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.errorprone.annotations.CanIgnoreReturnValue;
021import com.google.errorprone.annotations.DoNotMock;
022import java.time.Duration;
023import java.util.concurrent.Executor;
024import java.util.concurrent.TimeUnit;
025import java.util.concurrent.TimeoutException;
026
027/**
028 * An object with an operational state, plus asynchronous {@link #startAsync()} and {@link
029 * #stopAsync()} lifecycle methods to transition between states. Example services include
030 * webservers, RPC servers and timers.
031 *
032 * <p>The normal lifecycle of a service is:
033 *
034 * <ul>
035 *   <li>{@linkplain State#NEW NEW} -&gt;
036 *   <li>{@linkplain State#STARTING STARTING} -&gt;
037 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
038 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
039 *   <li>{@linkplain State#TERMINATED TERMINATED}
040 * </ul>
041 *
042 * <p>There are deviations from this if there are failures or if {@link Service#stopAsync} is called
043 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
044 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>,
045 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
046 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
047 * states it cannot ever leave them.
048 *
049 * <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes
050 * in this package which implement this interface and make the threading and state management
051 * easier.
052 *
053 * @author Jesse Wilson
054 * @author Luke Sandberg
055 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
056 */
057@DoNotMock("Create an AbstractIdleService")
058@GwtIncompatible
059@ElementTypesAreNonnullByDefault
060public interface Service {
061  /**
062   * If the service state is {@link State#NEW}, this initiates service startup and returns
063   * immediately. A stopped service may not be restarted.
064   *
065   * @return this
066   * @throws IllegalStateException if the service is not {@link State#NEW}
067   * @since 15.0
068   */
069  @CanIgnoreReturnValue
070  Service startAsync();
071
072  /** Returns {@code true} if this service is {@linkplain State#RUNNING running}. */
073  boolean isRunning();
074
075  /** Returns the lifecycle state of the service. */
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 {@linkplain
081   * State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been started nor
082   * stopped. If the service has already been stopped, this method returns immediately without
083   * 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 {@code
096   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
097   * @since 15.0
098   */
099  void awaitRunning();
100
101  /**
102   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
103   * than the given time.
104   *
105   * @param timeout the maximum time to wait
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 28.0
111   */
112  default void awaitRunning(Duration timeout) throws TimeoutException {
113    awaitRunning(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
114  }
115
116  /**
117   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
118   * than the given time.
119   *
120   * @param timeout the maximum time to wait
121   * @param unit the time unit of the timeout argument
122   * @throws TimeoutException if the service has not reached the given state within the deadline
123   * @throws IllegalStateException if the service reaches a state from which it is not possible to
124   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is {@code
125   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
126   * @since 15.0
127   */
128  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
129  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
130
131  /**
132   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
133   *
134   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
135   * @since 15.0
136   */
137  void awaitTerminated();
138
139  /**
140   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
141   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
142   *
143   * @param timeout the maximum time to wait
144   * @throws TimeoutException if the service has not reached the given state within the deadline
145   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
146   * @since 28.0
147   */
148  default void awaitTerminated(Duration timeout) throws TimeoutException {
149    awaitTerminated(toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
150  }
151
152  /**
153   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
154   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
155   *
156   * @param timeout the maximum time to wait
157   * @param unit the time unit of the timeout argument
158   * @throws TimeoutException if the service has not reached the given state within the deadline
159   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
160   * @since 15.0
161   */
162  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
163  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
164
165  /**
166   * Returns the {@link Throwable} that caused this service to fail.
167   *
168   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
169   * @since 14.0
170   */
171  Throwable failureCause();
172
173  /**
174   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
175   * executor. The listener will have the corresponding transition method called whenever the
176   * service changes state. The listener will not have previous state changes replayed, so it is
177   * suggested that listeners are added before the service starts.
178   *
179   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
180   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
181   * invoked in the same order as the underlying service enters those states. Additionally, at most
182   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
183   * may execute concurrently, and listeners may execute in an order different from the one in which
184   * they were registered.
185   *
186   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
187   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
188   * logged.
189   *
190   * @param listener the listener to run when the service changes state is complete
191   * @param executor the executor in which the listeners callback methods will be run. For fast,
192   *     lightweight listeners that would be safe to execute in any thread, consider {@link
193   *     MoreExecutors#directExecutor}.
194   * @since 13.0
195   */
196  void addListener(Listener listener, Executor executor);
197
198  /**
199   * The lifecycle states of a service.
200   *
201   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
202   * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
203   * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
204   * {@code A -> B}.
205   *
206   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
207   */
208  enum State {
209    /** A service in this state is inactive. It does minimal work and consumes minimal resources. */
210    NEW,
211
212    /** A service in this state is transitioning to {@link #RUNNING}. */
213    STARTING,
214
215    /** A service in this state is operational. */
216    RUNNING,
217
218    /** A service in this state is transitioning to {@link #TERMINATED}. */
219    STOPPING,
220
221    /**
222     * A service in this state has completed execution normally. It does minimal work and consumes
223     * minimal resources.
224     */
225    TERMINATED,
226
227    /**
228     * A service in this state has encountered a problem and may not be operational. It cannot be
229     * started nor stopped.
230     */
231    FAILED,
232  }
233
234  /**
235   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
236   *
237   * <p>All methods are no-ops by default, implementors should override the ones they care about.
238   *
239   * @author Luke Sandberg
240   * @since 15.0 (present as an interface in 13.0)
241   */
242  abstract class Listener {
243    /**
244     * Called when the service transitions from {@linkplain State#NEW NEW} to {@linkplain
245     * State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called the first
246     * time.
247     */
248    public void starting() {}
249
250    /**
251     * Called when the service transitions from {@linkplain State#STARTING STARTING} to {@linkplain
252     * State#RUNNING RUNNING}. This occurs when a service has successfully started.
253     */
254    public void running() {}
255
256    /**
257     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
258     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or {@linkplain
259     * State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
260     *
261     * @param from The previous state that is being transitioned from.
262     */
263    public void stopping(State from) {}
264
265    /**
266     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
267     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
268     * diagram. Therefore, if this method is called, no other methods will be called on the {@link
269     * Listener}.
270     *
271     * @param from The previous state that is being transitioned from. Failure can occur in any
272     *     state with the exception of {@linkplain State#FAILED FAILED} and {@linkplain
273     *     State#TERMINATED TERMINATED}.
274     */
275    public void terminated(State from) {}
276
277    /**
278     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
279     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
280     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
281     *
282     * @param from The previous state that is being transitioned from. Failure can occur in any
283     *     state with the exception of {@linkplain State#NEW NEW} or {@linkplain State#TERMINATED
284     *     TERMINATED}.
285     * @param failure The exception that caused the failure.
286     */
287    public void failed(State from, Throwable failure) {}
288  }
289}