​ 首先我们使用AsyncTask时，一般是：

new AsyncTask(...).execute()
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我们看new AsyncTask(),它走的是：

  public AsyncTask() {this((Looper) null);}public AsyncTask(@Nullable Looper callbackLooper) {//创建一个handler到后面将子线程处理的消息传递给主线程，这个handler是基于主线程创建的mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()? getMainHandler(): new Handler(callbackLooper);
//创建了一个WorkerRunnable,它实现了Callable，这个后面会讲到mWorker = new WorkerRunnable<Params, Result>() {public Result call() throws Exception {mTaskInvoked.set(true);Result result = null;try {Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);//在子线程中处理异步任务result = doInBackground(mParams);Binder.flushPendingCommands();} catch (Throwable tr) {mCancelled.set(true);throw tr;} finally {postResult(result);}//将处理结果传递出去return result;}};//FutureTask是实现了RunnableFuture，Runnable和Future接口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 occurred while executing doInBackground()",e.getCause());} catch (CancellationException e) {postResultIfNotInvoked(null);}}};}复制代码

主要是创建了三个对象，mHandler，mWorker和mFuture，mHandler是将后面处理的结果发送到主线程；mWorker主要是实现Callable接口，并在call中调用doInBackground()方法处理任务；mFuture则是实现了RunnableFuture，Runnable和Future接口，最后在done中调用postResultIfNotInvoked（）方法将处理结果传递出去。也就是主要弄清楚mWorker.call()以及mFuture.done()。下面我们分析他们分别在什么时候调用。接着往下看AsyncTask的execute（）：

 @MainThreadpublic final AsyncTask<Params, Progress, Result> execute(Params... params) {return executeOnExecutor(sDefaultExecutor, params);}@MainThreadpublic final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,Params... params) {......mStatus = Status.RUNNING;onPreExecute();mWorker.mParams = params;exec.execute(mFuture);return this;}
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我们可以看到调用AsyncTask.execute()是执行了executeOnExecutor(sDefaultExecutor, params);这个方法,我们看executeOnExecutor()方法，它执行了 onPreExecute()，对应我们应用中的异步任务前的准备操作，接着将params值传递 给mWorker，最后执行了 exec.execute(mFuture);这个exec就是上面传递进来的 sDefaultExecutor，也就是执行了sDefaultExecutor.execute()方法。sDefaultExecutor是什么呢？往下看:

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
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);}}}
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sDefaultExecutor其实就是SerialExecutor,所以就是执行了SerialExecutor的execute(mFuture)方法，然后调用了 r.run()也就是mFuture.run()，我们看FutureTask的run方法:

 public void run() {if (state != NEW ||!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))return;try {Callable<V> c = callable;if (c != null && state == NEW) {V result;boolean ran;try {result = c.call();ran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);}if (ran)set(result);}} 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 interruptsint s = state;if (s >= INTERRUPTING)handlePossibleCancellationInterrupt(s);}}
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Callable c = callable中的callable其实就是mWorker,因为我们在AsyncTask的构造方法中创建mFuture对象时将mWorker传递进去了： mFuture = new FutureTask(mWorker) ,我们看看FutureTask的构造方法：

  public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW;       // ensure visibility of callable}
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发现有 this.callable = callable，所以callable其实就是mWorker。我们接着看FutureTask的run方法，它调用了c.run(),也就是mWorker.run(),现在很清晰了，也就回调到最开始mWorker的run,调用doInBackground()方法等等。接着往下看，调用了 set(result)方法：

    protected void set(V v) {if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {outcome = v;U.putOrderedInt(this, STATE, NORMAL); // final statefinishCompletion();}}
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将result赋值给了outcome。继续往下看，调用了finishCompletion()：

  private void finishCompletion() {...done();...}
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调用了done()方法，是不是很熟悉，也就是我们之前mFuture的done()方法，接着在done()中调用postResultIfNotInvoked(get())：

public V get() throws InterruptedException, ExecutionException {int s = state;if (s <= COMPLETING)s = awaitDone(false, 0L);//返回report（）的结果return report(s);}private V report(int s) throws ExecutionException {Object x = outcome;if (s == NORMAL)//将outcome返回，也就是对应的resultreturn (V)x;if (s >= CANCELLED)throw new CancellationException();throw new ExecutionException((Throwable)x);}private void postResultIfNotInvoked(Result result) {final boolean wasTaskInvoked = mTaskInvoked.get();if (!wasTaskInvoked) {postResult(result);}}
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调用get()返回了outcome也就是result最后通过postResult()将结果传递出去:

  private Result postResult(Result result) {@SuppressWarnings("unchecked")Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,new AsyncTaskResult<Result>(this, result));message.sendToTarget();return result;}...public 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;}}...
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结果通过handler将消息传递到主线程，并执行了result.mTask.finish();我们接着看finish方法：

 private void finish(Result result) {if (isCancelled()) {onCancelled(result);} else {onPostExecute(result);}mStatus = Status.FINISHED;}
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如果取消了就执行onCancelled()，否则执行onPostExecute()将结果回调到我们的应用当中。 接着我们看在doInBackground()中回调处理进度的方法publishProgress():

protected final void publishProgress(Progress... values) {if (!isCancelled()) {getHandler().obtainMessage(MESSAGE_POST_PROGRESS,new AsyncTaskResult<Progress>(this, values)).sendToTarget();}}
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通过handler将任务进度传到主线程，我们看handleMessage(见上面的处理） result.mTask.onProgressUpdate(result.mData);是不是很熟悉，就是将进度回调到应用中，并且是在主线程中，可直接更新UI。很多很小的细节，可以再慢慢琢磨，先把大概的分析清楚，这就是AsyncTask的大致流程。