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
023 /**
024 * An object with an operational state, plus asynchronous {@link #start()} and
025 * {@link #stop()} lifecycle methods to transition between states.
026 * Example services include webservers, RPC servers and timers.
027 *
028 * <p>The normal lifecycle of a service is:
029 * <ul>
030 * <li>{@link State#NEW} -></li>
031 * <li>{@link State#STARTING} -></li>
032 * <li>{@link State#RUNNING} -></li>
033 * <li>{@link State#STOPPING} -></li>
034 * <li>{@link State#TERMINATED}</li>
035 * </ul>
036 *
037 * <p>The valid state transitions of a Service are:
038 * <ul>
039 * <li>{@link State#NEW} -> {@link State#STARTING}: This occurs when
040 * start() is called the first time and is the only valid state transition
041 * from the NEW state.</li>
042 * <li>{@link State#NEW} -> {@link State#TERMINATED}: This occurs when
043 * stop() is called from the NEW state.</li>
044 * <li>{@link State#STARTING} -> {@link State#RUNNING}: This occurs when
045 * a service has successfully started</li>
046 * <li>{@link State#STARTING} -> {@link State#FAILED}: This occurs when a
047 * service experiences an unrecoverable error while starting up</li>
048 * <li>{@link State#STARTING} -> {@link State#STOPPING}: This occurs when
049 * stop() is called while a service is starting up.</li>
050 * <li>{@link State#RUNNING} -> {@link State#STOPPING}: This occurs when
051 * stop() is called on a running service.</li>
052 * <li>{@link State#RUNNING} -> {@link State#FAILED}: This occurs when an
053 * unrecoverable error occurs while a service is running.</li>
054 * <li>{@link State#STOPPING} -> {@link State#FAILED}: This occurs when an
055 * unrecoverable error occurs while a service is stopping.</li>
056 * <li>{@link State#STOPPING} -> {@link State#TERMINATED}: This occurs
057 * when the service successfully stops.</li>
058 * </ul>
059 *
060 * <p>N.B. The {@link State#FAILED} and {@link State#TERMINATED} states are
061 * terminal states, once a service enters either of these states it cannot ever
062 * leave them.
063 *
064 * <p>Implementors of this interface are strongly encouraged to extend one of
065 * the abstract classes in this package which implement this interface and
066 * make the threading and state management easier.
067 *
068 * @author Jesse Wilson
069 * @since 9.0 (in 1.0 as
070 * {@code com.google.common.base.Service})
071 */
072 @Beta // TODO(kevinb): make abstract class?
073 public interface Service {
074 /**
075 * If the service state is {@link State#NEW}, this initiates service startup
076 * and returns immediately. If the service has already been started, this
077 * method returns immediately without taking action. A stopped service may not
078 * be restarted.
079 *
080 * @return a future for the startup result, regardless of whether this call
081 * initiated startup. Calling {@link ListenableFuture#get} will block
082 * until the service has finished starting, and returns one of {@link
083 * State#RUNNING}, {@link State#STOPPING} or {@link State#TERMINATED}. If
084 * the service fails to start, {@link ListenableFuture#get} will throw an
085 * {@link ExecutionException}, and the service's state will be {@link
086 * State#FAILED}. If it has already finished starting, {@link
087 * ListenableFuture#get} returns immediately. Cancelling this future has
088 * no effect on the service.
089 */
090 ListenableFuture<State> start();
091
092 /**
093 * Initiates service startup (if necessary), returning once the service has
094 * finished starting. Unlike calling {@code start().get()}, this method throws
095 * no checked exceptions, and it cannot be {@linkplain Thread#interrupt
096 * interrupted}.
097 *
098 * @throws UncheckedExecutionException if startup failed
099 * @return the state of the service when startup finished.
100 */
101 State startAndWait();
102
103 /**
104 * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
105 */
106 boolean isRunning();
107
108 /**
109 * Returns the lifecycle state of the service.
110 */
111 State state();
112
113 /**
114 * If the service is {@linkplain State#STARTING starting} or {@linkplain
115 * State#RUNNING running}, this initiates service shutdown and returns
116 * immediately. If the service is {@linkplain State#NEW new}, it is
117 * {@linkplain State#TERMINATED terminated} without having been started nor
118 * stopped. If the service has already been stopped, this method returns
119 * immediately without taking action.
120 *
121 * @return a future for the shutdown result, regardless of whether this call
122 * initiated shutdown. Calling {@link ListenableFuture#get} will block
123 * until the service has finished shutting down, and either returns
124 * {@link State#TERMINATED} or throws an {@link ExecutionException}. If
125 * it has already finished stopping, {@link ListenableFuture#get} returns
126 * immediately. Cancelling this future has no effect on the service.
127 */
128 ListenableFuture<State> stop();
129
130 /**
131 * Initiates service shutdown (if necessary), returning once the service has
132 * finished stopping. If this is {@link State#STARTING}, startup will be
133 * cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED
134 * terminated} without having been started nor stopped. Unlike calling {@code
135 * stop().get()}, this method throws no checked exceptions.
136 *
137 * @throws UncheckedExecutionException if shutdown failed
138 * @return the state of the service when shutdown finished.
139 */
140 State stopAndWait();
141
142 /**
143 * The lifecycle states of a service.
144 *
145 * @since 9.0 (in 1.0 as
146 * {@code com.google.common.base.Service.State})
147 */
148 @Beta // should come out of Beta when Service does
149 enum State {
150 /**
151 * A service in this state is inactive. It does minimal work and consumes
152 * minimal resources.
153 */
154 NEW,
155
156 /**
157 * A service in this state is transitioning to {@link #RUNNING}.
158 */
159 STARTING,
160
161 /**
162 * A service in this state is operational.
163 */
164 RUNNING,
165
166 /**
167 * A service in this state is transitioning to {@link #TERMINATED}.
168 */
169 STOPPING,
170
171 /**
172 * A service in this state has completed execution normally. It does minimal
173 * work and consumes minimal resources.
174 */
175 TERMINATED,
176
177 /**
178 * A service in this state has encountered a problem and may not be
179 * operational. It cannot be started nor stopped.
180 */
181 FAILED
182 }
183 }