Spring Boot Drools DRL: A Comprehensive Guide with Code Examples

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[Drools]( is a popular open-source Business Rules Management System (BRMS) that allows you to define and execute complex business rules in your application. It provides a flexible and declarative way to implement rules using a domain-specific language called DRL (Drools Rule Language).

In this article, we will explore how to integrate Drools with a Spring Boot application and use DRL to define and execute business rules.

Introduction to Drools

Drools is a rule engine that separates the business logic from the application code. It allows you to define rules to make decisions based on data or events. These rules can be modified at runtime without the need to modify and recompile the application code.

Drools provides a rule-based approach to implement complex business logic. It follows the principle of "Separation of Concerns" by separating the rules from the application logic. This makes the application more maintainable and flexible.

Setting up a Spring Boot Project

To get started, let's set up a basic Spring Boot project with Maven. Open your favorite IDE and create a new Spring Boot project with the following dependencies:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.kie</groupId>
        <artifactId>kie-api</artifactId>
        <version>7.59.0.Final</version>
    </dependency>
    <dependency>
        <groupId>org.kie</groupId>
        <artifactId>kie-spring</artifactId>
        <version>7.59.0.Final</version>
    </dependency>
    <dependency>
        <groupId>org.drools</groupId>
        <artifactId>drools-core</artifactId>
        <version>7.59.0.Final</version>
    </dependency>
</dependencies>

Creating a Drools Rule

Once the project is set up, let's create a simple Drools rule. Create a file named discountRule.drl in the resources directory with the following content:

import com.example.demo.Order;

rule "Discount Rule"
    when
        $order: Order(totalAmount > 100)
    then
        $order.applyDiscount(10);
end

In this rule, we define a condition using the when keyword and specify that the totalAmount of an Order object should be greater than 100. If the condition is met, the then block is executed, and a discount of 10% is applied to the order.

Defining a Fact

In Drools, a fact is an object that the rules operate on. In our example, the Order class is a fact. Let's create the Order class:

public class Order {
    private double totalAmount;
    // ... getters and setters

    public void applyDiscount(double discountPercentage) {
        totalAmount -= (totalAmount * discountPercentage / 100);
    }
}

Executing the Rule

Now, let's write some code to execute the Drools rule. Create a class named DroolsService with the following content:

import org.kie.api.KieServices;
import org.kie.api.runtime.KieContainer;
import org.kie.api.runtime.KieSession;
import org.springframework.stereotype.Service;

@Service
public class DroolsService {
    public void applyDiscount(Order order) {
        KieServices kieServices = KieServices.Factory.get();
        KieContainer kContainer = kieServices.getKieClasspathContainer();
        KieSession kSession = kContainer.newKieSession();

        kSession.insert(order);
        kSession.fireAllRules();
        kSession.dispose();
    }
}

In this class, we use the KieServices factory to create a KieContainer that loads the rules from the classpath. Then, we create a KieSession, insert the Order object as a fact, fire all the rules, and dispose of the session.

Integrating with Spring Boot

To integrate Drools with Spring Boot, we need to configure the KieContainer as a bean. Create a class named DroolsConfig with the following content:

import org.kie.api.KieServices;
import org.kie.api.runtime.KieContainer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class DroolsConfig {
    @Bean
    public KieContainer kieContainer() {
        return KieServices.Factory.get().getKieClasspathContainer();
    }
}

In this configuration class, we define a bean method that creates and returns the KieContainer using the KieServices factory.

Testing the Application

Let's write a simple test to verify that our Drools rule is working