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:

• http://techgrafitti.wordpress.com/2011/02/21/drools-%E2%80%93-getting-started-hello-world-example/
• http://dist.wso2.org/maven2/org/eclipse/jdt/core/3.4.2.v_883_R34x/
• http://docs.jboss.org/drools/release/5.3.0.Final/
• http://jcp.org/aboutJava/communityprocess/review/jsr094/