Use ternary operator in freemarker?

If you're using freemarker 2.3.23 or newer, you can use the then built-in:

<a href="${a?then('a.htm','b.html')}" target="${openTarget}">

If you're using an older version of freemarker, you can use instead the string built-in:

<a href="${a?string('a.htm','b.html')}" target="${openTarget}">

When applied to a boolean, the string built-in will act as a ternary operator.

If you are using older Freemarker versions before 2.3.23, then you can use this macro which provides a straightforward way to do ternary operations:

<#macro if if then else=""><#if if>${then}<#else>${else}</#if></#macro>

It's easy to use, looks nice and quite readable:

<@if someBoolean "yes" "no"/>

Note that it is @if - and not #if as in the built-in directive. Here are some more examples.

<!-- `else` is optional -->
<@if someBoolean "someBoolean is true"/>  

<!-- expressions -->
<@if (someBoolean||otherBoolean)  "hello,"+user.name  1+2+3 />  

<!-- with parameter names -->
<@if someBoolean then="yes" else="no" />  

<!-- first in list? -->
<#list seq as x>
    <@if (x_index==0)  "first"  "not first"/> 
<#list> 

For some reason you can't add parenthesis around nameless parameters, if they are non-boolean expressions. That could have increased readability even more.

You can define a custom function if that is declared like so:

<#function if cond then else="">
  <#if cond>
    <#return then>
  <#else>
    <#return else>
  </#if>
</#function>

The function can be used in any ${...} expression. Your code would look like so:

<a href="${if(a, 'a.htm', 'b.htm')}">

In contrast to @kapep, I think you should use a function, not a macro. Macros produce (textual) output, while functions return a value that can for example be assigned to a variable, but also written to the output, so using a function is more flexible. Furthermore, the way to apply the function is much closer to using a ternary operator, which would also be used inside ${...} expressions, not as a directive.

For example, if you need the conditional link target multiple times, it would make sense to assign it to a local variable:

<#assign targetUrl=if(a, 'a.htm', 'b.htm')/>
<a href="${targetUrl}">link 1</a>
...
<a href="${targetUrl}">link 2</a>

Using the function instead of the macro, @kapep's examples would look like this:

<!-- `else` is optional -->
${if(someBoolean, "someBoolean is true")}  

<!-- expressions -->
${if(someBoolean||otherBoolean, "hello,"+user.name, 1+2+3)} 

<!-- with parameter names: not possible with functions,
     but also not really helpful -->

<!-- first in list? -->
<#list seq as x>
    ${if(x_index==0, "first", "not first")}
<#list>