Velocity determines whether it is null and ""

Method 1: Valid for null and FALSE

#if (! $car. Fuel)


Method 2: null and "" are valid
#if ("$!car.fuel" = = "")
If you judge the character blank:
#if ("$car. Fuel" = = "")


Combination Method 1 and 2, the judging field can only be null
#if ((! $car. Fuel) && ("$!car.fuel" = = ""))


Method 4: Use tool detection, see: Http://wiki.apache.org/velocity/NullTool
#if ($null. IsNull ($car. Fuel))


Method 5: Use your own method to detect
#if ($car. Fuelempty)
The car class implements the Isfuelempty () method to
public boolean isfuelempty ()
{
Return true if fuel is empty.
}

Method 6: Use a custom directive. Cf. ifnulldirective, ifnotnulldirective
#ifnull ($car. Fuel)
#ifnotnull ($car. Fuel)
Must be registered in Velocity.properties.:
Userdirective = Org.apache.velocity.tools.generic.directive.Ifnull
Userdirective = Org.apache.velocity.tools.generic.directive.Ifnotnull