Getting started with Drools and Netbeans

Rule engines are a rather complex topic with potentially a steep learning curve. I am looking at a few options, Drools being one of them. As usual one can read all the theoretical papers, but rather I have a something to look at and play with fast. Ultimately I need to integrate the rule engine into my application, so I dont want to play with eclipse editors for rules but seeing a rule engine ticking inside my Netbeans project. I found one tutorial from Vishal Akhouri which I updated to the latest version of Drools and some minor fixes. Most of the sample source code is from Vishal.

Pre-Requirements:

Netbeans 6.x and later
Stable Drools from http://www.jboss.org/drools/downloads

Tutorial:

Create a new plain java project HelloDrools

New project

New project
Add below drools libraries from the distribution zip file drools-distribution-5.3.0.Final.zip
(except the core file which is not part of the zip file)

Drools Distribution file
We are operating outside the eclipse environment, we need to add this jdt core file from http://dist.wso2.org/maven2/org/eclipse/jdt/core/3.4.2.v_883_R34x/
Following Vishal’s tutorial we create 3 files

Simple Message Class

package hellodrools;

public class Message {

    private String type;
    private String msgtext;

    /**
     * @return the type
     */
    public String getType() {
        return type;
    }

    /**
     * @param type the type to set
     */
    public void setType(String type) {
        this.type = type;
    }

    /**
     * @return the msgtext
     */
    public String getMsgtext() {
        return msgtext;
    }

    /**
     * @param msgtext the msgtext to set
     */
    public void setMsgtext(String msgtext) {
        this.msgtext = msgtext;
    }
}

The rule file testrules.drl

import hellodrools.Message
rule "Hello World"
when
    message:Message (type=="Hello")
then
    System.out.println("Hello World, Drools!");
end

The main file HelloDrools.java
Update 2012-03-01: Check the updated code with new packages here.


package hellodrools;

import java.io.InputStreamReader;
import java.io.Reader;
import org.drools.RuleBase;
import org.drools.RuleBaseFactory;
import org.drools.StatefulSession;
import org.drools.compiler.PackageBuilder;
import org.drools.compiler.PackageBuilderErrors;

public class HelloDrools {

    private static RuleBase rbase = RuleBaseFactory.newRuleBase();
    private static PackageBuilder pbuilder = new PackageBuilder();
    private static StatefulSession sessionObject;
    private static String DRL_FILE = "/hellodrools/testrules.drl";

    public static void main(String[] args) {

        initialiseDrools();
        initiliseMessageObject();
        runRules();
    }

    private static void initialiseDrools() {
        //1. Read the DRL File and add to package builder
        try {
            Reader reader = new InputStreamReader(HelloDrools.class.getResourceAsStream(DRL_FILE));
            pbuilder.addPackageFromDrl(reader);
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }

        //2. Check for any errors
        PackageBuilderErrors errors = pbuilder.getErrors();

        if (errors.getErrors().length > 0) {
            System.out.println("Some errors exists in packageBuilder");
            for (int i = 0; i < errors.getErrors().length; i++) {
                System.out.println(errors.getErrors()[i]);
            }
            throw new IllegalArgumentException("Could not parse knowledge.");
        }

        //3. Add package to rule base
        try {
            rbase.addPackage(pbuilder.getPackage());
        } catch (Exception e) {
            System.out.println("Error: " + e);
        }
    }

    // Method to fire all rules
    private static void runRules() {
        sessionObject.fireAllRules();
    }

    // Method to insert message object in session
    private static void initiliseMessageObject() {
        Message msg = new Message();
        msg.setType("Hello");
        sessionObject = rbase.newStatefulSession();
        sessionObject.insert(msg);
    }
}

Run the project
HelloDrools project

Executed rule

Summary:

We know almost nothing about rules yet (other than ‘rule-when-then-end’), nor have any clue about the drools engine. Up to you to read up about the background. Refer to below references.
But the most simple rule executed in our beloved Netbeans environment. A starting point to integrate drools into your application and conquer more complex scenarios.
I recommend to verify if you really need a rules engines or not. Depends very much on the dynamic nature of your rules and complexity (branches,etc).

References:

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