Velocity nulls and empty strings

问题

In velocity I have a variable which its value is null. I don't want to display anything in that case.

Currently the template engine translates "" to null so I have to do.

#set ( $a = "")
#if ($a) 
   assert("never prints a neither gets here: " + $a)
#end

Is there a way I could do that directly? I'd like to be able to make something like:

This is the variable $a. ## in case that $a is null i don't want 'dollar a' to be displayed

回答1:

$!a does the trick. You can use this form directly without an if check.

回答2:

You want Quiet Reference Notation: $!a

Here's your example:

This is the variable $!a.

If $a is null or "", Velocity will render:

This is the variable .

Official Guide section: https://velocity.apache.org/engine/devel/user-guide.html#quietreferencenotation

回答3:

Another alternative is to modify your if statement per Checking for Null (thanks for the link @xavi-lópez):

Approach 2: Use the fact that null is evaluated as an empty string in quiet references. (cf. http://velocity.apache.org/engine/devel/user-guide.html#quietreferencenotation)

So, your code would be:

#set ( $a = "")
#if ("$a" != "") 
   assert("never prints a neither gets here: " + $a)
#end

来源：https://stackoverflow.com/questions/12263548/velocity-nulls-and-empty-strings