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Drools – Debugging

There are different ways to debug a Drools project. Here, we will write a Utility class to let you know which rules are being triggered or fired.

With this approach, you can check what all rules are getting triggered in your Drools project. Here is our Utility Class

Utility.java

package com.sample;
import org.drools.spi.KnowledgeHelper;

public class Utility {
   public static void help(final KnowledgeHelper drools, final String message){
      System.out.println(message);
      System.out.println("\nrule triggered: " + drools.getRule().getName());
   }
   public static void helper(final KnowledgeHelper drools){
      System.out.println("\nrule triggered: " + drools.getRule().getName());
   }
}

The first method help prints the rule triggered along with some extra information which you can pass as String via the DRL file.

The second rule helper prints whether the particular rule was triggered or not.

We have added one of the Utility methods in each DRL file. We have also added the import function in the DRL file (Pune.drl). In the then part of the rule, we have added the utility function call. The modified Pune.drl is given below. Changes are highlighted in blue.

Modified Pune.drl

//created on: Dec 24, 2014
package droolsexample

//list any import classes here.
import com.sample.ItemCity;
import java.math.BigDecimal;
import com.sample.HelloCity; 
import function com.sample.Utility.helper;

// declare any global variables here
dialect "java"
rule "Pune Medicine Item"
   when
      item : ItemCity(purchaseCity == ItemCity.City.PUNE, 
         typeofItem == ItemCity.Type.MEDICINES)
   
   then
      BigDecimal tax = new BigDecimal(0.0);
      item.setLocalTax(tax.multiply(item.getSellPrice()));
      HelloCity.writeHello(item.getPurchaseCity().toString()); 
      helper(drools);
end

rule "Pune Groceries Item"
   when
      item : ItemCity(purchaseCity == ItemCity.City.PUNE, 
         typeofItem == ItemCity.Type.GROCERIES)
   then
      BigDecimal tax = new BigDecimal(2.0);
      item.setLocalTax(tax.multiply(item.getSellPrice())); 
      helper(drools);
end

Similarly, we have added the other utility function in the second DRL file (Nagpur.drl). Here is the modified code −

Modified Nagpur.drl

// created on: Dec 26, 2014
package droolsexample

// list any import classes here.
import com.sample.ItemCity;
import java.math.BigDecimal; 
import function com.sample.Utility.help;

//declare any global variables here
dialect "java"

rule "Nagpur Medicine Item"
   when
      item : ItemCity(purchaseCity == ItemCity.City.NAGPUR, 
         typeofItem == ItemCity.Type.MEDICINES)
   
   then
      BigDecimal tax = new BigDecimal(0.0);
      item.setLocalTax(tax.multiply(item.getSellPrice())); 
      help(drools,"added info");
end

rule "Nagpur Groceries Item"
   when
      item : ItemCity(purchaseCity == ItemCity.City.NAGPUR, 
         typeofItem == ItemCity.Type.GROCERIES)
   then
      BigDecimal tax = new BigDecimal(1.0);
      item.setLocalTax(tax.multiply(item.getSellPrice())); 
      help(drools,"info");
end

Run the program again and it should produce the following output −

info

rule triggered: Nagpur Groceries Item
added info

rule triggered: Nagpur Medicine Item

rule triggered: Pune Groceries Item
HELLO PUNE!!!!!!

rule triggered: Pune Medicine Item
PUNE 0
PUNE 20
NAGPUR 0
NAGPUR 10

Both the utility functions are called and it shows whether the particular rule was called or not. In the above example, all the rules are being called, but in an enterprise application, this utility function can be really useful to debug and find out whether a particular rule was fired or not.

Using the Debug Perspective in Eclipse

You can debug the rules during the execution of your Drools application. You can add breakpoints in the consequences of your rules, and whenever such a breakpoint is encountered during the execution of the rules, execution is stopped temporarily. You can then inspect the variables known at that point as you do in a Java Application, and use the normal debugging options available in Eclipse.

To create a breakpoint in your DRL file, just double-click at the line where you want to create a breakpoint. Remember, you can only create a breakpoint in the then part of a rule. A breakpoint can be removed by double-clicking on the breakpoint in the DRL editor.

After applying the breakpoints, you need to debug your application as a Drools application. Drools breakpoints (breakpoints in DRL file) will only work if your application is being debugged as a Drools application. Here is how you need to do the same −

Once you debug your application as a Drools application, you would see the control on the DRL file as shown in the following screenshot −

You can see the variables and the current values of the object at that debug point. The same control of F6 to move to the next line and F8 to jump to the next debug point are applicable here as well. In this way, you can debug your Drools application.

Note − The debug perspective in Drools application works only if the dialect is MVEL until Drools 5.x.

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