大纲：
- 1.何为AsyncTask？
- 2.使用注意
- 3.结构&原理
- 4.任务执行过程
1、何为AsyncTask?
- android.os.AsyncTask
- 执行异步任务的帮助类，便于开发者在子线程中更新UI
- http://androidxref.com/7.1.1_r6/xref/frameworks/base/core/java/android/os/AsyncTask.java
2、使用注意
- 实例必须在UI线程中创建
- execute()必须在UI线程中调用
- 不要手动调用onPreExecute()，doInBackground()，onProgressUpdate()，onPostExecute()
- 不能在doInBackground()中更新UI
- 一个任务实例只能执行一次，如果执行第二次将会抛出异常
3、结构&原理
封装了线程池、Handler
线程池：
- 199 private static final BlockingQueue<Runnable> sPoolWorkQueue =
- 200 new LinkedBlockingQueue<Runnable>(128);//任务队列
- 205 public static final Executor THREAD_POOL_EXECUTOR;//可用于执行并行parallel任务的线程池
- 206
- 207 static {
- 208 ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
- 209 CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
- 210 sPoolWorkQueue, sThreadFactory);//KEEP_ALIVE_SECONDS = 30，非核心线程闲置超时时长，会被回收
- 211 threadPoolExecutor.allowCoreThreadTimeOut(true);//核心线程也有超时策略，同非核心线程
- 212 THREAD_POOL_EXECUTOR = threadPoolExecutor;
- 213 }
- ThreadPoolExecutor参数：核心线程数（默认一直存在）、最大线程数、非核心线程闲置超时时长、时长单位、任务队列、线程工厂；
- ThreadPoolExecutor执行过程：启动核心线程；核心线程数满，则插入任务队列排队等待；无法插入队列，且最大线程数未满，则立刻启动非核心线程；上述最大线程数满，则拒绝该任务，默认抛异常java.util.concurrent.RejectedExecutionException。
- 219 public static final Executor SERIAL_EXECUTOR = new SerialExecutor();//串行serial线程池
- 235 private static class SerialExecutor implements Executor {//以下封装了串行serial排队逻辑
- 236 final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
- 237 Runnable mActive;
- 238
- 239 public synchronized void execute(final Runnable r) {
- 240 mTasks.offer(new Runnable() {
- 241 public void run() {
- 242 try {
- 243 r.run();
- 244 } finally {
- 245 scheduleNext();
- 246 }
- 247 }
- 248 });
- 249 if (mActive == null) {
- 250 scheduleNext();
- 251 }
- 252 }
- 253
- 254 protected synchronized void scheduleNext() {
- 255 if ((mActive = mTasks.poll()) != null) {
- 256 THREAD_POOL_EXECUTOR.execute(mActive);
- 257 }
- 258 }
- 259 }
Handler:
225 private static InternalHandler sHandler;
- 672 private static class InternalHandler extends Handler {
- 673 public InternalHandler() {
- 674 super(Looper.getMainLooper());//主线程handler
- 675 }
4、任务执行过程
一些变量：
227 private final WorkerRunnable<Params, Result> mWorker;//mFuture = new FutureTask<Result>(mWorker){}，回调
- 228 private final FutureTask<Result> mFuture;//exec.execute(mFuture)，任务
- 232 private final AtomicBoolean mCancelled = new AtomicBoolean();//cancel标志位
- 233 private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
串行or并行：
- 224 private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;//默认串行
- 289 /** @hide */
- 290 public static void setDefaultExecutor(Executor exec) {
- 291 sDefaultExecutor = exec;
- 292 }
初始化：
297 public AsyncTask() {
- 298 mWorker = new WorkerRunnable<Params, Result>() {
- 299 public Result call() throws Exception {
- 300 mTaskInvoked.set(true);
- 301 Result result = null;
- 302 try {
- 303 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
- 304 //noinspection unchecked
- 305 result = doInBackground(mParams);//核心回调函数~~~
- 306 Binder.flushPendingCommands();
- 307 } catch (Throwable tr) {
- 308 mCancelled.set(true);//设置cancel标志位
- 309 throw tr;
- 310 } finally {
- 311 postResult(result);//返回结果
- 312 }
- 313 return result;
- 314 }
- 315 };
- 316
- 317 mFuture = new FutureTask<Result>(mWorker) {
- 318 @Override
- 319 protected void done() {//异常过程调用
- 320 try {
- 321 postResultIfNotInvoked(get());
- 322 } catch (InterruptedException e) {
- 323 android.util.Log.w(LOG_TAG, e);
- 324 } catch (ExecutionException e) {
- 325 throw new RuntimeException("An error occurred while executing doInBackground()",
- 326 e.getCause());
- 327 } catch (CancellationException e) {
- 328 postResultIfNotInvoked(null);
- 329 }
- 330 }
- 331 };
- 332 }
默认execute()执行过程：
- 566 public final AsyncTask<Params, Progress, Result> execute(Params... params) {
- 567 return executeOnExecutor(sDefaultExecutor, params);
- 568 }
- 604 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
- 605 Params... params) {
- 606 if (mStatus != Status.PENDING) {
- 607 switch (mStatus) {
- 608 case RUNNING:
- 609 throw new IllegalStateException("Cannot execute task:"
- 610 + " the task is already running.");
- 611 case FINISHED:
- 612 throw new IllegalStateException("Cannot execute task:"
- 613 + " the task has already been executed "
- 614 + "(a task can be executed only once)");
- 615 }
- 616 }
- 617
- 618 mStatus = Status.RUNNING;
- 619
- 620 onPreExecute();
- 621
- 622 mWorker.mParams = params;
- 623 exec.execute(mFuture);
- 624
- 625 return this;
- 626 }
改为并行调用的方式：
executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)
返回结果:
- 663 private void finish(Result result) {
- 664 if (isCancelled()) {
- 665 onCancelled(result);
- 666 } else {
- 667 onPostExecute(result);
- 668 }
- 669 mStatus = Status.FINISHED;
- 670 }
