Spring Framework Drools tutorial with Maven

Drools is a popular open source business rules and work flow engine. It helps you externalize the rules as opposed to embedding within the Java code. This tutorial contains a a number of artifacts highlighted below.


Step 1: Create a new Maven project with the standard src/main/java, src/main/resources, src/test/java, etc source folder directories. Create a new src/main/rules folder to define the rules using the "mvel" expresion language.
Update the pom.xml file as shown below with relevant dependencies and also add the build/resources to tell maven that sr/main/rules is a source folder.

 

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.myapp</groupId>
 <artifactId>drools-poc</artifactId>
 <version>0.0.1-SNAPSHOT</version>
 <packaging>war</packaging>

 <properties>
  <drools.version>5.3.1.Final</drools.version>
  <junit.version>4.7</junit.version>
 </properties>

 <dependencies>
  <dependency>
   <groupId>junit</groupId>
   <artifactId>junit</artifactId>
   <version>${junit.version}</version>
  </dependency>
  <!-- drools library -->
  <dependency>
   <groupId>org.drools</groupId>
   <artifactId>drools-core</artifactId>
   <version>${drools.version}</version>
  </dependency>
  <dependency>
   <groupId>org.drools</groupId>
   <artifactId>drools-core</artifactId>
   <version>${drools.version}</version>
  </dependency>
  <dependency>
   <groupId>org.drools</groupId>
   <artifactId>drools-compiler</artifactId>
   <version>${drools.version}</version>
  </dependency>
  <!-- required for drools and spring integration -->
  <dependency>
   <groupId>org.drools</groupId>
   <artifactId>drools-spring</artifactId>
   <version>${drools.version}</version>
  </dependency>
  <dependency>
   <groupId>com.thoughtworks.xstream</groupId>
   <artifactId>xstream</artifactId>
   <version>1.2.2</version>
  </dependency>
 </dependencies>
 
 
 <build>
    <resources>
        <resource>
           <directory>src/main/rules</directory>
        </resource>
    </resources>
 </build>

</project>



Step 2: Define the domain classes like Applicant.java.

 

package com.company.license;

public class Applicant {

 private Integer id;
 private String name;
 private int age;
 
 public Applicant(Integer id, String name, int age) {
  super();
  this.id = id;
  this.name = name;
  this.age = age;
 }

 public Integer getId() {
  return id;
 }

 public void setId(Integer id) {
  this.id = id;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public int getAge() {
  return age;
 }

 public void setAge(int age) {
  this.age = age;
 }

 
 @Override
 public String toString() {
  return "name=" + name + ", age=" + age;
 }

}


Step 3: Define the data access interface and implementation classes that retrieves data from a persistent source like file or database. The implementation is simplified to read from a pre-populated in-memory map.

 

package com.company.license;

public interface ApplicantDao {
    Applicant findApplicant(Integer applicantId);
}


The implementation of the above interface.

package com.company.license;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ApplicantDaoImpl implements ApplicantDao {
 
 private static final Map<Integer, Applicant> data = new ConcurrentHashMap<Integer, Applicant>();
 
 static {
  data.put(1, new Applicant(1, "John", 16));
  data.put(2, new Applicant(2, "Peter", 20));
 }

 public Applicant findApplicant(Integer applicantId) {
  Applicant applicant = data.get(applicantId);
  System.out.println("fetched applicant: " + applicant);
  return applicant;
 }
}



Step 4: Define the value object class that captures the user entry via a GUI. For example, ApplicantForm.java as shown below.

package com.company.license;

public class ApplicantForm {
 
 private Integer applicantId;
 private boolean eligible = true; // eligible by default

 public Integer getApplicantId() {
  return applicantId;
 }

 public void setApplicantId(Integer applicantId) {
  this.applicantId = applicantId;
 }

 public boolean isEligible() {
  return eligible;
 }

 public void setEligible(boolean eligible) {
  this.eligible = eligible;
 }
 
 @Override
 public String toString() {
  return "applicantId=" + applicantId + ", eligible=" + eligible;
 }
}

Step 5: Define the business rules via mvel expression language in a separate external file -- applicant-license.drl. 

 

package com.company.license;

import com.company.license.ApplicantForm;
import com.company.license.Applicant;
import com.company.license.ApplicantDao;
import com.company.license.ApplicantDaoImpl;

global ApplicantDao applicantDao;
 
rule "is eligible for licence"
when
   $a : ApplicantForm();
   $applicant: Applicant( age < 18 ) from applicantDao.findApplicant($a.getApplicantId());
then
   $a.setEligible( false );
end


Step 6: Finally, the junit test class that makes use of drools to validate applicants. We will create 2 applicants, one with age >= 18, and another one with age < 18, to test the business rules engine (aka expert systems) of drools in action.


 

package com.company.license;

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;
import org.drools.logger.KnowledgeRuntimeLogger;
import org.drools.logger.KnowledgeRuntimeLoggerFactory;
import org.drools.runtime.StatefulKnowledgeSession;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

public class ApplicantLicenseTest {

 private KnowledgeBase kbase;

 @Before
 public void setup() {
  KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
  kbuilder.add(ResourceFactory.newClassPathResource("com/company/license/applicant-license.drl"),
    ResourceType.DRL);
  KnowledgeBuilderErrors errors = kbuilder.getErrors();
  if (errors.size() > 0) {
   for (KnowledgeBuilderError error : errors) {
    System.err.println(error);
   }
   throw new IllegalArgumentException("Could not parse knowledge.");
  }
  kbase = KnowledgeBaseFactory.newKnowledgeBase();
  kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());

 }

 @Test
 public void testApplicantLicense() {
  StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
  KnowledgeRuntimeLogger logger = KnowledgeRuntimeLoggerFactory.newFileLogger(ksession, "test");
  KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
  kbase.newStatefulKnowledgeSession();

  ApplicantForm applicant1 = new ApplicantForm();
  applicant1.setApplicantId(1);
  
  ApplicantForm applicant2 = new ApplicantForm();
  applicant2.setApplicantId(2);
    
  ksession.setGlobal("applicantDao", new ApplicantDaoImpl()); // assign the global dao
  
  ksession.insert(applicant1);
  ksession.insert(applicant2);
  ksession.fireAllRules();
  
  ksession.dispose();

  Assert.assertTrue(applicant1.isEligible() == false);//John is not eligible
  Assert.assertTrue(applicant2.isEligible() == true); //Peter is eligible
 }

}





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