AsyncTask.java revision 23fdaf6fb62a9b5154b2508916a21c678462c5d0
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 */ 127public abstract class AsyncTask<Params, Progress, Result> { 128 private static final String LOG_TAG = "AsyncTask"; 129 130 private static final int CORE_POOL_SIZE = 5; 131 private static final int MAXIMUM_POOL_SIZE = 128; 132 private static final int KEEP_ALIVE = 10; 133 134 private static final BlockingQueue<Runnable> sWorkQueue = 135 new LinkedBlockingQueue<Runnable>(10); 136 137 private static final ThreadFactory sThreadFactory = new ThreadFactory() { 138 private final AtomicInteger mCount = new AtomicInteger(1); 139 140 public Thread newThread(Runnable r) { 141 return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 142 } 143 }; 144 145 private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, 146 MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); 147 148 private static final int MESSAGE_POST_RESULT = 0x1; 149 private static final int MESSAGE_POST_PROGRESS = 0x2; 150 private static final int MESSAGE_POST_CANCEL = 0x3; 151 152 private static final InternalHandler sHandler = new InternalHandler(); 153 154 private final WorkerRunnable<Params, Result> mWorker; 155 private final FutureTask<Result> mFuture; 156 157 private volatile Status mStatus = Status.PENDING; 158 159 /** 160 * Indicates the current status of the task. Each status will be set only once 161 * during the lifetime of a task. 162 */ 163 public enum Status { 164 /** 165 * Indicates that the task has not been executed yet. 166 */ 167 PENDING, 168 /** 169 * Indicates that the task is running. 170 */ 171 RUNNING, 172 /** 173 * Indicates that {@link AsyncTask#onPostExecute} has finished. 174 */ 175 FINISHED, 176 } 177 178 /** @hide Used to force static handler to be created. */ 179 public static void init() { 180 sHandler.getLooper(); 181 } 182 183 /** 184 * Creates a new asynchronous task. This constructor must be invoked on the UI thread. 185 */ 186 public AsyncTask() { 187 mWorker = new WorkerRunnable<Params, Result>() { 188 public Result call() throws Exception { 189 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 190 return doInBackground(mParams); 191 } 192 }; 193 194 mFuture = new FutureTask<Result>(mWorker) { 195 196 @Override 197 protected void set(Result v) { 198 super.set(v); 199 if (isCancelled()) { 200 Message message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 201 new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 202 message.sendToTarget(); 203 } 204 } 205 206 @Override 207 protected void done() { 208 Message message; 209 Result result = null; 210 211 try { 212 result = get(); 213 } catch (InterruptedException e) { 214 android.util.Log.w(LOG_TAG, e); 215 } catch (ExecutionException e) { 216 throw new RuntimeException("An error occured while executing doInBackground()", 217 e.getCause()); 218 } catch (CancellationException e) { 219 message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 220 new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 221 message.sendToTarget(); 222 return; 223 } catch (Throwable t) { 224 throw new RuntimeException("An error occured while executing " 225 + "doInBackground()", t); 226 } 227 228 message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 229 new AsyncTaskResult<Result>(AsyncTask.this, result)); 230 message.sendToTarget(); 231 } 232 }; 233 } 234 235 /** 236 * Returns the current status of this task. 237 * 238 * @return The current status. 239 */ 240 public final Status getStatus() { 241 return mStatus; 242 } 243 244 /** 245 * Override this method to perform a computation on a background thread. The 246 * specified parameters are the parameters passed to {@link #execute} 247 * by the caller of this task. 248 * 249 * This method can call {@link #publishProgress} to publish updates 250 * on the UI thread. 251 * 252 * @param params The parameters of the task. 253 * 254 * @return A result, defined by the subclass of this task. 255 * 256 * @see #onPreExecute() 257 * @see #onPostExecute 258 * @see #publishProgress 259 */ 260 protected abstract Result doInBackground(Params... params); 261 262 /** 263 * Runs on the UI thread before {@link #doInBackground}. 264 * 265 * @see #onPostExecute 266 * @see #doInBackground 267 */ 268 protected void onPreExecute() { 269 } 270 271 /** 272 * Runs on the UI thread after {@link #doInBackground}. The 273 * specified result is the value returned by {@link #doInBackground} 274 * or null if the task was cancelled or an exception occured. 275 * 276 * @param result The result of the operation computed by {@link #doInBackground}. 277 * 278 * @see #onPreExecute 279 * @see #doInBackground 280 */ 281 @SuppressWarnings({"UnusedDeclaration"}) 282 protected void onPostExecute(Result result) { 283 } 284 285 /** 286 * Runs on the UI thread after {@link #publishProgress} is invoked. 287 * The specified values are the values passed to {@link #publishProgress}. 288 * 289 * @param values The values indicating progress. 290 * 291 * @see #publishProgress 292 * @see #doInBackground 293 */ 294 @SuppressWarnings({"UnusedDeclaration"}) 295 protected void onProgressUpdate(Progress... values) { 296 } 297 298 /** 299 * Runs on the UI thread after {@link #cancel(boolean)} is invoked. 300 * 301 * @see #cancel(boolean) 302 * @see #isCancelled() 303 */ 304 protected void onCancelled() { 305 } 306 307 /** 308 * Returns <tt>true</tt> if this task was cancelled before it completed 309 * normally. 310 * 311 * @return <tt>true</tt> if task was cancelled before it completed 312 * 313 * @see #cancel(boolean) 314 */ 315 public final boolean isCancelled() { 316 return mFuture.isCancelled(); 317 } 318 319 /** 320 * Attempts to cancel execution of this task. This attempt will 321 * fail if the task has already completed, already been cancelled, 322 * or could not be cancelled for some other reason. If successful, 323 * and this task has not started when <tt>cancel</tt> is called, 324 * this task should never run. If the task has already started, 325 * then the <tt>mayInterruptIfRunning</tt> parameter determines 326 * whether the thread executing this task should be interrupted in 327 * an attempt to stop the task. 328 * 329 * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this 330 * task should be interrupted; otherwise, in-progress tasks are allowed 331 * to complete. 332 * 333 * @return <tt>false</tt> if the task could not be cancelled, 334 * typically because it has already completed normally; 335 * <tt>true</tt> otherwise 336 * 337 * @see #isCancelled() 338 * @see #onCancelled() 339 */ 340 public final boolean cancel(boolean mayInterruptIfRunning) { 341 return mFuture.cancel(mayInterruptIfRunning); 342 } 343 344 /** 345 * Waits if necessary for the computation to complete, and then 346 * retrieves its result. 347 * 348 * @return The computed result. 349 * 350 * @throws CancellationException If the computation was cancelled. 351 * @throws ExecutionException If the computation threw an exception. 352 * @throws InterruptedException If the current thread was interrupted 353 * while waiting. 354 */ 355 public final Result get() throws InterruptedException, ExecutionException { 356 return mFuture.get(); 357 } 358 359 /** 360 * Waits if necessary for at most the given time for the computation 361 * to complete, and then retrieves its result. 362 * 363 * @param timeout Time to wait before cancelling the operation. 364 * @param unit The time unit for the timeout. 365 * 366 * @return The computed result. 367 * 368 * @throws CancellationException If the computation was cancelled. 369 * @throws ExecutionException If the computation threw an exception. 370 * @throws InterruptedException If the current thread was interrupted 371 * while waiting. 372 * @throws TimeoutException If the wait timed out. 373 */ 374 public final Result get(long timeout, TimeUnit unit) throws InterruptedException, 375 ExecutionException, TimeoutException { 376 return mFuture.get(timeout, unit); 377 } 378 379 /** 380 * Executes the task with the specified parameters. The task returns 381 * itself (this) so that the caller can keep a reference to it. 382 * 383 * This method must be invoked on the UI thread. 384 * 385 * @param params The parameters of the task. 386 * 387 * @return This instance of AsyncTask. 388 * 389 * @throws IllegalStateException If {@link #getStatus()} returns either 390 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. 391 */ 392 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 393 if (mStatus != Status.PENDING) { 394 switch (mStatus) { 395 case RUNNING: 396 throw new IllegalStateException("Cannot execute task:" 397 + " the task is already running."); 398 case FINISHED: 399 throw new IllegalStateException("Cannot execute task:" 400 + " the task has already been executed " 401 + "(a task can be executed only once)"); 402 } 403 } 404 405 mStatus = Status.RUNNING; 406 407 onPreExecute(); 408 409 mWorker.mParams = params; 410 sExecutor.execute(mFuture); 411 412 return this; 413 } 414 415 /** 416 * This method can be invoked from {@link #doInBackground} to 417 * publish updates on the UI thread while the background computation is 418 * still running. Each call to this method will trigger the execution of 419 * {@link #onProgressUpdate} on the UI thread. 420 * 421 * {@link #onProgressUpdate} will note be called if the task has been 422 * canceled. 423 * 424 * @param values The progress values to update the UI with. 425 * 426 * @see #onProgressUpdate 427 * @see #doInBackground 428 */ 429 protected final void publishProgress(Progress... values) { 430 if (!isCancelled()) { 431 sHandler.obtainMessage(MESSAGE_POST_PROGRESS, 432 new AsyncTaskResult<Progress>(this, values)).sendToTarget(); 433 } 434 } 435 436 private void finish(Result result) { 437 if (isCancelled()) result = null; 438 onPostExecute(result); 439 mStatus = Status.FINISHED; 440 } 441 442 private static class InternalHandler extends Handler { 443 @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 444 @Override 445 public void handleMessage(Message msg) { 446 AsyncTaskResult result = (AsyncTaskResult) msg.obj; 447 switch (msg.what) { 448 case MESSAGE_POST_RESULT: 449 // There is only one result 450 result.mTask.finish(result.mData[0]); 451 break; 452 case MESSAGE_POST_PROGRESS: 453 result.mTask.onProgressUpdate(result.mData); 454 break; 455 case MESSAGE_POST_CANCEL: 456 result.mTask.onCancelled(); 457 break; 458 } 459 } 460 } 461 462 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 463 Params[] mParams; 464 } 465 466 @SuppressWarnings({"RawUseOfParameterizedType"}) 467 private static class AsyncTaskResult<Data> { 468 final AsyncTask mTask; 469 final Data[] mData; 470 471 AsyncTaskResult(AsyncTask task, Data... data) { 472 mTask = task; 473 mData = data; 474 } 475 } 476} 477