AsyncTask是后台执行操作在UI线程发布结果的对象，很轻松的使用UI线程，用于耗时较短的后台操作。

AsyncTask原理

AsyncTas类初始化sDefaultExecutor和sHandler变量，用于执行后台执行任务和分发消息给主线程

AsyncTask对象创建时实现后台执行函数doInBackground()和发布UI结果的回调函数onPostExecute()

构造AsyncTask时

构建类型为WorkerRunnable的mWork对象，执行doInBackground()，然后发送结束消息给sHandler

构建类型FutureTask的mFuture对象，以mWork为回调参数，并重写任务结束回调方法done()，如果上面没有发送消息没有成功会再次发给sHandler

mWork和mFuture中发送消息都调用postResult()，message的obj指向的是AsyncTaskResult对象，其包含AsyncTask对象和消息类型标记

execute()启动时在执行线程池处理任务之前先调用onPreExecute()

mFuture进入sDefaultExecutor线程池任务队列消费

任务执行结束调用FutureTask的done()方法，将执行完毕消息通过postResult()发送给sHandler

sHandler收到消息根据what标记分别调用asyncTask对象的发布结果回调方法和更新UI进度方法

注明： AsyncTask必须在主线程中创建后台执行回调调用publishProgress()就是发送更新进度消息给sHandler

动手实现

public abstract class AsyncTask
    
      {    private static final int MESSAGE_POST_RESULT = 1;//结束标记    private static final int MESSAGE_POST_PROGRESS = 2;//更新进度标记    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();//cpu核心数    //最少2个或者4个核心线程    //cpu核心数少一个线程，以免饱和    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;    private static final int KEEP_ALIVE_SECONDS = 30;    //线程创建工厂    private static final ThreadFactory sThreadFactory = new ThreadFactory() {        private final AtomicInteger mCount = new AtomicInteger(1);        public Thread newThread(Runnable r) {            参数2为线程名字            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());        }    };    //最多容纳128个任务，再多任务就会被阻塞    private static final BlockingQueue
     
       sPoolWorkQueue =            new LinkedBlockingQueue
      
       (128);    static {        //初始化线程执行器配置        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,                sPoolWorkQueue, sThreadFactory);        threadPoolExecutor.allowCoreThreadTimeOut(true);        sDefaultExecutor = threadPoolExecutor;//暂不使用哪个串行执行的执行器    }    private static volatile Executor sDefaultExecutor;    private static InternalHandler sHnadler;    //不适用懒加载直接使用    static {        sHnadler = new InternalHandler();    }    private final WorkerRunnable
       
         mWorker;    private final FutureTask
        
          mFuture;    public AsyncTask() {        //mWorker的参数在execute的时候补上        mWorker = new WorkerRunnable
         
          () { @Override public Result call() throws Exception { Result result = doInBackground(mParams);//调用后台执行操作 postResult(result);//通知handler调用发布结果回调 return result; } }; mFuture = new FutureTask
          
           (mWorker);//暂不包装二次检查发送 } //发送结束消息给handler private void postResult(Result result) { Message message = Message.obtain(); message.what = MESSAGE_POST_RESULT; message.obj = new AsyncTaskResult(this, result); } //发送更新消息给handler protected void publishProgress(Progress... values) { Message message = Message.obtain(); message.what = MESSAGE_POST_PROGRESS; message.obj = new AsyncTaskResult(this, values); } //执行任务 public void execute(Params... values) { onPreExecute(); mWorker.mParams = values; sDefaultExecutor.execute(mFuture); } protected void onPreExecute() { } //后台操作回调 protected abstract Result doInBackground(Params... var1); //发布结果回调 protected void onPostExecute(Result result) { } //进度回调 protected void onProgressUpdate(Progress... values) { } //主线程处理消息调用asyncTask的回调 private static class InternalHandler extends Handler { public InternalHandler() { super(Looper.getMainLooper()); } @Override public void handleMessage(Message msg) { AsyncTaskResult result = (AsyncTaskResult) msg.obj; switch (msg.what) { case MESSAGE_POST_RESULT: result.mTask.onPostExecute(result.mData[0]);//发布结果回调 break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData);//更新进度回调 break; } } } //通过线程传递给doInBackground用户定义好的参数 private static abstract class WorkerRunnable
           
             implements Callable
            
              { Params[] mParams; } //消息携带类 private static class AsyncTaskResult
             
               { AsyncTask mTask;//消息发送所属AsyncTask Result[] mData;//结果参数 public AsyncTaskResult(AsyncTask mTask, Result... mData) { this.mTask = mTask; this.mData = mData; } }}复制代码

使用

asyncTask.execute("http://www.baidu.com");...    AsyncTask
    
      asyncTask = new AsyncTask
     
      () {        @Override        protected String doInBackground(String... strings) {            try {                Thread.sleep(1000);            } catch (InterruptedException e) {                e.printStackTrace();            }            return "hello";        }        @Override        protected void onPostExecute(String value) {            super.onPostExecute(value);            Toast.makeText(MainActivity.this, value + " over", Toast.LENGTH_SHORT).show();        }    };复制代码