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.Executor;
022import java.util.concurrent.TimeUnit;
023import java.util.concurrent.TimeoutException;
024
025/**
026 * An object with an operational state, plus asynchronous {@link #startAsync()} and 
027 * {@link #stopAsync()} lifecycle methods to transition between states. Example services include 
028 * webservers, RPC servers and timers.
029 *
030 * <p>The normal lifecycle of a service is:
031 * <ul>
032 *   <li>{@linkplain State#NEW NEW} -&gt;
033 *   <li>{@linkplain State#STARTING STARTING} -&gt;
034 *   <li>{@linkplain State#RUNNING RUNNING} -&gt;
035 *   <li>{@linkplain State#STOPPING STOPPING} -&gt;
036 *   <li>{@linkplain State#TERMINATED TERMINATED}
037 * </ul>
038 *
039 * <p>There are deviations from this if there are failures or if {@link Service#stopAsync} is called
040 * before the {@link Service} reaches the {@linkplain State#RUNNING RUNNING} state. The set of legal
041 * transitions form a <a href="http://en.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>, 
042 * therefore every method of the listener will be called at most once. N.B. The {@link State#FAILED}
043 * and {@link State#TERMINATED} states are terminal states, once a service enters either of these
044 * states it cannot ever leave them.
045 *
046 * <p>Implementors of this interface are strongly encouraged to extend one of the abstract classes 
047 * in this package which implement this interface and make the threading and state management 
048 * easier.
049 *
050 * @author Jesse Wilson
051 * @author Luke Sandberg
052 * @since 9.0 (in 1.0 as {@code com.google.common.base.Service})
053 */
054@Beta
055public interface Service {
056  /**
057   * If the service state is {@link State#NEW}, this initiates service startup and returns
058   * immediately. A stopped service may not be restarted.
059   * 
060   * @return this
061   * @throws IllegalStateException if the service is not {@link State#NEW}
062   *
063   * @since 15.0
064   */
065  Service startAsync();
066
067  /**
068   * Returns {@code true} if this service is {@linkplain State#RUNNING running}.
069   */
070  boolean isRunning();
071
072  /**
073   * Returns the lifecycle state of the service.
074   */
075  State state();
076
077  /**
078   * If the service is {@linkplain State#STARTING starting} or {@linkplain State#RUNNING running},
079   * this initiates service shutdown and returns immediately. If the service is
080   * {@linkplain State#NEW new}, it is {@linkplain State#TERMINATED terminated} without having been
081   * started nor stopped. If the service has already been stopped, this method returns immediately
082   * without taking action.
083   *
084   * @return this
085   * @since 15.0
086   */
087  Service stopAsync();
088
089  /**
090   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state}.
091   *
092   * @throws IllegalStateException if the service reaches a state from which it is not possible to
093   *     enter the {@link State#RUNNING} state. e.g. if the {@code state} is 
094   *     {@code State#TERMINATED} when this method is called then this will throw an 
095   *     IllegalStateException.
096   *
097   * @since 15.0
098   */
099  void awaitRunning();
100
101  /**
102   * Waits for the {@link Service} to reach the {@linkplain State#RUNNING running state} for no 
103   * more than the given time.
104   *
105   * @param timeout the maximum time to wait
106   * @param unit the time unit of the timeout argument
107   * @throws TimeoutException if the service has not reached the given state within the deadline
108   * @throws IllegalStateException if the service reaches a state from which it is not possible to
109   *     enter the {@link State#RUNNING RUNNING} state. e.g. if the {@code state} is 
110   *     {@code State#TERMINATED} when this method is called then this will throw an 
111   *     IllegalStateException.
112   *
113   * @since 15.0
114   */
115  void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException;
116
117  /**
118   * Waits for the {@link Service} to reach the {@linkplain State#TERMINATED terminated state}.
119   * 
120   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
121   * 
122   * @since 15.0
123   */
124  void awaitTerminated();
125
126  /**
127   * Waits for the {@link Service} to reach a terminal state (either 
128   * {@link Service.State#TERMINATED terminated} or {@link Service.State#FAILED failed}) for no 
129   * more than the given time.
130   *
131   * @param timeout the maximum time to wait
132   * @param unit the time unit of the timeout argument
133   * @throws TimeoutException if the service has not reached the given state within the deadline
134   * @throws IllegalStateException if the service {@linkplain State#FAILED fails}.
135   * @since 15.0
136   */
137  void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException;
138
139  /**
140   * Returns the {@link Throwable} that caused this service to fail.
141   * 
142   * @throws IllegalStateException if this service's state isn't {@linkplain State#FAILED FAILED}.
143   *
144   * @since 14.0
145   */
146  Throwable failureCause();
147  
148  /**
149   * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given 
150   * executor.  The listener will have the corresponding transition method called whenever the 
151   * service changes state. The listener will not have previous state changes replayed, so it is 
152   * suggested that listeners are added before the service starts.
153   *
154   * <p>{@code addListener} guarantees execution ordering across calls to a given listener but not
155   * across calls to multiple listeners. Specifically, a given listener will have its callbacks
156   * invoked in the same order as the underlying service enters those states. Additionally, at most
157   * one of the listener's callbacks will execute at once. However, multiple listeners' callbacks
158   * may execute concurrently, and listeners may execute in an order different from the one in which
159   * they were registered.
160   *
161   * <p>RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown 
162   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
163   * logged.
164   * 
165   * @param listener the listener to run when the service changes state is complete
166   * @param executor the executor in which the listeners callback methods will be run. For fast,
167   *     lightweight listeners that would be safe to execute in any thread, consider 
168   *     {@link MoreExecutors#directExecutor}.
169   * @since 13.0
170   */
171  void addListener(Listener listener, Executor executor);
172  
173  /**
174   * The lifecycle states of a service.
175   * 
176   * <p>The ordering of the {@link State} enum is defined such that if there is a state transition
177   * from {@code A -> B} then {@code A.compareTo(B} < 0}.  N.B. The converse is not true, i.e. if
178   * {@code A.compareTo(B} < 0} then there is <b>not</b> guaranteed to be a valid state transition 
179   * {@code A -> B}.
180   *
181   * @since 9.0 (in 1.0 as {@code com.google.common.base.Service.State})
182   */
183  @Beta // should come out of Beta when Service does
184  enum State {
185    /**
186     * A service in this state is inactive. It does minimal work and consumes
187     * minimal resources.
188     */
189    NEW {
190      @Override boolean isTerminal() {
191        return false;
192      }
193    },
194
195    /**
196     * A service in this state is transitioning to {@link #RUNNING}.
197     */
198    STARTING {
199      @Override boolean isTerminal() {
200        return false;
201      }
202    },
203
204    /**
205     * A service in this state is operational.
206     */
207    RUNNING {
208      @Override boolean isTerminal() {
209        return false;
210      }
211    },
212
213    /**
214     * A service in this state is transitioning to {@link #TERMINATED}.
215     */
216    STOPPING {
217      @Override boolean isTerminal() {
218        return false;
219      }
220    },
221
222    /**
223     * A service in this state has completed execution normally. It does minimal work and consumes
224     * minimal resources.
225     */
226    TERMINATED {
227      @Override boolean isTerminal() {
228        return true;
229      }
230    },
231
232    /**
233     * A service in this state has encountered a problem and may not be operational. It cannot be
234     * started nor stopped.
235     */
236    FAILED {
237      @Override boolean isTerminal() {
238        return true;
239      }
240    };
241    
242    /** Returns true if this state is terminal. */
243    abstract boolean isTerminal();
244  }
245  
246  /**
247   * A listener for the various state changes that a {@link Service} goes through in its lifecycle.
248   *
249   * <p>All methods are no-ops by default, implementors should override the ones they care about.
250   *
251   * @author Luke Sandberg
252   * @since 15.0 (present as an interface in 13.0)
253   */
254  @Beta // should come out of Beta when Service does
255  abstract class Listener {
256    /**
257     * Called when the service transitions from {@linkplain State#NEW NEW} to 
258     * {@linkplain State#STARTING STARTING}. This occurs when {@link Service#startAsync} is called 
259     * the first time.
260     */
261    public void starting() {}
262    
263    /**
264     * Called when the service transitions from {@linkplain State#STARTING STARTING} to 
265     * {@linkplain State#RUNNING RUNNING}. This occurs when a service has successfully started.
266     */
267    public void running() {}
268    
269    /**
270     * Called when the service transitions to the {@linkplain State#STOPPING STOPPING} state. The 
271     * only valid values for {@code from} are {@linkplain State#STARTING STARTING} or 
272     * {@linkplain State#RUNNING RUNNING}.  This occurs when {@link Service#stopAsync} is called.
273     * 
274     * @param from The previous state that is being transitioned from.  
275     */
276    public void stopping(State from) {}
277    
278    /**
279     * Called when the service transitions to the {@linkplain State#TERMINATED TERMINATED} state. 
280     * The {@linkplain State#TERMINATED TERMINATED} state is a terminal state in the transition
281     * diagram.  Therefore, if this method is called, no other methods will be called on the 
282     * {@link Listener}.
283     * 
284     * @param from The previous state that is being transitioned from.  The only valid values for 
285     *     this are {@linkplain State#NEW NEW}, {@linkplain State#RUNNING RUNNING} or 
286     *     {@linkplain State#STOPPING STOPPING}.
287     */
288    public void terminated(State from) {}
289    
290    /**
291     * Called when the service transitions to the {@linkplain State#FAILED FAILED} state. The 
292     * {@linkplain State#FAILED FAILED} state is a terminal state in the transition diagram.  
293     * Therefore, if this method is called, no other methods will be called on the {@link Listener}.
294     * 
295     * @param from The previous state that is being transitioned from.  Failure can occur in any 
296     *     state with the exception of {@linkplain State#NEW NEW} or 
297     *     {@linkplain State#TERMINATED TERMINATED}.
298     * @param failure The exception that caused the failure.
299     */
300    public void failed(State from, Throwable failure) {}
301  }
302}