以下引用官方描述：
引用
The FreeMarker template language doesn't know the Java language null at all. It doesn't have null keyword, and it can't test if something is null or not.
1.判断是否存在,通过exists关键字或者"??"运算符。都将返回一个布尔值
user.name?exists
user.name??
<#if user.name?exists> //TO DO </#if> <#if user.age??> //TO DO </#if>
2.忽略null值
假设前提：user.name为null
${user.name}，异常
${user.name!},显示空白
${user.name!'vakin'}，若user.name不为空则显示本身的值，否则显示vakin
${user.name?default('vakin')}，同上
${user.name???string(user.name,'vakin')},同上