This post introduces a basic installation and example of Drools.

From https://www.drools.org/

Concepts

Basically, Drools allows to write business rules following this format:

rule R when
        $r : Result()
        $p1 : Person(name == "Jose")
        $p2 : Person(name != "Jose", age > $p1.age)
    then
        $r.setValue($p2.getName() + "is older than " + $p1.getName());
end

After having a set of business rules registered in the Drools Workbench - or server - and after an event is fired, then the RETE engine within Drools will choose and execute one of these rules.

Learn by example

1.- Configure the maven dependencies in your pom.xml:

• drools-core: this is the core engine, runtime component that also contains the RETE engine. This is the only runtime dependency if you are pre-compiling rules (and deploying via Package or RuleBase objects).

<dependency>
    <groupId>org.drools</groupId>
    <artifactId>drools-core</artifactId>
    <version>7.31.0.Final</version>
</dependency>

• drools-compiler: this contains the compiler/builder components to take rule source, and build executable rule bases. This is often a runtime dependency of your application, but it need not be if you are pre-compiling your rules.

<dependency>
    <groupId>org.drools</groupId>
    <artifactId>drools-compiler</artifactId>
    <version>7.31.0.Final</version>
</dependency>

• kie-ci and kie-api: these dependencies allow to run Drools locally and to work with Maven to resolve the rule files and other artifacts outside of the classpath.

<dependency>
    <groupId>org.kie</groupId>
    <artifactId>kie-api</artifactId>
    <version>7.31.0.Final</version>
</dependency>

<dependency>
    <groupId>org.kie</groupId>
    <artifactId>kie-ci</artifactId>
    <version>7.31.0.Final</version>
</dependency>

2.- Create the resource folders for your rules:

src/main/resources
    | META-INF
    | rules

3.- Create our rule and module:

• resources/rules/older.drl:

package rules

import com.sgitario.drools.model.Person

rule "Older" when
        $p1 : Person(name == "Jose")
        $p2 : Person(name != "Jose", age > $p1.age)
    then
        System.out.println($p2.getName() + " is older than " + $p1.getName());
end

• resources/META-INF/kmodule.xml:

<kmodule xmlns="http://www.drools.org/xsd/kmodule">
    <kbase name="rules" packages="rules">
        <ksession name="ksession-rules"/>
    </kbase>
</kmodule>

4.- And run our application:

public class App {
    public static void main(String[] args) {
        try {
            // load up the knowledge base
            KieServices ks = KieServices.Factory.get();
            KieContainer kContainer = ks.getKieClasspathContainer();
            KieSession kSession = kContainer.newKieSession("ksession-rules");

            // go !
            kSession.insert(new Person("Jose", 20));
            kSession.insert(new Person("Irene", 19));
            kSession.insert(new Person("Victor", 21));
            kSession.fireAllRules();
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }
}

Configure the manifest using Maven, pom.xml:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
            </configuration>
        </plugin>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>1.4.0</version>
            <configuration>
                <mainClass>com.sgitario.drools.App</mainClass>
            </configuration>
        </plugin>
    </plugins>
</build>

Output:

mvn clean package exec:java
> Victor is older than Jose

Conclusions

This post was based on this site, so greetings to them. Also, we worked with the Kie dependencies which are useful for testint purposes, but in order to take fully advantage of Drools, the rules must be integrated to Business Manager workbench (more in the next series in here).

The source code can be found here.