不懂就问，这个判断是啥意思？

String origin = request.getHeader("Origin");
if (!"".equals(origin) && origin != null) {
  response.setHeader("Access-Control-Allow-Origin", "xxx");
  response.setHeader("Access-Control-Allow-Methods", "POST, GET, HEAD, OPTIONS, DELETE");
  response.setHeader("Access-Control-Max-Age", "3600");
  response.setHeader("Access-Control-Allow-Credentials", "true");
  response.setHeader("Access-Control-Allow-Headers",
                     "Content-Type, Access-Control-Allow-Headers, Authorization, X-Requested-With");
}
filterChain.doFilter(request, response);

CORS 相关的代码，这里为啥要判断 origin 不等于空？