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
058public interface Service {
059  /**
060   * If the service state is {@link State#NEW}, this initiates service startup and returns
061   * immediately. A stopped service may not be restarted.
062   *
063   * @return this
064   * @throws IllegalStateException if the service is not {@link State#NEW}
065   * @since 15.0
066   */
067  @CanIgnoreReturnValue
068  Service startAsync();
069
070  /** Returns {@code true} if this service is {@linkplain State#RUNNING running}. */
071  boolean isRunning();
072
073  /** Returns the lifecycle state of the service. */
074  State state();
075
076  /**
077   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
078   * this initiates service shutdown and returns immediately. If the service is {@linkplain
079   * State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been started nor
080   * stopped. If the service has already been stopped, this method returns immediately without
081   * taking action.
082   *
083   * @return this
084   * @since 15.0
085   */
086  @CanIgnoreReturnValue
087  Service stopAsync();
088
089  /**
090   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
091   *
092   * @throws IllegalStateException if the service reaches a state from which it is not possible to
093   *     enter the {@link State#RUNNING} state. e.g. if the {@code state} is {@code
094   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
095   * @since 15.0
096   */
097  void awaitRunning();
098
099  /**
100   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no more
101   * than the given time.
102   *
103   * @param timeout the maximum time to wait
104   * @param unit the time unit of the timeout argument
105   * @throws TimeoutException if the service has not reached the given state within the deadline
106   * @throws IllegalStateException if the service reaches a state from which it is not possible to
107   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is {@code
108   *     State#TERMINATED} when this method is called then this will throw an IllegalStateException.
109   * @since 15.0
110   */
111  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
112  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
113
114  /**
115   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
116   *
117   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
118   * @since 15.0
119   */
120  void awaitTerminated();
121
122  /**
123   * Waits for the {@link Service} to reach a terminal state (either {@link Service.State#TERMINATED
124   * terminated} or {@link Service.State#FAILED failed}) for no more than the given time.
125   *
126   * @param timeout the maximum time to wait
127   * @param unit the time unit of the timeout argument
128   * @throws TimeoutException if the service has not reached the given state within the deadline
129   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
130   * @since 15.0
131   */
132  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
133  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
134
135  /**
136   * Returns the {@link Throwable} that caused this service to fail.
137   *
138   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
139   * @since 14.0
140   */
141  Throwable failureCause();
142
143  /**
144   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
145   * executor. The listener will have the corresponding transition method called whenever the
146   * service changes state. The listener will not have previous state changes replayed, so it is
147   * suggested that listeners are added before the service starts.
148   *
149   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
150   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
151   * invoked in the same order as the underlying service enters those states. Additionally, at most
152   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
153   * may execute concurrently, and listeners may execute in an order different from the one in which
154   * they were registered.
155   *
156   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown
157   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
158   * logged.
159   *
160   * @param listener the listener to run when the service changes state is complete
161   * @param executor the executor in which the listeners callback methods will be run. For fast,
162   *     lightweight listeners that would be safe to execute in any thread, consider {@link
163   *     MoreExecutors#directExecutor}.
164   * @since 13.0
165   */
166  void addListener(Listener listener, Executor executor);
167
168  /**
169   * The lifecycle states of a service.
170   *
171   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
172   * from {@code A -> B} then {@code A.compareTo(B) < 0}. N.B. The converse is not true, i.e. if
173   * {@code A.compareTo(B) < 0} then there is <b>not</b> guaranteed to be a valid state transition
174   * {@code A -> B}.
175   *
176   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
177   */
178  enum State {
179    /** A service in this state is inactive. It does minimal work and consumes minimal resources. */
180    NEW,
181
182    /** A service in this state is transitioning to {@link #RUNNING}. */
183    STARTING,
184
185    /** A service in this state is operational. */
186    RUNNING,
187
188    /** A service in this state is transitioning to {@link #TERMINATED}. */
189    STOPPING,
190
191    /**
192     * A service in this state has completed execution normally. It does minimal work and consumes
193     * minimal resources.
194     */
195    TERMINATED,
196
197    /**
198     * A service in this state has encountered a problem and may not be operational. It cannot be
199     * started nor stopped.
200     */
201    FAILED,
202  }
203
204  /**
205   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
206   *
207   * <p>All methods are no-ops by default, implementors should override the ones they care about.
208   *
209   * @author Luke Sandberg
210   * @since 15.0 (present as an interface in 13.0)
211   */
212  abstract class Listener {
213    /** Constructor for use by subclasses. */
214    public Listener() {}
215
216    /**
217     * Called when the service transitions from {@linkplain State#NEW NEW} to {@linkplain
218     * State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called the first
219     * time.
220     */
221    public void starting() {}
222
223    /**
224     * Called when the service transitions from {@linkplain State#STARTING STARTING} to {@linkplain
225     * State#RUNNING RUNNING}. This occurs when a service has successfully started.
226     */
227    public void running() {}
228
229    /**
230     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
231     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or {@linkplain
232     * State#RUNNING RUNNING}. This occurs when {@link Service#stopAsync} is called.
233     *
234     * @param from The previous state that is being transitioned from.
235     */
236    public void stopping(State from) {}
237
238    /**
239     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
240     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
241     * diagram. Therefore, if this method is called, no other methods will be called on the {@link
242     * Listener}.
243     *
244     * @param from The previous state that is being transitioned from. Failure can occur in any
245     *     state with the exception of {@linkplain State#FAILED FAILED} and {@linkplain
246     *     State#TERMINATED TERMINATED}.
247     */
248    public void terminated(State from) {}
249
250    /**
251     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
252     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
253     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
254     *
255     * @param from The previous state that is being transitioned from. Failure can occur in any
256     *     state with the exception of {@linkplain State#NEW NEW} or {@linkplain State#TERMINATED
257     *     TERMINATED}.
258     * @param failure The exception that caused the failure.
259     */
260    public void failed(State from, Throwable failure) {}
261  }
262}