How to fix the DROOL error "Field Reader does not exist for declaration '$emp_id' in '$emp_id : emp_id' in drool?

Question

I am working on a drool(drl) POC in which I create java beans at run time using reflection. I have set the below property in the config:

KnowledgeBuilderConfiguration config = KnowledgeBuilderFactory.newKnowledgeBuilderConfiguration();
config.setProperty("drools.dialect.default", "mvel");

//drl sample:
package script.demo
dialect "mvel"
import Employee;
rule "Rule - 1"
    when
        $emp: Employee($emp_id: emp_id)
    then
        System.out.println("emp id: "+$emp.emp_id);
end

But I am getting below error:

Field Reader does not exist for declaration '$emp_id' in '$emp_id: emp_id' in the rule 'Rule - 1' : [Rule name='Rule - 1'] @line [I@4cb9v654...............

Request help, how to solve this?

Answer 1

The error is trying to say that your Employee class has no field or method that it can access to map to the $emp_id variable you've declared.

It looks for either a public method prefixed with 'get', or a public variable named as-is.

An Employee class definition that looks like either of the following will resolve the error.

Option 1: declare a public variable emp_id.

public class Employee {
  public String emp_id;
}

Option 2: declare a public method called getEmp_id.

public class Employee {
  public String getEmp_id() { return "..."; }
}

Either of these options will resolve the issue and allow you to bind the $emp_id variable in your rule:

Employee( $emp_id: emp_id )

(I would recommend, of course, renaming to empId and then implementing either such a variable or a getEmpId method, since that follows Java naming conventions. You can still call your declared variable in the drools $emp_id of course.)