1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 38 /** 39 * A {@code Future} represents the result of an asynchronous 40 * computation. Methods are provided to check if the computation is 41 * complete, to wait for its completion, and to retrieve the result of 42 * the computation. The result can only be retrieved using method 43 * {@code get} when the computation has completed, blocking if 44 * necessary until it is ready. Cancellation is performed by the 45 * {@code cancel} method. Additional methods are provided to 46 * determine if the task completed normally or was cancelled. Once a 47 * computation has completed, the computation cannot be cancelled. 48 * If you would like to use a {@code Future} for the sake 49 * of cancellability but not provide a usable result, you can 50 * declare types of the form {@code Future<?>} and 51 * return {@code null} as a result of the underlying task. 52 * 53 * <p><b>Sample Usage</b> (Note that the following classes are all 54 * made-up.) 55 * 56 * <pre> {@code 57 * interface ArchiveSearcher { String search(String target); } 58 * class App { 59 * ExecutorService executor = ...; 60 * ArchiveSearcher searcher = ...; 61 * void showSearch(String target) throws InterruptedException { 62 * Callable<String> task = () -> searcher.search(target); 63 * Future<String> future = executor.submit(task); 64 * displayOtherThings(); // do other things while searching 65 * try { 66 * displayText(future.get()); // use future 67 * } catch (ExecutionException ex) { cleanup(); return; } 68 * } 69 * }}</pre> 70 * 71 * The {@link FutureTask} class is an implementation of {@code Future} that 72 * implements {@code Runnable}, and so may be executed by an {@code Executor}. 73 * For example, the above construction with {@code submit} could be replaced by: 74 * <pre> {@code 75 * FutureTask<String> future = new FutureTask<>(task); 76 * executor.execute(future);}</pre> 77 * 78 * <p>Memory consistency effects: Actions taken by the asynchronous computation 79 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 80 * actions following the corresponding {@code Future.get()} in another thread. 81 * 82 * @see FutureTask 83 * @see Executor 84 * @since 1.5 85 * @author Doug Lea 86 * @param <V> The result type returned by this Future's {@code get} method 87 */ 88 public interface Future<V> { 89 90 /** 91 * Attempts to cancel execution of this task. This method has no 92 * effect if the task is already completed or cancelled, or could 93 * not be cancelled for some other reason. Otherwise, if this 94 * task has not started when {@code cancel} is called, this task 95 * should never run. If the task has already started, then the 96 * {@code mayInterruptIfRunning} parameter determines whether the 97 * thread executing this task (when known by the implementation) 98 * is interrupted in an attempt to stop the task. 99 * 100 * <p>The return value from this method does not necessarily 101 * indicate whether the task is now cancelled; use {@link 102 * #isCancelled}. 103 * 104 * @param mayInterruptIfRunning {@code true} if the thread 105 * executing this task should be interrupted (if the thread is 106 * known to the implementation); otherwise, in-progress tasks are 107 * allowed to complete 108 * @return {@code false} if the task could not be cancelled, 109 * typically because it has already completed; {@code true} 110 * otherwise. If two or more threads cause a task to be cancelled, 111 * then at least one of them returns {@code true}. Implementations 112 * may provide stronger guarantees. 113 */ cancel(boolean mayInterruptIfRunning)114 boolean cancel(boolean mayInterruptIfRunning); 115 116 /** 117 * Returns {@code true} if this task was cancelled before it completed 118 * normally. 119 * 120 * @return {@code true} if this task was cancelled before it completed 121 */ isCancelled()122 boolean isCancelled(); 123 124 /** 125 * Returns {@code true} if this task completed. 126 * 127 * Completion may be due to normal termination, an exception, or 128 * cancellation -- in all of these cases, this method will return 129 * {@code true}. 130 * 131 * @return {@code true} if this task completed 132 */ isDone()133 boolean isDone(); 134 135 /** 136 * Waits if necessary for the computation to complete, and then 137 * retrieves its result. 138 * 139 * @return the computed result 140 * @throws CancellationException if the computation was cancelled 141 * @throws ExecutionException if the computation threw an 142 * exception 143 * @throws InterruptedException if the current thread was interrupted 144 * while waiting 145 */ get()146 V get() throws InterruptedException, ExecutionException; 147 148 /** 149 * Waits if necessary for at most the given time for the computation 150 * to complete, and then retrieves its result, if available. 151 * 152 * @param timeout the maximum time to wait 153 * @param unit the time unit of the timeout argument 154 * @return the computed result 155 * @throws CancellationException if the computation was cancelled 156 * @throws ExecutionException if the computation threw an 157 * exception 158 * @throws InterruptedException if the current thread was interrupted 159 * while waiting 160 * @throws TimeoutException if the wait timed out 161 */ get(long timeout, TimeUnit unit)162 V get(long timeout, TimeUnit unit) 163 throws InterruptedException, ExecutionException, TimeoutException; 164 165 /** 166 * Returns the computed result, without waiting. 167 * 168 * <p> This method is for cases where the caller knows that the task has 169 * already completed successfully, for example when filtering a stream 170 * of Future objects for the successful tasks and using a mapping 171 * operation to obtain a stream of results. 172 * {@snippet lang=java : 173 * results = futures.stream() 174 * .filter(f -> f.state() == Future.State.SUCCESS) 175 * .map(Future::resultNow) 176 * .toList(); 177 * } 178 * 179 * @implSpec 180 * The default implementation invokes {@code isDone()} to test if the task 181 * has completed. If done, it invokes {@code get()} to obtain the result. 182 * 183 * @return the computed result 184 * @throws IllegalStateException if the task has not completed or the task 185 * did not complete with a result 186 * @since 19 187 */ resultNow()188 default V resultNow() { 189 if (!isDone()) 190 throw new IllegalStateException("Task has not completed"); 191 boolean interrupted = false; 192 try { 193 while (true) { 194 try { 195 return get(); 196 } catch (InterruptedException e) { 197 interrupted = true; 198 } catch (ExecutionException e) { 199 throw new IllegalStateException("Task completed with exception"); 200 } catch (CancellationException e) { 201 throw new IllegalStateException("Task was cancelled"); 202 } 203 } 204 } finally { 205 if (interrupted) Thread.currentThread().interrupt(); 206 } 207 } 208 209 /** 210 * Returns the exception thrown by the task, without waiting. 211 * 212 * <p> This method is for cases where the caller knows that the task 213 * has already completed with an exception. 214 * 215 * @implSpec 216 * The default implementation invokes {@code isDone()} to test if the task 217 * has completed. If done and not cancelled, it invokes {@code get()} and 218 * catches the {@code ExecutionException} to obtain the exception. 219 * 220 * @return the exception thrown by the task 221 * @throws IllegalStateException if the task has not completed, the task 222 * completed normally, or the task was cancelled 223 * @since 19 224 */ exceptionNow()225 default Throwable exceptionNow() { 226 if (!isDone()) 227 throw new IllegalStateException("Task has not completed"); 228 if (isCancelled()) 229 throw new IllegalStateException("Task was cancelled"); 230 boolean interrupted = false; 231 try { 232 while (true) { 233 try { 234 get(); 235 throw new IllegalStateException("Task completed with a result"); 236 } catch (InterruptedException e) { 237 interrupted = true; 238 } catch (ExecutionException e) { 239 return e.getCause(); 240 } 241 } 242 } finally { 243 if (interrupted) Thread.currentThread().interrupt(); 244 } 245 } 246 247 /** 248 * Represents the computation state. 249 * @since 19 250 */ 251 enum State { 252 /** 253 * The task has not completed. 254 */ 255 RUNNING, 256 /** 257 * The task completed with a result. 258 * @see Future#resultNow() 259 */ 260 SUCCESS, 261 /** 262 * The task completed with an exception. 263 * @see Future#exceptionNow() 264 */ 265 FAILED, 266 /** 267 * The task was cancelled. 268 * @see #cancel(boolean) 269 */ 270 CANCELLED 271 } 272 273 /** 274 * {@return the computation state} 275 * 276 * @implSpec 277 * The default implementation uses {@code isDone()}, {@code isCancelled()}, 278 * and {@code get()} to determine the state. 279 * 280 * @since 19 281 */ state()282 default State state() { 283 if (!isDone()) 284 return State.RUNNING; 285 if (isCancelled()) 286 return State.CANCELLED; 287 boolean interrupted = false; 288 try { 289 while (true) { 290 try { 291 get(); // may throw InterruptedException when done 292 return State.SUCCESS; 293 } catch (InterruptedException e) { 294 interrupted = true; 295 } catch (ExecutionException e) { 296 return State.FAILED; 297 } 298 } 299 } finally { 300 if (interrupted) Thread.currentThread().interrupt(); 301 } 302 } 303 } 304