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.判斷是否存在,通過exists關(guān)鍵字或者"??"運算符裤唠。都將返回一個布爾值
user.name?exists
user.name??
Html代碼
<#if user.name?exists>
//TO DO
</#if>
<#if user.age??>
//TO DO
</#if>
2.忽略null值
假設(shè)前提：user.name為null
${user.name}种蘸，異常
${user.name!},顯示空白
${user.name!'vakin'}，若user.name不為空則顯示本身的值劈彪，否則顯示vakin
${user.name?default('vakin')}，同上
${user.name???string(user.name,'vakin')},同上
