public AsyncTask() {
    //[1]
    mWorker = new WorkerRunnable<Params, Result>() {
        public Result call() throws Exception {
            mTaskInvoked.set(true);
            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
            return postResult(doInBackground(mParams));//doInBackground(mParams)对外使用
        }
    };
    //[2]
    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);
            }
        }
    };
}

[1]

其中WorkerRunnable

@FunctionalInterface
public interface Callable<V> {
    V call() throws Exception;
}

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
    Params[] mParams;
}

将doInBackground的结果返回到postResult方法中

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

其中sHandler是InternalHandler的实例

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;
        }
    }
}

生成AsyncTaskResult

private static class AsyncTaskResult<Data> {
    final AsyncTask mTask;
    final Data[] mData;

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

这里在Handler中进程处理的是：

result.mTask.finish(result.mData[0]);

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

==到这里明白了在WorkerRunnable中回调方法执行doInBackground()并且将返回结果交给主线程Handler处理==

[2]

mFuture = new FutureTask<Result>(mWorker) {
    @Override
    protected void done() {
        try {
            postResultIfNotInvoked(get());
        } catch (Exception e) {
            postResultIfNotInvoked(null);
        }
    }
};

对于FutureTask来说,将mWorker设置进去命名callable

public FutureTask(Callable<V> callable) {
    if (callable == null)
        throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;       // ensure visibility of callable
}

此时构造完成，下面看看当我们使用的时候

String[] urls = {
                "http://blog.csdn.net/iispring/article/details/47115879",
                "http://blog.csdn.net/iispring/article/details/47180325",
                "http://blog.csdn.net/iispring/article/details/47300819",
                "http://blog.csdn.net/iispring/article/details/47320407",
                "http://blog.csdn.net/iispring/article/details/47622705"
        };

DownloadTask downloadTask = new DownloadTask();//自定义
downloadTask.execute(urls);

我们会执行execute方法

public static final Executor SERIAL_EXECUTOR = Utils.hasHoneycomb() ? new SerialExecutor() :
        Executors.newSingleThreadExecutor(sThreadFactory);

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

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

如果SDK版本大于11，生成SerialExecutor对象，否则用一个线程池

对于SerialExecutor是

@TargetApi(11)
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);
        }
    }
}

这里暂时不看对应这里的分析

我们继续看主逻辑

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();//刚进来走这里
    mWorker.mParams = params;//将参数给mWorker
    exec.execute(mFuture);//执行mFutrue

    return this;
}

==执行onPreExecute方法，这个实现是空，所以是对于子类进行复写的。也就是任务执行前的逻辑==

==着重看exec.execute(mFuture);==

所以执行的是SerialExecutor类的execute方法

public synchronized void execute(final Runnable r) {
    mTasks.offer(new Runnable() {
        public void run() {
            try {
                r.run();
            } finally {
                scheduleNext();
            }
        }
    });
    if (mActive == null) {
        scheduleNext();
    }
}

SerialExecutor将对应的mFuture添加到mTasks中，完了执行scheduleNext()

public static final Executor THREAD_POOL_EXECUTOR
        = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
        TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory,
        new ThreadPoolExecutor.DiscardOldestPolicy());

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

用一个线程池去执行刚才的任务，对于任务先执行run方法

也就是说执行的是FutureTask的run方法，然后执行下一个任务的run方法。

==也就是一个线程池执行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 = null;
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

run方法在调用c.call()，c是callable，是构造时候传递进来的mWorker,所以调用的是mWorker的call

mWorker = new WorkerRunnable<Params, Result>() {
    public Result call() throws Exception {
        mTaskInvoked.set(true);
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        return postResult(doInBackground(mParams));
    }
};

执行的是回调里面的东西。与此同时在run方法中有set(result);
也就是说将WorkerRunnable.call()的返回结果返进行set。

protected void set(V v) {
    if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
        outcome = v;//核心
        U.putOrderedInt(this, STATE, NORMAL); // final state
        finishCompletion();//[1]这个方法调用down
    }
}

==现在知道了outcome是我们在mWorker中call的返回结果==

[1]

private void finishCompletion() {
    for (WaitNode q; (q = waiters) != null;) {
        if (U.compareAndSwapObject(this, WAITERS, q, null)) {
            for (;;) {
                Thread t = q.thread;
                if (t != null) {
                    q.thread = null;
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }
    done();//调用
    callable = null;        // to reduce footprint
}

也就是执行

mFuture = new FutureTask<Result>(mWorker) {
    @Override
    protected void done() {
        try {
            postResultIfNotInvoked(get());
        } catch (Exception e) {
            postResultIfNotInvoked(null);
        }
    }
};

其中get

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
    return report(s);
}

private V report(int s) throws ExecutionException {
    Object x = outcome;//这个就是worker产生的结果
    if (s == NORMAL)
        return (V)x;
    if (s >= CANCELLED)
        throw new CancellationException();
    throw new ExecutionException((Throwable)x);
}

==所以在子线程中执行了FutureTask.run()，得到了传递构造中的worker调用了call()得到的返回值set到了outcome中，随后调用了FutureTask.done()，get得到的返回值传入postResultIfNotInvoked(get())==

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;
}

将结果放入了主线程中执行。