Velocity CheckingForNull, checkingfornull

Velocity CheckingForNull, checkingfornull

Q: I want to check for null, something like this:

#if ($car.fuel == null)

A: There are several approaches. Select the one most suitable depending on what you really want to do.

Approach 1:Use the fact that null is evaluated as a false conditional. (cf. http://velocity.apache.org/engine/devel/user-guide.html#Conditionals)

#if( ! $car.fuel )

• Note:The conditional will also pass if the result of $ car. fuel is the boolean false. What this approach is actually checking is whether the reference isNull or false.

 

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)

#if( "$!car.fuel" == "" )

• Note:The conditional will also pass if the result of $ car. fuel is an empty String. What this approach is actually checking is whether the reference isNull or empty.

BTW, just checkingEmptyCan be achieved:

#if( "$car.fuel" == "" )

 

Approach 3:Combine Approach 1 and 2. This will checkNull and null only.

#if ((! $car.fuel) && ("$!car.fuel" == ""))

• Note:The logic underlying here is that: "(null or false) and (null or> empty-string)" => if true, must be null. this is true because "false and empty-string and not null" is never true. IMHO, this makes the template too complicated to read.

 

Approach 4:Use a Tool that can check for null (NullTool, ViewNullTool ).

#if( $null.isNull($car.fuel) )

• Note:Of course, NullTool must be in the Context as $ null in this case.

 

Approach 5:Don't check for null directly, use a self-explaining method.

#if( $car.fuelEmpty )

• Note:This is my (Shinobu Kawai's) recommended solution. You have to implement the method, but it makes the template so easy-to-read.

 

public boolean isFuelEmpty(){  // return true if fuel is empty.}

Approach 6:Use a custom directive. cf. IfNullDirective, IfNotNullDirective

#ifnull( $car.fuel )#ifnotnull( $car.fuel )

• Note:You will have to register the directive in your velocity. properties.

userdirective = org.apache.velocity.tools.generic.directive.Ifnulluserdirective = org.apache.velocity.tools.generic.directive.Ifnotnull