001 /*
002 * Copyright (C) 2009 Google Inc.
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.base;
018
019 import com.google.common.annotations.Beta;
020
021 import java.util.concurrent.ExecutionException;
022 import java.util.concurrent.Future;
023
024 /**
025 * An object with an operational state, plus asynchronous {@link #start()} and
026 * {@link #stop()} lifecycle methods to transfer into and out of this state.
027 * Example services include webservers, RPC servers and timers. The normal
028 * 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 * If the service fails while starting, running or stopping, its state will be
038 * {@link State#FAILED}, and its behavior is undefined. Such a service cannot be
039 * started nor stopped.
040 *
041 * <p>Implementors of this interface are strongly encouraged to extend {@link
042 * com.google.common.util.concurrent.AbstractService} or {@link
043 * com.google.common.util.concurrent.AbstractExecutionThreadService}, which make
044 * the threading and state management easier.
045 *
046 * @author Jesse Wilson
047 * @since 1
048 */
049 @Beta // TODO: make it an abstract class and move to common.util.concurrent
050 public interface Service {
051 /**
052 * If the service state is {@link State#NEW}, this initiates service startup
053 * and returns immediately. If the service has already been started, this
054 * method returns immediately without taking action. A stopped service may not
055 * be restarted.
056 *
057 * @return a future for the startup result, regardless of whether this call
058 * initiated startup. Calling {@link Future#get} will block until the
059 * service has finished starting, and returns one of {@link
060 * State#RUNNING}, {@link State#STOPPING} or {@link State#TERMINATED}. If
061 * the service fails to start, {@link Future#get} will throw an {@link
062 * ExecutionException}, and the service's state will be {@link
063 * State#FAILED}. If it has already finished starting, {@link Future#get}
064 * returns immediately. Cancelling the returned future is unsupported and
065 * always returns {@code false}.
066 */
067 Future<State> start();
068
069 /**
070 * Initiates service startup (if necessary), returning once the service has
071 * finished starting. Unlike calling {@code start().get()}, this method throws
072 * no checked exceptions.
073 *
074 * @throws InterruptedRuntimeException if the thread was interrupted while
075 * waiting for the service to finish starting up.
076 * @throws RuntimeException if startup failed
077 * @return the state of the service when startup finished.
078 */
079 State startAndWait();
080
081 /**
082 * Returns {@code true} if this service is {@link State#RUNNING running}.
083 */
084 boolean isRunning();
085
086 /**
087 * Returns the lifecycle state of the service.
088 */
089 State state();
090
091 /**
092 * If the service is {@link State#STARTING} or {@link State#RUNNING}, this
093 * initiates service shutdown and returns immediately. If this is {@link
094 * State#NEW}, it is {@link State#TERMINATED terminated} without having been
095 * started nor stopped. If the service has already been stopped, this
096 * method returns immediately without taking action.
097 *
098 * @return a future for the shutdown result, regardless of whether this call
099 * initiated shutdown. Calling {@link Future#get} will block until the
100 * service has finished shutting down, and either returns {@link
101 * State#TERMINATED} or throws an {@link ExecutionException}. If it has
102 * already finished stopping, {@link Future#get} returns immediately.
103 * Cancelling this future is unsupported and always returns {@code
104 * false}.
105 */
106 Future<State> stop();
107
108 /**
109 * Initiates service shutdown (if necessary), returning once the service has
110 * finished stopping. If this is {@link State#STARTING}, startup will be
111 * cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED
112 * terminated} without having been started nor stopped. Unlike calling {@code
113 * stop().get()}, this method throws no checked exceptions.
114 *
115 * @throws InterruptedRuntimeException if the thread was interrupted while
116 * waiting for the service to finish shutting down.
117 * @throws RuntimeException if shutdown failed
118 * @return the state of the service when shutdown finished.
119 */
120 State stopAndWait();
121
122 /**
123 * The lifecycle states of a service.
124 */
125 public enum State {
126
127 /**
128 * A service in this state is inactive. It does minimal work and consumes
129 * minimal resources.
130 */
131 NEW,
132
133 /**
134 * A service in this state is transitioning to {@link #RUNNING}.
135 */
136 STARTING,
137
138 /**
139 * A service in this state is operational.
140 */
141 RUNNING,
142
143 /**
144 * A service in this state is transitioning to {@link #TERMINATED}.
145 */
146 STOPPING,
147
148 /**
149 * A service in this state has completed execution normally. It does minimal
150 * work and consumes minimal resources.
151 */
152 TERMINATED,
153
154 /**
155 * A service in this state has encountered a problem and may not be
156 * operational. It cannot be started nor stopped.
157 */
158 FAILED
159 }
160 }