如何检查FreeMarker模板中是否存在变量？

问题描述

我有一个Freemarker模板，其中包含一堆占位符，在处理模板时会为其提供值。如果提供了userName变量，我想有条件地包含模板的一部分，例如：

I have a Freemarker template which contains a bunch of placeholders for which values are supplied when the template is processed. I want to conditionally include part of the template if the userName variable is supplied, something like:

[#if_exists userName]
  Hi ${userName}, How are you?
[/#if_exists]

但是，FreeMarker手册似乎表明不推荐使用if_exists ，但我找不到另一种方法来实现这一目标。当然，我可以简单地提供一个额外的布尔变量isUserName并使用如下：

However, the FreeMarker manual seems to indicate that if_exists is deprecated, but I can't find another way to achieve this. Of course, I could simple providing an additional boolean variable isUserName and use that like this:

[#if isUserName]
  Hi ${userName}, How are you?
[/#if]

但是如果有办法检查userName是否存在那么我可以避免添加这个额外的变量。

But if there's a way of checking whether userName exists then I can avoid adding this extra variable.

推荐答案

检查价值是否存在：

[#if userName??]
   Hi ${userName}, How are you?
[/#if]

或者使用标准的freemarker语法：

Or with the standard freemarker syntax:

<#if userName??>
   Hi ${userName}, How are you?
</#if>

检查价值是否存在且不为空：

To check if the value exists and is not empty:

<#if userName?has_content>
    Hi ${userName}, How are you?
</#if>

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