BRE and Liferay

Business rule framework using Drools

In most of the software products the complexities imposed by requirements are behavioral and functional. Usually the business logic is implemented in a J2SE/J2EE application by writing Java code which in most cases are very intricate. The complexity of the code makes maintaining and updating the application's business logic very cumbersome and increases the possiblity of human errors and result is bugs in the product. Also any change incurs recompilation and redeployment costs.

Drools facilitates rule based validation. The advantage of using a rule engine is in the ability to externalise the business rules from the application and enable them to be edited without redeploying the application.

 About Drools

Drools is an open source rules engine, written in the Java language, that uses the Rete algorithm to evaluate the rules you write. Drools lets you express your business logic rules in a declarative way. You can write rules using a non-XML native language that is quite easy to learn and understand. And you can embed Java code directly in a rules file, which makes the experience of learning Drools even more attractive. Drools also has other advantages, such as:

• Supported by an active community
• Easy to use
• Quick to execute
• Gaining popularity among Java developers
• Compliant with the Java Rule Engine API (JSR 94)
• Free

Example

Following is a very basic example:

The POJO (.java file)

package com.company.license;

 public class Applicant {

  private String name;

  private int age;

  private boolean valid;

  // getter and setter methods here

}

The Drools rule file (.drl file).

package com.company.license;

rule "Is of valid age"

when

   $a : Applicant( age < 18 )

then

   $a.setValid( false );

end

The rule consists of conditions (when) and consequences (then). In this rule, when the Applicant age property is less than 18, the valid property is set to false.Often, business rules will need to access collaborators such as services or DAOs. These can be injected as "globals" in the rule definition. For example, the scenario above could be modified so that the applicant needs to be loaded from a DAO in order to check the age.

package com.acme.app.rules

import com.acme.app.form.ApplicationForm

import com.acme.app.model.Applicant

import com.acme.app.validation.Errors

import com.acme.app.dao.ApplicantDao

global ApplicantDao applicantDao

package com.acme.app.rules

import com.acme.app.form.ApplicationForm

import com.acme.app.model.Applicant

import com.acme.app.validation.Errors

import com.acme.app.dao.ApplicantDao

global ApplicantDao applicantDao

rule "Check applicant age"

   when

       $a : ApplicationForm()

       $errors : Errors()

       $applicant:Applicant( age < 18) from applicantDao.findApplicant($a.getApplicantId())

   then

       $errors.addError( $a, "applicantId", "Applicant age < 18" );

end

Integration with Liferay

Detailed steps of integrating Drools with Liferay can be found at Blog writetn by Venkata Mukku at http://www.liferay.com/web/venkata.mukku/blog/-/blogs/7871213