Question

I am having difficulty understanding the part value : value == 0? How does this code work?

rule "My rule"
   when
       m : MyClass( value : value == 0)
    then
end

Answer 1

Assuming you are using Drools 5.4 or a newer snapshot, you can write any boolean expression as a constraint, so value == 0 is a constraint where “value” is a field name in MyClass.

Drools also allows you do use “:” to bind an attribute to a variable name, like this:

<variable_name> : <fieldName>

So, you can write:

MyClass( $var : value == 0)

Finally, since Drools uses a “context-aware” parser, you can have a variable with the same name as the attribute name, because Drools knows what comes before the : is a variable name, not a field. So, in your example: the variable “value” will be bound to the attribute “value” and the constraint “value == 0” will be true if the value attribute is equal to 0.

Hope it helps.