自10年大量看源码后，很少看了，抽时间把最新的源码看看！
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 = 10;
private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10);
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 ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static final int MESSAGE_POST_CANCEL = 0x3;
private static final InternalHandler sHandler = new InternalHandler();
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private volatile Status mStatus = Status.PENDING;
/**
* 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,
}
/**
* 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 {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;
try {
result = 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) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
}
/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}
protected abstract Result doInBackground(Params... params);
protected void onPreExecute() {
}
@SuppressWarnings({"UnusedDeclaration"})
protected void onPostExecute(Result result) {
}
@SuppressWarnings({"UnusedDeclaration"})
protected void onProgressUpdate(Progress... values) {
}
protected void onCancelled() {
}
public final boolean isCancelled() {
return mFuture.isCancelled();
}
public final boolean cancel(boolean mayInterruptIfRunning) {
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);
}
public final AsyncTask<Params, Progress, Result> execute(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;
sExecutor.execute(mFuture);
return this;
}
protected final void publishProgress(Progress... values) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
private void finish(Result result) {
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;
case MESSAGE_POST_CANCEL:
result.mTask.onCancelled();
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;
}
}
}
以上是AsyncTask的全部代码。
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AsyncTask多用于Android的Context(Activity)处理后台逻辑同时又要兼顾主线程的一些逻辑(比如说Activity的UI更新)。
AsyncTask它其实是封装了一个多线程、向开发者屏蔽了考虑多线程的问题。开发人员只需去重写AsyncTask中的doInBackground、onProgressUpdate、onPreExecute、onPostExecute等方法，然后生成该对象并执行execute方法即可实现异步操作。
由于Android负责Context(Activity)的主线程(或者说是UI控制线程)不是线程安全的，也就是说开发者不能在自己生成的线程对象里面操作Activity的UI更新。
对于习惯了Java开发的开发人员来说，非要自己生成线程对象，在这个线程里操作一些时间较长的逻辑(比如下载文件)，完成之后再提醒用户下载完成(UI更新，比如在一个TextView中把文字修改成"下载完成")的话，那么UI更新的工作只能借助于Handler（这个Handler必须是在主线程中生成，或者说和主线程共用一个Looper和MessageQueue，通常的方法是在Activity中的onCreate方法中生成重写的Handler对象）。当开发者的线程完成逻辑操作之后，发送一个消息到Handler，再由Handler去处理。如果Handler由主线程生成，那么这个Handler的handleMessage()会在主线程中执行，因此在handleMessage()中可以访问Activity中的某个View并修改。如一下代码:
public class TestActivity extends Activity{
private static final int WHAT = 0x01;<br>
private Thread downloadThread;
private Handler refleshHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
setContentView(R.layout.main);
super.onCreate(savedInstanceState);
downloadThread = new Thread(new DownloadRunnable());
refleshHandler = new RefleshHandler();
downloadThread.start();
}
private class DownloadRunnable implements Runnable {
public void run() {
System.out.println("开始处理业务");
//耗时较长的逻辑代码省略
refleshHandler.sendEmptyMessage(WHAT);
}
}
private class RefleshHanler extends Handler{
@Override
public void handleMessage(Message msg) {
if(msg.what == WHAT){
//更新TestActivity UI
}
}
}
上述的方法也未尝不是一种好的方法。然而Android为开发者提供了一种新的解决方案，那就是AsyncTask，分析完源代码会发现，其实AsyncTask为我们封装了上述的方法。
对于我们重写AsyncTask的doInBackground()的方法，AsyncTask会将它封装成一个实现Callable接口的对象，在线程池中找出一个线程是实现它，因此我们所需要的耗时较长的逻辑可以放在这个方法里面。
AsyncTask也维护着一个静态的Handler，这个Handler属于创建AsyncTask的线程，而创建AsyncTask一般都是主线程(UI线程)，因此这个Handler可以访问并Activity的UI。
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;
case MESSAGE_POST_CANCEL:
result.mTask.onCancelled();
break;
}
}
}
可以看到这个Handler操作了AsyncTask的三个方法：finish() , onProgressUpdate() , onCancelled(),而finish() 又调用onPostExecute()方法。
可以这么说，onProgressUpdate() ,onCancelled(),onPostExecute() 用来被开发者重写，去更新UI的，这3个方法会涉及到UI的操作，因此doInBackground()方法里不能调用这几个方法，也就是说开发者可以重写这些方法，但是又不能直接调用这些方法，只能通过发送消息给Handler的方式来隐式的调用。
onProgressUpdate()是一个更新处理进度的方法，开发者可以重写它，可以将它关联到Activity的一个进度条控件上，在doInBackground()里可以用publishProgress()去间接调用它(其实这个函数也是通过发送what为MESSAGE_POST_PROGRESS的Message给Handler的方式来调用onProgressUpdate()的)，这样就能在UI上显示进度信息。
finish()方法，也就是onPostExecute()，是处理完doInBackground()得到结果之后的调用。doInBackground()的函数封装在实现Callable接口的名叫一个WorkerRunnable的抽象类中，再将这个类封装在Future中，并重写Future的done()方法，这个方法会在Callable的call方法执行完之后调用，就是doInBackground()方法执行完之后调用，它可以获得执行完的结果,Future.get()方法（可能阻塞），并将得到的结果封装在what为MESSAGE_POST_RESULT的Message中，并发送。上诉的核心代码如下：
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;
try {
result = 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) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
同理，onCancelled()是在AsyncTask被取消时的调用，也是通过Handler的方法。
有一点要注意到，还有一个onPreExecute()方法，虽然这个方法是未被Handler加入到消息处理的方法里，但是这个方法是在execute()里执行的，execute是主线程(UI线程)才会去执行的，所以这个方法也能访问和修改UI。
总结： 开发者可以重写AsyncTask的onProgressUpdate() , onCancelled(), onPostExecute(), onPreExecute(), doInBackground()方法，以得到异步实现后台逻辑并更新UI的操作。
其中onProgressUpdate() , onCancelled(), onPostExecute(), onPreExecute() 可以直接访问并修改UI。
但是doInBackground()不能出现涉及UI的操作，也不能直接调用onProgressUpdate() , onCancelled(), onPostExecute(), onPreExecute() 这四个方法，后三者不需要调用，可以通过publishProgress()去间接的调用onProgressUpdate()方法。
最后要说的是AsyncTask的对象一旦生成之后,execute()方法只能被调用一次，即使是同样的操作，也需要重新生成AsyncTask对象才行。