001 /*
002 * Copyright (C) 2007 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 static com.google.common.base.Preconditions.checkNotNull;
020
021 import java.util.concurrent.CancellationException;
022 import java.util.concurrent.ExecutionException;
023 import java.util.concurrent.Executor;
024 import java.util.concurrent.TimeUnit;
025 import java.util.concurrent.TimeoutException;
026 import java.util.concurrent.locks.AbstractQueuedSynchronizer;
027
028 import javax.annotation.Nullable;
029
030 /**
031 * An abstract implementation of the {@link ListenableFuture} interface. This
032 * class is preferable to {@link java.util.concurrent.FutureTask} for two
033 * reasons: It implements {@code ListenableFuture}, and it does not implement
034 * {@code Runnable}. (If you want a {@code Runnable} implementation of {@code
035 * ListenableFuture}, create a {@link ListenableFutureTask}, or submit your
036 * tasks to a {@link ListeningExecutorService}.)
037 *
038 * <p>This class implements all methods in {@code ListenableFuture}.
039 * Subclasses should provide a way to set the result of the computation through
040 * the protected methods {@link #set(Object)} and
041 * {@link #setException(Throwable)}. Subclasses may also override {@link
042 * #interruptTask()}, which will be invoked automatically if a call to {@link
043 * #cancel(boolean) cancel(true)} succeeds in canceling the future.
044 *
045 * <p>{@code AbstractFuture} uses an {@link AbstractQueuedSynchronizer} to deal
046 * with concurrency issues and guarantee thread safety.
047 *
048 * <p>The state changing methods all return a boolean indicating success or
049 * failure in changing the future's state. Valid states are running,
050 * completed, failed, or cancelled.
051 *
052 * <p>This class uses an {@link ExecutionList} to guarantee that all registered
053 * listeners will be executed, either when the future finishes or, for listeners
054 * that are added after the future completes, immediately.
055 * {@code Runnable}-{@code Executor} pairs are stored in the execution list but
056 * are not necessarily executed in the order in which they were added. (If a
057 * listener is added after the Future is complete, it will be executed
058 * immediately, even if earlier listeners have not been executed. Additionally,
059 * executors need not guarantee FIFO execution, or different listeners may run
060 * in different executors.)
061 *
062 * @author Sven Mawson
063 * @since 1.0
064 */
065 public abstract class AbstractFuture<V> implements ListenableFuture<V> {
066
067 /** Synchronization control for AbstractFutures. */
068 private final Sync<V> sync = new Sync<V>();
069
070 // The execution list to hold our executors.
071 private final ExecutionList executionList = new ExecutionList();
072
073 /**
074 * Constructor for use by subclasses.
075 */
076 protected AbstractFuture() {}
077
078 /*
079 * Improve the documentation of when InterruptedException is thrown. Our
080 * behavior matches the JDK's, but the JDK's documentation is misleading.
081 */
082 /**
083 * {@inheritDoc}
084 *
085 * <p>The default {@link AbstractFuture} implementation throws {@code
086 * InterruptedException} if the current thread is interrupted before or during
087 * the call, even if the value is already available.
088 *
089 * @throws InterruptedException if the current thread was interrupted before
090 * or during the call (optional but recommended).
091 * @throws CancellationException {@inheritDoc}
092 */
093 public V get(long timeout, TimeUnit unit) throws InterruptedException,
094 TimeoutException, ExecutionException {
095 return sync.get(unit.toNanos(timeout));
096 }
097
098 /*
099 * Improve the documentation of when InterruptedException is thrown. Our
100 * behavior matches the JDK's, but the JDK's documentation is misleading.
101 */
102 /**
103 * {@inheritDoc}
104 *
105 * <p>The default {@link AbstractFuture} implementation throws {@code
106 * InterruptedException} if the current thread is interrupted before or during
107 * the call, even if the value is already available.
108 *
109 * @throws InterruptedException if the current thread was interrupted before
110 * or during the call (optional but recommended).
111 * @throws CancellationException {@inheritDoc}
112 */
113 public V get() throws InterruptedException, ExecutionException {
114 return sync.get();
115 }
116
117 public boolean isDone() {
118 return sync.isDone();
119 }
120
121 public boolean isCancelled() {
122 return sync.isCancelled();
123 }
124
125 public boolean cancel(boolean mayInterruptIfRunning) {
126 if (!sync.cancel()) {
127 return false;
128 }
129 executionList.execute();
130 if (mayInterruptIfRunning) {
131 interruptTask();
132 }
133 return true;
134 }
135
136 /**
137 * Subclasses can override this method to implement interruption of the
138 * future's computation. The method is invoked automatically by a successful
139 * call to {@link #cancel(boolean) cancel(true)}.
140 *
141 * <p>The default implementation does nothing.
142 *
143 * @since 10.0
144 */
145 protected void interruptTask() {
146 }
147
148 /**
149 * {@inheritDoc}
150 *
151 * @since 10.0
152 */
153 public void addListener(Runnable listener, Executor exec) {
154 executionList.add(listener, exec);
155 }
156
157 /**
158 * Subclasses should invoke this method to set the result of the computation
159 * to {@code value}. This will set the state of the future to
160 * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
161 * state was successfully changed.
162 *
163 * @param value the value that was the result of the task.
164 * @return true if the state was successfully changed.
165 */
166 protected boolean set(@Nullable V value) {
167 boolean result = sync.set(value);
168 if (result) {
169 executionList.execute();
170 }
171 return result;
172 }
173
174 /**
175 * Subclasses should invoke this method to set the result of the computation
176 * to an error, {@code throwable}. This will set the state of the future to
177 * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
178 * state was successfully changed.
179 *
180 * @param throwable the exception that the task failed with.
181 * @return true if the state was successfully changed.
182 * @throws Error if the throwable was an {@link Error}.
183 */
184 protected boolean setException(Throwable throwable) {
185 boolean result = sync.setException(checkNotNull(throwable));
186 if (result) {
187 executionList.execute();
188 }
189
190 // If it's an Error, we want to make sure it reaches the top of the
191 // call stack, so we rethrow it.
192 if (throwable instanceof Error) {
193 throw (Error) throwable;
194 }
195 return result;
196 }
197
198 /**
199 * <p>Following the contract of {@link AbstractQueuedSynchronizer} we create a
200 * private subclass to hold the synchronizer. This synchronizer is used to
201 * implement the blocking and waiting calls as well as to handle state changes
202 * in a thread-safe manner. The current state of the future is held in the
203 * Sync state, and the lock is released whenever the state changes to either
204 * {@link #COMPLETED} or {@link #CANCELLED}.
205 *
206 * <p>To avoid races between threads doing release and acquire, we transition
207 * to the final state in two steps. One thread will successfully CAS from
208 * RUNNING to COMPLETING, that thread will then set the result of the
209 * computation, and only then transition to COMPLETED or CANCELLED.
210 *
211 * <p>We don't use the integer argument passed between acquire methods so we
212 * pass around a -1 everywhere.
213 */
214 static final class Sync<V> extends AbstractQueuedSynchronizer {
215
216 private static final long serialVersionUID = 0L;
217
218 /* Valid states. */
219 static final int RUNNING = 0;
220 static final int COMPLETING = 1;
221 static final int COMPLETED = 2;
222 static final int CANCELLED = 4;
223
224 private V value;
225 private Throwable exception;
226
227 /*
228 * Acquisition succeeds if the future is done, otherwise it fails.
229 */
230
231 @Override
232 protected int tryAcquireShared(int ignored) {
233 if (isDone()) {
234 return 1;
235 }
236 return -1;
237 }
238
239 /*
240 * We always allow a release to go through, this means the state has been
241 * successfully changed and the result is available.
242 */
243
244 @Override
245 protected boolean tryReleaseShared(int finalState) {
246 setState(finalState);
247 return true;
248 }
249
250 /**
251 * Blocks until the task is complete or the timeout expires. Throws a
252 * {@link TimeoutException} if the timer expires, otherwise behaves like
253 * {@link #get()}.
254 */
255 V get(long nanos) throws TimeoutException, CancellationException,
256 ExecutionException, InterruptedException {
257
258 // Attempt to acquire the shared lock with a timeout.
259 if (!tryAcquireSharedNanos(-1, nanos)) {
260 throw new TimeoutException("Timeout waiting for task.");
261 }
262
263 return getValue();
264 }
265
266 /**
267 * Blocks until {@link #complete(Object, Throwable, int)} has been
268 * successfully called. Throws a {@link CancellationException} if the task
269 * was cancelled, or a {@link ExecutionException} if the task completed with
270 * an error.
271 */
272 V get() throws CancellationException, ExecutionException,
273 InterruptedException {
274
275 // Acquire the shared lock allowing interruption.
276 acquireSharedInterruptibly(-1);
277 return getValue();
278 }
279
280 /**
281 * Implementation of the actual value retrieval. Will return the value
282 * on success, an exception on failure, a cancellation on cancellation, or
283 * an illegal state if the synchronizer is in an invalid state.
284 */
285 private V getValue() throws CancellationException, ExecutionException {
286 int state = getState();
287 switch (state) {
288 case COMPLETED:
289 if (exception != null) {
290 throw new ExecutionException(exception);
291 } else {
292 return value;
293 }
294
295 case CANCELLED:
296 throw new CancellationException("Task was cancelled.");
297
298 default:
299 throw new IllegalStateException(
300 "Error, synchronizer in invalid state: " + state);
301 }
302 }
303
304 /**
305 * Checks if the state is {@link #COMPLETED} or {@link #CANCELLED}.
306 */
307 boolean isDone() {
308 return (getState() & (COMPLETED | CANCELLED)) != 0;
309 }
310
311 /**
312 * Checks if the state is {@link #CANCELLED}.
313 */
314 boolean isCancelled() {
315 return getState() == CANCELLED;
316 }
317
318 /**
319 * Transition to the COMPLETED state and set the value.
320 */
321 boolean set(@Nullable V v) {
322 return complete(v, null, COMPLETED);
323 }
324
325 /**
326 * Transition to the COMPLETED state and set the exception.
327 */
328 boolean setException(Throwable t) {
329 return complete(null, t, COMPLETED);
330 }
331
332 /**
333 * Transition to the CANCELLED state.
334 */
335 boolean cancel() {
336 return complete(null, null, CANCELLED);
337 }
338
339 /**
340 * Implementation of completing a task. Either {@code v} or {@code t} will
341 * be set but not both. The {@code finalState} is the state to change to
342 * from {@link #RUNNING}. If the state is not in the RUNNING state we
343 * return {@code false} after waiting for the state to be set to a valid
344 * final state ({@link #COMPLETED} or {@link #CANCELLED}).
345 *
346 * @param v the value to set as the result of the computation.
347 * @param t the exception to set as the result of the computation.
348 * @param finalState the state to transition to.
349 */
350 private boolean complete(@Nullable V v, @Nullable Throwable t,
351 int finalState) {
352 boolean doCompletion = compareAndSetState(RUNNING, COMPLETING);
353 if (doCompletion) {
354 // If this thread successfully transitioned to COMPLETING, set the value
355 // and exception and then release to the final state.
356 this.value = v;
357 this.exception = t;
358 releaseShared(finalState);
359 } else if (getState() == COMPLETING) {
360 // If some other thread is currently completing the future, block until
361 // they are done so we can guarantee completion.
362 acquireShared(-1);
363 }
364 return doCompletion;
365 }
366 }
367 }