Drools

In order to get started writing DRL files, you'll need to learn about DRL syntax, semantic modules and how to think like a declarative programmer. For now, we'll show you a simple example to get you started.

This is what a basic DRL file looks like:

<rule-set name="cheese rules"
    xmlns="http://drools.org/rules"    
    xmlns:java="http://drools.org/semantics/java"
    xmlns:xs="http://www.w3.org/2001/XMLSchema-instance"
    xs:schemaLocation="http://drools.org/rules rules.xsd
    				   http://drools.org/semantics/java java.xsd">

    <rule name="Bob Likes Cheese">
        <parameter identifier="bob">
            <class>org.drools.examples.simple.Bob</class>
        </parameter>
        <java:condition>bob.likesCheese() == true</java:condition>
        <java:consequence>
            System.out.println( "Bob likes cheese." );
        </java:consequence>
    </rule>

</rule-set>

It uses two semantic modules--the Base Semantic Module and the Java semantic module. It's impossible to get by with only base semantics. You must use either one of the three language modules that ship with drools or write your own domain specific language.

Our rule-set has a single rule element, named "Bob Likes Cheese". This rule looks in the working memory for instances of the org.drools.examples.simple.Bob object. If it finds an instance of the Bob object, and if that instance likes cheese, the rule will fire off a consequence, which prints "Bob likes cheese" to the command line. All rule elements are composed of one or more parameters, one or more conditions and a consequence. The form those conditions and consequences take are different for each semantic module.

The above consequence will fire off each time an instance of Bob that likes cheese is passed into the rule execution set.

//load the rule base from the classpath
        //in this example it expects that there is a CheeseRules.java.drl file in the class path as shown (containing the DRL above)
        RuleBase ruleBase = RuleBaseLoader.loadFromStream(
            this.getClass().getResourceAsStream( "/rules/CheeseRules.java.drl" ) );

        WorkingMemory workingMemory = ruleBase.newWorkingMemory( );

        //put "bob" in working memory
        workingMemory.assertObject( bob );

        workingMemory.fireAllRules( );