Rule engine

This rule engine is heavily inspired by Easy Rules.

Some design decisions have been changed, such as:

• Rules are provided to engines when they are built, not at runtime, use different engines for different rule namespaces
• Engines do not ensure rule ordering, Rules abstraction can, so they do not have to be ordered
• Engines cannot be parametrized, RuleEngineListener can control when to stop further evaluation without adding such specifics to engines
• The default RuleEngine uses a single listener, but the core library provides a CompositeRuleEngineListener to encapsulate multiple listeners

Core features

• Zero runtime dependencies for the core module
• Easy to use and extend API
• Different ways to define rules
• Different rule engine implementations

Example

1. Define rules

Annotations

@Rule(name = "weather rule", description = "if it rains then take an umbrella")
public class WeatherRule {

    @When
    public boolean itRains(@Given("rain") boolean rain) {
        return rain;
    }
    
    @Then
    public void takeAnUmbrella() {
        System.out.println("It rains, take an umbrella!");
    }
}

Programmatic

Rule weatherRule = newRule("weather rule")
        .description("if it rains then take an umbrella")
        .<Boolean>when("rain", rain -> rain == true)
        .then(facts -> System.out.println("It rains, take an umbrella!"))
        .build();

Expression language

Rule weatherRule = newMVELRule("weather rule")
        .description("if it rains then take an umbrella")
        .when("rain == true")
        .then("System.out.println(\"It rains, take an umbrella!\");")
        .build();

Files

weather-rule.yaml file:

---
- name: Weather rule
  description: if it rains then take an umbrella
  when: "#{['rain'] == true}"
  then:
    - "#{T(java.lang.System).out.println('It rains, take an umbrella!')}"

RulesFactory factory = new SpELRulesFactory(new JacksonRuleDefinitionReader(new ObjectMapper(new YAMLFactory())));
Rules rules = factory.create(new FileInputStream("/weather-rule.yaml"));

2. Evaluate rules

public class ExampleApplication {
    public static void main(String[] args) {
       var engine = new DefaultRuleEngineBuilder()
               .rule(new WeatherRule())
               .build();

       var facts = SimpleFacts.builder()
               .fact("rain", true)
               .build();

       engine.evaluate(facts); 
    }
}

Importing into your project using Maven

Add the JitPack repository to your pom.xml.

<repositories>
    <repository>
        <id>jitpack.io</id>
        <url>https://jitpack.io</url>
    </repository>
</repositories>

Add the following under your <dependencies>:

<dependency>
    <groupId>com.github.alturkovic</groupId>
    <artifactId>rule-engine</artifactId>
    <version>1.0.0</version>
</dependency>