AsyncTask工作原理

AsyncTask封装了Thread,Handler,用于方便的执行后台任务以及在主线程中访问UI。
类的声明如下：

public abstract class AsyncTask 

首先看execute()方法，execute()直接调用了executeOnExecutor()方法：

可以看到，首先调用了onPreExecute(),然后将参数传给mWorker，然后执行sDefaultExecutor.execute(mFuture)

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

继续看mWorker是什么：
为实现Callable的AsyncTask的内部抽象类WorkerRunnable，增加了参数属性。

private final WorkerRunnable mWorker;
...
private static abstract class WorkerRunnable implements Callable {
        Params[] mParams;
}

搜索代码，发现在构造方法中实现了WorkerRunnable：
重写了call()方法，调用doInBackground()并返回result。
mFuture也在构造方法中被实现，FutureTask继承自RunnableFuture，为Future和Runnable结合体，同时将mWorker作为参数传给mFuture。可得，现在我们的任务被放在了mFuture中。

public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);

        mWorker = new WorkerRunnable() {
            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(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);
                }
            }
        };
    }

分析exec.execute(mFuture);
最终发现exec为SerialExecutor类型，如下：
持有一个Runnable类型的mActive，可看出来，此runnable为将要执行的runable。
持有一个Runnable类型的list，execute()中新建一个Runnable，它的run()为mFuture的run()方法、取出并执行runnable的scheduleNext()，将之插入list。即每次执行SerialExecutor.execute()都会将mFuture放入等待执行的list，然后依靠THREAD_POOL_EXECUTOR.execute()执行。
由于SerialExecutor.execute()为synchronized方法，即排队等待过程为串行进行。但可以通过AsyncTask.executeOnExecutor()指定并行线程池THREAD_POOL_EXECUTOR执行runnable。

private static class SerialExecutor implements Executor {
        final ArrayDeque mTasks = new ArrayDeque();
        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);
            }
        }
    }

THREAD_POOL_EXECUTOR的定义如下：

 static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

执行到此结束，由于执行的最终为mWorker的call()方法，再次看call()会在最终完成后台任务后调用postResult()

mWorker = new WorkerRunnable() {
            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;
            }
        };

postResult()使用Handler发出任务已完成的消息，并携带result

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

消息的处理由如下mHandler解决，当指定Looper且该Looper不是MainLooper时，就new Handler()，否则为如下类型.
当任务完成时，结束，当进度更新时，更新进度

 private static class InternalHandler extends Handler {
        public InternalHandler(Looper looper) {
            super(looper);
        }

        @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;
            }
        }
    }