How to define variable in side drools and add values to them - drools

Question

How to define variable and add values to it inside DRL file that can be used between the rules as a static resource.
I tried to use global keyword but when add I add values to it i will not be effected inside Working Memory as it mentioned in documentation. And in my case i conn't add it from Application side.
Example :
global java.util.List myList;
function java.util.List initMyList() {
List list = new ArrayList();
list.add(1);
return list;
}
rule "initListRule"
salience 1001
when
eval(myList == null)
then
myList = initMyList();
end
rule "checkIfListIsStillEmptyRule"
salience 1000
when
eval(myList != null)
then
System.out.println("MyList is Not null");
end
as global are not stored in woking memory then myList will be always null as it is not provided from Application side. is there any alternative to define variables and fill them in DRL ?

Answer 1

A DRL global is not evaluated dynamically and therefore your rule "checkIfListIsStillEmptyRule" will not fire.
You can do
rule "initListFact"
when
not List()
then
insert( new ArrayList() );
end
rule "checkThatThereIsAnEmptyList"
when
$list: List( size == 0 )
then
modify( $list ){ add( "something" ) }
end
If you don't need to observe changes of a global you can initialize it in a rule with very high salience. It will be available as a resource but you cannot base rule conditions on its state.
global list myList
rule "initListRule"
salience 9999999999
when
then
myList = new ArrayList();
end
You can use more than one global:
global List myListOne
global List myListTwo
rule "initListsRule"
salience 9999999999
when
then
myListOne = new ArrayList();
myListTwo = new ArrayList();
end
If you need to react to changes, there's no way around facts.
declare NamedList
name: String
list: ArrayList
end
rule createListOne
when
not NamedList( name == "one" )
then
insert( new NamedList( "one", new ArrayList() ) );
end
rule "checkIfListOneIsStillEmpty"
when
$nl: NamedList( name == "one", eval( list.size() == 0 ) )
then
modify( $nl ){vgetList().add( "something" ) }
end