Android中AsyncTask的使用与源码分析

本文参考Mr.Simple的：Android中AsyncTask的使用与源码分析

在Android中实现异步任务机制有两种方式，Handler和AsyncTask。

     Handler模式需要为每一个任务创建一个新的线程，任务完成后通过Handler实例向UI线程发送消息，完成界面的更新，这种方式对于整个过程的控制比较精细，但也是有缺点的，例如代码相对臃肿，在多个任务同时执行时，不易对线程进行精确的控制。

为了简化操作，Android1.5提供了工具类android.os.AsyncTask，它使创建异步任务变得更加简单，不再需要编写任务线程和Handler实例即可完成相同的任务，但其内部也是使用Handler来传递消息，而且基于线程池。因此明显的AsyncTask比Handler要重量级。

     先来看看AsyncTask的定义：

public abstract class AsyncTask<Params, Progress, Result> {

      三种泛型类型分别代表“启动任务执行的输入参数”、“后台任务执行的进度”、“后台计算结果的类型”。在特定场合下，并不是所有类型都被使用，如果没有被使用，可以用java.lang.Void类型代替。

一个异步任务的执行一般包括以下几个步骤：

1.execute(Params... params)，执行一个异步任务，需要我们在代码中调用此方法，触发异步任务的执行。

2.onPreExecute()，在execute(Params... params)被调用后立即执行，一般用来在执行后台任务前对UI做一些标记。

3.doInBackground(Params... params)，在onPreExecute()完成后立即执行，用于执行较为费时的操作，此方法将接收输入参数和返回计算结果。在执行过程中可以调用publishProgress(Progress... values)来更新进度信息。

4.onProgressUpdate(Progress... values)，在调用publishProgress(Progress... values)时，此方法被执行，直接将进度信息更新到UI组件上。

5.onPostExecute(Result result)，当后台操作结束时，此方法将会被调用，计算结果将做为参数传递到此方法中，直接将结果显示到UI组件上。

在使用的时候，有几点需要格外注意：

1.异步任务的实例必须在UI线程中创建。

2.execute(Params... params)方法必须在UI线程中调用。

3.不能在doInBackground(Params... params)中更改UI组件的信息。

4.一个任务实例只能执行一次，如果执行第二次将会抛出异常。

一 、 AsyncTask的使用示例

接下来，我们来看看如何使用AsyncTask执行异步任务操作，我们先建立一个项目，结构如下：



结构相对简单一些，让我们先看看MainActivity.java的代码：

package com.silion.asynctaskdemo;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;
import okhttp3.ResponseBody;

/**
* Created by silion on 2016/7/7.
*/
public class MainActivity extends Activity {
private Button btExecute;
private Button btCancel;
private ProgressBar pb;
private TextView tvContent;

private DownloadTask mTask;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btExecute = (Button) findViewById(R.id.execute);
btExecute.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
mTask = new DownloadTask();
mTask.execute("http://www.baidu.com");

btExecute.setEnabled(false);
btCancel.setEnabled(true);
}
});
btCancel = (Button
4000
) findViewById(R.id.cancel);
btCancel.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (mTask.getStatus() == AsyncTask.Status.RUNNING) {
mTask.cancel(true);
}
}
});
pb = (ProgressBar) findViewById(R.id.progress_bar);
tvContent = (TextView) findViewById(R.id.text_view);
}

public class DownloadTask extends AsyncTask<String, Integer, String> {

private OkHttpClient mClient;

@Override
protected void onPreExecute() {
Log.i("silion", "onPreExecute called");
tvContent.setText("loading...");
super.onPreExecute();
}

@Override
protected String doInBackground(String... params) {
Log.i("silion", "doInBackground called");
mClient = new OkHttpClient();
InputStream is = null;
ByteArrayOutputStream baos = new ByteArrayOutputStream();
try {
ResponseBody body = run(params[0]);
long total = body.contentLength();
is = body.byteStream();
byte[] buf = new byte[1024];
int count = 0;
int length;
while ((length = is.read(buf)) != -1) {
baos.write(buf, 0, length);
count += length;
//调用publishProgress公布进度,最后onProgressUpdate方法将被执行
publishProgress((int) ((count / (float) total) * 100));
//为了演示进度,休眠500毫秒
Thread.sleep(100);
}
return new String(baos.toByteArray(), "utf-8");
} catch (IOException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (is != null) {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (baos != null) {
try {
baos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return null;
}

@Override
protected void onProgressUpdate(Integer... progresses) {
Log.i("silion", "onProgressUpdate called");
pb.setProgress(progresses[0]);
tvContent.setText("longding..." + progresses[0] + "%");
super.onProgressUpdate(progresses);
}

@Override
protected void onPostExecute(String result) {
Log.i("silion", "onPostExecute called");
tvContent.setText(result);

btExecute.setEnabled(true);
btCancel.setEnabled(false);
super.onPostExecute(result);
}

@Override
protected void onCancelled() {
Log.i("silion", "onCancelled called");
tvContent.setText("cancelled");
pb.setProgress(0);

btExecute.setEnabled(true);
btCancel.setEnabled(false);
super.onCancelled();
}

ResponseBody run(String url) throws IOException {
/**
* 把HttpClient替换成OKHttp之后，有时会获取不到content-length
* 经常抓包分析，发现服务器会随机的对下发的资源做GZip操作，而此时就没有相应的content-length
* 在Header中加入”Accept-Encoding”, “identity”，这样强迫服务器不走压缩。问题就得到了解决
*/
Request request = new Request.Builder().url(url).header("Accept-Encoding", "identity").build();
Response response = mClient.newCall(request).execute();
if (response.isSuccessful()) {
return response.body();
} else {
throw new IOException("Unexpected code" + response);
}
}
}
}

布局文件activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical">

<Button
android:id="@+id/execute"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="execute"/>

<Button
android:id="@+id/cancel"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:enabled="false"
android:text="cancel"/>

<ProgressBar
android:id="@+id/progress_bar"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:max="100"
android:progress="0"/>

<ScrollView
android:layout_width="fill_parent"
android:layout_height="wrap_content">

<TextView
android:id="@+id/text_view"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
</ScrollView>
</LinearLayout>

因为需要访问网络，所以我们还需要在AndroidManifest.xml中加入访问网络的权限：

<uses-permission android:name="android.permission.INTERNET"/>

我们来看一下运行时的界面：

以上几个截图分别是初始界面、执行异步任务时界面、执行成功后界面、取消任务后界面。执行成功后，整个过程日志打印如下：

07-27 15:36:40.941  17249-17249/com.silion.asynctaskdemo I/silion﹕ onPreExecute called
07-27 15:36:40.941   17249-3258/com.silion.asynctaskdemo I/silion﹕ doInBackground called
07-27 15:36:43.341  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
...
07-27 15:36:53.231  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:36:53.341  17249-17249/com.silion.asynctaskdemo I/silion﹕ onPostExecute called

如果我们在执行任务时按下了"cancel"按钮，日志打印如下：

07-27 15:38:29.861  17249-17249/com.silion.asynctaskdemo D/ViewRootImpl﹕ ViewPostImeInputStage ACTION_DOWN
07-27 15:38:29.931  17249-17249/com.silion.asynctaskdemo I/silion﹕ onPreExecute called
07-27 15:38:29.931   17249-4130/com.silion.asynctaskdemo I/silion﹕ doInBackground called
07-27 15:38:30.591  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:30.691  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:30.791  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:30.891  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:30.991  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.091  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.201  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.301  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.401  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.501  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.601  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.701  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.741  17249-17249/com.silion.asynctaskdemo D/ViewRootImpl﹕ ViewPostImeInputStage ACTION_DOWN
07-27 15:38:31.801  17249-17249/com.silion.asynctaskdemo I/silion﹕ onProgressUpdate called
07-27 15:38:31.821   17249-4130/com.silion.asynctaskdemo W/System.err﹕ java.lang.InterruptedException
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.lang.Thread.sleep(Native Method)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.lang.Thread.sleep(Thread.java:1031)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.lang.Thread.sleep(Thread.java:985)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at com.silion.asynctaskdemo.MainActivity$DownloadTask.doInBackground(MainActivity.java:90)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at com.silion.asynctaskdemo.MainActivity$DownloadTask.doInBackground(MainActivity.java:60)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at android.os.AsyncTask$2.call(AsyncTask.java:292)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.util.concurrent.FutureTask.run(FutureTask.java:237)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
07-27 15:38:31.831   17249-4130/com.silion.asynctaskdemo W/System.err﹕ at java.lang.Thread.run(Thread.java:818)
07-27 15:38:31.841  17249-17249/com.silion.asynctaskdemo I/silion﹕ onCancelled called

可以看到onCancelled()方法将会被调用，onPostExecute(Result result)方法将不再被调用。

二、 AsyncTask的实现基本原理

     上面介绍了AsyncTask的基本应用，有些朋友也许会有疑惑，AsyncTask内部是怎么执行的呢，它执行的过程跟我们使用Handler又有什么区别呢？答案是：AsyncTask是对Thread+Handler良好的封装，在android.os.AsyncTask代码里仍然可以看到Thread和Handler的踪迹。下面就向大家详细介绍一下AsyncTask的执行原理。

我们先看一下AsyncTask的大纲视图：



源代码如下：

/*
* 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.
*/

package android.os;

import android.annotation.MainThread;
import android.annotation.WorkerThread;

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;

/**
* <p>AsyncTask enables proper and easy use of the UI thread. This class allows to
* perform background operations and publish results on the UI thread without
* having to manipulate threads and/or handlers.</p>
*
* <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
* and does not constitute a generic threading framework. AsyncTasks should ideally be
* used for short operations (a few seconds at the most.) If you need to keep threads
* running for long periods of time, it is highly recommended you use the various APIs
* provided by the <code>java.util.concurrent</code> package such as {@link Executor},
* {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
*
* <p>An asynchronous task is defined by a computation that runs on a background thread and
* whose result is published on the UI thread. An asynchronous task is defined by 3 generic
* types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
* and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
* <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
*
* <div class="special reference">
* <h3>Developer Guides</h3>
* <p>For more information about using tasks and threads, read the
* <a href="{@docRoot}guide/topics/fundamentals/processes-and-threads.html">Processes and
* Threads</a> developer guide.</p>
* </div>
*
* <h2>Usage</h2>
* <p>AsyncTask must be subclassed to be used. The subclass will override at least
* one method ({@link #doInBackground}), and most often will override a
* second one ({@link #onPostExecute}.)</p>
*
* <p>Here is an example of subclassing:</p>
* <pre class="prettyprint">
* private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
*     protected Long doInBackground(URL... urls) {
*         int count = urls.length;
*         long totalSize = 0;
*         for (int i = 0; i < count; i++) {
*             totalSize += Downloader.downloadFile(urls[i]);
*             publishProgress((int) ((i / (float) count) * 100));
*             // Escape early if cancel() is called
*             if (isCancelled()) break;
*         }
*         return totalSize;
*     }
*
*     protected void onProgressUpdate(Integer... progress) {
*         setProgressPercent(progress[0]);
*     }
*
*     protected void onPostExecute(Long result) {
*         showDialog("Downloaded " + result + " bytes");
*     }
* }
* </pre>
*
* <p>Once created, a task is executed very simply:</p>
* <pre class="prettyprint">
* new DownloadFilesTask().execute(url1, url2, url3);
* </pre>
*
* <h2>AsyncTask's generic types</h2>
* <p>The three types used by an asynchronous task are the following:</p>
* <ol>
*     <li><code>Params</code>, the type of the parameters sent to the task upon
*     execution.</li>
*     <li><code>Progress</code>, the type of the progress units published during
*     the background computation.</li>
*     <li><code>Result</code>, the type of the result of the background
*     computation.</li>
* </ol>
* <p>Not all types are always used by an asynchronous task. To mark a type as unused,
* simply use the type {@link Void}:</p>
* &l
16531
t;pre>
* private class MyTask extends AsyncTask<Void, Void, Void> { ... }
* </pre>
*
* <h2>The 4 steps</h2>
* <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
* <ol>
*     <li>{@link #onPreExecute()}, invoked on the UI thread before the task
*     is executed. This step is normally used to setup the task, for instance by
*     showing a progress bar in the user interface.</li>
*     <li>{@link #doInBackground}, invoked on the background thread
*     immediately after {@link #onPreExecute()} finishes executing. This step is used
*     to perform background computation that can take a long time. The parameters
*     of the asynchronous task are passed to this step. The result of the computation must
*     be returned by this step and will be passed back to the last step. This step
*     can also use {@link #publishProgress} to publish one or more units
*     of progress. These values are published on the UI thread, in the
*     {@link #onProgressUpdate} step.</li>
*     <li>{@link #onProgressUpdate}, invoked on the UI thread after a
*     call to {@link #publishProgress}. The timing of the execution is
*     undefined. This method is used to display any form of progress in the user
*     interface while the background computation is still executing. For instance,
*     it can be used to animate a progress bar or show logs in a text field.</li>
*     <li>{@link #onPostExecute}, invoked on the UI thread after the background
*     computation finishes. The result of the background computation is passed to
*     this step as a parameter.</li>
* </ol>
*
* <h2>Cancelling a task</h2>
* <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
* this method will cause subsequent calls to {@link #isCancelled()} to return true.
* After invoking this method, {@link #onCancelled(Object)}, instead of
* {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
* returns. To ensure that a task is cancelled as quickly as possible, you should always
* check the return value of {@link #isCancelled()} periodically from
* {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
*
* <h2>Threading rules</h2>
* <p>There are a few threading rules that must be followed for this class to
* work properly:</p>
* <ul>
*     <li>The AsyncTask class must be loaded on the UI thread. This is done
*     automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
*     <li>The task instance must be created on the UI thread.</li>
*     <li>{@link #execute} must be invoked on the UI thread.</li>
*     <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
*     {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
*     <li>The task can be executed only once (an exception will be thrown if
*     a second execution is attempted.)</li>
* </ul>
*
* <h2>Memory observability</h2>
* <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
* operations are safe without explicit synchronizations.</p>
* <ul>
*     <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
*     in {@link #doInBackground}.
*     <li>Set member fields in {@link #doInBackground}, and refer to them in
*     {@link #onProgressUpdate} and {@link #onPostExecute}.
* </ul>
*
* <h2>Order of execution</h2>
* <p>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. Starting with
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
* thread to avoid common application errors caused by parallel execution.</p>
* <p>If you truly want parallel execution, you can invoke
* {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
* {@link #THREAD_POOL_EXECUTOR}.</p>
*/
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";

private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
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>(128);

/**
* 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 volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
private static InternalHandler sHandler;

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;

private volatile Status mStatus = Status.PENDING;

private final AtomicBoolean mCancelled = new AtomicBoolean();
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,
}

private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler();
}
return sHandler;
}
}

/** @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);
//noinspection unchecked
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(result);
}
};

mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}

private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}

private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().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
*/
@WorkerThread
protected abstract Result doInBackground(Params... params);

/**
* Runs on the UI thread before {@link #doInBackground}.
*
* @see #onPostExecute
* @see #doInBackground
*/
@MainThread
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"})
@MainThread
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"})
@MainThread
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"})
@MainThread
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()
*/
@MainThread
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 mCancelled.get();
}

/**
* <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) {
mCancelled.set(true);
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. Starting
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
* executed on 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}.
*
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
* @see #execute(Runnable)
*/
@MainThread
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}.
*
* @see #execute(Object[])
*/
@MainThread
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. See {@link #execute(Object[])} for more
* information on the order of execution.
*
* @see #execute(Object[])
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
*/
@MainThread
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 not be called if the task has been
* canceled.
*
* @param values The progress values to update the UI with.
*
* @see #onProgressUpdate
* @see #doInBackground
*/
@WorkerThread
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().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 {
public InternalHandler() {
super(Looper.getMainLooper());
}

@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;
}
}
}

我们可以看到关键几个步骤的方法都在其中。

1、doInBackground(Params... params)是一个抽象方法，我们继承AsyncTask时必须覆写此方法；

2、onPreExecute()、onProgressUpdate(Progress... values)、onPostExecute(Result result)、onCancelled()这几个方法体都是空的，我们需要的时候可以选择性的覆写它们；

3、publishProgress(Progress... values)是final修饰的，不能覆写，只能去调用，我们一般会在doInBackground(Params... params)中调用此方法来更新进度条；
4、另外，我们可以看到有一个Status的枚举类和getStatus()方法，Status枚举类代码段如下：

private volatile Status mStatus = Status.PENDING; //初始状态

/**
* 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,
}

/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}

可以看到，AsyncTask的初始状态为PENDING，代表待定状态，RUNNING代表执行状态，FINISHED代表结束状态，这几种状态在AsyncTask一次生命周期内的很多地方被使用，非常重要。

执行任务execute(Params... params)

/**
* 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. Starting
* {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
* executed on 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}.
*
* @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
* @see #execute(Runnable)
*/
@MainThread
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}

其实是调用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}.
*
* @see #execute(Object[])
*/
@MainThread
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(); //调用onPreExecute,<span style="font-family: Arial; font-size: 14px; line-height: 26px;">在执行后台任务前对UI做一些初始化或标记</span>

mWorker.mParams = params;
exec.execute(mFuture);

return this;
}

这里涉及到三个变量：mWorker(保存了参数)、mFeature、exec(即传进来的sDefaultExecutor)，我们先看一下他们的庐山真面目：

关于sDefaultExecutor, 它的初始值是AsyncTask的一个内部类SerialExecutor的实例,但如果是HONEYCOMB_MR1之前的版本，会重新设置为java.util.concurrent.ThradPoolExecutor的实例，用于管理线程的执行。代码如下：

/**
* 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 volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

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);
}
}
}

// If the app is Honeycomb MR1 or earlier, switch its AsyncTask
// implementation to use the pool executor.  Normally, we use the
// serialized executor as the default. This has to happen in the
// main thread so the main looper is set right.
if (data.appInfo.targetSdkVersion <= android.os.Build.VERSION_CODES.HONEYCOMB_MR1) {
AsyncTask.setDefaultExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}

mWorker实际上是AsyncTask的一个的抽象内部类的实现对象实例，它实现了Callable<Result>接口中的call()方法，代码如下：

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}

public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);

Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
return postResult(result);
}
};
...
}

而mFuture实际上是java.util.concurrent.FutureTask的实例,代码如下：

/**
* Creates a new asynchronous task. This constructor must be invoked on the UI thread.
*/
public AsyncTask() {
...
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}

下面是它的FutureTask类的相关信息：

public class FutureTask<V> implements RunnableFuture<V> {
...
}

/**
* A {@link Future} that is {@link Runnable}. Successful execution of
* the {@code run} method causes completion of the {@code Future}
* and allows access to its results.
* @see FutureTask
* @see Executor
* @since 1.6
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}

可以看到FutureTask是一个可以中途取消的用于异步计算的类。

回到exec.execute(mFeture),
进入到SerialExecutor的execute函数，如下：

final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); //双队列列表

public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() { //添加到队列，参考offer的注释Inserts the specified element at the end of this deque.
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}

在SerialExecutor的execute主要是将异步任务mFuture(r.run)加入到将要执行的双队列列表。

r.run执行后，进入到FutureTask的run, 如下：

/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Callable}.
*
* @param  callable the callable task
* @throws NullPointerException if the callable is null
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;       // ensure visibility of callable
}

public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable; //就是在AsyncTask()初始化mFuture传入的mWorker
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call(); //调用mWorker的call(),并在call中才真正调用了doInBackground函数，至此线程真正启动了
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result); //最后会调用在AsyncTask构造方法中创建的mFuture对象覆写了的done()方法
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}

回调mWorker的call()方法以及调用在AsyncTask构造方法中创建的mFuture对象覆写了的done()方法。

现在再回过头看一下mWorker的call()方法和mFuture的done()方法：

/**
* 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); //设置线程为后台线程
//noinspection unchecked
Result result = doInBackground(mParams); //调用doInBackground
Binder.flushPendingCommands();
return postResult(result); //发送处理结果消息
}
};

mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get()); //如果还没发送处理结果消息，则发送
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}

在mWorker的call()中将线程设为后台线程，调用doInBackground并通过postResult(result)发送处理结果消息。
如果没有执行mWorker的postResult, 则在mFuture的done()方法里会通过postResultIfNotInvoked(get())发送处理结果消息。

再来看一下AsyncTask是如何处理消息的，代码如下：

private static Handler getHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler(); //AsyncTask内部类InternalHandler的实例
}
return sHandler;
}
}

private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}

处理消息的sHandler是AsyncTask内部类InternalHandler的实例，继承了Handler, 看一下代码：
private static class InternalHandler extends Handler {
public InternalHandler() {
super(Looper.getMainLooper());
}

@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]); //onCancelled或者onPosstExecute
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData); //调用onProgressUpdate更新UI
break;
}
}
}

根据传进来的Message，如果是MESSAGE_POST_RESULT, 调用finish，如果是MESSAGE_POST_PROGRESS, 调用onProgressUpdate更新UI。
是取消还是执行完成AsyncTask，就要看finish()方法了

private void finish(Result result) {
if (isCancelled()) {
onCancelled(result); //调用onCancelled取消AsyncTask
} else {
onPostExecute(result); //调用onPostExecute将结果传递回去
}
mStatus = Status.FINISHED;
}

概括来说，当我们调用execute(Params... params)方法后，execute方法会调用onPreExecute()方法，然后由ThreadPoolExecutor实例sExecutor执行一个FutureTask任务，这个过程中doInBackground(Params... params)将被调用，如果被开发者覆写的doInBackground(Params... params)方法中调用了publishProgress(Progress... values)方法，则通过InternalHandler实例sHandler发送一条MESSAGE_POST_PROGRESS消息，更新进度，sHandler处理消息时onProgressUpdate(Progress...
values)方法将被调用；如果遇到异常，则发送一条MESSAGE_POST_CANCEL的消息，取消任务，sHandler处理消息时onCancelled()方法将被调用；如果执行成功，则发送一条MESSAGE_POST_RESULT的消息，显示结果，sHandler处理消息时onPostExecute(Result result)方法被调用。

经过上面的介绍，相信朋友们都已经认识到AsyncTask的本质了，它对Thread+Handler的良好封装，减少了开发者处理问题的复杂度，提高了开发效率，希望朋友们能多多体会一下。