Drools Using Rules from Excel Files

1. Overview

Drools has support for managing business rules in a spreadsheet format.

In this article, we’ll see a quick example of using Drools to manage business rules using an Excel file.

2. Maven Dependencies

Let’s add the required Drools dependencies into our application:

<dependency>
    <groupId>org.kie</groupId>
    <artifactId>kie-ci</artifactId>
    <version>8.32.0.Final</version>
</dependency>
<dependency>
    <groupId>org.drools</groupId>
    <artifactId>drools-decisiontables</artifactId>
    <version>8.32.0.Final</version>
</dependency>

The latest version of these dependencies can be found at kie-ci and drools-decisiontables.

3. Defining Rules in Excel

For our example, let’s define rules to determine discounts based on customer type and the number of years as a customer:

• Individual customers with greater than 3 years get a 15% discount
• Individual customers with less than 3 years get a 5% discount
• All business customers get a 20% discount

3.1. The Excel File

Let’s begin with creating our excel file as per the specific structure and keywords required by Drools:

For our simple example, we have used the most relevant set of keywords:

• RuleSet – indicates the beginning of the decision table
• Import – Java classes used in the rules
• RuleTable – indicates the beginning of the set of rules
• Name – Name of the rule
• CONDITION – the code snippet of the condition to be checked against the input data. A rule should contain at least one condition
• ACTION – the code snippet of the action to be taken if the conditions of the rule are met. A rule should contain at least one action. In the example, we are calling setDiscount on the Customer object

In addition, we have used the Customer class in the Excel file. So, let’s create that now.

3.2. The Customer Class

As can be seen from the CONDITIONs and ACTION in the excel sheet, we are using an object of the Customer class for the input data (type and years) and to store the result (discount).

The Customer class:

public class Customer {
    private CustomerType type;

    private int years;

    private int discount;

    // Standard getters and setters

    public enum CustomerType {
        INDIVIDUAL,
        BUSINESS;
    }
}

4. Creating Drools Rule Engine Instance

Before we can execute the rules that we have defined, we have to work with an instance of the Drools rule engine. For that, we have to use Kie core components.

4.1. KieServices

The KieServices class provides access to all the Kie build and runtime facilities. It provides several factories, services, and utility methods. So, let’s first get hold of a KieServices instance:

KieServices kieServices = KieServices.Factory.get();

Using the KieServices, we are going to create new instances of KieFileSystem, KieBuilder, and KieContainer.

4.2. KieFileSystem

KieFileSystem is a virtual file system. Let’s add our Excel spreadsheet to it:

Resource dt 
  = ResourceFactory
    .newClassPathResource("com/baeldung/drools/rules/Discount.drl.xls",
      getClass());

KieFileSystem kieFileSystem = kieServices.newKieFileSystem().write(dt);

4.3. KieBuilder

Now, build the content of the KieFileSystem by passing it to KieBuilder:

KieBuilder kieBuilder = kieServices.newKieBuilder(kieFileSystem);
kieBuilder.buildAll();

If successfully built, it creates a KieModule (any Maven produced jar with a kmodule.xml in it is a KieModule).

4.4. KieRepository

The framework automatically adds the KieModule (resulting from the build) to KieRepository:

KieRepository kieRepository = kieServices.getRepository();

4.5. KieContainer

It is now possible to create a new KieContainer with this KieModule using its ReleaseId. In this case, Kie assigns a default ReleaseId:

ReleaseId krDefaultReleaseId = kieRepository.getDefaultReleaseId();
KieContainer kieContainer 
  = kieServices.newKieContainer(krDefaultReleaseId);

4.6. KieSession

We can now obtain KieSession from the KieContainer. Our application interacts with the KieSession, which stores and executes on the runtime data:

KieSession kieSession = kieContainer.newKieSession();

5. Executing the Rules

Finally, it is time to provide input data and fire the rules:

Customer customer = new Customer(CustomerType.BUSINESS, 2);
kieSession.insert(customer);

kieSession.fireAllRules();

6. Test Cases

Let’s now add some test cases:

public class DiscountExcelIntegrationTest {

    private KieSession kSession;

    @Before
    public void setup() {
        Resource dt 
          = ResourceFactory
            .newClassPathResource("com/baeldung/drools/rules/Discount.drl.xls",
              getClass());
        kSession = new DroolsBeanFactory().getKieSession(dt);
    }

    @Test
    public void 
      giveIndvidualLongStanding_whenFireRule_thenCorrectDiscount() 
        throws Exception {
        Customer customer = new Customer(CustomerType.INDIVIDUAL, 5);
        kSession.insert(customer);

        kSession.fireAllRules();

        assertEquals(customer.getDiscount(), 15);
    }

    @Test
    public void 
      giveIndvidualRecent_whenFireRule_thenCorrectDiscount() 
      throws Exception {
        Customer customer = new Customer(CustomerType.INDIVIDUAL, 1);
        kSession.insert(customer);

        kSession.fireAllRules();

        assertEquals(customer.getDiscount(), 5);
    }

    @Test
    public void 
      giveBusinessAny_whenFireRule_thenCorrectDiscount() 
        throws Exception {
        Customer customer = new Customer(CustomerType.BUSINESS, 0);
        kSession.insert(customer);

        kSession.fireAllRules();

        assertEquals(customer.getDiscount(), 20);
    }
}

7. Troubleshooting

Drools convert the decision table to DRL. Due to that, dealing with errors and typos in the Excel file can be hard. Often the errors refer to the content of the DRL. So to troubleshoot, it helps to print and analyze the DRL:

Resource dt 
  = ResourceFactory
    .newClassPathResource("com/baeldung/drools/rules/Discount.drl.xls",
      getClass());

DecisionTableProviderImpl decisionTableProvider 
  = new DecisionTableProviderImpl();
 
String drl = decisionTableProvider.loadFromResource(dt, null);

8. Conclusion

In this article, we have seen a quick example of using Drools to manage business rules in an Excel spreadsheet. We have seen the structure and the minimal set of keywords to be used in defining rules in an Excel file. Next, we used Kie components to read and fire the rules. Finally, we wrote test cases to verify the results.

As always, the example used in this article can be found in the GitHub project.