code: public void addInterceptor(Interceptor interceptor) {
      if (interceptor == null) {
        throw new NullPointerException("interceptor can't be null");
      }
      if (interceptorChain == null) {
        interceptorChain = new InterceptorChain();
      }
      interceptorChain.addInterceptor(interceptor);
    }
docs: 添加拦截器