App启动的入口
ActivityThread.java

public static void main(String[] args) {
  ...
  Looper.prepareMainLooper();
  ...
  ActivityThread thread = new ActivityThread();
  thread.attach(false, startSeq);

  if (sMainThreadHandler == null) {
    sMainThreadHandler = thread.getHandler();
  } 
  Looper.loop();
}

Looper.java

    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
    ......
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mThread = Thread.currentThread();
    }
    ......
    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

    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 static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }