AsyncTask.java revision aa60a02f7934ac23cb88f16ed2d726157bc9f31c
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>Threading rules</h2> 116 * <p>There are a few threading rules that must be followed for this class to 117 * work properly:</p> 118 * <ul> 119 * <li>The task instance must be created on the UI thread.</li> 120 * <li>{@link #execute} must be invoked on the UI thread.</li> 121 * <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute}, 122 * {@link #doInBackground}, {@link #onProgressUpdate} manually.</li> 123 * <li>The task can be executed only once (an exception will be thrown if 124 * a second execution is attempted.)</li> 125 * </ul> 126 * 127 * <h2>Memory observability</h2> 128 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following 129 * operations are safe without explicit synchronizations.</p> 130 * <ul> 131 * <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them 132 * in {@link #doInBackground}. 133 * <li>Set member fields in {@link #doInBackground}, and refer to them in 134 * {@link #onProgressUpdate} and {@link #onPostExecute}. 135 * </ul> 136 */ 137public abstract class AsyncTask<Params, Progress, Result> { 138 private static final String LOG_TAG = "AsyncTask"; 139 140 private static final int CORE_POOL_SIZE = 5; 141 private static final int MAXIMUM_POOL_SIZE = 128; 142 private static final int KEEP_ALIVE = 10; 143 144 private static final BlockingQueue<Runnable> sWorkQueue = 145 new LinkedBlockingQueue<Runnable>(10); 146 147 private static final ThreadFactory sThreadFactory = new ThreadFactory() { 148 private final AtomicInteger mCount = new AtomicInteger(1); 149 150 public Thread newThread(Runnable r) { 151 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 152 } 153 }; 154 155 private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, 156 MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); 157 158 private static final int MESSAGE_POST_RESULT = 0x1; 159 private static final int MESSAGE_POST_PROGRESS = 0x2; 160 private static final int MESSAGE_POST_CANCEL = 0x3; 161 162 private static final InternalHandler sHandler = new InternalHandler(); 163 164 private final WorkerRunnable<Params, Result> mWorker; 165 private final FutureTask<Result> mFuture; 166 167 private volatile Status mStatus = Status.PENDING; 168 169 /** 170 * Indicates the current status of the task. Each status will be set only once 171 * during the lifetime of a task. 172 */ 173 public enum Status { 174 /** 175 * Indicates that the task has not been executed yet. 176 */ 177 PENDING, 178 /** 179 * Indicates that the task is running. 180 */ 181 RUNNING, 182 /** 183 * Indicates that {@link AsyncTask#onPostExecute} has finished. 184 */ 185 FINISHED, 186 } 187 188 /** @hide Used to force static handler to be created. */ 189 public static void init() { 190 sHandler.getLooper(); 191 } 192 193 /** 194 * Creates a new asynchronous task. This constructor must be invoked on the UI thread. 195 */ 196 public AsyncTask() { 197 mWorker = new WorkerRunnable<Params, Result>() { 198 public Result call() throws Exception { 199 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 200 return doInBackground(mParams); 201 } 202 }; 203 204 mFuture = new FutureTask<Result>(mWorker) { 205 206 @Override 207 protected void set(Result v) { 208 super.set(v); 209 if (isCancelled()) { 210 Message message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 211 new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 212 message.sendToTarget(); 213 } 214 } 215 216 @Override 217 protected void done() { 218 Message message; 219 Result result = null; 220 221 try { 222 result = get(); 223 } catch (InterruptedException e) { 224 android.util.Log.w(LOG_TAG, e); 225 } catch (ExecutionException e) { 226 throw new RuntimeException("An error occured while executing doInBackground()", 227 e.getCause()); 228 } catch (CancellationException e) { 229 message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 230 new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 231 message.sendToTarget(); 232 return; 233 } catch (Throwable t) { 234 throw new RuntimeException("An error occured while executing " 235 + "doInBackground()", t); 236 } 237 238 message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 239 new AsyncTaskResult<Result>(AsyncTask.this, result)); 240 message.sendToTarget(); 241 } 242 }; 243 } 244 245 /** 246 * Returns the current status of this task. 247 * 248 * @return The current status. 249 */ 250 public final Status getStatus() { 251 return mStatus; 252 } 253 254 /** 255 * Override this method to perform a computation on a background thread. The 256 * specified parameters are the parameters passed to {@link #execute} 257 * by the caller of this task. 258 * 259 * This method can call {@link #publishProgress} to publish updates 260 * on the UI thread. 261 * 262 * @param params The parameters of the task. 263 * 264 * @return A result, defined by the subclass of this task. 265 * 266 * @see #onPreExecute() 267 * @see #onPostExecute 268 * @see #publishProgress 269 */ 270 protected abstract Result doInBackground(Params... params); 271 272 /** 273 * Runs on the UI thread before {@link #doInBackground}. 274 * 275 * @see #onPostExecute 276 * @see #doInBackground 277 */ 278 protected void onPreExecute() { 279 } 280 281 /** 282 * Runs on the UI thread after {@link #doInBackground}. The 283 * specified result is the value returned by {@link #doInBackground} 284 * or null if the task was cancelled or an exception occured. 285 * 286 * @param result The result of the operation computed by {@link #doInBackground}. 287 * 288 * @see #onPreExecute 289 * @see #doInBackground 290 */ 291 @SuppressWarnings({"UnusedDeclaration"}) 292 protected void onPostExecute(Result result) { 293 } 294 295 /** 296 * Runs on the UI thread after {@link #publishProgress} is invoked. 297 * The specified values are the values passed to {@link #publishProgress}. 298 * 299 * @param values The values indicating progress. 300 * 301 * @see #publishProgress 302 * @see #doInBackground 303 */ 304 @SuppressWarnings({"UnusedDeclaration"}) 305 protected void onProgressUpdate(Progress... values) { 306 } 307 308 /** 309 * Runs on the UI thread after {@link #cancel(boolean)} is invoked. 310 * 311 * @see #cancel(boolean) 312 * @see #isCancelled() 313 */ 314 protected void onCancelled() { 315 } 316 317 /** 318 * Returns <tt>true</tt> if this task was cancelled before it completed 319 * normally. 320 * 321 * @return <tt>true</tt> if task was cancelled before it completed 322 * 323 * @see #cancel(boolean) 324 */ 325 public final boolean isCancelled() { 326 return mFuture.isCancelled(); 327 } 328 329 /** 330 * Attempts to cancel execution of this task. This attempt will 331 * fail if the task has already completed, already been cancelled, 332 * or could not be cancelled for some other reason. If successful, 333 * and this task has not started when <tt>cancel</tt> is called, 334 * this task should never run. If the task has already started, 335 * then the <tt>mayInterruptIfRunning</tt> parameter determines 336 * whether the thread executing this task should be interrupted in 337 * an attempt to stop the task. 338 * 339 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 340 * task should be interrupted; otherwise, in-progress tasks are allowed 341 * to complete. 342 * 343 * @return <tt>false</tt> if the task could not be cancelled, 344 * typically because it has already completed normally; 345 * <tt>true</tt> otherwise 346 * 347 * @see #isCancelled() 348 * @see #onCancelled() 349 */ 350 public final boolean cancel(boolean mayInterruptIfRunning) { 351 return mFuture.cancel(mayInterruptIfRunning); 352 } 353 354 /** 355 * Waits if necessary for the computation to complete, and then 356 * retrieves its result. 357 * 358 * @return The computed result. 359 * 360 * @throws CancellationException If the computation was cancelled. 361 * @throws ExecutionException If the computation threw an exception. 362 * @throws InterruptedException If the current thread was interrupted 363 * while waiting. 364 */ 365 public final Result get() throws InterruptedException, ExecutionException { 366 return mFuture.get(); 367 } 368 369 /** 370 * Waits if necessary for at most the given time for the computation 371 * to complete, and then retrieves its result. 372 * 373 * @param timeout Time to wait before cancelling the operation. 374 * @param unit The time unit for the timeout. 375 * 376 * @return The computed result. 377 * 378 * @throws CancellationException If the computation was cancelled. 379 * @throws ExecutionException If the computation threw an exception. 380 * @throws InterruptedException If the current thread was interrupted 381 * while waiting. 382 * @throws TimeoutException If the wait timed out. 383 */ 384 public final Result get(long timeout, TimeUnit unit) throws InterruptedException, 385 ExecutionException, TimeoutException { 386 return mFuture.get(timeout, unit); 387 } 388 389 /** 390 * Executes the task with the specified parameters. The task returns 391 * itself (this) so that the caller can keep a reference to it. 392 * 393 * This method must be invoked on the UI thread. 394 * 395 * @param params The parameters of the task. 396 * 397 * @return This instance of AsyncTask. 398 * 399 * @throws IllegalStateException If {@link #getStatus()} returns either 400 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 401 */ 402 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 403 if (mStatus != Status.PENDING) { 404 switch (mStatus) { 405 case RUNNING: 406 throw new IllegalStateException("Cannot execute task:" 407 + " the task is already running."); 408 case FINISHED: 409 throw new IllegalStateException("Cannot execute task:" 410 + " the task has already been executed " 411 + "(a task can be executed only once)"); 412 } 413 } 414 415 mStatus = Status.RUNNING; 416 417 onPreExecute(); 418 419 mWorker.mParams = params; 420 sExecutor.execute(mFuture); 421 422 return this; 423 } 424 425 /** 426 * This method can be invoked from {@link #doInBackground} to 427 * publish updates on the UI thread while the background computation is 428 * still running. Each call to this method will trigger the execution of 429 * {@link #onProgressUpdate} on the UI thread. 430 * 431 * {@link #onProgressUpdate} will note be called if the task has been 432 * canceled. 433 * 434 * @param values The progress values to update the UI with. 435 * 436 * @see #onProgressUpdate 437 * @see #doInBackground 438 */ 439 protected final void publishProgress(Progress... values) { 440 if (!isCancelled()) { 441 sHandler.obtainMessage(MESSAGE_POST_PROGRESS, 442 new AsyncTaskResult<Progress>(this, values)).sendToTarget(); 443 } 444 } 445 446 private void finish(Result result) { 447 if (isCancelled()) result = null; 448 onPostExecute(result); 449 mStatus = Status.FINISHED; 450 } 451 452 private static class InternalHandler extends Handler { 453 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 454 @Override 455 public void handleMessage(Message msg) { 456 AsyncTaskResult result = (AsyncTaskResult) msg.obj; 457 switch (msg.what) { 458 case MESSAGE_POST_RESULT: 459 // There is only one result 460 result.mTask.finish(result.mData[0]); 461 break; 462 case MESSAGE_POST_PROGRESS: 463 result.mTask.onProgressUpdate(result.mData); 464 break; 465 case MESSAGE_POST_CANCEL: 466 result.mTask.onCancelled(); 467 break; 468 } 469 } 470 } 471 472 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 473 Params[] mParams; 474 } 475 476 @SuppressWarnings({"RawUseOfParameterizedType"}) 477 private static class AsyncTaskResult<Data> { 478 final AsyncTask mTask; 479 final Data[] mData; 480 481 AsyncTaskResult(AsyncTask task, Data... data) { 482 mTask = task; 483 mData = data; 484 } 485 } 486} 487