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 workflow 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

Related Posts
Drools 5 Case Study 2 - Complex Event Processing
Rule Engine Stress Testing
Brainteaser Drools: Testing Objects
Feedback by the Drools Team
Drools 5 - Complex Event Processing
Drools - tutorial on writing DSL template
Drools - Stop executing current agenda group and all rules
Drools - working with Stateless session

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