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