AsyncTask源码学习总结

前言

AsyncTask作为Android常用的异步消息机制，内部核心原理其实就是Handler。本小记不对AsyncTask基本使用作介绍，学习了AsyncTask的源码，拜读郭神文章Android AsyncTask完全解析，带你从源码的角度彻底理解. ，趁热来一发，呸，作文以记之。基于android-28。

先贴一张复习图

流程概要

开搞

1、构造方法

AsyncTask task=new AsyncTask()

写在前头
总共就三步：

创建Handler对象
创建WorkerRunnable对象
创建FutureTask对象

WorkerRunnable是何物，继承于Callable，内置call方法，浅尝辄止~
FutureTask是何物，继承于Runnable，内置run方法，且引用了WorkerRunnable对象，什么作用暂且不谈，先mark~

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

        mWorker = new WorkerRunnable<Params, Result>() {
            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<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 occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

2、执行

task.execute(params);

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

默认执行方法的本质是调用了

executeOnExecutor（Executor exec, Params... params）//其第一个参数为线程池对象

不看源码，先总结executeOnExecutor核心步骤

执行onPreExecute()，也就很好地说明了onPreExecute在任务启动前执行
赋值params给WorkerRunnable对象，也就相当于丢到了FutureTask里（因为他们有PY交易关系）
线程池启动FutureTask
这里有两个重点，线程池对象和FutureTask对象，源码里sDefaultExecutor是默认的线程池对象

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

然后贴出executeOnExecutor部分代码

 @MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
       ...ignore...
        onPreExecute();

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

        return this;
    }

3、默认线程池sDefaultExecutor

sDefaultExecutor是静态内部类SerialExecutor的一个实例，不看源码直接总结其execute方法

将任务加入队列->offer
出列任务->poll，如果还有任务，则让另一个小弟来执行任务->THREAD_POOL_EXECUTOR.execute
说白了sDefaultExecutor它就是个甩锅的，棵棵

    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);
            }
        }
    }

另一个小弟THREAD_POOL_EXECUTOR也是线程池对象，暂不深究，反正接下来的动作是执行于子线程中，啊仿佛闻到了doInBackground的香气。最终要执行FutureTask.run，来了，FutureTask终于来了

4、FutureTask&WorkerRunnable

直接干核心run方法，其实里面就主要是callable.call()，callable对象哪里来，当然是那位有点♂哲学关系的WorkerRunnable兄弟，直接进入WorkerRunnable.call（）一看究竟

FutureTask.run部分代码，c即callable实例

public void run() {
       ...ignore...
        try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
        } finally {
          ...ignore...
        }
    }

上面创建AsyncTask构造方法时，也创建了WorkerRunnable。
先给call方法作个总结:

设置线程优先级
执行doInBackground
结束后执行postResult

mWorker = new WorkerRunnable<Params, Result>() {
            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;
            }
        };

其中doInBackground就是我们很熟悉的，需要重写的抽象函数
进入postResult(result)，发现了handler的踪迹

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

进入getHandler，层层上翻，发现这里的handler对象是InternalHandler是示例

   private static Handler getMainHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler(Looper.getMainLooper());
            }
            return sHandler;
        }
    }

且看InternalHandler

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;
            }
        }

handleMessage里有两个what，一个是MESSAGE_POST_RESULT即上面postResult发送的，进入.mTask.finish，任务终于收网了，若中途被取消则执行onCancelled(result)，否则执行熟悉的 onPostExecute(result)

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

handleMessage的空一格what，MESSAGE_POST_PROGRESS很显然是更新进度，调用熟悉的onProgressUpdate，在哪里发送呢？必须是publishProgress

protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

补充

任务调度的默认线程池是SerialExecutor

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

分析其代码，执行完一个任务后，在finally进行下一个任务的检查，当有多个任务

        new MyAsyncTask("AsyncTask#1").execute("");  
        new MyAsyncTask("AsyncTask#2").execute("");  
        new MyAsyncTask("AsyncTask#3").execute("");  
        new MyAsyncTask("AsyncTask#4").execute("");  
        new MyAsyncTask("AsyncTask#5").execute("");

那么
AsyncTask是默认串行工作的（依次排队挨打）
如果不需要考虑并发带来的问题，可以
自定义线程池对象，调用executeOnExecutor达到并行效果(一排同时挨打)

AsyncTask源码学习总结

前言

开搞

1、构造方法

2、执行

3、默认线程池sDefaultExecutor

4、FutureTask&WorkerRunnable

补充

END