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;
    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;
    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;
    
    private final AtomicBoolean mCancelled = new AtomicBoolean();
    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();
                    }
                }
            });
            if (mActive == null) {
            	//如果当前没有任务在执行，直接进入执行逻辑  
                scheduleNext();
            }
        }

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

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    /**
     * 任务的三种状态
     */  
    public enum Status {
        /** 任务等待执行
         * Indicates that the task has not been executed yet.
         */
        PENDING,
        /**任务正在执行
         * Indicates that the task is running.
         */
        RUNNING,
        /**任务已经执行结束
         * Indicates that {@link AsyncTask#onPostExecute} has finished.
         */
        FINISHED,
    }

    /** @hide Used to force static handler to be created. 
     * 隐藏API：在UI线程中调用，用来初始化Handler 
     * */
    public static void init() {
//        sHandler.getLooper();
    }

    /** @hide  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);
        }
    }

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

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

    /**
     * Override this method to perform a computation on a background thread. The
     * specified parameters are the parameters passed to {@link #execute}
     * by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates
     * on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    protected abstract Result doInBackground(Params... params);

    /**
     * Runs on the UI thread before {@link #doInBackground}.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    protected void onPreExecute() {
    }

    /**
     * <p>Runs on the UI thread after {@link #doInBackground}. The
     * specified result is the value returned by {@link #doInBackground}.</p>
     * 
     * <p>This method won't be invoked if the task was cancelled.</p>
     *
     * @param result The result of the operation computed by {@link #doInBackground}.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object) 
     */
    @SuppressWarnings({"UnusedDeclaration"})
    protected void onPostExecute(Result result) {
    }

    /**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     * The specified values are the values passed to {@link #publishProgress}.
     *
     * @param values The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    @SuppressWarnings({"UnusedDeclaration"})
    protected void onProgressUpdate(Progress... values) {
    }

    /**
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     * 
     * <p>The default implementation simply invokes {@link #onCancelled()} and
     * ignores the result. If you write your own implementation, do not call
     * <code>super.onCancelled(result)</code>.</p>
     *
     * @param result The result, if any, computed in
     *               {@link #doInBackground(Object[])}, can be null
     * 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @SuppressWarnings({"UnusedParameters"})
    protected void onCancelled(Result result) {
        onCancelled();
    }    
    
    /**
     * <p>Applications should preferably override {@link #onCancelled(Object)}.
     * This method is invoked by the default implementation of
     * {@link #onCancelled(Object)}.</p>
     * 
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     *
     * @see #onCancelled(Object) 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled() {
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally. If you are calling {@link #cancel(boolean)} on the task,
     * the value returned by this method should be checked periodically from
     * {@link #doInBackground(Object[])} to end the task as soon as possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return mCancelled.get();
    }

    /**
     * <p>Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when <tt>cancel</tt> is called,
     * this task should never run. If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.</p>
     * 
     * <p>Calling this method will result in {@link #onCancelled(Object)} being
     * invoked on the UI thread after {@link #doInBackground(Object[])}
     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}
     * is never invoked. After invoking this method, you should check the
     * value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground(Object[])} to finish the task as early as
     * possible.</p>
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     *        task should be interrupted; otherwise, in-progress tasks are allowed
     *        to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled,
     *         typically because it has already completed normally;
     *         <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     */
    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result.
     *
     * @param timeout Time to wait before cancelling the operation.
     * @param unit The time unit for the timeout.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     * @throws TimeoutException If the wait timed out.
     */
    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     * 
     * <p>Note: this function schedules the task on a queue for a single background
     * thread or pool of threads depending on the platform version.  When first
     * introduced, AsyncTasks were executed serially on a single background thread.
     * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
     * to a pool of threads allowing multiple tasks to operate in parallel. Starting
     * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
     * executed on a single thread to avoid common application errors caused
     * by parallel execution.  If you truly want parallel execution, you can use
     * the {@link #executeOnExecutor} version of this method
     * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
     * on its use.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     * @see #execute(Runnable)
     */ /**
     * 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); 
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     * 
     * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to
     * allow multiple tasks to run in parallel on a pool of threads managed by
     * AsyncTask, however you can also use your own {@link Executor} for custom
     * behavior.
     * 
     * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
     * a thread pool is generally <em>not</em> what one wants, because the order
     * of their operation is not defined.  For example, if these tasks are used
     * to modify any state in common (such as writing a file due to a button click),
     * there are no guarantees on the order of the modifications.
     * Without careful work it is possible in rare cases for the newer version
     * of the data to be over-written by an older one, leading to obscure data
     * loss and stability issues.  Such changes are best
     * executed in serial; to guarantee such work is serialized regardless of
     * platform version you can use this function with {@link #SERIAL_EXECUTOR}.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param exec The executor to use.  {@link #THREAD_POOL_EXECUTOR} is available as a
     *              convenient process-wide thread pool for tasks that are loosely coupled.
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #execute(Object[])
     */
    /**
     * 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;

        onPreExecute();

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

    /**
     * Convenience version of {@link #execute(Object...)} for use with
     * a simple Runnable object. See {@link #execute(Object[])} for more
     * information on the order of execution.
     *
     * @see #execute(Object[])
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     *//**
     * 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);
    }

    /**
     * This method can be invoked from {@link #doInBackground} to
     * publish updates on the UI thread while the background computation is
     * still running. Each call to this method will trigger the execution of
     * {@link #onProgressUpdate} on the UI thread.
     *
     * {@link #onProgressUpdate} will note be called if the task has been
     * canceled.
     *
     * @param values The progress values to update the UI with.
     *
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

    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;
        }
    }
}