001 /*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.util.concurrent;
018
019 import com.google.common.annotations.Beta;
020
021 import java.util.concurrent.ExecutionException;
022 import java.util.concurrent.Executor;
023
024 /**
025 * An object with an operational state, plus asynchronous {@link #start()} and {@link #stop()}
026 * lifecycle methods to transition between states. Example services include webservers, RPC servers
027 * and timers.
028 *
029 * <p>The normal lifecycle of a service is:
030 * <ul>
031 * <li>{@linkplain State#NEW NEW} ->
032 * <li>{@linkplain State#STARTING STARTING} ->
033 * <li>{@linkplain State#RUNNING RUNNING} ->
034 * <li>{@linkplain State#STOPPING STOPPING} ->
035 * <li>{@linkplain State#TERMINATED TERMINATED}
036 * </ul>
037 *
038 * <p>There are deviations from this if there are failures or if {@link Service#stop} 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 public interface Service {
055 /**
056 * If the service state is {@link State#NEW}, this initiates service startup and returns
057 * immediately. If the service has already been started, this method returns immediately without
058 * taking action. A stopped service may not be restarted.
059 *
060 * @return a future for the startup result, regardless of whether this call initiated startup.
061 * Calling {@link ListenableFuture#get} will block until the service has finished
062 * starting, and returns one of {@link State#RUNNING}, {@link State#STOPPING} or
063 * {@link State#TERMINATED}. If the service fails to start, {@link ListenableFuture#get}
064 * will throw an {@link ExecutionException}, and the service's state will be
065 * {@link State#FAILED}. If it has already finished starting, {@link ListenableFuture#get}
066 * returns immediately. Cancelling this future has no effect on the service.
067 */
068 ListenableFuture<State> start();
069
070 /**
071 * Initiates service startup (if necessary), returning once the service has finished starting.
072 * Unlike calling {@code start().get()}, this method throws no checked exceptions, and it cannot
073 * be {@linkplain Thread#interrupt interrupted}.
074 *
075 * @throws UncheckedExecutionException if startup failed
076 * @return the state of the service when startup finished.
077 */
078 State startAndWait();
079
080 /**
081 * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
082 */
083 boolean isRunning();
084
085 /**
086 * Returns the lifecycle state of the service.
087 */
088 State state();
089
090 /**
091 * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
092 * this initiates service shutdown and returns immediately. If the service is
093 * {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been
094 * started nor stopped. If the service has already been stopped, this method returns immediately
095 * without taking action.
096 *
097 * @return a future for the shutdown result, regardless of whether this call initiated shutdown.
098 * Calling {@link ListenableFuture#get} will block until the service has finished shutting
099 * down, and either returns {@link State#TERMINATED} or throws an
100 * {@link ExecutionException}. If it has already finished stopping,
101 * {@link ListenableFuture#get} returns immediately. Cancelling this future has no effect
102 * on the service.
103 */
104 ListenableFuture<State> stop();
105
106 /**
107 * Initiates service shutdown (if necessary), returning once the service has finished stopping. If
108 * this is {@link State#STARTING}, startup will be cancelled. If this is {@link State#NEW}, it is
109 * {@link State#TERMINATED terminated} without having been started nor stopped. Unlike calling
110 * {@code stop().get()}, this method throws no checked exceptions.
111 *
112 * @throws UncheckedExecutionException if the service has failed or fails during shutdown
113 * @return the state of the service when shutdown finished.
114 */
115 State stopAndWait();
116
117 /**
118 * Returns the {@link Throwable} that caused this service to fail.
119 *
120 * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
121 * @since 13.0
122 */
123 Throwable failureCause();
124
125 /**
126 * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
127 * executor. The listener will have the corresponding transition method called whenever the
128 * service changes state. The listener will not have previous state changes replayed, so it is
129 * suggested that listeners are added before the service starts.
130 *
131 * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
132 * this method is guaranteed to be called whenever there is a state change.
133 *
134 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
135 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
136 * thrown by {@linkplain MoreExecutors#sameThreadExecutor inline execution}) will be caught and
137 * logged.
138 *
139 * @param listener the listener to run when the service changes state is complete
140 * @param executor the executor in which the the listeners callback methods will be run. For fast,
141 * lightweight listeners that would be safe to execute in any thread, consider
142 * {@link MoreExecutors#sameThreadExecutor}.
143 * @since 13.0
144 */
145 void addListener(Listener listener, Executor executor);
146
147 /**
148 * The lifecycle states of a service.
149 *
150 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
151 */
152 @Beta // should come out of Beta when Service does
153 enum State {
154 /**
155 * A service in this state is inactive. It does minimal work and consumes
156 * minimal resources.
157 */
158 NEW,
159
160 /**
161 * A service in this state is transitioning to {@link #RUNNING}.
162 */
163 STARTING,
164
165 /**
166 * A service in this state is operational.
167 */
168 RUNNING,
169
170 /**
171 * A service in this state is transitioning to {@link #TERMINATED}.
172 */
173 STOPPING,
174
175 /**
176 * A service in this state has completed execution normally. It does minimal work and consumes
177 * minimal resources.
178 */
179 TERMINATED,
180
181 /**
182 * A service in this state has encountered a problem and may not be operational. It cannot be
183 * started nor stopped.
184 */
185 FAILED
186 }
187
188 /**
189 * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
190 *
191 * @author Luke Sandberg
192 * @since 13.0
193 */
194 @Beta // should come out of Beta when Service does
195 interface Listener {
196 /**
197 * Called when the service transitions from {@linkplain State#NEW NEW} to
198 * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#start} or
199 * {@link Service#startAndWait} is called the first time.
200 */
201 void starting();
202
203 /**
204 * Called when the service transitions from {@linkplain State#STARTING STARTING} to
205 * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
206 */
207 void running();
208
209 /**
210 * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The
211 * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or
212 * {@linkplain State#RUNNING RUNNING}. This occurs when {@link Service#stop} is called.
213 *
214 * @param from The previous state that is being transitioned from.
215 */
216 void stopping(State from);
217
218 /**
219 * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state.
220 * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
221 * diagram. Therefore, if this method is called, no other methods will be called on the
222 * {@link Listener}.
223 *
224 * @param from The previous state that is being transitioned from. The only valid values for
225 * this are {@linkplain State#NEW NEW} or {@linkplain State#STOPPING STOPPING}.
226 */
227 void terminated(State from);
228
229 /**
230 * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The
231 * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.
232 * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
233 *
234 * @param from The previous state that is being transitioned from. Failure can occur in any
235 * state with the exception of {@linkplain State#NEW NEW}.
236 * @param failure The exception that caused the failure.
237 */
238 void failed(State from, Throwable failure);
239 }
240 }