//AsyncTask是个抽象类
public abstract class AsyncTask<Params, Progress, Result> {
  private static final String LOG_TAG = "AsyncTask";

  //Cpu数量，创建线程池使用
  private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
  //核心线程，创建线程池使用
    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
  //最大线程数量，创建线程池使用
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
  //存活时间，创建线程池使用
    private static final int KEEP_ALIVE = 1;

  //创建线程的工厂sThreadFactory，创建线程池使用
  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());
      }
  };

  //存放Runnable任务的队列，创建线程池使用
  private static final BlockingQueue<Runnable> sPoolWorkQueue =
          new LinkedBlockingQueue<Runnable>(128);


  //创建线程池THREAD_POOL_EXECUTOR, 实现多线程并行
  public static final Executor THREAD_POOL_EXECUTOR
          = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                  TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

  //自定义的串行线程池
  public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

  //Message种类:发送结果
  private static final int MESSAGE_POST_RESULT = 0x1;
  //Message种类:发送进度
  private static final int MESSAGE_POST_PROGRESS = 0x2;

  //默认的线程池:串行的线程池
  private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    
  //内部的Handler
  private static InternalHandler sHandler;

  //WorkerRunnable是自定义的Callable，从参数猜测是用来存放传进来的参数和传出去的结果
  private final WorkerRunnable<Params, Result> mWorker;
  
  //FutureTask是继承至RunableFuture，可接收一个callable为参数，可以有返回结果，而且里面cancel(boolean mayInterruptIfRunning)可以取消任务
  private final FutureTask<Result> mFuture;

  //事件状态
  private volatile Status mStatus = Status.PENDING;
  
  //是否取消任务
  private final AtomicBoolean mCancelled = new AtomicBoolean();
  
  //标识任务是否被执行过  
  private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
    
  //串行的线程池，保证串行顺序执行任务
  private static class SerialExecutor implements Executor {
      final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
      Runnable mActive;

      public synchronized void execute(final Runnable r) {//将 Runnable加入到队列中，再THREAD_POOL_EXECUTOR处理Runnable
          mTasks.offer(new Runnable() {
              public void run() {
                  try {
                      r.run();
                  } finally {//上面执行完毕后才能执行下一个任务
                      scheduleNext();
                  }
              }
          });
          if (mActive == null) {
              scheduleNext();
          }
      }

      protected synchronized void scheduleNext() {
          if ((mActive = mTasks.poll()) != null) {
              THREAD_POOL_EXECUTOR.execute(mActive);//使用THREAD_POOL_EXECUTOR处理Runnable
          }
      }
  }

  /**
   * Indicates the current status of the task. Each status will be set only once
   * during the lifetime of a task.
   * 每一种状态只能改变一次，也就是说整个生命周期只能执行一次任务
   */
  public enum Status {
      /**
       * Indicates that the task has not been executed yet.
       */
      PENDING,//待定状态
      /**
       * Indicates that the task is running.
       */
      RUNNING,//执行状态
      /**
       * Indicates that {@link AsyncTask#onPostExecute} has finished.
       */
      FINISHED,//结束状态
  }

  private static Handler getHandler() {
      synchronized (AsyncTask.class) {
          if (sHandler == null) {
              sHandler = new InternalHandler();//这里直接new Handler(),说明是在UI线程中进行的，所以AsyncTask要在主线程初始化
          }
          return sHandler;
      }
  }

  /** @hide */
  public static void setDefaultExecutor(Executor exec) {//设置默认线程池，可选串行和并行
      sDefaultExecutor = exec;
  }

  /**
   * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
   */
  public AsyncTask() {  
      
      //mWork是继承自callable，封装成执行doInBackground的单位
      mWorker = new WorkerRunnable<Params, Result>() {  
          public Result call() throws Exception {
              mTaskInvoked.set(true);//任务被执行了

              Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);//设置线程优先级是后台线程
              //noinspection unchecked
              Result result = doInBackground(mParams);//执行doInBackground(),并保存结果
              Binder.flushPendingCommands();
              return postResult(result);//将结果发送出去
          }
      };
      //FutureTask使用Callable作为构造函数的参数
      mFuture = new FutureTask<Result>(mWorker) {
          @Override
          protected void done() {
              try {
                  postResultIfNotInvoked(get());//返回结果: 如果mTaskInvoked==true，则不会返回结果，因为在mWorker里面已经返回了
              } 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);//取消执行
              }
          }
      };
  }

  private void postResultIfNotInvoked(Result result) {
      final boolean wasTaskInvoked = mTaskInvoked.get();//检查是否执行过
      if (!wasTaskInvoked) {//未执行过
          postResult(result);
      }
  }

  private Result postResult(Result result) {//将结果封装在Message，发送给Handler处理
      @SuppressWarnings("unchecked")
      Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
              new AsyncTaskResult<Result>(this, result));//将结果给内部Handler处理
      message.sendToTarget();
      return result;
  }

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

  @WorkerThread //执行的方法，在线程池处理
  protected abstract Result doInBackground(Params... params);


  @MainThread //执行前调用，在UI线程执行
  protected void onPreExecute() {
  }

  @SuppressWarnings({"UnusedDeclaration"})
  @MainThread  //执行后调用，在UI线程执行
  protected void onPostExecute(Result result) {
  }

  @SuppressWarnings({"UnusedDeclaration"})
  @MainThread //进度改变，在UI线程执行
  protected void onProgressUpdate(Progress... values) {
  }


  @SuppressWarnings({"UnusedParameters"})
  //  //执行取消后调用的方法
  @MainThread 
  protected void onCancelled(Result result) {
      onCancelled();
  }    
  
  //执行取消后调用的方法
  @MainThread  
  protected void onCancelled() {
  }

  /**
   * 检查任务是否取消
   */
  public final boolean isCancelled() {
      return mCancelled.get();
  }

  /**
   * 取消任务
   */
  public final boolean cancel(boolean mayInterruptIfRunning) {
      mCancelled.set(true);
      return mFuture.cancel(mayInterruptIfRunning);
      //取消mFuture任务，不需要发送信息给handler，因为mFuture结束后，也会发送postResult给Handler，
      //Handler就可以根据mCancelled进行处理
  }

  /**
   * 获取任务执行的结果
   */
  public final Result get() throws InterruptedException, ExecutionException {
      return mFuture.get();
  }

  /**
   * 最多等待为使计算完成所给定的时间之后,检索其结果(如果结果可用),则返回
   */
  public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
          ExecutionException, TimeoutException {
      return mFuture.get(timeout, unit);
  }

  /**
   * execute调用executeOnExecutor
   */
  @MainThread
  public final AsyncTask<Params, Progress, Result> execute(Params... params) {
      return executeOnExecutor(sDefaultExecutor, params);//使用默认的线程池
  }

  /**
   * 分析的入口
   */
  @MainThread
  public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
          Params... params) {
      //检查状态
      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)");
          }
      }
      //将状态设置为运行，在workRunnable执行的时候也会设置为running状态
      mStatus = Status.RUNNING;
      
      //可见onPreExecute是在主线程进行
      onPreExecute();
      //将参数也赋进去
      mWorker.mParams = params;
      
      //加入线程池执行
      exec.execute(mFuture);

      return this;
  }
  
//如果参数是runnable，直接通过默认的线程池执行，跟普通线程池运行差不多
  @MainThread
  public static void execute(Runnable runnable) {
      sDefaultExecutor.execute(runnable);
  }

  /**
   * 改变进度，可以在doInbackground调用
   */
  @WorkerThread
  protected final void publishProgress(Progress... values) {
      if (!isCancelled()) {
          getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                  new AsyncTaskResult<Progress>(this, values)).sendToTarget();
      }
  }

  //任务结束，这个方法是在Handler里面调用
  private void finish(Result result) {
      if (isCancelled()) {
          onCancelled(result);//执行取消
      } else {
          onPostExecute(result);//任务完成
      }
      mStatus = Status.FINISHED;
  }

  //内部定义的Handler
  private static class InternalHandler extends Handler {
      public InternalHandler() {
          super(Looper.getMainLooper());//获取Looper
      }

      @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
      @Override
      public void handleMessage(Message msg) {//对AsyncTaskResult进行处理
          AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
          switch (msg.what) {
              case MESSAGE_POST_RESULT://任务结束
                  // There is only one result
                  result.mTask.finish(result.mData[0]);
                  break;
              case MESSAGE_POST_PROGRESS://改变进度
                  result.mTask.onProgressUpdate(result.mData);
                  break;
          }
      }
  }

  //自定义的带有参数的Callable
  private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
      Params[] mParams;
  }
  
  //封装的返回结果，主要给handler使用的
  @SuppressWarnings({"RawUseOfParameterizedType"})
  private static class AsyncTaskResult<Data> {
      final AsyncTask mTask;
      final Data[] mData;

      AsyncTaskResult(AsyncTask task, Data... data) {
          mTask = task;
          mData = data;
      }
  }
}

总结: 好累，下次再总结吧...