在Java LinkedList上执行Drools insertLogical

Question

I have a LinkedList created (instantiated) in Java, which have several nodes (items). This list is also used in Drools, and to be exactly, modified by Drools. I need to insert a new item in the list, but this insertion must be only temporary. I do not know when the removal of this item could occur, so I'm obliged to use insertLogical statement (right?).

This is my code, i add three "Nodo" item in the list called "lista". Nodo is the class that follow, which has the constructor and the set/get method:

package com.sample;

public class Nodo {

    private int valore;

    public Nodo(){}

    public Nodo(int valore){
        this.valore=valore;
    }

    public int getValore() {
        return valore;
    }

    public void setValore(int valore) {
        this.valore = valore;
    }

}

this is java Main class instead, which call fireAllRules():

package com.sample;

/**
* This is a sample class to launch a rule.
*/

import java.util.LinkedList;
import java.util.List;

import org.kie.api.KieServices;
import org.kie.api.runtime.KieContainer;
import org.kie.api.runtime.KieSession;


public class DroolsTest {

public static final void main(String[] args) {
    try {
       // load up the knowledge base
       KieServices ks = KieServices.Factory.get();
       KieContainer kContainer = ks.getKieClasspathContainer();
       KieSession kSession = kContainer.newKieSession("ksession-rules");           

       List<Nodo> lista = new LinkedList();
       Nodo n1 = new Nodo(11);
       Nodo n2 = new Nodo(12);
       Nodo n3 = new Nodo(13);

       lista.add(n1);
       lista.add(n2);
       lista.add(n3);

       kSession.insert(lista);
       kSession.fireAllRules();

       for(Nodo nodo : lista){
           System.out.println( nodo.getValore());
       }

       System.out.println( "End");

    } catch (Throwable t) {
        t.printStackTrace();
    }
}
}

The output of Main class is

11 12 13 50

but what i really wanted was only

11 12 13

because the "50" value is inserted logically in Drools and then immediately removed. The "hello" message is not printed, because the 50 item isn't in the list anymore. (correctly, because the list items was removed).

The Drools rule file is that which follows:

rule "Rule 1"
no-loop true
when
    $l : LinkedList(get(0).getValore() == 11)
then
    Nodo a = new Nodo(50);
    $l.add(0,a);
    insertLogical($l);
end


rule "Rule 2"
no-loop true
when
    $l : LinkedList(get(0).getValore() == 50)
then
    modify($l){remove(0)};
end


rule "Rule 3"
when
    $p: Nodo(valore==50)
then
   System.out.println("hello");
end

I tried this way, and I established the list item is correctly retracted from Drools working memory but the item on the list on Java side still continues to exists. I would like to know why the list item remains, how to remove it (without explicitly destroying).

Is this a Drools problem with LinkedList or I have not correctly understood insertLogical usage?

I also tried to use LinkedList items defined not n the java.util package, but in the drools one "org.drools.core.util.LinkedList" with some error compilation.

What is the correct solutions ? Thanks

Answer 1

There is Java: with objects of all kinds. They have their methods, and with collections (like LinkedList) you can call methods to change the their composition, adding or removing elements.

There is Drools: it has a big pool of Facts (Working Memory, WM), each of which is a Java object. Promotion to a Fact is done by an insert operation, and a Fact can be demoted to its nothing-but-POJO existence by a retract: automatically or explicitly.

No add to a collection has any effect on the added object being or not being a Fact. Reciprocally, neither insertion into or removal from working memory changes anything in the Java-ish existence of the object.

Moreover, a list is one object, and its elements are other objects. Each object is jealously safeguarding its own status as a Fact, and a collection doesn't care two hoots about any of its elements becoming promoted or demoted - and you can bet that a list element couldn't care less about its container being in the WM or not.

Later

Rule 1 is weird: its right hand side destroys the truth of the left hand side, due to the addition of a Nodo(50) at index 0. There is no modify or update on the list object, and therefore the logically inserted fact isn't retracted right away. Even weirder: the RHS inserts the list object that is already in WM.

Rule 2 is never executed, because the add in Rule 1 is not signalled to the Engine.

Rule 3 is never executed, because the Nodo(50) is never inserted as a Fact anyway.