AsyncTask.java revision e95003e4a15eea2d5f93950fc46f99ff2b9c973a
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.concurrent.BlockingQueue; 20import java.util.concurrent.Callable; 21import java.util.concurrent.CancellationException; 22import java.util.concurrent.ExecutionException; 23import java.util.concurrent.FutureTask; 24import java.util.concurrent.LinkedBlockingQueue; 25import java.util.concurrent.ThreadFactory; 26import java.util.concurrent.ThreadPoolExecutor; 27import java.util.concurrent.TimeUnit; 28import java.util.concurrent.TimeoutException; 29import java.util.concurrent.atomic.AtomicInteger; 30 31/** 32 * <p>AsyncTask enables proper and easy use of the UI thread. This class allows to 33 * perform background operations and publish results on the UI thread without 34 * having to manipulate threads and/or handlers.</p> 35 * 36 * <p>An asynchronous task is defined by a computation that runs on a background thread and 37 * whose result is published on the UI thread. An asynchronous task is defined by 3 generic 38 * types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>, 39 * and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>, 40 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p> 41 * 42 * <h2>Usage</h2> 43 * <p>AsyncTask must be subclassed to be used. The subclass will override at least 44 * one method ({@link #doInBackground}), and most often will override a 45 * second one ({@link #onPostExecute}.)</p> 46 * 47 * <p>Here is an example of subclassing:</p> 48 * <pre class="prettyprint"> 49 * private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> { 50 * protected Long doInBackground(URL... urls) { 51 * int count = urls.length; 52 * long totalSize = 0; 53 * for (int i = 0; i < count; i++) { 54 * totalSize += Downloader.downloadFile(urls[i]); 55 * publishProgress((int) ((i / (float) count) * 100)); 56 * } 57 * return totalSize; 58 * } 59 * 60 * protected void onProgressUpdate(Integer... progress) { 61 * setProgressPercent(progress[0]); 62 * } 63 * 64 * protected void onPostExecute(Long result) { 65 * showDialog("Downloaded " + result + " bytes"); 66 * } 67 * } 68 * </pre> 69 * 70 * <p>Once created, a task is executed very simply:</p> 71 * <pre class="prettyprint"> 72 * new DownloadFilesTask().execute(url1, url2, url3); 73 * </pre> 74 * 75 * <h2>AsyncTask's generic types</h2> 76 * <p>The three types used by an asynchronous task are the following:</p> 77 * <ol> 78 * <li><code>Params</code>, the type of the parameters sent to the task upon 79 * execution.</li> 80 * <li><code>Progress</code>, the type of the progress units published during 81 * the background computation.</li> 82 * <li><code>Result</code>, the type of the result of the background 83 * computation.</li> 84 * </ol> 85 * <p>Not all types are always used by an asynchronous task. To mark a type as unused, 86 * simply use the type {@link Void}:</p> 87 * <pre> 88 * private class MyTask extends AsyncTask<Void, Void, Void> { ... } 89 * </pre> 90 * 91 * <h2>The 4 steps</h2> 92 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p> 93 * <ol> 94 * <li>{@link #onPreExecute()}, invoked on the UI thread immediately after the task 95 * is executed. This step is normally used to setup the task, for instance by 96 * showing a progress bar in the user interface.</li> 97 * <li>{@link #doInBackground}, invoked on the background thread 98 * immediately after {@link #onPreExecute()} finishes executing. This step is used 99 * to perform background computation that can take a long time. The parameters 100 * of the asynchronous task are passed to this step. The result of the computation must 101 * be returned by this step and will be passed back to the last step. This step 102 * can also use {@link #publishProgress} to publish one or more units 103 * of progress. These values are published on the UI thread, in the 104 * {@link #onProgressUpdate} step.</li> 105 * <li>{@link #onProgressUpdate}, invoked on the UI thread after a 106 * call to {@link #publishProgress}. The timing of the execution is 107 * undefined. This method is used to display any form of progress in the user 108 * interface while the background computation is still executing. For instance, 109 * it can be used to animate a progress bar or show logs in a text field.</li> 110 * <li>{@link #onPostExecute}, invoked on the UI thread after the background 111 * computation finishes. The result of the background computation is passed to 112 * this step as a parameter.</li> 113 * </ol> 114 * 115 * <h2>Cancelling a task</h2> 116 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking 117 * this method will cause subsequent calls to {@link #isCancelled()} to return true. 118 * After invoking this method, {@link #onCancelled()}, instead of {@link #onPostExecute(Object)} 119 * will be invoked after {@link #doInBackground(Object[])} returns. To ensure that 120 * a task is cancelled as quickly as possible, you should always check the return 121 * value of {@link #isCancelled()} periodically from {@link #doInBackground(Object[])}, 122 * if possible (inside a loop for instance.)</p> 123 * 124 * <h2>Threading rules</h2> 125 * <p>There are a few threading rules that must be followed for this class to 126 * work properly:</p> 127 * <ul> 128 * <li>The task instance must be created on the UI thread.</li> 129 * <li>{@link #execute} must be invoked on the UI thread.</li> 130 * <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute}, 131 * {@link #doInBackground}, {@link #onProgressUpdate} manually.</li> 132 * <li>The task can be executed only once (an exception will be thrown if 133 * a second execution is attempted.)</li> 134 * </ul> 135 * 136 * <h2>Memory observability</h2> 137 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following 138 * operations are safe without explicit synchronizations.</p> 139 * <ul> 140 * <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them 141 * in {@link #doInBackground}. 142 * <li>Set member fields in {@link #doInBackground}, and refer to them in 143 * {@link #onProgressUpdate} and {@link #onPostExecute}. 144 * </ul> 145 */ 146public abstract class AsyncTask<Params, Progress, Result> { 147 private static final String LOG_TAG = "AsyncTask"; 148 149 private static final int CORE_POOL_SIZE = 5; 150 private static final int MAXIMUM_POOL_SIZE = 128; 151 private static final int KEEP_ALIVE = 1; 152 153 private static final BlockingQueue<Runnable> sWorkQueue = 154 new LinkedBlockingQueue<Runnable>(10); 155 156 private static final ThreadFactory sThreadFactory = new ThreadFactory() { 157 private final AtomicInteger mCount = new AtomicInteger(1); 158 159 public Thread newThread(Runnable r) { 160 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 161 } 162 }; 163 164 private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, 165 MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); 166 167 private static final int MESSAGE_POST_RESULT = 0x1; 168 private static final int MESSAGE_POST_PROGRESS = 0x2; 169 170 private static final InternalHandler sHandler = new InternalHandler(); 171 172 private final WorkerRunnable<Params, Result> mWorker; 173 private final FutureTask<Result> mFuture; 174 175 private volatile Status mStatus = Status.PENDING; 176 177 /** 178 * Indicates the current status of the task. Each status will be set only once 179 * during the lifetime of a task. 180 */ 181 public enum Status { 182 /** 183 * Indicates that the task has not been executed yet. 184 */ 185 PENDING, 186 /** 187 * Indicates that the task is running. 188 */ 189 RUNNING, 190 /** 191 * Indicates that {@link AsyncTask#onPostExecute} has finished. 192 */ 193 FINISHED, 194 } 195 196 /** @hide Used to force static handler to be created. */ 197 public static void init() { 198 sHandler.getLooper(); 199 } 200 201 /** 202 * Creates a new asynchronous task. This constructor must be invoked on the UI thread. 203 */ 204 public AsyncTask() { 205 mWorker = new WorkerRunnable<Params, Result>() { 206 public Result call() throws Exception { 207 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 208 209 Result result = doInBackground(mParams); 210 211 Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 212 new AsyncTaskResult<Result>(AsyncTask.this, result)); 213 message.sendToTarget(); 214 215 return result; 216 } 217 }; 218 219 mFuture = new FutureTask<Result>(mWorker) { 220 @Override 221 protected void done() { 222 try { 223 get(); 224 } catch (InterruptedException e) { 225 android.util.Log.w(LOG_TAG, e); 226 } catch (ExecutionException e) { 227 throw new RuntimeException("An error occured while executing doInBackground()", 228 e.getCause()); 229 } catch (CancellationException e) { 230 // Taken care of in the WorkerRunnable 231 } catch (Throwable t) { 232 throw new RuntimeException("An error occured while executing " 233 + "doInBackground()", t); 234 } 235 } 236 }; 237 } 238 239 /** 240 * Returns the current status of this task. 241 * 242 * @return The current status. 243 */ 244 public final Status getStatus() { 245 return mStatus; 246 } 247 248 /** 249 * Override this method to perform a computation on a background thread. The 250 * specified parameters are the parameters passed to {@link #execute} 251 * by the caller of this task. 252 * 253 * This method can call {@link #publishProgress} to publish updates 254 * on the UI thread. 255 * 256 * @param params The parameters of the task. 257 * 258 * @return A result, defined by the subclass of this task. 259 * 260 * @see #onPreExecute() 261 * @see #onPostExecute 262 * @see #publishProgress 263 */ 264 protected abstract Result doInBackground(Params... params); 265 266 /** 267 * Runs on the UI thread before {@link #doInBackground}. 268 * 269 * @see #onPostExecute 270 * @see #doInBackground 271 */ 272 protected void onPreExecute() { 273 } 274 275 /** 276 * <p>Runs on the UI thread after {@link #doInBackground}. The 277 * specified result is the value returned by {@link #doInBackground}.</p> 278 * 279 * <p>This method won't be invoked if the task was cancelled.</p> 280 * 281 * @param result The result of the operation computed by {@link #doInBackground}. 282 * 283 * @see #onPreExecute 284 * @see #doInBackground 285 * @see #onCancelled() 286 */ 287 @SuppressWarnings({"UnusedDeclaration"}) 288 protected void onPostExecute(Result result) { 289 } 290 291 /** 292 * Runs on the UI thread after {@link #publishProgress} is invoked. 293 * The specified values are the values passed to {@link #publishProgress}. 294 * 295 * @param values The values indicating progress. 296 * 297 * @see #publishProgress 298 * @see #doInBackground 299 */ 300 @SuppressWarnings({"UnusedDeclaration"}) 301 protected void onProgressUpdate(Progress... values) { 302 } 303 304 /** 305 * Runs on the UI thread after {@link #cancel(boolean)} is invoked and 306 * {@link #doInBackground(Object[])} has finished. 307 * 308 * @see #cancel(boolean) 309 * @see #isCancelled() 310 */ 311 protected void onCancelled() { 312 } 313 314 /** 315 * Returns <tt>true</tt> if this task was cancelled before it completed 316 * normally. If you are calling {@link #cancel(boolean)} on the task, 317 * the value returned by this method should be checked periodically from 318 * {@link #doInBackground(Object[])} to end the task as soon as possible. 319 * 320 * @return <tt>true</tt> if task was cancelled before it completed 321 * 322 * @see #cancel(boolean) 323 */ 324 public final boolean isCancelled() { 325 return mFuture.isCancelled(); 326 } 327 328 /** 329 * <p>Attempts to cancel execution of this task. This attempt will 330 * fail if the task has already completed, already been cancelled, 331 * or could not be cancelled for some other reason. If successful, 332 * and this task has not started when <tt>cancel</tt> is called, 333 * this task should never run. If the task has already started, 334 * then the <tt>mayInterruptIfRunning</tt> parameter determines 335 * whether the thread executing this task should be interrupted in 336 * an attempt to stop the task.</p> 337 * 338 * <p>Calling this method will result in {@link #onCancelled()} being 339 * invoked on the UI thread after {@link #doInBackground(Object[])} 340 * returns. Calling this method guarantees that {@link #onPostExecute(Object)} 341 * is never invoked. After invoking this method, you should check the 342 * value returned by {@link #isCancelled()} periodically from 343 * {@link #doInBackground(Object[])} to finish the task as early as 344 * possible.</p> 345 * 346 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 347 * task should be interrupted; otherwise, in-progress tasks are allowed 348 * to complete. 349 * 350 * @return <tt>false</tt> if the task could not be cancelled, 351 * typically because it has already completed normally; 352 * <tt>true</tt> otherwise 353 * 354 * @see #isCancelled() 355 * @see #onCancelled() 356 */ 357 public final boolean cancel(boolean mayInterruptIfRunning) { 358 return mFuture.cancel(mayInterruptIfRunning); 359 } 360 361 /** 362 * Waits if necessary for the computation to complete, and then 363 * retrieves its result. 364 * 365 * @return The computed result. 366 * 367 * @throws CancellationException If the computation was cancelled. 368 * @throws ExecutionException If the computation threw an exception. 369 * @throws InterruptedException If the current thread was interrupted 370 * while waiting. 371 */ 372 public final Result get() throws InterruptedException, ExecutionException { 373 return mFuture.get(); 374 } 375 376 /** 377 * Waits if necessary for at most the given time for the computation 378 * to complete, and then retrieves its result. 379 * 380 * @param timeout Time to wait before cancelling the operation. 381 * @param unit The time unit for the timeout. 382 * 383 * @return The computed result. 384 * 385 * @throws CancellationException If the computation was cancelled. 386 * @throws ExecutionException If the computation threw an exception. 387 * @throws InterruptedException If the current thread was interrupted 388 * while waiting. 389 * @throws TimeoutException If the wait timed out. 390 */ 391 public final Result get(long timeout, TimeUnit unit) throws InterruptedException, 392 ExecutionException, TimeoutException { 393 return mFuture.get(timeout, unit); 394 } 395 396 /** 397 * Executes the task with the specified parameters. The task returns 398 * itself (this) so that the caller can keep a reference to it. 399 * 400 * This method must be invoked on the UI thread. 401 * 402 * @param params The parameters of the task. 403 * 404 * @return This instance of AsyncTask. 405 * 406 * @throws IllegalStateException If {@link #getStatus()} returns either 407 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 408 */ 409 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 410 if (mStatus != Status.PENDING) { 411 switch (mStatus) { 412 case RUNNING: 413 throw new IllegalStateException("Cannot execute task:" 414 + " the task is already running."); 415 case FINISHED: 416 throw new IllegalStateException("Cannot execute task:" 417 + " the task has already been executed " 418 + "(a task can be executed only once)"); 419 } 420 } 421 422 mStatus = Status.RUNNING; 423 424 onPreExecute(); 425 426 mWorker.mParams = params; 427 sExecutor.execute(mFuture); 428 429 return this; 430 } 431 432 /** 433 * This method can be invoked from {@link #doInBackground} to 434 * publish updates on the UI thread while the background computation is 435 * still running. Each call to this method will trigger the execution of 436 * {@link #onProgressUpdate} on the UI thread. 437 * 438 * {@link #onProgressUpdate} will note be called if the task has been 439 * canceled. 440 * 441 * @param values The progress values to update the UI with. 442 * 443 * @see #onProgressUpdate 444 * @see #doInBackground 445 */ 446 protected final void publishProgress(Progress... values) { 447 if (!isCancelled()) { 448 sHandler.obtainMessage(MESSAGE_POST_PROGRESS, 449 new AsyncTaskResult<Progress>(this, values)).sendToTarget(); 450 } 451 } 452 453 private void finish(Result result) { 454 if (isCancelled()) { 455 onCancelled(); 456 } else { 457 onPostExecute(result); 458 } 459 mStatus = Status.FINISHED; 460 } 461 462 private static class InternalHandler extends Handler { 463 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 464 @Override 465 public void handleMessage(Message msg) { 466 AsyncTaskResult result = (AsyncTaskResult) msg.obj; 467 switch (msg.what) { 468 case MESSAGE_POST_RESULT: 469 // There is only one result 470 result.mTask.finish(result.mData[0]); 471 break; 472 case MESSAGE_POST_PROGRESS: 473 result.mTask.onProgressUpdate(result.mData); 474 break; 475 } 476 } 477 } 478 479 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 480 Params[] mParams; 481 } 482 483 @SuppressWarnings({"RawUseOfParameterizedType"}) 484 private static class AsyncTaskResult<Data> { 485 final AsyncTask mTask; 486 final Data[] mData; 487 488 AsyncTaskResult(AsyncTask task, Data... data) { 489 mTask = task; 490 mData = data; 491 } 492 } 493} 494