One of the blog readers, who posted a comment in my previous post Drools – tutorial on writing DSL template asked to me to help him with creating DSL for the following rule, so I decided to use his example as a small case study:

For readability, I added some comments to the original rule:

rule 'Rank accomodation name'
salience 90
when
     //Matches every AccomodationBase
     $accBase: AccomodationBase()

     //Uses inline eval to evaluate that there is
     //no AccomodationBase of type AccomodationRank
     //in the session.

     //To remind: inline eval evaluated only once
     //and then it is cached by Drools.

     not AccomodationBase(eval($accBase
                       instanceof AccomodationRank))

     //Matches every AccomodationRank that has the same
     //level and the description as the AccomodationBase
     $accRank: AccomodationRank(
            level == $accBase.level,
	    description == $accBase.description)
then
     //Increments the score
     $accRank.setScore($accRank.getScore()+1);
end

For my solution, I created two POJOs, tester class DSL and DSLR files. The following is my DSL:

[when]AccomodationBaseObj = $accBase: AccomodationBase()
[when]There is Accomodation base object of type Accomodation rank = eval($accBase instanceof AccomodationRank)
[when]AccomodationRankObj = $accRank: AccomodationRank(level == $accBase.level, description == $accBase.description)
[then]IncrementScore = $accRank.setScore($accRank.score+1);
[then]PrintScore = System.out.println("Rank score: " + $accRank.score);
[then]PrintLevel = System.out.println("Base level: " + $accBase.level);

and this is the DSLR file:

package net.javabeansdotasia.casestudy;

expander accomodation.dsl

import net.javabeansdotasia.casestudy.pojo.AccomodationBase;
import net.javabeansdotasia.casestudy.pojo.AccomodationRank;

rule "Rank"
dialect "mvel"
when
   AccomodationBaseObj
   not (There is Accomodation base object of type Accomodation rank)
   AccomodationRankObj
then
   IncrementScore
   PrintScore
   PrintLevel
end

In the following class I load DSL and DSLR files in to the KnowledgeBuilder and get a KnowledgeBase object. Once I have the KnowledgeBase object, I can get StatefulKnowledgeSession or StatelessKnowledgeSession, depends on what I want to do.

package net.javabeansdotasia.casestudy.utils;

import org.drools.KnowledgeBase;
import org.drools.KnowledgeBaseFactory;
import org.drools.builder.KnowledgeBuilder;
import org.drools.builder.KnowledgeBuilderError;
import org.drools.builder.KnowledgeBuilderErrors;
import org.drools.builder.KnowledgeBuilderFactory;
import org.drools.builder.ResourceType;
import org.drools.io.ResourceFactory;

public class MyKnowledgeBaseFactory {
  public static KnowledgeBase
                createKnowledgeBaseFromDSL(String dslr,
			String dsl) throws Exception {

   KnowledgeBuilder builder =
            KnowledgeBuilderFactory
				.newKnowledgeBuilder();

     //Attention!!!!
     //Add DSL BEFORE DSLR
     builder.add(
              ResourceFactory.newClassPathResource(dsl),
						ResourceType.DSL);

     builder.add(
              ResourceFactory.newClassPathResource(dslr),
				ResourceType.DSLR);

     KnowledgeBuilderErrors errors = builder.getErrors();

      if (errors.size() > 0) {
	for (KnowledgeBuilderError error : errors) {
		System.err.println(error);
	}
	throw new IllegalArgumentException("Could not parse knowledge.");
      }
      KnowledgeBase knowledgeBase =
                KnowledgeBaseFactory.newKnowledgeBase();
      knowledgeBase.addKnowledgePackages(
                    builder.getKnowledgePackages());
      return knowledgeBase;
   }
}

Below is my Tester class:

package net.javabeansdotasia.casestudy.test;

import net.javabeansdotasia.casestudy.pojo.AccomodationBase;
import net.javabeansdotasia.casestudy.pojo.AccomodationRank;
import net.javabeansdotasia.casestudy.utils.MyKnowledgeBaseFactory;

import org.drools.KnowledgeBase;
import org.drools.runtime.StatefulKnowledgeSession;

public class Test {

   public static final void main(String[] args) {
     try {
        KnowledgeBase kbase =
               MyKnowledgeBaseFactory
		   .createKnowledgeBaseFromDSL(
                        "accomodation.dslr",
			 "accomodation.dsl");
	StatefulKnowledgeSession ksession =
              kbase.newStatefulKnowledgeSession();
	AccomodationBase accomBase =
                  new AccomodationBase(9, "Just Demo");
	AccomodationRank accomRank =
                  new AccomodationRank(9, "Just Demo");
	ksession.insert(accomBase);
	ksession.insert(accomRank);
	ksession.fireAllRules();
      } catch (Throwable t) {
	 t.printStackTrace();
      }
    }
}

As you may have noticed, in Drools 5 the process of loading the rule files (DSL and DSLR) and getting a working session is different to Drools 4. In Drools 5 there is a whole new set of APIs. Basically, main change is that Drools now is knowledge oriented, instead of rule oriented. It was a real step forward in order to support other forms of logic, such as work-flow and event processing. You can read about it in Drools 5.0 docs.

That it. Please note that the above example was tested by me and its working fine. I also included source files for the above example as Eclipse project. You can simply create a new Java project from this existing Ant build.xml file.

Also, I wanted to point out that in my Eclipse project setup my Drools binaries located under JAVA_HOME/lib/drools-5.0-bin/ (have a look at the build.xml)

Cheers