1、首先在AsyncTask被初始化时构建一个WorkerRunnable（mWorker）和一个FutureTask（mFuture），后面在执行的过程中会将封装好的mFuture放入一个线程池中执行。
2、在外界调用AsyncTask.execute方法之后，开始启动AsyncTask任务，根据代码查看调用过程如下：
@MainThread public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); }3、在execute方法中又调用了executeOnExecutor方法：
@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; }我们需要了解的是，一个AsyncTask任务有三种状态：PENDING（未开始）、RUNNING（运行中）、FINISHED（已完成）。
从上面源码我们可以看到，一个AsyncTask实例任务只能运行一次，只有是在PENDING状态下，任务才能正常运行，否则就会抛出异常。
接着在执行任务前先调用了onPreExecute方法，并将参数赋值给mWorker的参数数组，然后使用exec执行在AsyncTask初始化阶段封装好的mFuture。
4、从第二步可以知道exec就是sDefaultExecutor，它是一个Executor对象，相关的源码如下：
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; /** * 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 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); } } }从源码可以进一步看出，sDefaultExecutor是SerialExecutor对象，是一个串行的线程池，同一个进程中的所有的AsyncTask任务都在这个线程池中排队执行。
SerialExecutor的execute方法首先会将FutureTask任务加入到mTasks队列中，此时如果没有正在活动的任务就会调用scheduleNext执行下一个AsyncTask任务。
5、接下来我们再回过头来看AsyncTask的构造器中在创建mWorker和mFuture时都做了些什么：
public AsyncTask(@Nullable Looper callbackLooper) { mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper() ? getMainHandler() : new Handler(callbackLooper); mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Result result = null; try { Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked result = doInBackground(mParams); Binder.flushPendingCommands(); } catch (Throwable tr) { mCancelled.set(true); throw tr; } finally { postResult(result); } return 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); } } }; }6、先来分析一下上面源码中的WorkerRunnable的call方法：