一、什么是AsyncTask

AsyncTask是轻量级的用于处理简单的异步任务，不需要借助线程和Handler即可实现。除了之外还有以下方法解决线程不能更新UI组件问题；

使用Handler实现线程间通讯
Activity.runOnUIThread(Runnable);
View.post(Runnable)
View.postDelayed(Runnable);

二、AsyncTask的使用方法

1.创建AsyncTask的子类，并为三个泛型指定类型的参数，如果不需要制定则Void.
2.根据需要调用onPreExecute()，onPostExecute(Void aVoid)，doInBackground(Void... voids)，onProgressUpdate(Void... values) 等方法
3.调用AsyncTask子类的execute(Param params)执行耗时任务

  PAsyncTask pAsyncTask=new PAsyncTask();
  pAsyncTask.execute("广州");


    private class PAsyncTask extends AsyncTask<String,String,String>{

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
        }

        @Override
        protected void onProgressUpdate(String... values) {
            super.onProgressUpdate(values);
        }

        @Override
        protected String doInBackground(String... strings) {
            return null;
        }
    }

四、AsyncTask机制原理

  /**
     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
     *
     * @hide
     */
    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);
                }
            }
        };
    }

此处是AsyncTask的构造方法，创建了WorkerRunnable、FutureTask 的实例mWorker和mFuture，其中mWorker实现了call()的接口，处理doInBackground()数据，mFuture则是实现了Runnable接口的子线程，里面调用了mWorker的call()接口

 public final AsyncTask<Params, Progress, Result> 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;
    }

这里先是在UI线程中执行了onPreExecute（）； exec.execute(mFuture);方法中exec是实现了Executor接口的（如下）, r.run();是实现了Runable接口的mFuture子线程，具体执行子线程实在线程队列mTasks 中执行。

  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);
            }
        }
    }

而后在postResult（）方法中使用Handler发送消息通知UI线程

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

总结：

AsyncTask使用了FutureTask和Handler的结合，其中FutureTask实现了runnable接口，SerialExecutor则采用线程池的方式管理线程。
AsyncTask在代码上比Handler要轻量级别，但实际上比Handler更耗资源，因为AsyncTask底层是一个线程池，而Handler仅仅就是发送了一个消息队列。但是，如果异步任务的数据特别庞大，AsyncTask线程池比Handler节省开销，因为Handler需要不停的new Thread执行。
AsyncTask的实例化只能在主线程，Handler可以随意，只和Looper有关系,因此为什么开发中比较少用AsyncTask，而Handler+Runnable的方式更加灵活多变来适应不同的业务需求。