068  @CanIgnoreReturnValue
069  Service startAsync();
070
071  /** Returns {@code true} if this service is {@linkplain State#RUNNING running}. */
072  boolean isRunning();
073
074  /** Returns the lifecycle state of the service. */
075  State state();
076
077  /**
078   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
079   * this initiates service shutdown and returns immediately. If the service is {@linkplain
080   * State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been started nor
081   * stopped. If the service has already been stopped, this method returns immediately without
082   * taking action.
083   *
084   * @return this
085   * @since 15.0
086   */
087  @CanIgnoreReturnValue
088  Service stopAsync();
089
090  /**
091   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
092   *
093   * @throws IllegalStateException if the service reaches a state from which it is not possible to
094   *     enter the {@link State#RUNNING} state. e.g. if the {@code state} is {@code
095   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
096   * @since 15.0
097   */
098  void awaitRunning();
099
100  /**
101   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
102   * than the given time.
103   *
104   * @param timeout the maximum time to wait
105   * @param unit the time unit of the timeout argument
106   * @throws TimeoutException if the service has not reached the given state within the deadline
107   * @throws IllegalStateException if the service reaches a state from which it is not possible to
108   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is {@code
109   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
110   * @since 15.0
111   */
112  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
113  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
114
115  /**
116   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
117   *
118   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
119   * @since 15.0
120   */
121  void awaitTerminated();
122
123  /**
124   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
125   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
126   *
127   * @param timeout the maximum time to wait
128   * @param unit the time unit of the timeout argument
129   * @throws TimeoutException if the service has not reached the given state within the deadline
130   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
131   * @since 15.0
132   */
133  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
134  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
135
136  /**
137   * Returns the {@link Throwable} that caused this service to fail.
138   *
139   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
140   * @since 14.0
141   */
142  Throwable failureCause();
143
144  /**
145   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
146   * executor. The listener will have the corresponding transition method called whenever the
147   * service changes state. The listener will not have previous state changes replayed, so it is
148   * suggested that listeners are added before the service starts.
149   *
150   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
151   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
152   * invoked in the same order as the underlying service enters those states. Additionally, at most
153   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
154   * may execute concurrently, and listeners may execute in an order different from the one in which
155   * they were registered.
156   *
157   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
158   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
159   * logged.
160   *
161   * @param listener the listener to run when the service changes state is complete
162   * @param executor the executor in which the listeners callback methods will be run. For fast,
163   *     lightweight listeners that would be safe to execute in any thread, consider {@link
164   *     MoreExecutors#directExecutor}.
165   * @since 13.0
166   */
167  void addListener(Listener listener, Executor executor);
168
169  /**
170   * The lifecycle states of a service.
171   *
172   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
173   * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
174   * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
175   * {@code A -> B}.
176   *
177   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
178   */
179  enum State {
180    /** A service in this state is inactive. It does minimal work and consumes minimal resources. */
181    NEW,
182
183    /** A service in this state is transitioning to {@link #RUNNING}. */
184    STARTING,
185
186    /** A service in this state is operational. */
187    RUNNING,
188
189    /** A service in this state is transitioning to {@link #TERMINATED}. */
190    STOPPING,
191
192    /**
193     * A service in this state has completed execution normally. It does minimal work and consumes
194     * minimal resources.
195     */
196    TERMINATED,
197
198    /**
199     * A service in this state has encountered a problem and may not be operational. It cannot be
200     * started nor stopped.
201     */
202    FAILED,
203  }
204
205  /**
206   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
207   *
208   * <p>All methods are no-ops by default, implementors should override the ones they care about.
209   *
210   * @author Luke Sandberg
211   * @since 15.0 (present as an interface in 13.0)
212   */
213  abstract class Listener {
214    /**
215     * Called when the service transitions from {@linkplain State#NEW NEW} to {@linkplain
216     * State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called the first
217     * time.
218     */
219    public void starting() {}
220
221    /**
222     * Called when the service transitions from {@linkplain State#STARTING STARTING} to {@linkplain
223     * State#RUNNING RUNNING}. This occurs when a service has successfully started.
224     */
225    public void running() {}
226
227    /**
228     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
229     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or {@linkplain
230     * State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
231     *
232     * @param from The previous state that is being transitioned from.
233     */
234    public void stopping(State from) {}
235
236    /**
237     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
238     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
239     * diagram. Therefore, if this method is called, no other methods will be called on the {@link
240     * Listener}.
241     *
242     * @param from The previous state that is being transitioned from. Failure can occur in any
243     *     state with the exception of {@linkplain State#FAILED FAILED} and {@linkplain
244     *     State#TERMINATED TERMINATED}.
245     */
246    public void terminated(State from) {}
247
248    /**
249     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
250     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
251     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
252     *
253     * @param from The previous state that is being transitioned from. Failure can occur in any
254     *     state with the exception of {@linkplain State#NEW NEW} or {@linkplain State#TERMINATED
255     *     TERMINATED}.
256     * @param failure The exception that caused the failure.
257     */
258    public void failed(State from, Throwable failure) {}
259  }
260}