Getting Started with Drools 7: A Hello World Rule Engine Example

In the previous chapter we used Drools 5.x, where the KnowledgeBase class was central; Drools 7 marks that class as @Deprecated. This section shows how to use Drools 7, and all subsequent examples assume this version.

Project Structure

The Drools project follows the standard Maven layout. Key files include pom.xml for dependencies, src/main/resources for rule files, and a new kmodule.xml configuration.

Maven Dependency

Add the Drools core and compiler dependencies to pom.xml as shown in the screenshot.

Domain Model

The fact object remains a Product class, representing items with a type (e.g., DIAMOND, GOLD) and a discount field.

Rule File

The rule definitions are stored in Rules.drl. The content is unchanged from the Drools 5 example.

kmodule.xml Configuration

Drools 7 introduces kmodule.xml. Its main elements are:

One or more kbase entries, each linking to one or more DRL packages.

Each kbase must have a unique name attribute.

The packages attribute lists the resource paths of DRL files; multiple packages are comma‑separated. By default, all resources under src/main/resources are scanned.

The default attribute marks a KieBase as the default, allowing it to be retrieved without specifying a name (only one default per module).

Each kbase can contain one or more ksession elements, each requiring a unique name.

Unit Test Example

The example is implemented as a JUnit test. The test creates a Product fact, sets its type to GOLD, inserts it into a KieSession, fires all rules, and prints the resulting discount.

The test output shows that one rule was fired and the product received the expected discount.

Key Takeaways

This Hello World example demonstrates the complete workflow for Drools 7: Maven setup, kmodule configuration, rule definition, and execution via a unit test. It also highlights that Drools 7 uses a completely new API compared to Drools 5, requiring developers to adapt their code accordingly.