如何檢查FreeMarker模板中是否存在變量？ - How to check if a variable exists in a FreeMarker template?

問題：

I have a Freemarker template which contains a bunch of placeholders for which values are supplied when the template is processed. 我有一個Freemarker模板，其中包含一堆佔位符，在處理模板時會爲其提供值。 I want to conditionally include part of the template if the userName variable is supplied, something like: 如果提供了userName變量，我想有條件地包含模板的一部分，如：

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

However, the FreeMarker manual seems to indicate that if_exists is deprecated, but I can't find another way to achieve this. 但是，FreeMarker手冊似乎表明if_exists已被棄用，但我找不到另一種方法來實現這一點。 Of course, I could simple providing an additional boolean variable isUserName and use that like this: 當然，我可以簡單地提供一個額外的布爾變量isUserName並使用如下：

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

But if there's a way of checking whether userName exists then I can avoid adding this extra variable. 但是如果有一種檢查userName是否存在的方法，那麼我可以避免添加這個額外的變量。

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解決方案：

參考一： https://en.stackoom.com/question/1HnI
參考二： https://stackoom.com/question/1HnI