API級別11之前的AsyncTask.executeOnExecutor（）

Question

我們在Android中執行AsyncTask的正常方式是，從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);
     }
   }

 }

我的緊張任務是松散耦合的，執行順序並不重要，通過這種方式，分配一個線程來運行一系列緊張的任務。 我個人認為這是一種中途解決方案。 是的，緊張的工作不再在UI線程中運行，但仍然需要逐個執行（在很多情況下，我們面臨着一份緊張的工作列表，我認為這也是為什么AsyncTask中的方法是多參數化的原因）。 谷歌應該使API更具可重用性，以解決不同類型的場景。

我真正喜歡的是由線程池（例如poolSize = 5）並行運行多個doIntenseJob（）。 看起來google確實通過AsyncTask.executeOnExecutor（）提供了一個解決方案，但遺憾的是，自API級別11以來才可用。我正在開發移動應用程序，並想知道是否有一個解決方法，我可以在API級別11下實現相同的行為。

提前致謝
ÿ

Answer 1

如果您的構建目標設置為API級別11或更高級別，並且您希望專門使用並行任務，則需要在代碼中明確指出，類似於：

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

Answer 2

自從我提出這個問題已經有很長一段時間了，我不時會在StackOverflow中找到類似的問題而沒有可靠的答案，所以我決定做一些進一步的研究並嘗試自己回答。

我應該首先指出的一點是，在大多數常見情況下，底層線程池實現的默認行為隨着AsyncTask API的出現就足夠了，並且不必使用AsyncTask.executeOnExecutor（）來改變它，特別是在HoneyComb之前定位早期版本時已經在CommonsWare的回答中說明了這一點。 但是，如果您確實需要使用AsyncTask.executeOnExecutor（）對早期版本的SDK上的底層線程池進行精細控制，那么您可能會感興趣。

一般來說，我的解決方案只是將新版本的AsyncTask（從API級別11）復制到我們自己的AsyncTask實現中，並使其適用於早期的Android SDK（直至API級別3）。 首先， 在這里閱讀AsyncTask源代碼，並確保您了解它是如何實現的。

從源代碼中，您可以看到AsyncTask導入和使用的幾乎所有類都是從API級別1引入的，即java.util.concurrent。*中的所有類以及來自android.os的其他三個類（Handler，Message和Process）。 。*，唯一的除外是java.util.ArrayDeque ，它自API級別9引入Android SDK .ArrayDeque僅用於實現AsyncTask附帶的默認Executor SERIAL_EXECUTOR之一，以使我們的AsyncTask.executeOnExecutor（）可用於舊版本Android SDK，從源代碼中簡單刪除SERIAL_EXECUTOR實現，使用singleThreadPoolExecutor可以實現相同的行為。

修改后的源代碼附在最后（在Gingerbread上測試）。 你現在需要做的是從這個com.example.AsyncTask而不是android.os.AsyncTask擴展你的AsyncTask，它支持AsyncTask.executeOnExecutor（）到API級別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;
        }
    }
}

Answer 3

我的緊張任務是松散耦合的，執行順序並不重要，通過這種方式，分配一個線程來運行一系列緊張的任務。

AsyncTask目前使用具有多個線程的線程池。 在未來，它可能僅限於一個線程 - 谷歌已暗示這將是這種情況。

想知道是否有一種解決方法可以在API級別11下實現相同的行為。

默認行為是您想要的行為。 如果你檢查AsyncTask的源代碼 ，你會看到，從Gingerbread開始，它使用了一個至少包含5個線程且最多為128個的線程池。

現在，請記住，目前使用的絕大多數 Android設備都是單核的。 因此，除非您的“緊張任務”在網絡I / O上阻塞不是很大，否則您不希望並行執行它們，因為線程之間的上下文切換會讓您進一步降低速度。

Answer 4

我創建了一個抽象幫助器類來確定構建號，並適當地選擇execute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);
        }
    }
}

抽象類示例的實現：

 public class MyTask extends MyAsyncTask<String, Void, Void> {
    @Override
    protected Void doInBackground(String... params) {
         //do work
         return null;
    }
}

創建類的實例

 new MyTask().executeContent("go");

Answer 5

在兼容性庫中存在AsyncTaskCompat。 該類包含一個靜態方法executeInParallel。

此方法等於方法executeOrExecutor，您可以將此方法與API 4一起使用

查看使用此示例的示例：

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);
                }
            }
        }
    });

請享用