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

          
        }.execute("leavesC");

AsyncTask构造方法

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<Params, Result>() {
            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<Result>(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<Result>(this, result));
        message.sendToTarget();
        return result;
    }

AsyncTaskResult 异步结果包装类 使用了范型

 private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = 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<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }

execute()方法

  public final AsyncTask<Params, Progress, Result> 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<Params, Progress, Result> 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<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);
            }
        }
    }
//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<Runnable>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。