How to check if a variable exists in a FreeMarker template?

Question

To check if the value exists:

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

Or with the standard freemarker syntax:

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

To check if the value exists and is not empty:

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

This one seems to be a better fit:

<#if userName?has_content>
... do something
</#if>

http://freemarker.sourceforge.net/docs/ref_builtins_expert.html

Also I think if_exists was used like:

Hi ${userName?if_exists}, How are you?

which will not break if userName is null, the result if null would be:

Hi , How are you?

if_exists is now deprecated and has been replaced with the default operator ! as in

Hi ${userName!}, How are you?

the default operator also supports a default value, such as:

Hi ${userName!"John Doe"}, How are you?

I think a lot of people are wanting to be able to check to see if their variable is not empty as well as if it exists. I think that checking for existence and emptiness is a good idea in a lot of cases, and makes your template more robust and less prone to silly errors. In other words, if you check to make sure your variable is not null AND not empty before using it, then your template becomes more flexible, because you can throw either a null variable or an empty string into it, and it will work the same in either case.

<#if p?? && p?has_content>1</#if>

Let's say you want to make sure that p is more than just whitespace. Then you could trim it before checking to see if it has_content.

<#if p?? && p?trim?has_content>1</#if>

UPDATE

Please ignore my suggestion -- has_content is all that is needed, as it does a null check along with the empty check. Doing p?? && p?has_content is equivalent to p?has_content, so you may as well just use has_content.

For versions previous to FreeMarker 2.3.7

You can not use ?? to handle missing values, the old syntax is:

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