面试官还问Handler？那我要给你讲个故事

ActivityThread.main();//初始化入口
    1. Looper.prepareMainLooper(); //初始化
          Looper.prepare(false); //设置不可关闭
              Looper.sThreadLocal.set(new Looper(quitAllowed)); //跟线程绑定
                    1.1.Looper.mQueue = new MessageQueue(quitAllowed); //Looper和MessageQueue绑定
                    1.2.Looper.mThread = Thread.currentThread();
    2. Looper.loop();
        2.1.myLooper().mQueue.next(); //循环获取MessageQueue中的消息
              nativePollOnce(); //阻塞队列
                  native -> pollInner() //底层阻塞实现
                        native -> epoll_wait();
        2.2.Handler.dispatchMessage(msg);//消息分发

1.Handler handler = new Handler();//初始化Handler
        1.Handler.mLooper = Looper.myLooper();//获取当前线程Looper。
        2.Handler.mQueue = mLooper.mQueue;//获取Looper绑定的MessageQueue对象。

2.handler.post(Runnable);//发送消息
        sendMessageDelayed(Message msg, long delayMillis);
            sendMessageAtTime(Message msg, long uptimeMillis);
                Handler.enqueueMessage();//Message.target 赋值为this。
                    Handler.mQueue.enqueueMessage();//添加消息到MessageQueue。

       new Thread(new Runnable() {
           @Override
           public void run() {
              Handler handler = new Handler();
           }
       }).start();


       以上代码会报以下下错误

java.lang.RuntimeException: Can't create handler inside thread Thread[Thread-2,5,main] that has not called Looper.prepare()
       at android.os.Handler.<init>(Handler.java:207)
       at android.os.Handler.<init>(Handler.java:119)
       at com.example.test.MainActivity$2.run(MainActivity.java:21)
       at java.lang.Thread.run(Thread.java:919)

 public Handler(Callback callback, boolean async) {
        ...省略若干代码

       //通过 Looper.myLooper()获取了mLooper 对象，如果mLooper ==null则抛异常
        mLooper = Looper.myLooper();
        if (mLooper == null) {
             //可以看到异常就是从这报出去的
            throw new RuntimeException(
                "Can't create handler inside thread " + Thread.currentThread()
                        + " that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

 public static @Nullable Looper myLooper() {
        //而myLooper()是通过sThreadLocal.get()获取的，那sThreadLocal又是个什么鬼？
        return sThreadLocal.get();
    }

 //可以看到sThreadLocal 是一个ThreadLocal<Looper>对象，那ThreadLocal值从哪赋值的？
 static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

//sThreadLocal 的值就是在这个方法里赋值的
 private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        //具体的赋值点
        sThreadLocal.set(new Looper(quitAllowed));
    }

    //我们看下ActivityMain的入口main方法，调用了 Looper.prepareMainLooper();
    public static void main(String[] args) {
       ...
        Looper.prepareMainLooper();
        ...
    }

  //看到这一下就明白了，原来主线程在启动的时候默认就调用了prepareMainLooper(),而在这个方法中调用了prepare()。  
 //提前将sThreadLocal 进行赋值了。
  public static void prepareMainLooper() {
        prepare(false);
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

public class HandlerActivity extends AppCompatActivity {
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_handler);
        handler.sendEmptyMessageDelayed(1,5000);
    }
}

public class HandlerActivity extends AppCompatActivity {
      MyHandler handler = new MyHandler(this);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_handler);
        handler.sendEmptyMessageDelayed(1,5000);
    }
    private static class MyHandler extends Handler{
        private WeakReference<HandlerActivity> activityWeakReference;

        public MyHandler(HandlerActivity activity) {
            activityWeakReference = new WeakReference<>(activity);
        }

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
        }
    }
}