Springboot整合html 报java.lang.IllegalArgumentException: Root element name cannot be null
解决：
<!DOCTYPE> <html> 改为
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml">
