Android AsyncTask 源码解析（任玉刚版）

转载请标明出处：http://blog.csdn.net/singwhatiwanna/article/details/17596225，本文出自：【任玉刚的博客】

前言

什么是AsyncTask，相信搞过Android开发的朋友们都不陌生。AsyncTask内部封装了Thread和Handler，可以让我们在后台进行计算并且把计算的结果及时更新到UI上，而这些正是Thread+Handler所做的事情，没错，AsyncTask的作用就是简化Thread+Handler，让我们能够通过更少的代码来完成一样的功能，这里，我要说明的是：AsyncTask只是简化Thread+Handler而不是替代，实际上它也替代不了。同时，AsyncTask从最开始到现在已经经过了几次代码修改，任务的执行逻辑慢慢地发生了改变，并不是大家所想象的那样：AsyncTask是完全并行执行的就像多个线程一样，其实不是的，所以用AsyncTask的时候还是要注意，下面会一一说明。另外本文主要是分析AsyncTask的源代码以及使用时候的一些注意事项，如果你还不熟悉AsyncTask，请先阅读android之AsyncTask 来了解其基本用法。

这里先给出AsyncTask的一个例子：

[java] view plaincopyprint?

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”);
}
}

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 calledif (isCancelled()) break;}return totalSize;}protected void onProgressUpdate(Integer... progress) {setProgressPercent(progress[0]);}protected void onPostExecute(Long result) {showDialog("Downloaded " + result + " bytes");}}

使用AsyncTask的规则

AsyncTask的类必须在UI线程加载（从4.1开始系统会帮我们自动完成）
AsyncTask对象必须在UI线程创建
execute方法必须在UI线程调用
不要在你的程序中去直接调用onPreExecute(), onPostExecute, doInBackground, onProgressUpdate方法
一个AsyncTask对象只能执行一次，即只能调用一次execute方法，否则会报运行时异常
AsyncTask不是被设计为处理耗时操作的，耗时上限为几秒钟，如果要做长耗时操作，强烈建议你使用Executor，ThreadPoolExecutor以及FutureTask
在1.6之前，AsyncTask是串行执行任务的，1.6的时候AsyncTask开始采用线程池里处理并行任务，但是从3.0开始，为了避免AsyncTask所带来的并发错误，AsyncTask又采用一个线程来串行执行任务

AsyncTask到底是串行还是并行？

给大家做一下实验，请看如下实验代码：代码很简单，就是点击按钮的时候同时执行5个AsyncTask，每个AsyncTask休眠3s，同时把每个AsyncTask执行结束的时间打印出来，这样我们就能观察出到底是串行执行还是并行执行。

[java] view plaincopyprint?

@Override
public void onClick(View v) {
if (v == mButton) {
new MyAsyncTask(“AsyncTask#1”).execute(“”);
new MyAsyncTask(“AsyncTask#2”).execute(“”);
new MyAsyncTask(“AsyncTask#3”).execute(“”);
new MyAsyncTask(“AsyncTask#4”).execute(“”);
new MyAsyncTask(“AsyncTask#5”).execute(“”);
}
}
private static class MyAsyncTask extends AsyncTask<String, Integer, String> {
private String mName = “AsyncTask”;
public MyAsyncTask(String name) {
super();
mName = name;
}
@Override
protected String doInBackground(String… params) {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return mName;
}
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
SimpleDateFormat df = new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss”);
Log.e(TAG, result + ”execute finish at ” + df.format(new Date()));
}
}

    @Overridepublic void onClick(View v) {if (v == mButton) {new MyAsyncTask("AsyncTask#1").execute("");new MyAsyncTask("AsyncTask#2").execute("");new MyAsyncTask("AsyncTask#3").execute("");new MyAsyncTask("AsyncTask#4").execute("");new MyAsyncTask("AsyncTask#5").execute("");}}private static class MyAsyncTask extends AsyncTask<String, Integer, String> {private String mName = "AsyncTask";public MyAsyncTask(String name) {super();mName = name;}@Overrideprotected String doInBackground(String... params) {try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}return mName;}@Overrideprotected void onPostExecute(String result) {super.onPostExecute(result);SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Log.e(TAG, result + "execute finish at " + df.format(new Date()));}}

我找了2个手机，系统分别是4.1.1和2.3.3，按照我前面的描述，AsyncTask在4.1.1应该是串行的，在2.3.3应该是并行的，到底是不是这样呢？请看Log

Android 4.1.1上执行：从下面Log可以看出，5个AsyncTask共耗时15s且时间间隔为3s，很显然是串行执行的

Android 2.3.3上执行：从下面Log可以看出，5个AsyncTask的结束时间是一样的，很显然是并行执行

结论：从上面的两个Log可以看出，我前面的描述是完全正确的。下面请看源码，让我们去了解下其中的原理。

源码分析

[java] view plaincopyprint?

/*
* 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 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;
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = “AsyncTask”;
//获取当前的cpu核心数
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//线程池核心容量
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
//线程池最大容量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//过剩的空闲线程的存活时间
private static final int KEEP_ALIVE = 1;
//ThreadFactory 线程工厂，通过工厂方法newThread来获取新线程
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
//原子整数，可以在超高并发下正常工作
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, “AsyncTask #” + mCount.getAndIncrement());
}
};
//静态阻塞式队列，用来存放待执行的任务，初始容量：128个
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
/**
* 静态并发线程池，可以用来并行执行任务，尽管从3.0开始，AsyncTask默认是串行执行任务
* 但是我们仍然能构造出并行的AsyncTask
*/
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
/**
* 静态串行任务执行器，其内部实现了串行控制，
* 循环的取出一个个任务交给上述的并发线程池去执行
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
//消息类型：发送结果
private static final int MESSAGE_POST_RESULT = 0x1;
//消息类型：更新进度
private static final int MESSAGE_POST_PROGRESS = 0x2;
/**静态Handler，用来发送上述两种通知，采用UI线程的Looper来处理消息
* 这就是为什么AsyncTask必须在UI线程调用，因为子线程
* 默认没有Looper无法创建下面的Handler，程序会直接Crash
*/
private static final InternalHandler sHandler = new InternalHandler();
//默认任务执行器，被赋值为串行任务执行器，就是它，AsyncTask变成串行的了
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
//如下两个变量我们先不要深究，不影响我们对整体逻辑的理解
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
//任务的状态默认为挂起，即等待执行，其类型标识为易变的（volatile）
private volatile Status mStatus = Status.PENDING;
//原子布尔型，支持高并发访问，标识任务是否被取消
private final AtomicBoolean mCancelled = new AtomicBoolean();
//原子布尔型，支持高并发访问，标识任务是否被执行过
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
/*串行执行器的实现，我们要好好看看，它是怎么把并行转为串行的
*目前我们需要知道，asyncTask.execute(Params …)实际上会调用
*SerialExecutor的execute方法，这一点后面再说明。也就是说：当你的asyncTask执行的时候，
*首先你的task会被加入到任务队列，然后排队，一个个执行
*/
private static class SerialExecutor implements Executor {
//线性双向队列，用来存储所有的AsyncTask任务
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
//当前正在执行的AsyncTask任务
Runnable mActive;
public synchronized void execute(final Runnable r) {
//将新的AsyncTask任务加入到双向队列中
mTasks.offer(new Runnable() {
public void run() {
try {
//执行AsyncTask任务
r.run();
} finally {
//当前AsyncTask任务执行完毕后，进行下一轮执行，如果还有未执行任务的话
//这一点很明显体现了AsyncTask是串行执行任务的，总是一个任务执行完毕才会执行下一个任务
scheduleNext();
}
}
});
//如果当前没有任务在执行，直接进入执行逻辑
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
//从任务队列中取出队列头部的任务，如果有就交给并发线程池去执行
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
/**
* 任务的三种状态
*/
public enum Status {
/**
* 任务等待执行
*/
PENDING,
/**
* 任务正在执行
*/
RUNNING,
/**
* 任务已经执行结束
*/
FINISHED,
}
/** 隐藏API：在UI线程中调用，用来初始化Handler */
public static void init() {
sHandler.getLooper();
}
/** 隐藏API：为AsyncTask设置默认执行器 */
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
return postResult(doInBackground(mParams));
}
};
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 occured while executing doInBackground()”,
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
postResult(result);
}
}
//doInBackground执行完毕，发送消息
private Result postResult(Result result) {
@SuppressWarnings(“unchecked”)
Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
/**
* 返回任务的状态
*/
public final Status getStatus() {
return mStatus;
}
/**
* 这个方法是我们必须要重写的，用来做后台计算
* 所在线程：后台线程
*/
protected abstract Result doInBackground(Params… params);
/**
* 在doInBackground之前调用，用来做初始化工作
* 所在线程：UI线程
*/
protected void onPreExecute() {
}
/**
* 在doInBackground之后调用，用来接受后台计算结果更新UI
* 所在线程：UI线程
*/
protected void onPostExecute(Result result) {
}
/**
* Runs on the UI thread after {@link #publishProgress} is invoked.
/**
* 在publishProgress之后调用，用来更新计算进度
* 所在线程：UI线程
*/
protected void onProgressUpdate(Progress… values) {
}
/**
* cancel被调用并且doInBackground执行结束，会调用onCancelled，表示任务被取消
* 这个时候onPostExecute不会再被调用，二者是互斥的，分别表示任务取消和任务执行完成
* 所在线程：UI线程
*/
@SuppressWarnings({“UnusedParameters”})
protected void onCancelled(Result result) {
onCancelled();
}
protected void onCancelled() {
}
public final boolean isCancelled() {
return mCancelled.get();
}
public final boolean cancel(boolean mayInterruptIfRunning) {
mCancelled.set(true);
return mFuture.cancel(mayInterruptIfRunning);
}
public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
ExecutionException, TimeoutException {
return mFuture.get(timeout, unit);
}
/**
* 这个方法如何执行和系统版本有关，在AsyncTask的使用规则里已经说明，如果你真的想使用并行AsyncTask，
* 也是可以的，只要稍作修改
* 必须在UI线程调用此方法
*/
public final AsyncTask<Params, Progress, Result> execute(Params… params) {
//串行执行
return executeOnExecutor(sDefaultExecutor, params);
//如果我们想并行执行，这样改就行了，当然这个方法我们没法改
//return executeOnExecutor(THREAD_POOL_EXECUTOR, params);
}
/**
* 通过这个方法我们可以自定义AsyncTask的执行方式，串行or并行，甚至可以采用自己的Executor
* 为了实现并行，我们可以在外部这么用AsyncTask：
* asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params… params);
* 必须在UI线程调用此方法
*/
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;
//这里#onPreExecute会最先执行
onPreExecute();
mWorker.mParams = params;
//然后后台计算#doInBackground才真正开始
exec.execute(mFuture);
//接着会有#onProgressUpdate被调用，最后是#onPostExecute
return this;
}
/**
* 这是AsyncTask提供的一个静态方法，方便我们直接执行一个runnable
*/
public static void execute(Runnable runnable) {
sDefaultExecutor.execute(runnable);
}
/**
* 打印后台计算进度，onProgressUpdate会被调用
*/
protected final void publishProgress(Progress… values) {
if (!isCancelled()) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
//任务结束的时候会进行判断，如果任务没有被取消，则onPostExecute会被调用
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
//AsyncTask内部Handler，用来发送后台计算进度更新消息和计算完成消息
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;
}
}
}

/** 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 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;public abstract class AsyncTask<Params, Progress, Result> {private static final String LOG_TAG = "AsyncTask";//获取当前的cpu核心数private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();//线程池核心容量private static final int CORE_POOL_SIZE = CPU_COUNT + 1;//线程池最大容量private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;//过剩的空闲线程的存活时间private static final int KEEP_ALIVE = 1;//ThreadFactory 线程工厂，通过工厂方法newThread来获取新线程private static final ThreadFactory sThreadFactory = new ThreadFactory() {//原子整数，可以在超高并发下正常工作private final AtomicInteger mCount = new AtomicInteger(1);public Thread newThread(Runnable r) {return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());}};//静态阻塞式队列，用来存放待执行的任务，初始容量：128个private static final BlockingQueue<Runnable> sPoolWorkQueue =new LinkedBlockingQueue<Runnable>(128);/*** 静态并发线程池，可以用来并行执行任务，尽管从3.0开始，AsyncTask默认是串行执行任务* 但是我们仍然能构造出并行的AsyncTask*/public static final Executor THREAD_POOL_EXECUTOR= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);/*** 静态串行任务执行器，其内部实现了串行控制，* 循环的取出一个个任务交给上述的并发线程池去执行*/public static final Executor SERIAL_EXECUTOR = new SerialExecutor();//消息类型：发送结果private static final int MESSAGE_POST_RESULT = 0x1;//消息类型：更新进度private static final int MESSAGE_POST_PROGRESS = 0x2;/**静态Handler，用来发送上述两种通知，采用UI线程的Looper来处理消息* 这就是为什么AsyncTask必须在UI线程调用，因为子线程* 默认没有Looper无法创建下面的Handler，程序会直接Crash*/private static final InternalHandler sHandler = new InternalHandler();//默认任务执行器，被赋值为串行任务执行器，就是它，AsyncTask变成串行的了private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;//如下两个变量我们先不要深究，不影响我们对整体逻辑的理解private final WorkerRunnable<Params, Result> mWorker;private final FutureTask<Result> mFuture;//任务的状态 默认为挂起，即等待执行，其类型标识为易变的（volatile）private volatile Status mStatus = Status.PENDING;//原子布尔型，支持高并发访问，标识任务是否被取消private final AtomicBoolean mCancelled = new AtomicBoolean();//原子布尔型，支持高并发访问，标识任务是否被执行过private final AtomicBoolean mTaskInvoked = new AtomicBoolean();/*串行执行器的实现，我们要好好看看，它是怎么把并行转为串行的*目前我们需要知道，asyncTask.execute(Params ...)实际上会调用*SerialExecutor的execute方法，这一点后面再说明。也就是说：当你的asyncTask执行的时候，*首先你的task会被加入到任务队列，然后排队，一个个执行*/private static class SerialExecutor implements Executor {//线性双向队列，用来存储所有的AsyncTask任务final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();//当前正在执行的AsyncTask任务Runnable mActive;public synchronized void execute(final Runnable r) {//将新的AsyncTask任务加入到双向队列中mTasks.offer(new Runnable() {public void run() {try {//执行AsyncTask任务r.run();} finally {//当前AsyncTask任务执行完毕后，进行下一轮执行，如果还有未执行任务的话//这一点很明显体现了AsyncTask是串行执行任务的，总是一个任务执行完毕才会执行下一个任务scheduleNext();}}});//如果当前没有任务在执行，直接进入执行逻辑if (mActive == null) {scheduleNext();}}protected synchronized void scheduleNext() {//从任务队列中取出队列头部的任务，如果有就交给并发线程池去执行if ((mActive = mTasks.poll()) != null) {THREAD_POOL_EXECUTOR.execute(mActive);}}}/*** 任务的三种状态*/public enum Status {/*** 任务等待执行*/PENDING,/*** 任务正在执行*/RUNNING,/*** 任务已经执行结束*/FINISHED,}/** 隐藏API：在UI线程中调用，用来初始化Handler */public static void init() {sHandler.getLooper();}/** 隐藏API：为AsyncTask设置默认执行器 */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 uncheckedreturn postResult(doInBackground(mParams));}};mFuture = new FutureTask<Result>(mWorker) {@Overrideprotected void done() {try {postResultIfNotInvoked(get());} 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);}}};}private void postResultIfNotInvoked(Result result) {final boolean wasTaskInvoked = mTaskInvoked.get();if (!wasTaskInvoked) {postResult(result);}}//doInBackground执行完毕，发送消息private Result postResult(Result result) {@SuppressWarnings("unchecked")Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,new AsyncTaskResult<Result>(this, result));message.sendToTarget();return result;}/*** 返回任务的状态*/public final Status getStatus() {return mStatus;}/*** 这个方法是我们必须要重写的，用来做后台计算* 所在线程：后台线程*/protected abstract Result doInBackground(Params... params);/*** 在doInBackground之前调用，用来做初始化工作* 所在线程：UI线程*/protected void onPreExecute() {}/*** 在doInBackground之后调用，用来接受后台计算结果更新UI* 所在线程：UI线程*/protected void onPostExecute(Result result) {}/*** Runs on the UI thread after {@link #publishProgress} is invoked./*** 在publishProgress之后调用，用来更新计算进度* 所在线程：UI线程*/protected void onProgressUpdate(Progress... values) {}/*** cancel被调用并且doInBackground执行结束，会调用onCancelled，表示任务被取消* 这个时候onPostExecute不会再被调用，二者是互斥的，分别表示任务取消和任务执行完成* 所在线程：UI线程*/@SuppressWarnings({"UnusedParameters"})protected void onCancelled(Result result) {onCancelled();}    protected void onCancelled() {}public final boolean isCancelled() {return mCancelled.get();}public final boolean cancel(boolean mayInterruptIfRunning) {mCancelled.set(true);return mFuture.cancel(mayInterruptIfRunning);}public final Result get() throws InterruptedException, ExecutionException {return mFuture.get();}public final Result get(long timeout, TimeUnit unit) throws InterruptedException,ExecutionException, TimeoutException {return mFuture.get(timeout, unit);}/*** 这个方法如何执行和系统版本有关，在AsyncTask的使用规则里已经说明，如果你真的想使用并行AsyncTask，* 也是可以的，只要稍作修改* 必须在UI线程调用此方法*/public final AsyncTask<Params, Progress, Result> execute(Params... params) {//串行执行return executeOnExecutor(sDefaultExecutor, params);//如果我们想并行执行，这样改就行了，当然这个方法我们没法改//return executeOnExecutor(THREAD_POOL_EXECUTOR, params);}/*** 通过这个方法我们可以自定义AsyncTask的执行方式，串行or并行，甚至可以采用自己的Executor* 为了实现并行，我们可以在外部这么用AsyncTask：* asyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, Params... params);* 必须在UI线程调用此方法*/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;//这里#onPreExecute会最先执行onPreExecute();mWorker.mParams = params;//然后后台计算#doInBackground才真正开始exec.execute(mFuture);//接着会有#onProgressUpdate被调用，最后是#onPostExecutereturn this;}/*** 这是AsyncTask提供的一个静态方法，方便我们直接执行一个runnable*/public static void execute(Runnable runnable) {sDefaultExecutor.execute(runnable);}/*** 打印后台计算进度，onProgressUpdate会被调用*/protected final void publishProgress(Progress... values) {if (!isCancelled()) {sHandler.obtainMessage(MESSAGE_POST_PROGRESS,new AsyncTaskResult<Progress>(this, values)).sendToTarget();}}//任务结束的时候会进行判断，如果任务没有被取消，则onPostExecute会被调用private void finish(Result result) {if (isCancelled()) {onCancelled(result);} else {onPostExecute(result);}mStatus = Status.FINISHED;}//AsyncTask内部Handler，用来发送后台计算进度更新消息和计算完成消息private static class InternalHandler extends Handler {@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})@Overridepublic void handleMessage(Message msg) {AsyncTaskResult result = (AsyncTaskResult) msg.obj;switch (msg.what) {case MESSAGE_POST_RESULT:// There is only one resultresult.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;}}
}

让你的AsyncTask在3.0以上的系统中并行起来

通过上面的源码分析，我已经给出了在3.0以上系统中让AsyncTask并行执行的方法，现在，让我们来试一试，代码还是之前采用的测试代码，我们要稍作修改，调用AsyncTask的executeOnExecutor方法而不是execute，请看：

[java] view plaincopyprint?

@TargetApi(Build.VERSION_CODES.HONEYCOMB)
@Override
public void onClick(View v) {
if (v == mButton) {
new MyAsyncTask(“AsyncTask#1”).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,“”);
new MyAsyncTask(“AsyncTask#2”).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,“”);
new MyAsyncTask(“AsyncTask#3”).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,“”);
new MyAsyncTask(“AsyncTask#4”).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,“”);
new MyAsyncTask(“AsyncTask#5”).executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,“”);
}
}
private static class MyAsyncTask extends AsyncTask<String, Integer, String> {
private String mName = “AsyncTask”;
public MyAsyncTask(String name) {
super();
mName = name;
}
@Override
protected String doInBackground(String… params) {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return mName;
}
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
SimpleDateFormat df = new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss”);
Log.e(TAG, result + ”execute finish at ” + df.format(new Date()));
}
}

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)@Overridepublic void onClick(View v) {if (v == mButton) {new MyAsyncTask("AsyncTask#1").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");new MyAsyncTask("AsyncTask#2").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");new MyAsyncTask("AsyncTask#3").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");new MyAsyncTask("AsyncTask#4").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");new MyAsyncTask("AsyncTask#5").executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,"");}}private static class MyAsyncTask extends AsyncTask<String, Integer, String> {private String mName = "AsyncTask";public MyAsyncTask(String name) {super();mName = name;}@Overrideprotected String doInBackground(String... params) {try {Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}return mName;}@Overrideprotected void onPostExecute(String result) {super.onPostExecute(result);SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");Log.e(TAG, result + "execute finish at " + df.format(new Date()));}}

下面是系统为4.1.1手机打印出的Log：很显然，我们的目的达到了，成功的让AsyncTask在4.1.1的手机上并行起来了，很高兴吧！希望这篇文章对你有用。