AsyncTask 工作原理分析 - HelloWorld开发者社区

AsyncTask 相信大家对他的使用并不陌生,它是谷歌官方提供的一种轻量级别的异步任务类,底层封装的是两个线程池和一个Handler.sDefaultExecutor=SERIAL_EXECUTOR 用来Runnable排队用的, THREAD_POOL_EXECUTOR 是用来真正执行任务的.

我们先从 execute()开始分析,executeOnExecutor(Executor exec, Params... params) 真正执行的代码如下:

    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;
}

如果执行状态是Running(正在执行)或Finish(已经执行完了),抛出异常,所以一个AsyncTask 只能执行一次. 接着调用onPreExcute(),这就是为什么onPreEcxute是在执行实际的后台操作前被UI thread调用. 任务的真正执行是在exec.execute(mFuture) 中, exec 是 SerialExecutor 的一个对象,我们就去看看SerialExecutor 是如何工作的,代码如下:

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);
        }
    }
}

我们可以知道AsyncTask在构造方法中把Params 参数转化为一个FutureTask,接着SerialExecutor 会把FutureTask对象插入到ArrayDeque 任务队列,ArrayDeque 内部使用数组存储数据,大小自增长的队列.从这里可以看书AsyncTask 是串行执行的,最终交给scheduleNext()去执行.

public AsyncTask() {
    mWorker = new WorkerRunnable<Params, Result>() {
        public Result call() throws Exception {
            mTaskInvoked.set(true);

            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
            //noinspection unchecked
            Result result = doInBackground(mParams);
            Binder.flushPendingCommands();
            return postResult(result);
        }
    };

    mFuture = new FutureTask<Result>(mWorker) {
        [@Override](https://my.oschina.net/u/1162528)
        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);
            }
        }
    };
}

我们在public AsyncTask() {}构造方法中可以得知mWorker 首先将mTaskInvoked.set(true) 接着调用doInBackground(mParams) 返回Result,最后将其返回值传递给postResult(result).

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

postResult ()获取一个InternalHandler ,通过InternalHandler 发送MESSAGE_POST_RESULT的消息,最终由InternalHandler 来处理消息

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

    @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
    [@Override](https://my.oschina.net/u/1162528)
    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;
        }
    }
}

为了能让执行环境切换到主线程,这就要求InternalHandler 必须在主线中,所以AsyncTask必须在主线程中创建.

以上就是AsyncTask的工作原理分析. 通过以上分析我们可以得知:

1. Task的实例必须在UI thread中创建
execute方法必须在UI thread中调用
不要手动的调用onPreExecute(), onPostExecute(Result)，doInBackground(Params...), onProgressUpdate(Progress...)这几个方法
该task只能被执行一次，否则多次调用时将会出现异常。如果在子线程中创建调用 onPreExecute()也在创建AsyncTask的子线程中执行，doInBackground(Params...)在子线程中执行，onPostExecute(Result)和onProgressUpdate(Progress...)在主线程中

由于本人第一次写源码分析的文章,再加上本人表达能力有限,如若哪里有误,,还望多多拍砖,请与指正,谢谢