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 * Improve the documentation of when InterruptedException is thrown. Our
075 * behavior matches the JDK's, but the JDK's documentation is misleading.
076 */
077 /**
078 * {@inheritDoc}
079 *
080 * <p>The default {@link AbstractFuture} implementation throws {@code
081 * InterruptedException} if the current thread is interrupted before or during
082 * the call, even if the value is already available.
083 *
084 * @throws InterruptedException if the current thread was interrupted before
085 * or during the call (optional but recommended).
086 * @throws CancellationException {@inheritDoc}
087 */
088 @Override
089 public V get(long timeout, TimeUnit unit) throws InterruptedException,
090 TimeoutException, ExecutionException {
091 return sync.get(unit.toNanos(timeout));
092 }
093
094 /*
095 * Improve the documentation of when InterruptedException is thrown. Our
096 * behavior matches the JDK's, but the JDK's documentation is misleading.
097 */
098 /**
099 * {@inheritDoc}
100 *
101 * <p>The default {@link AbstractFuture} implementation throws {@code
102 * InterruptedException} if the current thread is interrupted before or during
103 * the call, even if the value is already available.
104 *
105 * @throws InterruptedException if the current thread was interrupted before
106 * or during the call (optional but recommended).
107 * @throws CancellationException {@inheritDoc}
108 */
109 @Override
110 public V get() throws InterruptedException, ExecutionException {
111 return sync.get();
112 }
113
114 @Override
115 public boolean isDone() {
116 return sync.isDone();
117 }
118
119 @Override
120 public boolean isCancelled() {
121 return sync.isCancelled();
122 }
123
124 @Override
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 @Override
154 public void addListener(Runnable listener, Executor exec) {
155 executionList.add(listener, exec);
156 }
157
158 /**
159 * Subclasses should invoke this method to set the result of the computation
160 * to {@code value}. This will set the state of the future to
161 * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
162 * state was successfully changed.
163 *
164 * @param value the value that was the result of the task.
165 * @return true if the state was successfully changed.
166 */
167 protected boolean set(@Nullable V value) {
168 boolean result = sync.set(value);
169 if (result) {
170 executionList.execute();
171 }
172 return result;
173 }
174
175 /**
176 * Subclasses should invoke this method to set the result of the computation
177 * to an error, {@code throwable}. This will set the state of the future to
178 * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
179 * state was successfully changed.
180 *
181 * @param throwable the exception that the task failed with.
182 * @return true if the state was successfully changed.
183 * @throws Error if the throwable was an {@link Error}.
184 */
185 protected boolean setException(Throwable throwable) {
186 boolean result = sync.setException(checkNotNull(throwable));
187 if (result) {
188 executionList.execute();
189 }
190
191 // If it's an Error, we want to make sure it reaches the top of the
192 // call stack, so we rethrow it.
193 if (throwable instanceof Error) {
194 throw (Error) throwable;
195 }
196 return result;
197 }
198
199 /**
200 * <p>Following the contract of {@link AbstractQueuedSynchronizer} we create a
201 * private subclass to hold the synchronizer. This synchronizer is used to
202 * implement the blocking and waiting calls as well as to handle state changes
203 * in a thread-safe manner. The current state of the future is held in the
204 * Sync state, and the lock is released whenever the state changes to either
205 * {@link #COMPLETED} or {@link #CANCELLED}.
206 *
207 * <p>To avoid races between threads doing release and acquire, we transition
208 * to the final state in two steps. One thread will successfully CAS from
209 * RUNNING to COMPLETING, that thread will then set the result of the
210 * computation, and only then transition to COMPLETED or CANCELLED.
211 *
212 * <p>We don't use the integer argument passed between acquire methods so we
213 * pass around a -1 everywhere.
214 */
215 static final class Sync<V> extends AbstractQueuedSynchronizer {
216
217 private static final long serialVersionUID = 0L;
218
219 /* Valid states. */
220 static final int RUNNING = 0;
221 static final int COMPLETING = 1;
222 static final int COMPLETED = 2;
223 static final int CANCELLED = 4;
224
225 private V value;
226 private Throwable exception;
227
228 /*
229 * Acquisition succeeds if the future is done, otherwise it fails.
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 @Override
244 protected boolean tryReleaseShared(int finalState) {
245 setState(finalState);
246 return true;
247 }
248
249 /**
250 * Blocks until the task is complete or the timeout expires. Throws a
251 * {@link TimeoutException} if the timer expires, otherwise behaves like
252 * {@link #get()}.
253 */
254 V get(long nanos) throws TimeoutException, CancellationException,
255 ExecutionException, InterruptedException {
256
257 // Attempt to acquire the shared lock with a timeout.
258 if (!tryAcquireSharedNanos(-1, nanos)) {
259 throw new TimeoutException("Timeout waiting for task.");
260 }
261
262 return getValue();
263 }
264
265 /**
266 * Blocks until {@link #complete(Object, Throwable, int)} has been
267 * successfully called. Throws a {@link CancellationException} if the task
268 * was cancelled, or a {@link ExecutionException} if the task completed with
269 * an error.
270 */
271 V get() throws CancellationException, ExecutionException,
272 InterruptedException {
273
274 // Acquire the shared lock allowing interruption.
275 acquireSharedInterruptibly(-1);
276 return getValue();
277 }
278
279 /**
280 * Implementation of the actual value retrieval. Will return the value
281 * on success, an exception on failure, a cancellation on cancellation, or
282 * an illegal state if the synchronizer is in an invalid state.
283 */
284 private V getValue() throws CancellationException, ExecutionException {
285 int state = getState();
286 switch (state) {
287 case COMPLETED:
288 if (exception != null) {
289 throw new ExecutionException(exception);
290 } else {
291 return value;
292 }
293
294 case CANCELLED:
295 throw new CancellationException("Task was cancelled.");
296
297 default:
298 throw new IllegalStateException(
299 "Error, synchronizer in invalid state: " + state);
300 }
301 }
302
303 /**
304 * Checks if the state is {@link #COMPLETED} or {@link #CANCELLED}.
305 */
306 boolean isDone() {
307 return (getState() & (COMPLETED | CANCELLED)) != 0;
308 }
309
310 /**
311 * Checks if the state is {@link #CANCELLED}.
312 */
313 boolean isCancelled() {
314 return getState() == CANCELLED;
315 }
316
317 /**
318 * Transition to the COMPLETED state and set the value.
319 */
320 boolean set(@Nullable V v) {
321 return complete(v, null, COMPLETED);
322 }
323
324 /**
325 * Transition to the COMPLETED state and set the exception.
326 */
327 boolean setException(Throwable t) {
328 return complete(null, t, COMPLETED);
329 }
330
331 /**
332 * Transition to the CANCELLED state.
333 */
334 boolean cancel() {
335 return complete(null, null, CANCELLED);
336 }
337
338 /**
339 * Implementation of completing a task. Either {@code v} or {@code t} will
340 * be set but not both. The {@code finalState} is the state to change to
341 * from {@link #RUNNING}. If the state is not in the RUNNING state we
342 * return {@code false} after waiting for the state to be set to a valid
343 * final state ({@link #COMPLETED} or {@link #CANCELLED}).
344 *
345 * @param v the value to set as the result of the computation.
346 * @param t the exception to set as the result of the computation.
347 * @param finalState the state to transition to.
348 */
349 private boolean complete(@Nullable V v, @Nullable Throwable t,
350 int finalState) {
351 boolean doCompletion = compareAndSetState(RUNNING, COMPLETING);
352 if (doCompletion) {
353 // If this thread successfully transitioned to COMPLETING, set the value
354 // and exception and then release to the final state.
355 this.value = v;
356 this.exception = t;
357 releaseShared(finalState);
358 } else if (getState() == COMPLETING) {
359 // If some other thread is currently completing the future, block until
360 // they are done so we can guarantee completion.
361 acquireShared(-1);
362 }
363 return doCompletion;
364 }
365 }
366 }