AsyncTask的基本用法這裡就不在贅述了，是個安卓開發者就會。

1.android 3.0以前的 AsyncTask

private static final int CORE_POOL_SIZE = 5;  
private static final int MAXIMUM_POOL_SIZE = 128;  
private static final it KEEP_ALIVE = 10;  
……  
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,  
        MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);  
 

在這裡同一時刻能夠執行的執行緒數為5個，執行緒池總大小為128，當執行緒數大於核心時，終止前多餘的空閒執行緒等待新任務的最長時間為10秒。在3.0之前的AsyncTask可以同時有5個任務在執行，而3.0之後的AsyncTask同時只能有1個任務在執行。

2.讓我們來看看android 4.3版本的 AsyncTask

AsyncTask建構函式：

 /**
     * 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);
                }
            }
        };
    }
 

這段程式碼初始化了兩個變數，mWorker和mFuture，並在初始化mFuture的時候將mWorker作為引數傳入。mWorker是一個Callable物件，mFuture是一個FutureTask物件，這兩個變數會暫時儲存在記憶體中，稍後才會用到它們。

我們要運用AsyncTask時，大多時候會呼叫execute()方法，來看看execute()的原始碼：

    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);}
 

返回了executeOnExecutor並傳進去sDefaultExecutor（預設的執行緒池）。先看看executeOnExecutor的原始碼：

 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;
    }

傳入的執行緒池exec呼叫了execute方法並將上文提到的mFuture傳了進去。

這個傳進來的執行緒池sDefaultExecutor就是預設的執行緒池SerialExecutor也就是呼叫了SerialExecutor的execute()方法：

  public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
  private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

SerialExecutor的原始碼：

        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);
            }
        }
    }

呼叫SerialExecutor的execute方法這裡可以看到傳進來一個Runnable，這個Runnable就是上文提到的mFuture(FutureTask)，第九行執行了FutureTask的run方法：

 public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         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 interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

在run方法中執行了c.call,這裡的c就是我們上文提到的mWorker（WorkerRunnable）。執行WorkerRunnable的call方法：

 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));
            }

最後一行postResult()方法原始碼：

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

我們發現就是傳送了一個訊息，上面的程式碼傳送的訊息由這裡接受：

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;
            }
        }
    }

訊息是MESSAGE_POST_RESULT所以會執行 result.mTask.finish(result.mData[0])  ，finish原始碼：

  private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

當被取消時會執行 onCancelled(result);否則就會呼叫 onPostExecute(result);這樣我們就可以在onPostExecute方發中得到我們需要的結果result來進行下一步的處理了。

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);
            }
        }

  private static final int CORE_POOL_SIZE = 5;
    private static final int MAXIMUM_POOL_SIZE = 128;
    private static final int KEEP_ALIVE = 1;
...
  public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

AsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, null);

                LikeListTask mLikeListTask=new LikeListTask();
		mLikeListTask.executeOnExecutor(Executors.newCachedThreadPool(), null);

Executor exec =new ThreadPoolExecutor(0,  Integer.MAX_VALUE,     
				  0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>());    
		new LikeListTask().executeOnExecutor(exec, null);