AsyncTask.java revision 96438cd658f91fed9d8fc651c4eb1e55dc6dbf80
1/* 2 * Copyright (C) 2008 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17package android.os; 18 19import java.util.ArrayDeque; 20import java.util.concurrent.BlockingQueue; 21import java.util.concurrent.Callable; 22import java.util.concurrent.CancellationException; 23import java.util.concurrent.Executor; 24import java.util.concurrent.ExecutionException; 25import java.util.concurrent.FutureTask; 26import java.util.concurrent.LinkedBlockingQueue; 27import java.util.concurrent.ThreadFactory; 28import java.util.concurrent.ThreadPoolExecutor; 29import java.util.concurrent.TimeUnit; 30import java.util.concurrent.TimeoutException; 31import java.util.concurrent.atomic.AtomicBoolean; 32import java.util.concurrent.atomic.AtomicInteger; 33 34/** 35 * <p>AsyncTask enables proper and easy use of the UI thread. This class allows to 36 * perform background operations and publish results on the UI thread without 37 * having to manipulate threads and/or handlers.</p> 38 * 39 * <p>An asynchronous task is defined by a computation that runs on a background thread and 40 * whose result is published on the UI thread. An asynchronous task is defined by 3 generic 41 * types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>, 42 * and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>, 43 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p> 44 * 45 * <h2>Usage</h2> 46 * <p>AsyncTask must be subclassed to be used. The subclass will override at least 47 * one method ({@link #doInBackground}), and most often will override a 48 * second one ({@link #onPostExecute}.)</p> 49 * 50 * <p>Here is an example of subclassing:</p> 51 * <pre class="prettyprint"> 52 * private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> { 53 * protected Long doInBackground(URL... urls) { 54 * int count = urls.length; 55 * long totalSize = 0; 56 * for (int i = 0; i < count; i++) { 57 * totalSize += Downloader.downloadFile(urls[i]); 58 * publishProgress((int) ((i / (float) count) * 100)); 59 * } 60 * return totalSize; 61 * } 62 * 63 * protected void onProgressUpdate(Integer... progress) { 64 * setProgressPercent(progress[0]); 65 * } 66 * 67 * protected void onPostExecute(Long result) { 68 * showDialog("Downloaded " + result + " bytes"); 69 * } 70 * } 71 * </pre> 72 * 73 * <p>Once created, a task is executed very simply:</p> 74 * <pre class="prettyprint"> 75 * new DownloadFilesTask().execute(url1, url2, url3); 76 * </pre> 77 * 78 * <h2>AsyncTask's generic types</h2> 79 * <p>The three types used by an asynchronous task are the following:</p> 80 * <ol> 81 * <li><code>Params</code>, the type of the parameters sent to the task upon 82 * execution.</li> 83 * <li><code>Progress</code>, the type of the progress units published during 84 * the background computation.</li> 85 * <li><code>Result</code>, the type of the result of the background 86 * computation.</li> 87 * </ol> 88 * <p>Not all types are always used by an asynchronous task. To mark a type as unused, 89 * simply use the type {@link Void}:</p> 90 * <pre> 91 * private class MyTask extends AsyncTask<Void, Void, Void> { ... } 92 * </pre> 93 * 94 * <h2>The 4 steps</h2> 95 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p> 96 * <ol> 97 * <li>{@link #onPreExecute()}, invoked on the UI thread immediately after the task 98 * is executed. This step is normally used to setup the task, for instance by 99 * showing a progress bar in the user interface.</li> 100 * <li>{@link #doInBackground}, invoked on the background thread 101 * immediately after {@link #onPreExecute()} finishes executing. This step is used 102 * to perform background computation that can take a long time. The parameters 103 * of the asynchronous task are passed to this step. The result of the computation must 104 * be returned by this step and will be passed back to the last step. This step 105 * can also use {@link #publishProgress} to publish one or more units 106 * of progress. These values are published on the UI thread, in the 107 * {@link #onProgressUpdate} step.</li> 108 * <li>{@link #onProgressUpdate}, invoked on the UI thread after a 109 * call to {@link #publishProgress}. The timing of the execution is 110 * undefined. This method is used to display any form of progress in the user 111 * interface while the background computation is still executing. For instance, 112 * it can be used to animate a progress bar or show logs in a text field.</li> 113 * <li>{@link #onPostExecute}, invoked on the UI thread after the background 114 * computation finishes. The result of the background computation is passed to 115 * this step as a parameter.</li> 116 * </ol> 117 * 118 * <h2>Cancelling a task</h2> 119 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking 120 * this method will cause subsequent calls to {@link #isCancelled()} to return true. 121 * After invoking this method, {@link #onCancelled(Object)}, instead of 122 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} 123 * returns. To ensure that a task is cancelled as quickly as possible, you should always 124 * check the return value of {@link #isCancelled()} periodically from 125 * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p> 126 * 127 * <h2>Threading rules</h2> 128 * <p>There are a few threading rules that must be followed for this class to 129 * work properly:</p> 130 * <ul> 131 * <li>The task instance must be created on the UI thread.</li> 132 * <li>{@link #execute} must be invoked on the UI thread.</li> 133 * <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute}, 134 * {@link #doInBackground}, {@link #onProgressUpdate} manually.</li> 135 * <li>The task can be executed only once (an exception will be thrown if 136 * a second execution is attempted.)</li> 137 * </ul> 138 * 139 * <h2>Memory observability</h2> 140 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following 141 * operations are safe without explicit synchronizations.</p> 142 * <ul> 143 * <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them 144 * in {@link #doInBackground}. 145 * <li>Set member fields in {@link #doInBackground}, and refer to them in 146 * {@link #onProgressUpdate} and {@link #onPostExecute}. 147 * </ul> 148 */ 149public abstract class AsyncTask<Params, Progress, Result> { 150 private static final String LOG_TAG = "AsyncTask"; 151 152 private static final int CORE_POOL_SIZE = 5; 153 private static final int MAXIMUM_POOL_SIZE = 128; 154 private static final int KEEP_ALIVE = 1; 155 156 157 private static final ThreadFactory sThreadFactory = new ThreadFactory() { 158 private final AtomicInteger mCount = new AtomicInteger(1); 159 160 public Thread newThread(Runnable r) { 161 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 162 } 163 }; 164 165 private static final BlockingQueue<Runnable> sPoolWorkQueue = 166 new LinkedBlockingQueue<Runnable>(10); 167 168 /** 169 * An {@link Executor} that can be used to execute tasks in parallel. 170 */ 171 public static final Executor THREAD_POOL_EXECUTOR 172 = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, 173 TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); 174 175 /** 176 * An {@link Executor} that executes tasks one at a time in serial 177 * order. This serialization is global to a particular process. 178 */ 179 public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); 180 181 private static final int MESSAGE_POST_RESULT = 0x1; 182 private static final int MESSAGE_POST_PROGRESS = 0x2; 183 184 private static final InternalHandler sHandler = new InternalHandler(); 185 186 private final WorkerRunnable<Params, Result> mWorker; 187 private final FutureTask<Result> mFuture; 188 189 private volatile Status mStatus = Status.PENDING; 190 191 private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); 192 193 private static class SerialExecutor implements Executor { 194 final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); 195 Runnable mActive; 196 197 public synchronized void execute(final Runnable r) { 198 mTasks.offer(new Runnable() { 199 public void run() { 200 try { 201 r.run(); 202 } finally { 203 scheduleNext(); 204 } 205 } 206 }); 207 if (mActive == null) { 208 scheduleNext(); 209 } 210 } 211 212 protected synchronized void scheduleNext() { 213 if ((mActive = mTasks.poll()) != null) { 214 THREAD_POOL_EXECUTOR.execute(mActive); 215 } 216 } 217 } 218 219 /** 220 * Indicates the current status of the task. Each status will be set only once 221 * during the lifetime of a task. 222 */ 223 public enum Status { 224 /** 225 * Indicates that the task has not been executed yet. 226 */ 227 PENDING, 228 /** 229 * Indicates that the task is running. 230 */ 231 RUNNING, 232 /** 233 * Indicates that {@link AsyncTask#onPostExecute} has finished. 234 */ 235 FINISHED, 236 } 237 238 /** @hide Used to force static handler to be created. */ 239 public static void init() { 240 sHandler.getLooper(); 241 } 242 243 /** 244 * Creates a new asynchronous task. This constructor must be invoked on the UI thread. 245 */ 246 public AsyncTask() { 247 mWorker = new WorkerRunnable<Params, Result>() { 248 public Result call() throws Exception { 249 mTaskInvoked.set(true); 250 251 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 252 return postResult(doInBackground(mParams)); 253 } 254 }; 255 256 mFuture = new FutureTask<Result>(mWorker) { 257 @Override 258 protected void done() { 259 try { 260 final Result result = get(); 261 262 postResultIfNotInvoked(result); 263 } catch (InterruptedException e) { 264 android.util.Log.w(LOG_TAG, e); 265 } catch (ExecutionException e) { 266 throw new RuntimeException("An error occured while executing doInBackground()", 267 e.getCause()); 268 } catch (CancellationException e) { 269 postResultIfNotInvoked(null); 270 } catch (Throwable t) { 271 throw new RuntimeException("An error occured while executing " 272 + "doInBackground()", t); 273 } 274 } 275 }; 276 } 277 278 private void postResultIfNotInvoked(Result result) { 279 final boolean wasTaskInvoked = mTaskInvoked.get(); 280 if (!wasTaskInvoked) { 281 postResult(result); 282 } 283 } 284 285 private Result postResult(Result result) { 286 Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 287 new AsyncTaskResult<Result>(this, result)); 288 message.sendToTarget(); 289 return result; 290 } 291 292 /** 293 * Returns the current status of this task. 294 * 295 * @return The current status. 296 */ 297 public final Status getStatus() { 298 return mStatus; 299 } 300 301 /** 302 * Override this method to perform a computation on a background thread. The 303 * specified parameters are the parameters passed to {@link #execute} 304 * by the caller of this task. 305 * 306 * This method can call {@link #publishProgress} to publish updates 307 * on the UI thread. 308 * 309 * @param params The parameters of the task. 310 * 311 * @return A result, defined by the subclass of this task. 312 * 313 * @see #onPreExecute() 314 * @see #onPostExecute 315 * @see #publishProgress 316 */ 317 protected abstract Result doInBackground(Params... params); 318 319 /** 320 * Runs on the UI thread before {@link #doInBackground}. 321 * 322 * @see #onPostExecute 323 * @see #doInBackground 324 */ 325 protected void onPreExecute() { 326 } 327 328 /** 329 * <p>Runs on the UI thread after {@link #doInBackground}. The 330 * specified result is the value returned by {@link #doInBackground}.</p> 331 * 332 * <p>This method won't be invoked if the task was cancelled.</p> 333 * 334 * @param result The result of the operation computed by {@link #doInBackground}. 335 * 336 * @see #onPreExecute 337 * @see #doInBackground 338 * @see #onCancelled(Object) 339 */ 340 @SuppressWarnings({"UnusedDeclaration"}) 341 protected void onPostExecute(Result result) { 342 } 343 344 /** 345 * Runs on the UI thread after {@link #publishProgress} is invoked. 346 * The specified values are the values passed to {@link #publishProgress}. 347 * 348 * @param values The values indicating progress. 349 * 350 * @see #publishProgress 351 * @see #doInBackground 352 */ 353 @SuppressWarnings({"UnusedDeclaration"}) 354 protected void onProgressUpdate(Progress... values) { 355 } 356 357 /** 358 * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and 359 * {@link #doInBackground(Object[])} has finished.</p> 360 * 361 * <p>The default implementation simply invokes {@link #onCancelled()} and 362 * ignores the result. If you write your own implementation, do not call 363 * <code>super.onCancelled(result)</code>.</p> 364 * 365 * @param result The result, if any, computed in 366 * {@link #doInBackground(Object[])}, can be null 367 * 368 * @see #cancel(boolean) 369 * @see #isCancelled() 370 */ 371 @SuppressWarnings({"UnusedParameters"}) 372 protected void onCancelled(Result result) { 373 onCancelled(); 374 } 375 376 /** 377 * <p>Applications should preferably override {@link #onCancelled(Object)}. 378 * This method is invoked by the default implementation of 379 * {@link #onCancelled(Object)}.</p> 380 * 381 * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and 382 * {@link #doInBackground(Object[])} has finished.</p> 383 * 384 * @see #onCancelled(Object) 385 * @see #cancel(boolean) 386 * @see #isCancelled() 387 */ 388 protected void onCancelled() { 389 } 390 391 /** 392 * Returns <tt>true</tt> if this task was cancelled before it completed 393 * normally. If you are calling {@link #cancel(boolean)} on the task, 394 * the value returned by this method should be checked periodically from 395 * {@link #doInBackground(Object[])} to end the task as soon as possible. 396 * 397 * @return <tt>true</tt> if task was cancelled before it completed 398 * 399 * @see #cancel(boolean) 400 */ 401 public final boolean isCancelled() { 402 return mFuture.isCancelled(); 403 } 404 405 /** 406 * <p>Attempts to cancel execution of this task. This attempt will 407 * fail if the task has already completed, already been cancelled, 408 * or could not be cancelled for some other reason. If successful, 409 * and this task has not started when <tt>cancel</tt> is called, 410 * this task should never run. If the task has already started, 411 * then the <tt>mayInterruptIfRunning</tt> parameter determines 412 * whether the thread executing this task should be interrupted in 413 * an attempt to stop the task.</p> 414 * 415 * <p>Calling this method will result in {@link #onCancelled(Object)} being 416 * invoked on the UI thread after {@link #doInBackground(Object[])} 417 * returns. Calling this method guarantees that {@link #onPostExecute(Object)} 418 * is never invoked. After invoking this method, you should check the 419 * value returned by {@link #isCancelled()} periodically from 420 * {@link #doInBackground(Object[])} to finish the task as early as 421 * possible.</p> 422 * 423 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 424 * task should be interrupted; otherwise, in-progress tasks are allowed 425 * to complete. 426 * 427 * @return <tt>false</tt> if the task could not be cancelled, 428 * typically because it has already completed normally; 429 * <tt>true</tt> otherwise 430 * 431 * @see #isCancelled() 432 * @see #onCancelled(Object) 433 */ 434 public final boolean cancel(boolean mayInterruptIfRunning) { 435 return mFuture.cancel(mayInterruptIfRunning); 436 } 437 438 /** 439 * Waits if necessary for the computation to complete, and then 440 * retrieves its result. 441 * 442 * @return The computed result. 443 * 444 * @throws CancellationException If the computation was cancelled. 445 * @throws ExecutionException If the computation threw an exception. 446 * @throws InterruptedException If the current thread was interrupted 447 * while waiting. 448 */ 449 public final Result get() throws InterruptedException, ExecutionException { 450 return mFuture.get(); 451 } 452 453 /** 454 * Waits if necessary for at most the given time for the computation 455 * to complete, and then retrieves its result. 456 * 457 * @param timeout Time to wait before cancelling the operation. 458 * @param unit The time unit for the timeout. 459 * 460 * @return The computed result. 461 * 462 * @throws CancellationException If the computation was cancelled. 463 * @throws ExecutionException If the computation threw an exception. 464 * @throws InterruptedException If the current thread was interrupted 465 * while waiting. 466 * @throws TimeoutException If the wait timed out. 467 */ 468 public final Result get(long timeout, TimeUnit unit) throws InterruptedException, 469 ExecutionException, TimeoutException { 470 return mFuture.get(timeout, unit); 471 } 472 473 /** 474 * Executes the task with the specified parameters. The task returns 475 * itself (this) so that the caller can keep a reference to it. 476 * 477 * <p>Note: this function schedules the task on a queue for a single background 478 * thread or pool of threads depending on the platform version. When first 479 * introduced, AsyncTasks were executed serially on a single background thread. 480 * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed 481 * to a pool of threads allowing multiple tasks to operate in parallel. After 482 * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, it is planned to change this 483 * back to a single thread to avoid common application errors caused 484 * by parallel execution. If you truly want parallel execution, you can use 485 * the {@link #executeOnExecutor} version of this method 486 * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings on 487 * its use. 488 * 489 * <p>This method must be invoked on the UI thread. 490 * 491 * @param params The parameters of the task. 492 * 493 * @return This instance of AsyncTask. 494 * 495 * @throws IllegalStateException If {@link #getStatus()} returns either 496 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 497 */ 498 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 499 return executeOnExecutor(THREAD_POOL_EXECUTOR, params); 500 } 501 502 /** 503 * Executes the task with the specified parameters. The task returns 504 * itself (this) so that the caller can keep a reference to it. 505 * 506 * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to 507 * allow multiple tasks to run in parallel on a pool of threads managed by 508 * AsyncTask, however you can also use your own {@link Executor} for custom 509 * behavior. 510 * 511 * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from 512 * a thread pool is generally <em>not</em> what one wants, because the order 513 * of their operation is not defined. For example, if these tasks are used 514 * to modify any state in common (such as writing a file due to a button click), 515 * there are no guarantees on the order of the modifications. 516 * Without careful work it is possible in rare cases for the newer version 517 * of the data to be over-written by an older one, leading to obscure data 518 * loss and stability issues. Such changes are best 519 * executed in serial; to guarantee such work is serialized regardless of 520 * platform version you can use this function with {@link #SERIAL_EXECUTOR}. 521 * 522 * <p>This method must be invoked on the UI thread. 523 * 524 * @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a 525 * convenient process-wide thread pool for tasks that are loosely coupled. 526 * @param params The parameters of the task. 527 * 528 * @return This instance of AsyncTask. 529 * 530 * @throws IllegalStateException If {@link #getStatus()} returns either 531 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 532 */ 533 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, 534 Params... params) { 535 if (mStatus != Status.PENDING) { 536 switch (mStatus) { 537 case RUNNING: 538 throw new IllegalStateException("Cannot execute task:" 539 + " the task is already running."); 540 case FINISHED: 541 throw new IllegalStateException("Cannot execute task:" 542 + " the task has already been executed " 543 + "(a task can be executed only once)"); 544 } 545 } 546 547 mStatus = Status.RUNNING; 548 549 onPreExecute(); 550 551 mWorker.mParams = params; 552 exec.execute(mFuture); 553 554 return this; 555 } 556 557 /** 558 * Convenience version of {@link #execute(Object...)} for use with 559 * a simple Runnable object. 560 */ 561 public static void execute(Runnable runnable) { 562 THREAD_POOL_EXECUTOR.execute(runnable); 563 } 564 565 /** 566 * This method can be invoked from {@link #doInBackground} to 567 * publish updates on the UI thread while the background computation is 568 * still running. Each call to this method will trigger the execution of 569 * {@link #onProgressUpdate} on the UI thread. 570 * 571 * {@link #onProgressUpdate} will note be called if the task has been 572 * canceled. 573 * 574 * @param values The progress values to update the UI with. 575 * 576 * @see #onProgressUpdate 577 * @see #doInBackground 578 */ 579 protected final void publishProgress(Progress... values) { 580 if (!isCancelled()) { 581 sHandler.obtainMessage(MESSAGE_POST_PROGRESS, 582 new AsyncTaskResult<Progress>(this, values)).sendToTarget(); 583 } 584 } 585 586 private void finish(Result result) { 587 if (isCancelled()) { 588 onCancelled(result); 589 } else { 590 onPostExecute(result); 591 } 592 mStatus = Status.FINISHED; 593 } 594 595 private static class InternalHandler extends Handler { 596 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 597 @Override 598 public void handleMessage(Message msg) { 599 AsyncTaskResult result = (AsyncTaskResult) msg.obj; 600 switch (msg.what) { 601 case MESSAGE_POST_RESULT: 602 // There is only one result 603 result.mTask.finish(result.mData[0]); 604 break; 605 case MESSAGE_POST_PROGRESS: 606 result.mTask.onProgressUpdate(result.mData); 607 break; 608 } 609 } 610 } 611 612 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 613 Params[] mParams; 614 } 615 616 @SuppressWarnings({"RawUseOfParameterizedType"}) 617 private static class AsyncTaskResult<Data> { 618 final AsyncTask mTask; 619 final Data[] mData; 620 621 AsyncTaskResult(AsyncTask task, Data... data) { 622 mTask = task; 623 mData = data; 624 } 625 } 626} 627