Android|Android AsyncTask源码阅读 AndroidAsyncTask源码阅读

AsyncTask 封装了新建后台工作线程再用Handler更新UI的过程。在官方文档也说明这个类最好用于时间短的操作，如果要保持线程长连接，还是推荐使用Executor、ThreadPoolExecutor等APIs.
AsynTask有三个范型参数（Params, Progress ，Result）和四个顺序调用的方法。

onPreExecute
doInBackground 此方法是虚方法，必须实现的。
onProgressUpdate
onPostExecute

AsyncTask 一般用法如下

AsyncTask execute = new AsyncTask() { @Override protected String doInBackground(String... strings) { Log.e(TAG, "doInBackground"); return "" + strings; }}.execute("leavesC");

AsyncTask构造方法 【Android|Android AsyncTask源码阅读】AsyncTask有三个构造方法：

public AsyncTask() public AsyncTask(Handler) public AsyncTask(Looper)

最后都会调用下面这个构造方法，核心代码重要思路都在这个方法里面

public AsyncTask(@Nullable Looper callbackLooper) { //初始化mHandler mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper() ? getMainHandler() : new Handler(callbackLooper); //WokerRunable 是抽象类，实现Callable接口。源码如下文 mWorker = new WorkerRunnable() { public Result call() throws Exception { //AtomicBoolean 实例 mTaskInvoked mTaskInvoked.set(true); Result result = null; try { //设置线程优先线 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked //调用doInBackground并且获得返回 result = doInBackground(mParams); //刷新Pending命名 Binder.flushPendingCommands(); } catch (Throwable tr) { //异常处理 //AtomicBoolean mCancelled = new AtomicBoolean(); mCancelled.set(true); //抛出异常 throw tr; } finally { postResult(result); } return result; } }; //java的FutureTask 实例化 mFuture = new FutureTask(mWorker) { @Override protected void done() { try {//发送结果 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); } } }; }

postResultIfNotInvoked() 方法

private void postResultIfNotInvoked(Result result) { // private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); //原子类AtomicBoolean 实例mTaskInvoked final boolean wasTaskInvoked = mTaskInvoked.get(); if (!wasTaskInvoked) { //发送结果 postResult(result); } } //我们熟悉的handler发送消息 private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult(this, result)); message.sendToTarget(); return result; }

AsyncTaskResult 异步结果包装类使用了范型

private static class AsyncTaskResult { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = https://www.it610.com/article/data; } }

WokerRunable 类实现Callable接口至于Callable的作用笔者前面的文章有仔细说明。请自行查阅。

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

execute()方法

public final AsyncTask execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); public final AsyncTask executeOnExecutor(Executor exec, Params... params) { //异常状态处理 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)"); } } //设置当前状态是running mStatus = Status.RUNNING; //调用onPreExecute(); onPreExecute(); mWorker.mParams = params; exec.execute(mFuture); return this; }

SerialExecutor继承了Executor,并且使用ArrayDeque来保证了线程的顺序执行

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定义 public static final Executor THREAD_POOL_EXECUTOR; 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; }

InternalHandler 处理与UI线程

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: //更新进度 result.mTask.onProgressUpdate(result.mData); break; } } }

总结

1，AsyncTask 使用ThreadPoolExecutor 实现线程池来管理线程

2，AsyncTask的SerialExecutor使用 ArrayDeque来保证顺序执行线程。

3，使用WorkerRunable和FutureTask来实现子线程和获得子线程的执行的结果。

4，使用Handler用于线程与主线程之间的通信和回调结果。