What Is The Rule Engine?

A Rule Engine is a Logic, or a Condition, like “When some conditions are evaluated to true, then do some task.”A rule engine may be viewed as a sophisticated if/then statement interpreter. The if/then statements that are interpreted are called rules. Imagine a rule engine as a system that takes data and rules as input. It will apply those rules to the data and will give us an output based on the rule definition. In this article we will learn detailed about rule engine and some popular rule engines for java. Let’s take the example of online shopping where we like to give customers certain promotions or discounts. The if portions of rules contain conditions such as cart.getTotal() > 1000. The then portions of rules contain actions such as addDiscount(amount).

if (cart.getTotal() > 1000)

Above define shopping cart and customer as the data where rules will be executed if they meet the conditions defined in the ruleset.


Advantages of Rule Engine

Greater Flexibility

Keeping your rules into a Knowledge Base lets you adapt easily to your decisions when they are changing.

Easy to Understand

Rules are easy to understand as compared to other business logic. And this also creates a bridge between Business Analyst and Developers to understand and implement the business logic.

Reduced Complexity

It reduced the complexity as we no longer need to build the rule engine logic in the source.


By keeping rules in one place leads to greater reusability of your business rules.

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Some of the Most Popular Rule Engines for Java


Drools is a Business Rules Management System (BRMS) solution. It provides a core Business Rules Engine (BRE), a web authoring and rules management application (Drools Workbench), full runtime support for Decision Model and Notation (DMN) models at Conformance level 3, and an Eclipse IDE plugin for core development.

OpenL Tablets

EasyRule is a lightweight rule engine API. It provides the Rule abstraction to create rules with conditions and actions, and the RulesEngine API that runs through a set of rules to test conditions and execute actions. Here is some core feature of the EasyRule:

  • A lightweight rule engine API
  • Annotation and POJO based.
  • Supports composite rule constructions.
  • Supports expression languages (Like MVEL and SpEL) to define rules.

Easy Rules

Easy Rules is a simple Java rules engine providing a lightweight and POJO based framework to define the business. It can create complex rules from primitive ones by using the composite pattern. This framework, in contrast to the most traditional rules engines, doesn’t make use of XML files or any Domain Specific Language files to segregate rules from the application. It uses annotation-based classes and methods for injecting business logic into the application. Easy Rules can be handy for developers to create and maintain applications with business logic that’s entirely separated from the application itself. On the other hand, as this framework doesn’t implement the JSR94 standard and the business logic has to be coded straight to Java code.


RuleBook provides a simple and intuitive DSL that is flexible. If you have a larger collection of Rules, we can build them as annotated POJOs and RuleBook can transform the whole package into a RuleBook instantly. It provides an easy-to-use Lambda enabled Domain Specific Language or using POJO.
In this blog, we are going to cover the Drools Rule Engine. Let’s take a real-time example so that we will get a better idea. Consider Amazon live sale is going on, in that Amazon is offering multiple discounts on different bank cards.


In the above diagram, the basic flow is if the customer is having an HDFC card then the customer will get a 10% discount on the purchase of > 10000. Similarly, DBS cardholders will get a 10% discount on the purchase of > 5000, and ICICI cardholders will get a 10% discount on the purchase of > 15000.


So far so good. Now let’s start doing some code,

  1. Create a spring boot project from STS or http://start.spring.io/
  2. Add following dependencies

a. drools-decision tables

b.  drools-core

c. drools-compiler

3.  We will be going to keep the example simple so we will just create one package in that there will be basically 4 files.

a. DroolsConfig – To configure our drools setting.

b. OfferController – For rest service call.

c. OrderEntity – Pojo class

d. SpringDroolsApplication – Main class to run the project

4.  In resources

a. application.properties – For changing the port

b. offer.xls – Decision table

Now we are done with the project setup, let’s add some code in class files.


package com.drools.example;

import org.kie.api.KieServices;
import org.kie.api.builder.KieBuilder;
import org.kie.api.builder.KieFileSystem;
import org.kie.api.builder.KieModule;
import org.kie.api.builder.KieRepository;
import org.kie.api.builder.ReleaseId;
import org.kie.api.runtime.KieContainer;
import org.kie.api.runtime.KieSession;
import org.kie.internal.io.ResourceFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

public class DroolConfig {

 private KieServices kieServices = KieServices.Factory.get();

 private KieFileSystem getKieFileSystem() {
 KieFileSystem kieFileSystem = kieServices.newKieFileSystem();
 return kieFileSystem;


 public KieContainer getKieContainer() {
 // Container created
 KieBuilder kb = kieServices.newKieBuilder(getKieFileSystem());
 KieModule kieModule = kb.getKieModule();
 return kieServices.newKieContainer(kieModule.getReleaseId());

 private void getKieRepository() {
 final KieRepository kieRepository = kieServices.getRepository();
 kieRepository.addKieModule(new KieModule() {
 public ReleaseId getReleaseId() {
 return kieRepository.getDefaultReleaseId();

 public KieSession getKieSession(){
 //session created
 return getKieContainer().newKieSession();




package com.drools.example;

import org.kie.api.runtime.KieSession;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;

public class OfferController {
 private KieSession session;

 public OrderEntity orderNow(@RequestBody OrderEntity order) {
 return order;



 package com.drools.example;
public class OrderEntity {
private String name;
private String cardType;
private int discount;
private int price;
//getters and setters


 package com.drools.example;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
public class SpringDroolsApplication {
public static void main(String[] args) {
SpringApplication.run(SpringDroolsApplication.class, args);

We are done with the coding part, now let’s check the API.

We Build A Meditation Application For A Non-Profit Organization

Case 1

The customer has HDFC card and the order amount is less than 10000

Request Body


Response Body

"name": "Mobile",
"cardType": "HDFC",
"discount": 0,
"price": 9000

Case 2

The customer has an HDFC card and the order amount is greater than 10000

API URL: http://localhost:9091/order

Request Body


Response Body

"name": "Mobile",
"cardType": "HDFC",
"discount": 10,
"price": 15000

That’s it. We have successfully implemented the Rule Engine using the Drools decision table. The complete project you can find here.



In this article we understand the concept of rule engine and Some popular rule engines for java. Hope you like this article. If you have any doubts or suggestions regarding this please comment us.

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