API Level 11之前的AsyncTask.executeOnExecutor（）
我们在Android中执行AsyncTask的正常方法是，从Android API：
private class DoIntenseTask extends AsyncTask<Object, Object, Void> { protected Void doInBackground(Object... params) { for (Object param : params) { Object rtnObj = doIntenseJob(param); publishProgress(rtnObj); } return null; } protected void onProgressUpdate(Object... progress) { for (Object rtnObj : progress) { updateActivityUI(rtnObj); } } } 我的激烈任务是松散耦合的，执行顺序并不重要，通过这样做，一个单独的线程被分配来运行一系列激烈的任务。 我个人认为这是一种半途而废的解决scheme。 是的，紧张的工作不再是在UI线程中运行，而是仍然需要逐一执行（在许多情况下，我们正面临着一份紧张的工作列表，我认为这也是AsyncTask中的方法是多参数化的原因）。 Google应该使API更加可重用，以解决不同types的场景。
我真正想要的是在线程池（例如poolSize = 5）中并行运行一些doIntenseJob（）。 看起来像谷歌提供一个由AsyncTask.executeOnExecutor（）的解决scheme，但不幸的是只有API级别11后才可用。我正在开发移动应用程序，并想知道是否有一种解决方法，我可以在API级别11下实现相同的行为。
提前致谢
ÿ
我的激烈任务是松散耦合的，执行顺序并不重要，通过这样做，一个单独的线程被分配来运行一系列激烈的任务。
AsyncTask目前使用具有多个线程的线程池。 未来，它可能会被限制在一个单一的线程 – 谷歌已经暗示，情况就是这样。
想知道是否有一个解决方法，我可以在API级别11下实现相同的行为。
默认行为是你想要的行为。 如果您检查AsyncTask的源代码 ，您将看到从Gingerbread开始，它使用了一个至less有5个线程和最多128个线程池。
现在，请记住，目前使用的绝大多数 Android设备都是单核的。 因此，除非你的“激烈的任务”在networkingI / O上强度不大，而是阻塞，否则你不希望并行地执行它们，因为线程之间的上下文切换会使你进一步放慢速度。
如果您的构build目标设置为API级别11或更高级别，并且您希望专门使用并行任务，则需要在代码中明确指出，类似于：
if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.HONEYCOMB) { myTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, (Void[]) null); } else { myTask.execute((Void) null); }
http://commonsware.com/blog/2012/04/20/asynctask-threading-regression-confirmed.html
自从我问这个问题已经很长时间了，我不时可以在StackOverflow中find类似的问题而没有确切的答案，所以我决定做一些进一步的研究并尝试自己回答。
我首先应该指出的一件事情是，在最常见的情况下，底层线程池实现的默认行为来自AsyncTask API就足够了，并且没有必要通过使用AsyncTask.executeOnExecutor（）来更改它，特别是当您在早期版本上定位到HoneyComb这已经在CommonsWare的答案中说过了。 但是，如果您确实需要使用AsyncTask.executeOnExecutor（）来更好地控制早期版本的SDK上的underdying线程池，则可以参考以下答案。
一般来说，我的解决scheme是将新版本的AsyncTask（从API Level 11）复制到我们自己的AsyncTask实现中，并使其与早期的Android SDK（直到API Level 3）一起工作。 首先， 在这里阅读AsyncTask源代码，并确保你了解它是如何实现的。
从源代码中可以看到，几乎所有由AsyncTask导入和使用的类都是从API级别1开始引入的，也就是java.util.concurrent。*中的所有类以及来自android.os的其他三个类（Handler，Message和Process） 。*，唯一的例外是java.util.ArrayDeque ，它是从API Level 9开始引入到Android SDK中的.ArrayDeque仅用于实现默认执行程序SERIAL_EXECUTOR附带的AsyncTask，以使我们的AsyncTask.executeOnExecutor（） Android SDK中，简单地从源代码中删除SERIAL_EXECUTOR实现，使用singleThreadPoolExecutor可以实现同样的行为。
修改后的源代码附在最后（在姜饼上testing）。 你现在需要做的是扩展你的AsyncTask从这个com.example.AsyncTask，而不是android.os.AsyncTask，它支持AsyncTask.executeOnExecutor（）下降到API级别3。
package com.example; /* * Copyright (C) 2008 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ //import java.util.ArrayDeque; import java.util.concurrent.BlockingQueue; import java.util.concurrent.Callable; import java.util.concurrent.CancellationException; import java.util.concurrent.Executor; import java.util.concurrent.ExecutionException; import java.util.concurrent.FutureTask; import java.util.concurrent.LinkedBlockingQueue; import java.util.concurrent.ThreadFactory; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; import java.util.concurrent.atomic.AtomicBoolean; import java.util.concurrent.atomic.AtomicInteger; import android.os.Handler; import android.os.Message; import android.os.Process; /** * ### I delete this comments as it make the answer too long to submit ### */ public abstract class AsyncTask<Params, Progress, Result> { private static final String LOG_TAG = "AsyncTask"; private static final int CORE_POOL_SIZE = 5; private static final int MAXIMUM_POOL_SIZE = 128; private static final int KEEP_ALIVE = 1; private static final ThreadFactory sThreadFactory = new ThreadFactory() { private final AtomicInteger mCount = new AtomicInteger(1); public Thread newThread(Runnable r) { return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); } }; private static final BlockingQueue<Runnable> sPoolWorkQueue = new LinkedBlockingQueue<Runnable>(10); /** * An {@link Executor} that can be used to execute tasks in parallel. */ public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); /** * An {@link Executor} that executes tasks one at a time in serial * order. This serialization is global to a particular process. */ // public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static final int MESSAGE_POST_RESULT = 0x1; private static final int MESSAGE_POST_PROGRESS = 0x2; private static final InternalHandler sHandler = new InternalHandler(); // private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; private static volatile Executor sDefaultExecutor = THREAD_POOL_EXECUTOR; private final WorkerRunnable<Params, Result> mWorker; private final FutureTask<Result> mFuture; private volatile Status mStatus = Status.PENDING; private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); // private static class SerialExecutor implements Executor { // final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); // Runnable mActive; // // public synchronized void execute(final Runnable r) { // mTasks.offer(new Runnable() { // public void run() { // try { // r.run(); // } finally { // scheduleNext(); // } // } // }); // if (mActive == null) { // scheduleNext(); // } // } // // protected synchronized void scheduleNext() { // if ((mActive = mTasks.poll()) != null) { // THREAD_POOL_EXECUTOR.execute(mActive); // } // } // } /** * Indicates the current status of the task. Each status will be set only once * during the lifetime of a task. */ public enum Status { /** * Indicates that the task has not been executed yet. */ PENDING, /** * Indicates that the task is running. */ RUNNING, /** * Indicates that {@link AsyncTask#onPostExecute} has finished. */ FINISHED, } /** @hide Used to force static handler to be created. */ public static void init() { sHandler.getLooper(); } /** @hide */ public static void setDefaultExecutor(Executor exec) { sDefaultExecutor = exec; } /** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); return postResult(doInBackground(mParams)); } }; mFuture = new FutureTask<Result>(mWorker) { @Override protected void done() { try { final Result result = get(); postResultIfNotInvoked(result); } catch (InterruptedException e) { android.util.Log.w(LOG_TAG, e); } catch (ExecutionException e) { throw new RuntimeException("An error occured while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } catch (Throwable t) { throw new RuntimeException("An error occured while executing " + "doInBackground()", t); } } }; } private void postResultIfNotInvoked(Result result) { final boolean wasTaskInvoked = mTaskInvoked.get(); if (!wasTaskInvoked) { postResult(result); } } private Result postResult(Result result) { Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; } /** * Returns the current status of this task. * * @return The current status. */ public final Status getStatus() { return mStatus; } /** * Override this method to perform a computation on a background thread. The * specified parameters are the parameters passed to {@link #execute} * by the caller of this task. * * This method can call {@link #publishProgress} to publish updates * on the UI thread. * * @param params The parameters of the task. * * @return A result, defined by the subclass of this task. * * @see #onPreExecute() * @see #onPostExecute * @see #publishProgress */ protected abstract Result doInBackground(Params... params); /** * Runs on the UI thread before {@link #doInBackground}. * * @see #onPostExecute * @see #doInBackground */ protected void onPreExecute() { } /** * <p>Runs on the UI thread after {@link #doInBackground}. The * specified result is the value returned by {@link #doInBackground}.</p> * * <p>This method won't be invoked if the task was cancelled.</p> * * @param result The result of the operation computed by {@link #doInBackground}. * * @see #onPreExecute * @see #doInBackground * @see #onCancelled(Object) */ @SuppressWarnings({"UnusedDeclaration"}) protected void onPostExecute(Result result) { } /** * Runs on the UI thread after {@link #publishProgress} is invoked. * The specified values are the values passed to {@link #publishProgress}. * * @param values The values indicating progress. * * @see #publishProgress * @see #doInBackground */ @SuppressWarnings({"UnusedDeclaration"}) protected void onProgressUpdate(Progress... values) { } /** * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * <p>The default implementation simply invokes {@link #onCancelled()} and * ignores the result. If you write your own implementation, do not call * <code>super.onCancelled(result)</code>.</p> * * @param result The result, if any, computed in * {@link #doInBackground(Object[])}, can be null * * @see #cancel(boolean) * @see #isCancelled() */ @SuppressWarnings({"UnusedParameters"}) protected void onCancelled(Result result) { onCancelled(); } /** * <p>Applications should preferably override {@link #onCancelled(Object)}. * This method is invoked by the default implementation of * {@link #onCancelled(Object)}.</p> * * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * @see #onCancelled(Object) * @see #cancel(boolean) * @see #isCancelled() */ protected void onCancelled() { } /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. If you are calling {@link #cancel(boolean)} on the task, * the value returned by this method should be checked periodically from * {@link #doInBackground(Object[])} to end the task as soon as possible. * * @return <tt>true</tt> if task was cancelled before it completed * * @see #cancel(boolean) */ public final boolean isCancelled() { return mFuture.isCancelled(); } /** * <p>Attempts to cancel execution of this task. This attempt will * fail if the task has already completed, already been cancelled, * or could not be cancelled for some other reason. If successful, * and this task has not started when <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task.</p> * * <p>Calling this method will result in {@link #onCancelled(Object)} being * invoked on the UI thread after {@link #doInBackground(Object[])} * returns. Calling this method guarantees that {@link #onPostExecute(Object)} * is never invoked. After invoking this method, you should check the * value returned by {@link #isCancelled()} periodically from * {@link #doInBackground(Object[])} to finish the task as early as * possible.</p> * * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete. * * @return <tt>false</tt> if the task could not be cancelled, * typically because it has already completed normally; * <tt>true</tt> otherwise * * @see #isCancelled() * @see #onCancelled(Object) */ public final boolean cancel(boolean mayInterruptIfRunning) { return mFuture.cancel(mayInterruptIfRunning); } /** * Waits if necessary for the computation to complete, and then * retrieves its result. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws InterruptedException If the current thread was interrupted * while waiting. */ public final Result get() throws InterruptedException, ExecutionException { return mFuture.get(); } /** * Waits if necessary for at most the given time for the computation * to complete, and then retrieves its result. * * @param timeout Time to wait before cancelling the operation. * @param unit The time unit for the timeout. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws InterruptedException If the current thread was interrupted * while waiting. * @throws TimeoutException If the wait timed out. */ public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { return mFuture.get(timeout, unit); } /** * Executes the task with the specified parameters. The task returns * itself (this) so that the caller can keep a reference to it. * * <p>Note: this function schedules the task on a queue for a single background * thread or pool of threads depending on the platform version. When first * introduced, AsyncTasks were executed serially on a single background thread. * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed * to a pool of threads allowing multiple tasks to operate in parallel. After * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, it is planned to change this * back to a single thread to avoid common application errors caused * by parallel execution. If you truly want parallel execution, you can use * the {@link #executeOnExecutor} version of this method * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings on * its use. * * <p>This method must be invoked on the UI thread. * * @param params The parameters of the task. * * @return This instance of AsyncTask. * * @throws IllegalStateException If {@link #getStatus()} returns either * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. */ public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } /** * Executes the task with the specified parameters. The task returns * itself (this) so that the caller can keep a reference to it. * * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to * allow multiple tasks to run in parallel on a pool of threads managed by * AsyncTask, however you can also use your own {@link Executor} for custom * behavior. * * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from * a thread pool is generally <em>not</em> what one wants, because the order * of their operation is not defined. For example, if these tasks are used * to modify any state in common (such as writing a file due to a button click), * there are no guarantees on the order of the modifications. * Without careful work it is possible in rare cases for the newer version * of the data to be over-written by an older one, leading to obscure data * loss and stability issues. Such changes are best * executed in serial; to guarantee such work is serialized regardless of * platform version you can use this function with {@link #SERIAL_EXECUTOR}. * * <p>This method must be invoked on the UI thread. * * @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a * convenient process-wide thread pool for tasks that are loosely coupled. * @param params The parameters of the task. * * @return This instance of AsyncTask. * * @throws IllegalStateException If {@link #getStatus()} returns either * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}. */ public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } } mStatus = Status.RUNNING; onPreExecute(); mWorker.mParams = params; exec.execute(mFuture); return this; } /** * Convenience version of {@link #execute(Object...)} for use with * a simple Runnable object. */ public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } /** * This method can be invoked from {@link #doInBackground} to * publish updates on the UI thread while the background computation is * still running. Each call to this method will trigger the execution of * {@link #onProgressUpdate} on the UI thread. * * {@link #onProgressUpdate} will note be called if the task has been * canceled. * * @param values The progress values to update the UI with. * * @see #onProgressUpdate * @see #doInBackground */ protected final void publishProgress(Progress... values) { if (!isCancelled()) { sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget(); } } private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; } private static class InternalHandler extends Handler { @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) @Override public void handleMessage(Message msg) { AsyncTaskResult result = (AsyncTaskResult) msg.obj; switch (msg.what) { case MESSAGE_POST_RESULT: // There is only one result result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData); break; } } } private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; } @SuppressWarnings({"RawUseOfParameterizedType"}) private static class AsyncTaskResult<Data> { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }
我创build了一个抽象助手类来确定内部版本号，并适当地selectexecute或executeOnExecutor。 它似乎工作得很好
public abstract class MyAsyncTask<T, V, Q> extends AsyncTask<T, V, Q> { public void executeContent(T... content) { if (Build.VERSION.SDK_INT>=Build.VERSION_CODES.HONEYCOMB) { this.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, content); } else { this.execute(content); } } }
抽象类实例的实现：
public class MyTask extends MyAsyncTask<String, Void, Void> { @Override protected Void doInBackground(String... params) { //do work return null; } }
创build类的实例
new MyTask().executeContent("go");
在兼容库中存在一个AsyncTaskCompat。 这个类包含一个静态方法executeInParallel。
这个方法等于方法executeOrExecutor，你可以使用这个方法与API 4
看到一个使用这个例子：
AsyncTaskCompat.executeParallel(new AsyncTask<Void, Void, Bitmap>() { @Override protected Bitmap doInBackground(Void... params) { return MediaStore.Images.Thumbnails.getThumbnail( imageView.getContext().getContentResolver(), id, MediaStore.Images.Thumbnails.MINI_KIND, null); } @Override protected void onPostExecute(Bitmap bitmap) { imageView.setImageBitmap(bitmap); if (bitmap != null) { // Add the image to the memory cache first CACHE.put(id, bitmap); if (listener != null) { listener.onImageLoaded(bitmap); } } } });
请享用
