I have this rule that for some reason does not execute even though logically it should:

rule "Getting a List from a declared variable"
  when
    $object_with_var = Foo()
    $class_var: List() from $object_with_var.getListVar()
  then
    print("Rule fired!")
end

And the object that I insert into the KieSession:

class Foo() {
  @Getter
  List() listVar;

  public void addToListVar(Object something) {
      if (this.listVar == null) {
          this.listVar = new ArrayList<>();
      }
      this.listVar.add(something);
  }
}

When I execute the rule I make sure to instantiate Foo and add the correct information to the list so it is not null and not empty. The Foo object is then added to the KieSession. When I match just for Foo in the rule such that:

...
when
    $object_with_var = Foo()
then
...

The rule does execute. But for some reason simply getting/declaring the List variable seems to give problems:

...
when
    $object_with_var = Foo()
    $class_var: List() from $object_with_var.getListVar()
then
...

• I’ve tried changing the type to Arraylist.

• I’ve tried updating my Drools eng to the most up to date version.

• Something that did work was:

rule ""
  when
    Foo(listVar != null , !listVar.isEmpty())
  then
    ...something...
end

But I want to understand why declaring the variable doesn’t work.