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