FreeMarker 对null值的处理

以下引用官方描述：

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?? 
Html代码  收藏代码
<#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

eg:当outCar.startNumberKm为空时则取startNumberKmByLast作用域中的值

<div class="grid_5 lable-right">
		     <label class="form-lbl">起始公里数(km)：</label>
			</div>
			<div class="grid_6">
				<input type="text" name="startNumberKm" id="startNumberKm" maxlength="10"
			      value="${outCar.startNumberKm!'${startNumberKmByLast}'}"  class="form-txt" />
			</div>
			<div class="grid_1"><em class="form-req">*</em></div>