Learn Drools: Part II (Cross Product)

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Learn Drools: Part II (Cross Product)

If Drools seems foreign, try thinking of it as an RDBMS. See how their structures compare and why you can invoke one Drools Rule from another.

· Java Zone ·
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In the previous article, we got a basic idea what Drools is and how it works. In this article, we'll pay attention to the details of how Rules work.

When we write Rules, we often face the question, "How can I invoke one Rule from another like we call a method?"

Well, the answer is we can’t call a Rule from another Rule. Drools matches the Rules with incoming data/facts, and if the data satisfies the Rule condition, it stores the data in an Agenda. It might be possible for the same data or facts to be matched by different rules, so it stores matching facts in an Agenda. After firing all the rules, then it takes the action on said Agenda.

So, as the rules apply to data/facts, it is not possible to call a rule from another, as we can’t pass any arguments/facts beforehand — the facts are coming at runtime.

So, rather than confusing the Rules with functional/OOP programming, consider it with a RDBMS.

Here's how they compare:

Drools Component RDBMS Component
Data/Facts/Domain Object data structure Table
Facts/Domain Objects Rows
Fact /Domain Object attributes Column
Domain Objects HAS-A relationship Joining/Cartesian Product
Drools operators Filter Criterias
Drools When When in SQL
Drools Then Trigger
Drools Rule SQL Query

Let's try to understand the above relationship with a simple example. Suppose we have two domain objects, say Department and Employee, and each Employee is associated with a Department.

Step 1: Creating the Data Structure for Department and Employee

This is nothing but two Simple POJOs.


package com.example.droolsExample.pojo;
public class Department {
   String name;
   public String getName() {
      return name;

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


package com.example.droolsExample.pojo;
public class Employee {
   String name;
   boolean manager;
   String message;
   Department dept;
   public String getName() {
      return name;
   public void setName(String name) {
      this.name = name;
   public boolean isManager() {
      return manager;
   public void setManager(boolean manager) {
      this.manager = manager;
   public String getMessage() {
      return message;

   public void setMessage(String message) {
      this.message = message;

   public Department getDept() {
      return dept;

   public void setDept(Department dept) {
      this.dept = dept;

Step 2: Create a Simple Rule


package com.rules
import com.example.droolsExample.pojo.Employee
import com.example.droolsExample.pojo.Department

rule "print cross product"
      emp: Employee();
      dept: Department();
   System.out.println("Fire print cross product Rule");
   System.out.println(dept.getName() + "::" + emp.getName()+ ": "+emp.getDept().getName());

Here, we create a simple rule. It tells us that for every instance of Employee and Department in working memory, print the Department name and Employee name and the Employee's Department name.

It is same as the following SQL

 Select * from Employee emp Department dept where emp.deptId=dept.deptId 

So it will create all Cartesian products in working memory. If we have two departments and two employees in working memory, it creates four combinations and then fires the rules on these combinations.

Step 3: Create Working Memory and Insert Objects to Show the Result

package com.example.droolsExample;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import org.drools.compiler.compiler.DroolsParserException;
import org.drools.compiler.compiler.PackageBuilder;
import org.drools.core.RuleBase;
import org.drools.core.RuleBaseFactory;
import org.drools.core.WorkingMemory;
import com.example.droolsExample.pojo.Department;
import com.example.droolsExample.pojo.Employee;

public class DroolsTest {
   public static void main(String[] args) throws DroolsParserException,
          IOException {
      DroolsTest droolsTest = new DroolsTest();
   public void executeDroolsEmployee() throws DroolsParserException, IOException {
      PackageBuilder packageBuilder = new PackageBuilder();
      String ruleFile = "/com/rules/employee.drl";
      InputStream resourceAsStream = getClass().getResourceAsStream(ruleFile);
      Reader reader = new InputStreamReader(resourceAsStream);
      org.drools.core.rule.Package rulesPackage = packageBuilder.getPackage();
      RuleBase ruleBase = RuleBaseFactory.newRuleBase();
      WorkingMemory workingMemory = ruleBase.newStatefulSession();
      Department dep = new Department();
      Department dep1 = new Department();
      Employee emp = new Employee();
      emp.setName("Shamik Mitra");
      Employee emp1 = new Employee();
      emp1.setName("Samir Mitra");

Step 4: Checking the Output

Civil::Shamik Mitra: IT
IT::Shamik Mitra: IT
Civil::Samir Mitra: Civil
IT::Samir Mitra: Civil

Please note that as we have inserted two Employees objects and two Department objects in working memory, so it creates four different combinations. In the next article, we will discuss  how to filter the resultset using Drools.

drools, java, rdbms, rules

Published at DZone with permission of Shamik Mitra , DZone MVB. See the original article here.

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