java - How to check if a variable exists in a FreeMarker template? - Stack Overflow
https://stackoverflow.com/questions/306732/how-to-check-if-a-variable-exists-in-a-freemarker-template
FAQ - Apache FreeMarker Manual
https://freemarker.apache.org/docs/app_faq.html
FreeMarker : How to check if a variable exists in a FreeMarker template
原文：https://www.cnblogs.com/rgqancy/p/12195924.html