Drools - Debugging

There are different ways to debug a Drools project. Here, we will write a Utipty 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 Utipty Class

Utipty.java

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

pubpc class Utipty {
   pubpc static void help(final KnowledgeHelper drools, final String message){
      System.out.println(message);
      System.out.println("
rule triggered: " + drools.getRule().getName());
   }
   pubpc static void helper(final KnowledgeHelper drools){
      System.out.println("
rule 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 Utipty 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 utipty function call. The modified Pune.drl is given below. Changes are highpghted in blue.

Modified Pune.drl

//created on: Dec 24, 2014
package droolsexample

//pst any import classes here.
import com.sample.ItemCity;
import java.math.BigDecimal;
import com.sample.HelloCity; 
import function com.sample.Utipty.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 utipty function in the second DRL file (Nagpur.drl). Here is the modified code −

Modified Nagpur.drl

// created on: Dec 26, 2014
package droolsexample

// pst any import classes here.
import com.sample.ItemCity;
import java.math.BigDecimal; 
import function com.sample.Utipty.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 utipty 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 apppcation, this utipty function can be really useful to debug and find out whether a particular rule was fired or not.

Using the Debug Perspective in Ecppse

You can debug the rules during the execution of your Drools apppcation. 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 Apppcation, and use the normal debugging options available in Ecppse.

To create a breakpoint in your DRL file, just double-cpck at the pne 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-cpcking on the breakpoint in the DRL editor.

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

Once you debug your apppcation as a Drools apppcation, 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 pne and F8 to jump to the next debug point are apppcable here as well. In this way, you can debug your Drools apppcation.

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