How to set null to a variable in freemarker

问题

I can't find anything related to this on any question, and it is something really basic, but I can't figure it out.

So my problem is that I don't know how to set null to a variable in freemarker. Example:

${hi!"bye"}          <#-- Prints "bye" because hi is undefined -->
<#assign hi="hi">    <#-- Sets a value to the var hi -->
${hi!"bye"}          <#-- Prints "hi" because hi has a value -->
<#assign hi=null>    <#-- This does not work but is what I am looking for -->
${hi!"bye"}          <#-- I want it to print "bye" because hi should be undefined -->

I have this problem because I iterate over a list and set a var if some logic to the specific item validates, and then check if the var exists, but if the first item creates the var, then I will have the var set for the rest of the items in the list.

回答1:

No, there's no "unassign", nor the concept of null exists in FreeMarker (until 2.4.0 at least, but that's far away anyway). It only have missing variables (maybe technically a null, maybe doesn't exist at all) and those that are there. I don't really get why is that needed in your case. Can you show a simplified example of the situation?

回答2:

You could assign an empty string to your variable and check with the buit-in ?has_content if it is set:

${hi?has_content?then(hi, "bye")}
<#assign hi="hi">
${hi?has_content?then(hi, "bye")}
<#assign hi="">
${hi?has_content?then(hi, "bye")}

This will render:

bye
hi
bye

回答3:

Depending on what you need it for, you can use a different type to indicate a "missing" value.

For instance, if you have myVariable that is normally a number, assign false to it, and then instead of checking myVariable??, check myVariable!false?is_number. This will cover both cases (non-existent and "unset").

${ (myVariable!false?is_number)?c }

<#assign myVariable = 12 >
${ (myVariable!false?is_number)?c }

<#assign myVariable = false >
${ (myVariable!false?is_number)?c }

Result:

false
12
false

Go try.

来源：https://stackoverflow.com/questions/25190559/how-to-set-null-to-a-variable-in-freemarker

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freemarker