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:

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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12 thoughts on “Getting started with Drools and Netbeans

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  6. I am getting this error:

    java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
    at org.drools.RuleBaseConfiguration.(RuleBaseConfiguration.java:123)
    at org.drools.common.AbstractRuleBase.(AbstractRuleBase.java:155)
    at org.drools.reteoo.ReteooRuleBase.(ReteooRuleBase.java:151)
    at org.drools.reteoo.ReteooRuleBase.(ReteooRuleBase.java:128)
    at org.drools.RuleBaseFactory.newRuleBase(RuleBaseFactory.java:86)
    at org.drools.RuleBaseFactory.newRuleBase(RuleBaseFactory.java:74)
    at org.drools.RuleBaseFactory.newRuleBase(RuleBaseFactory.java:37)
    at hellodrools.HelloDrools.(HelloDrools.java:21)
    Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    … 8 more
    Could not find the main class: hellodrools.HelloDrools. Program will exit.
    Exception in thread “main” Java Result: 1
    BUILD SUCCESSFUL (total time: 0 seconds)

    what have I done! :(

  7. hey nice tutorial, but i am having one problem that my Main.java is not able to pick up testRules.drl file and throwing NullPointerException. Please Help.

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