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 017package com.google.common.util.concurrent; 018 019import com.google.common.annotations.Beta; 020 021import java.util.concurrent.ExecutionException; 022import 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 054public 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 * 122 * @since 14.0 123 */ 124 Throwable failureCause(); 125 126 /** 127 * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given 128 * executor. The listener will have the corresponding transition method called whenever the 129 * service changes state. The listener will not have previous state changes replayed, so it is 130 * suggested that listeners are added before the service starts. 131 * 132 * <p>There is no guaranteed ordering of execution of listeners, but any listener added through 133 * this method is guaranteed to be called whenever there is a state change. 134 * 135 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown 136 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception 137 * thrown by {@linkplain MoreExecutors#sameThreadExecutor inline execution}) will be caught and 138 * logged. 139 * 140 * @param listener the listener to run when the service changes state is complete 141 * @param executor the executor in which the the listeners callback methods will be run. For fast, 142 * lightweight listeners that would be safe to execute in any thread, consider 143 * {@link MoreExecutors#sameThreadExecutor}. 144 * @since 13.0 145 */ 146 void addListener(Listener listener, Executor executor); 147 148 /** 149 * The lifecycle states of a service. 150 * 151 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State}) 152 */ 153 @Beta // should come out of Beta when Service does 154 enum State { 155 /** 156 * A service in this state is inactive. It does minimal work and consumes 157 * minimal resources. 158 */ 159 NEW, 160 161 /** 162 * A service in this state is transitioning to {@link #RUNNING}. 163 */ 164 STARTING, 165 166 /** 167 * A service in this state is operational. 168 */ 169 RUNNING, 170 171 /** 172 * A service in this state is transitioning to {@link #TERMINATED}. 173 */ 174 STOPPING, 175 176 /** 177 * A service in this state has completed execution normally. It does minimal work and consumes 178 * minimal resources. 179 */ 180 TERMINATED, 181 182 /** 183 * A service in this state has encountered a problem and may not be operational. It cannot be 184 * started nor stopped. 185 */ 186 FAILED 187 } 188 189 /** 190 * A listener for the various state changes that a {@link Service} goes through in its lifecycle. 191 * 192 * @author Luke Sandberg 193 * @since 13.0 194 */ 195 @Beta // should come out of Beta when Service does 196 interface Listener { 197 /** 198 * Called when the service transitions from {@linkplain State#NEW NEW} to 199 * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#start} or 200 * {@link Service#startAndWait} is called the first time. 201 */ 202 void starting(); 203 204 /** 205 * Called when the service transitions from {@linkplain State#STARTING STARTING} to 206 * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started. 207 */ 208 void running(); 209 210 /** 211 * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The 212 * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or 213 * {@linkplain State#RUNNING RUNNING}. This occurs when {@link Service#stop} is called. 214 * 215 * @param from The previous state that is being transitioned from. 216 */ 217 void stopping(State from); 218 219 /** 220 * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state. 221 * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition 222 * diagram. Therefore, if this method is called, no other methods will be called on the 223 * {@link Listener}. 224 * 225 * @param from The previous state that is being transitioned from. The only valid values for 226 * this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or 227 * {@linkplain State#STOPPING STOPPING}. 228 */ 229 void terminated(State from); 230 231 /** 232 * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The 233 * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram. 234 * Therefore, if this method is called, no other methods will be called on the {@link Listener}. 235 * 236 * @param from The previous state that is being transitioned from. Failure can occur in any 237 * state with the exception of {@linkplain State#NEW NEW} or 238 * {@linkplain State#TERMINATED TERMINATED}. 239 * @param failure The exception that caused the failure. 240 */ 241 void failed(State from, Throwable failure); 242 } 243}