AsyncTask.java revision b1ec5ef460ac62fa8d3e621794787336e513a8ef
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 * <div class="special reference"> 46 * <h3>Developer Guides</h3> 47 * <p>For more information about using tasks and threads, read the 48 * <a href="{@docRoot}guide/topics/fundamentals/processes-and-threads.html">Processes and 49 * Threads</a> developer guide.</p> 50 * </div> 51 * 52 * <h2>Usage</h2> 53 * <p>AsyncTask must be subclassed to be used. The subclass will override at least 54 * one method ({@link #doInBackground}), and most often will override a 55 * second one ({@link #onPostExecute}.)</p> 56 * 57 * <p>Here is an example of subclassing:</p> 58 * <pre class="prettyprint"> 59 * private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> { 60 * protected Long doInBackground(URL... urls) { 61 * int count = urls.length; 62 * long totalSize = 0; 63 * for (int i = 0; i < count; i++) { 64 * totalSize += Downloader.downloadFile(urls[i]); 65 * publishProgress((int) ((i / (float) count) * 100)); 66 * } 67 * return totalSize; 68 * } 69 * 70 * protected void onProgressUpdate(Integer... progress) { 71 * setProgressPercent(progress[0]); 72 * } 73 * 74 * protected void onPostExecute(Long result) { 75 * showDialog("Downloaded " + result + " bytes"); 76 * } 77 * } 78 * </pre> 79 * 80 * <p>Once created, a task is executed very simply:</p> 81 * <pre class="prettyprint"> 82 * new DownloadFilesTask().execute(url1, url2, url3); 83 * </pre> 84 * 85 * <h2>AsyncTask's generic types</h2> 86 * <p>The three types used by an asynchronous task are the following:</p> 87 * <ol> 88 * <li><code>Params</code>, the type of the parameters sent to the task upon 89 * execution.</li> 90 * <li><code>Progress</code>, the type of the progress units published during 91 * the background computation.</li> 92 * <li><code>Result</code>, the type of the result of the background 93 * computation.</li> 94 * </ol> 95 * <p>Not all types are always used by an asynchronous task. To mark a type as unused, 96 * simply use the type {@link Void}:</p> 97 * <pre> 98 * private class MyTask extends AsyncTask<Void, Void, Void> { ... } 99 * </pre> 100 * 101 * <h2>The 4 steps</h2> 102 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p> 103 * <ol> 104 * <li>{@link #onPreExecute()}, invoked on the UI thread immediately after the task 105 * is executed. This step is normally used to setup the task, for instance by 106 * showing a progress bar in the user interface.</li> 107 * <li>{@link #doInBackground}, invoked on the background thread 108 * immediately after {@link #onPreExecute()} finishes executing. This step is used 109 * to perform background computation that can take a long time. The parameters 110 * of the asynchronous task are passed to this step. The result of the computation must 111 * be returned by this step and will be passed back to the last step. This step 112 * can also use {@link #publishProgress} to publish one or more units 113 * of progress. These values are published on the UI thread, in the 114 * {@link #onProgressUpdate} step.</li> 115 * <li>{@link #onProgressUpdate}, invoked on the UI thread after a 116 * call to {@link #publishProgress}. The timing of the execution is 117 * undefined. This method is used to display any form of progress in the user 118 * interface while the background computation is still executing. For instance, 119 * it can be used to animate a progress bar or show logs in a text field.</li> 120 * <li>{@link #onPostExecute}, invoked on the UI thread after the background 121 * computation finishes. The result of the background computation is passed to 122 * this step as a parameter.</li> 123 * </ol> 124 * 125 * <h2>Cancelling a task</h2> 126 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking 127 * this method will cause subsequent calls to {@link #isCancelled()} to return true. 128 * After invoking this method, {@link #onCancelled(Object)}, instead of 129 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])} 130 * returns. To ensure that a task is cancelled as quickly as possible, you should always 131 * check the return value of {@link #isCancelled()} periodically from 132 * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p> 133 * 134 * <h2>Threading rules</h2> 135 * <p>There are a few threading rules that must be followed for this class to 136 * work properly:</p> 137 * <ul> 138 * <li>The AsyncTask class must be loaded on the UI thread. This is done 139 * automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li> 140 * <li>The task instance must be created on the UI thread.</li> 141 * <li>{@link #execute} must be invoked on the UI thread.</li> 142 * <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute}, 143 * {@link #doInBackground}, {@link #onProgressUpdate} manually.</li> 144 * <li>The task can be executed only once (an exception will be thrown if 145 * a second execution is attempted.)</li> 146 * </ul> 147 * 148 * <h2>Memory observability</h2> 149 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following 150 * operations are safe without explicit synchronizations.</p> 151 * <ul> 152 * <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them 153 * in {@link #doInBackground}. 154 * <li>Set member fields in {@link #doInBackground}, and refer to them in 155 * {@link #onProgressUpdate} and {@link #onPostExecute}. 156 * </ul> 157 */ 158public abstract class AsyncTask<Params, Progress, Result> { 159 private static final String LOG_TAG = "AsyncTask"; 160 161 private static final int CORE_POOL_SIZE = 5; 162 private static final int MAXIMUM_POOL_SIZE = 128; 163 private static final int KEEP_ALIVE = 1; 164 165 private static final ThreadFactory sThreadFactory = new ThreadFactory() { 166 private final AtomicInteger mCount = new AtomicInteger(1); 167 168 public Thread newThread(Runnable r) { 169 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 170 } 171 }; 172 173 private static final BlockingQueue<Runnable> sPoolWorkQueue = 174 new LinkedBlockingQueue<Runnable>(10); 175 176 /** 177 * An {@link Executor} that can be used to execute tasks in parallel. 178 */ 179 public static final Executor THREAD_POOL_EXECUTOR 180 = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, 181 TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); 182 183 /** 184 * An {@link Executor} that executes tasks one at a time in serial 185 * order. This serialization is global to a particular process. 186 */ 187 public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); 188 189 private static final int MESSAGE_POST_RESULT = 0x1; 190 private static final int MESSAGE_POST_PROGRESS = 0x2; 191 192 private static final InternalHandler sHandler = new InternalHandler(); 193 194 private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; 195 private final WorkerRunnable<Params, Result> mWorker; 196 private final FutureTask<Result> mFuture; 197 198 private volatile Status mStatus = Status.PENDING; 199 200 private final AtomicBoolean mCancelled = new AtomicBoolean(); 201 private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); 202 203 private static class SerialExecutor implements Executor { 204 final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); 205 Runnable mActive; 206 207 public synchronized void execute(final Runnable r) { 208 mTasks.offer(new Runnable() { 209 public void run() { 210 try { 211 r.run(); 212 } finally { 213 scheduleNext(); 214 } 215 } 216 }); 217 if (mActive == null) { 218 scheduleNext(); 219 } 220 } 221 222 protected synchronized void scheduleNext() { 223 if ((mActive = mTasks.poll()) != null) { 224 THREAD_POOL_EXECUTOR.execute(mActive); 225 } 226 } 227 } 228 229 /** 230 * Indicates the current status of the task. Each status will be set only once 231 * during the lifetime of a task. 232 */ 233 public enum Status { 234 /** 235 * Indicates that the task has not been executed yet. 236 */ 237 PENDING, 238 /** 239 * Indicates that the task is running. 240 */ 241 RUNNING, 242 /** 243 * Indicates that {@link AsyncTask#onPostExecute} has finished. 244 */ 245 FINISHED, 246 } 247 248 /** @hide Used to force static handler to be created. */ 249 public static void init() { 250 sHandler.getLooper(); 251 } 252 253 /** @hide */ 254 public static void setDefaultExecutor(Executor exec) { 255 sDefaultExecutor = exec; 256 } 257 258 /** 259 * Creates a new asynchronous task. This constructor must be invoked on the UI thread. 260 */ 261 public AsyncTask() { 262 mWorker = new WorkerRunnable<Params, Result>() { 263 public Result call() throws Exception { 264 mTaskInvoked.set(true); 265 266 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 267 //noinspection unchecked 268 return postResult(doInBackground(mParams)); 269 } 270 }; 271 272 mFuture = new FutureTask<Result>(mWorker) { 273 @Override 274 protected void done() { 275 try { 276 postResultIfNotInvoked(get()); 277 } catch (InterruptedException e) { 278 android.util.Log.w(LOG_TAG, e); 279 } catch (ExecutionException e) { 280 throw new RuntimeException("An error occured while executing doInBackground()", 281 e.getCause()); 282 } catch (CancellationException e) { 283 postResultIfNotInvoked(null); 284 } 285 } 286 }; 287 } 288 289 private void postResultIfNotInvoked(Result result) { 290 final boolean wasTaskInvoked = mTaskInvoked.get(); 291 if (!wasTaskInvoked) { 292 postResult(result); 293 } 294 } 295 296 private Result postResult(Result result) { 297 @SuppressWarnings("unchecked") 298 Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 299 new AsyncTaskResult<Result>(this, result)); 300 message.sendToTarget(); 301 return result; 302 } 303 304 /** 305 * Returns the current status of this task. 306 * 307 * @return The current status. 308 */ 309 public final Status getStatus() { 310 return mStatus; 311 } 312 313 /** 314 * Override this method to perform a computation on a background thread. The 315 * specified parameters are the parameters passed to {@link #execute} 316 * by the caller of this task. 317 * 318 * This method can call {@link #publishProgress} to publish updates 319 * on the UI thread. 320 * 321 * @param params The parameters of the task. 322 * 323 * @return A result, defined by the subclass of this task. 324 * 325 * @see #onPreExecute() 326 * @see #onPostExecute 327 * @see #publishProgress 328 */ 329 protected abstract Result doInBackground(Params... params); 330 331 /** 332 * Runs on the UI thread before {@link #doInBackground}. 333 * 334 * @see #onPostExecute 335 * @see #doInBackground 336 */ 337 protected void onPreExecute() { 338 } 339 340 /** 341 * <p>Runs on the UI thread after {@link #doInBackground}. The 342 * specified result is the value returned by {@link #doInBackground}.</p> 343 * 344 * <p>This method won't be invoked if the task was cancelled.</p> 345 * 346 * @param result The result of the operation computed by {@link #doInBackground}. 347 * 348 * @see #onPreExecute 349 * @see #doInBackground 350 * @see #onCancelled(Object) 351 */ 352 @SuppressWarnings({"UnusedDeclaration"}) 353 protected void onPostExecute(Result result) { 354 } 355 356 /** 357 * Runs on the UI thread after {@link #publishProgress} is invoked. 358 * The specified values are the values passed to {@link #publishProgress}. 359 * 360 * @param values The values indicating progress. 361 * 362 * @see #publishProgress 363 * @see #doInBackground 364 */ 365 @SuppressWarnings({"UnusedDeclaration"}) 366 protected void onProgressUpdate(Progress... values) { 367 } 368 369 /** 370 * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and 371 * {@link #doInBackground(Object[])} has finished.</p> 372 * 373 * <p>The default implementation simply invokes {@link #onCancelled()} and 374 * ignores the result. If you write your own implementation, do not call 375 * <code>super.onCancelled(result)</code>.</p> 376 * 377 * @param result The result, if any, computed in 378 * {@link #doInBackground(Object[])}, can be null 379 * 380 * @see #cancel(boolean) 381 * @see #isCancelled() 382 */ 383 @SuppressWarnings({"UnusedParameters"}) 384 protected void onCancelled(Result result) { 385 onCancelled(); 386 } 387 388 /** 389 * <p>Applications should preferably override {@link #onCancelled(Object)}. 390 * This method is invoked by the default implementation of 391 * {@link #onCancelled(Object)}.</p> 392 * 393 * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and 394 * {@link #doInBackground(Object[])} has finished.</p> 395 * 396 * @see #onCancelled(Object) 397 * @see #cancel(boolean) 398 * @see #isCancelled() 399 */ 400 protected void onCancelled() { 401 } 402 403 /** 404 * Returns <tt>true</tt> if this task was cancelled before it completed 405 * normally. If you are calling {@link #cancel(boolean)} on the task, 406 * the value returned by this method should be checked periodically from 407 * {@link #doInBackground(Object[])} to end the task as soon as possible. 408 * 409 * @return <tt>true</tt> if task was cancelled before it completed 410 * 411 * @see #cancel(boolean) 412 */ 413 public final boolean isCancelled() { 414 return mCancelled.get(); 415 } 416 417 /** 418 * <p>Attempts to cancel execution of this task. This attempt will 419 * fail if the task has already completed, already been cancelled, 420 * or could not be cancelled for some other reason. If successful, 421 * and this task has not started when <tt>cancel</tt> is called, 422 * this task should never run. If the task has already started, 423 * then the <tt>mayInterruptIfRunning</tt> parameter determines 424 * whether the thread executing this task should be interrupted in 425 * an attempt to stop the task.</p> 426 * 427 * <p>Calling this method will result in {@link #onCancelled(Object)} being 428 * invoked on the UI thread after {@link #doInBackground(Object[])} 429 * returns. Calling this method guarantees that {@link #onPostExecute(Object)} 430 * is never invoked. After invoking this method, you should check the 431 * value returned by {@link #isCancelled()} periodically from 432 * {@link #doInBackground(Object[])} to finish the task as early as 433 * possible.</p> 434 * 435 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 436 * task should be interrupted; otherwise, in-progress tasks are allowed 437 * to complete. 438 * 439 * @return <tt>false</tt> if the task could not be cancelled, 440 * typically because it has already completed normally; 441 * <tt>true</tt> otherwise 442 * 443 * @see #isCancelled() 444 * @see #onCancelled(Object) 445 */ 446 public final boolean cancel(boolean mayInterruptIfRunning) { 447 mCancelled.set(true); 448 return mFuture.cancel(mayInterruptIfRunning); 449 } 450 451 /** 452 * Waits if necessary for the computation to complete, and then 453 * retrieves its result. 454 * 455 * @return The computed result. 456 * 457 * @throws CancellationException If the computation was cancelled. 458 * @throws ExecutionException If the computation threw an exception. 459 * @throws InterruptedException If the current thread was interrupted 460 * while waiting. 461 */ 462 public final Result get() throws InterruptedException, ExecutionException { 463 return mFuture.get(); 464 } 465 466 /** 467 * Waits if necessary for at most the given time for the computation 468 * to complete, and then retrieves its result. 469 * 470 * @param timeout Time to wait before cancelling the operation. 471 * @param unit The time unit for the timeout. 472 * 473 * @return The computed result. 474 * 475 * @throws CancellationException If the computation was cancelled. 476 * @throws ExecutionException If the computation threw an exception. 477 * @throws InterruptedException If the current thread was interrupted 478 * while waiting. 479 * @throws TimeoutException If the wait timed out. 480 */ 481 public final Result get(long timeout, TimeUnit unit) throws InterruptedException, 482 ExecutionException, TimeoutException { 483 return mFuture.get(timeout, unit); 484 } 485 486 /** 487 * Executes the task with the specified parameters. The task returns 488 * itself (this) so that the caller can keep a reference to it. 489 * 490 * <p>Note: this function schedules the task on a queue for a single background 491 * thread or pool of threads depending on the platform version. When first 492 * introduced, AsyncTasks were executed serially on a single background thread. 493 * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed 494 * to a pool of threads allowing multiple tasks to operate in parallel. After 495 * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, it is planned to change this 496 * back to a single thread to avoid common application errors caused 497 * by parallel execution. If you truly want parallel execution, you can use 498 * the {@link #executeOnExecutor} version of this method 499 * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings on 500 * its use. 501 * 502 * <p>This method must be invoked on the UI thread. 503 * 504 * @param params The parameters of the task. 505 * 506 * @return This instance of AsyncTask. 507 * 508 * @throws IllegalStateException If {@link #getStatus()} returns either 509 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 510 */ 511 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 512 return executeOnExecutor(sDefaultExecutor, params); 513 } 514 515 /** 516 * Executes the task with the specified parameters. The task returns 517 * itself (this) so that the caller can keep a reference to it. 518 * 519 * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to 520 * allow multiple tasks to run in parallel on a pool of threads managed by 521 * AsyncTask, however you can also use your own {@link Executor} for custom 522 * behavior. 523 * 524 * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from 525 * a thread pool is generally <em>not</em> what one wants, because the order 526 * of their operation is not defined. For example, if these tasks are used 527 * to modify any state in common (such as writing a file due to a button click), 528 * there are no guarantees on the order of the modifications. 529 * Without careful work it is possible in rare cases for the newer version 530 * of the data to be over-written by an older one, leading to obscure data 531 * loss and stability issues. Such changes are best 532 * executed in serial; to guarantee such work is serialized regardless of 533 * platform version you can use this function with {@link #SERIAL_EXECUTOR}. 534 * 535 * <p>This method must be invoked on the UI thread. 536 * 537 * @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a 538 * convenient process-wide thread pool for tasks that are loosely coupled. 539 * @param params The parameters of the task. 540 * 541 * @return This instance of AsyncTask. 542 * 543 * @throws IllegalStateException If {@link #getStatus()} returns either 544 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 545 */ 546 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, 547 Params... params) { 548 if (mStatus != Status.PENDING) { 549 switch (mStatus) { 550 case RUNNING: 551 throw new IllegalStateException("Cannot execute task:" 552 + " the task is already running."); 553 case FINISHED: 554 throw new IllegalStateException("Cannot execute task:" 555 + " the task has already been executed " 556 + "(a task can be executed only once)"); 557 } 558 } 559 560 mStatus = Status.RUNNING; 561 562 onPreExecute(); 563 564 mWorker.mParams = params; 565 exec.execute(mFuture); 566 567 return this; 568 } 569 570 /** 571 * Convenience version of {@link #execute(Object...)} for use with 572 * a simple Runnable object. 573 */ 574 public static void execute(Runnable runnable) { 575 sDefaultExecutor.execute(runnable); 576 } 577 578 /** 579 * This method can be invoked from {@link #doInBackground} to 580 * publish updates on the UI thread while the background computation is 581 * still running. Each call to this method will trigger the execution of 582 * {@link #onProgressUpdate} on the UI thread. 583 * 584 * {@link #onProgressUpdate} will note be called if the task has been 585 * canceled. 586 * 587 * @param values The progress values to update the UI with. 588 * 589 * @see #onProgressUpdate 590 * @see #doInBackground 591 */ 592 protected final void publishProgress(Progress... values) { 593 if (!isCancelled()) { 594 sHandler.obtainMessage(MESSAGE_POST_PROGRESS, 595 new AsyncTaskResult<Progress>(this, values)).sendToTarget(); 596 } 597 } 598 599 private void finish(Result result) { 600 if (isCancelled()) { 601 onCancelled(result); 602 } else { 603 onPostExecute(result); 604 } 605 mStatus = Status.FINISHED; 606 } 607 608 private static class InternalHandler extends Handler { 609 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 610 @Override 611 public void handleMessage(Message msg) { 612 AsyncTaskResult result = (AsyncTaskResult) msg.obj; 613 switch (msg.what) { 614 case MESSAGE_POST_RESULT: 615 // There is only one result 616 result.mTask.finish(result.mData[0]); 617 break; 618 case MESSAGE_POST_PROGRESS: 619 result.mTask.onProgressUpdate(result.mData); 620 break; 621 } 622 } 623 } 624 625 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 626 Params[] mParams; 627 } 628 629 @SuppressWarnings({"RawUseOfParameterizedType"}) 630 private static class AsyncTaskResult<Data> { 631 final AsyncTask mTask; 632 final Data[] mData; 633 634 AsyncTaskResult(AsyncTask task, Data... data) { 635 mTask = task; 636 mData = data; 637 } 638 } 639} 640