Now that we have our project structure ready, we can write our first rule. For that, we will create a new empty text file. This will be a static resource, therefore, we need to place it in the src/main/resources directory. This text file needs to have the .drl extension so that it can be picked up as a rule file. In the .drl files, we will write as many rules as we want. Now, we will start easy with just one rule.

Let's write our first rule to classify the items based on what they cost us. Our rules.drl text file will look similar to the following:

package myfirstproject.rules
import org.drools.devguide.eshop.model.Item;
import org.drools.devguide.eshop.model.Item.Category;
rule "Classify Item - Low Range"
    when
        $i: Item(cost < 200)
    then
        $i.setCategory(Category.LOW_RANGE);
end

This rule checks for each item that costs less than 200 USD and automatically tags it with a category, in this case, LOW_RANGE. For our shop, it makes sense to differentiate our items in different ranges so that we can apply different discounts and marketing strategies for them. This classification process can be done automatically using rules, which centralize the point where we have this business definition of what LOW_RANGE, MID_RANGE or HIGH_RANGE items they are.

In general, these files will be structured as follow:

Before analyzing further, let's try to execute the rule and see what happens. In order to execute and test our new rule, we need to create a Java class, where we bootstrap the Rule Engine and provide the required information for it to work. We can just create an empty class with a main method, bootstrap the engine, and start using it right away.

In order to do this, we just create an App.java class for this example and place it in the src/main/java/ directory so that it can be compiled by Maven. For this example, we have created the org/drools6/book directory that follows the standard Java package structure, as follows:

package org.drools6.book;
import ...

public class App {
  public static void main( String[] args ) {
    System.out.println( "Bootstrapping the Rule Engine ..." );
    //1) Bootstrapping a Rule Engine Session
    KieServices ks = KieServices.Factory.get();
    KieContainer kContainer = ks.getKieClasspathContainer();
    KieSession kSession =  kContainer.newKieSession();
        
    Item item = new Item("A", 123.0,234.0);  
    System.out.println( "Item Category: " + item.getCategory());
    //2) Provide information to the Rule Engine Context
    kSession.insert(item);
    //3) Execute the rules that are matching
    int fired = kSession.fireAllRules();
    System.out.println( "Number of Rules executed = " + fired );
    System.out.println( "Item Category: " + item.getCategory());
  }
}

As you can see in the previous example, there are three main stages, as shown in the following:

This will compile and package your project, look for the Build Success output in the terminal. After executing this line, you will find the /target directory containing a myfirst-drools-project-1.0.0.jar jar file that you can use to execute the previously compiled class using the following line:

mvn exec:java -Dexec.mainClass="org.drools6.book.App"

You will see the following output on the terminal:

user$ mvn exec:java -Dexec.mainClass="org.drools6.book.App"

[INFO] Scanning for projects...

[INFO]

[INFO] ------------------------------------------------------------------------

[INFO] Building myfirst-drools-project 1.0.0

[INFO] ------------------------------------------------------------------------

[INFO]

[INFO] --- exec-maven-plugin:1.4.0:java (default-cli) @ myfirstproject ---

Bootstrapping the Rule Engine ...

...

Item Category: NA

Number of Rules executed = 1

Item Category: LOW_RANGE

[INFO] ------------------------------------------------------------------------

[INFO] BUILD SUCCESS

[INFO] ------------------------------------------------------------------------

[INFO] Total time: 2.945 s

[INFO] Finished at: 2015-04-02T09:55:08+01:00

[INFO] Final Memory: 20M/257M

[INFO] ------------------------------------------------------------------------

As you can see, the rule was executed as the category of the item was changed accordingly with what the rule says. I would encourage you to change the cost of the item to effectively see that the rule will not be executed if the cost is over the value defined in the rule. You can also add more items with different costs to KieSession and see what happens.

<dependencies>
  ...
  <dependency>
    <groupId>javax.enterprise</groupId>
    <artifactId>cdi-api</artifactId>
  </dependency>
  <dependency>
    <groupId>org.jboss.weld.se</groupId>
    <artifactId>weld-se-core</artifactId>
  </dependency>
</dependencies>

<beans xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/javaee  http://java.sun.com/xml/ns/javaee/beans_1_0.xsd">
</beans>

public class App {
    @Inject
    @KSession 
    KieSession kSession;
    
    public void go(PrintStream out){
        Item item = new Item("A", 123.0,234.0);
        out.println( "Item Category: " + item.getCategory());
        kSession.insert(item);
        int fired = kSession.fireAllRules();
        out.println( "Number of Rules executed = " + fired );
        out.println( "Item Category: " + item.getCategory());
    }
    
    public static void main( String[] args )
    {
        // Bootstraping the WELD CDI container
        Weld w = new Weld();
        WeldContainer wc = w.initialize();
        App bean = wc.instance().select(App.class).get();
        bean.go(System.out);
        w.shutdown();
    }
}