http://oncodesign.io/2015/08/10/dynamically-create-rules-using-drools--rule-templates

instance.drl

package org.ossandme;
global org.ossandme.AlertDecision alertDecision;

rule "alert_0"
when
    org.ossandme.event.OrderEvent(price > 5000.0 && customer == 'Widgets Inc.')
then
    alertDecision.setDoAlert(Boolean.TRUE);
end

Program(1).java

Rule highValueOrderRule = new Rule();

Condition highValueOrderCondition = new Condition();
highValueOrderCondition.setField("price");
highValueOrderCondition.setOperator(Condition.Operator.GREATER_THAN);
highValueOrderCondition.setValue(5000.0);

// In reality, you would have multiple rules for different types of events. 
// The eventType property would be used to find rules relevant to the event
highValueOrderRule.setEventType(Rule.eventType.ORDER);

highValueOrderRule.setCondition(highValueOrderCondition);

Program(2).java

Rule highValueOrderWidgetsIncRule = new Rule();

Condition highValueOrderCondition = new Condition();
highValueOrderCondition.setField("price");
highValueOrderCondition.setOperator(Condition.Operator.GREATER_THAN);
highValueOrderCondition.setValue(5000.0);

Condition widgetsIncCustomerCondition = new Condition();
widgetsIncCustomerCondition.setField("customer");
widgetsIncCustomerCondition.setOperator(Condition.Operator.EQUAL_TO);
widgetsIncCustomerCondition.setValue("Widgets Inc.");

// In reality, you would have multiple rules for different types of events. 
// The eventType property would be used to find rules relevant to the event
highValueOrderWidgetsIncRule.setEventType(Rule.eventType.ORDER);

highValueOrderWidgetsIncRule.setConditions(Arrays.asList(highValueOrderCondition, widgetsIncCustomerCondition));

Program(3).java

package org.ossandme;

import org.drools.template.ObjectDataCompiler;
...

public final class Program {
    ...
    // Event parameter is an interface and OrderEvent implements it
    static private String applyRuleTemplate(Event event, Rule rule) throws Exception {
        Map<String, Object> data = new HashMap<String, Object>();
        ObjectDataCompiler objectDataCompiler = new ObjectDataCompiler();

        data.put("rule", rule);
        data.put("eventType", event.getClass().getName());

        return objectDataCompiler.compile(Arrays.asList(data), Thread.currentThread().getContextClassLoader().getResourceAsStream("template.drl"));
    }
}

Program(4).java

package org.ossandme;

...

public final class Program {
    ...
    static private AlertDecision evaluate(String drl, Event event) throws Exception {
        KieServices kieServices = KieServices.Factory.get();
        KieFileSystem kieFileSystem = kieServices.newKieFileSystem();
        kieFileSystem.write("src/main/resources/rule.drl", drl);
        kieServices.newKieBuilder(kieFileSystem).buildAll();

        KieContainer kieContainer = kieServices.newKieContainer(kieServices.getRepository().getDefaultReleaseId());
        StatelessKieSession statelessKieSession = kieContainer.getKieBase().newStatelessKieSession();

        AlertDecision alertDecision = new AlertDecision();
        statelessKieSession.getGlobals().set("alertDecision", alertDecision);
        statelessKieSession.execute(event);

        return alertDecision;
    }
    ...
}

Program.java

package org.ossandme;

...

public final class Program {

    static public void main(String[] args) throws Exception {

        // Create an event that will be tested against the rule. 
        // Realistically, the event would be read from some external source. 
        OrderEvent orderEvent = new OrderEvent();
        orderEvent.setPrice(5000.1);
        orderEvent.setCustomer("Widgets Inc.");

        // Create rule
        Rule highValueOrderWidgetsIncRule = new Rule();
        
        // Create condition
        Condition highValueOrderCondition = new Condition();
        highValueOrderCondition.setField("price");
        highValueOrderCondition.setOperator(Condition.Operator.GREATER_THAN);
        highValueOrderCondition.setValue(5000.0);

        // Create another condition
        Condition widgetsIncCustomerCondition = new Condition();
        widgetsIncCustomerCondition.setField("customer");
        widgetsIncCustomerCondition.setOperator(Condition.Operator.EQUAL_TO);
        widgetsIncCustomerCondition.setValue("Widgets Inc.");

        // In reality, you would have multiple rules for different types of events. 
        // The eventType property would be used to find rules relevant to the event
        highValueOrderWidgetsIncRule.setEventType(Rule.eventType.ORDER);
        
        // Add conditions to rule
        highValueOrderWidgetsIncRule.setConditions(Arrays.asList(highValueOrderCondition, widgetsIncCustomerCondition));

        String drl = applyRuleTemplate(orderEvent, highValueOrderWidgetsIncRule);
        AlertDecision alertDecision = evaluate(drl, orderEvent);

        System.out.println(alertDecision.getDoAlert());

       // doAlert is false by default
        if (alertDecision.getDoAlert()) {
            // do notification
            // ...
        }
    }

    // OrderEvent implements Event interface
    static private AlertDecision evaluate(String drl, Event event) throws Exception {
        KieServices kieServices = KieServices.Factory.get();
        KieFileSystem kieFileSystem = kieServices.newKieFileSystem();
        kieFileSystem.write("src/main/resources/rule.drl", drl);
        kieServices.newKieBuilder(kieFileSystem).buildAll();

        KieContainer kieContainer = kieServices.newKieContainer(kieServices.getRepository().getDefaultReleaseId());
        StatelessKieSession statelessKieSession = kieContainer.getKieBase().newStatelessKieSession();

        AlertDecision alertDecision = new AlertDecision();
        statelessKieSession.getGlobals().set("alertDecision", alertDecision);
        statelessKieSession.execute(event);

        return alertDecision;
    }

    // Event parameter is an interface and OrderEvent implements it
    static private String applyRuleTemplate(Event event, Rule rule) throws Exception {
        Map<String, Object> data = new HashMap<String, Object>();
        ObjectDataCompiler objectDataCompiler = new ObjectDataCompiler();

        data.put("rule", rule);
        data.put("eventType", event.getClass().getName());

        return objectDataCompiler.compile(Arrays.asList(data), Thread.currentThread().getContextClassLoader().getResourceAsStream("rule-template.drl"));
    }
}

Rule.java

package org.ossandme.rule;

...

public class Rule {
    ...
    @Override
    public String toString(){
        StringBuilder statementBuilder = new StringBuilder();

        for (Condition condition : getConditions()) {

            String operator = null;

            switch (condition.getOperator()) {
                case EQUAL_TO:
                    operator = "==";
                    break;
                case NOT_EQUAL_TO:
                    operator = "!=";
                    break;
                case GREATER_THAN:
                    operator = ">";
                    break;
                case LESS_THAN:
                    operator = "<";
                    break;
                case GREATER_THAN_OR_EQUAL_TO:
                    operator = ">=";
                    break;
                case LESS_THAN_OR_EQUAL_TO:
                    operator = "<=";
                    break;
            }

            statementBuilder.append(condition.getField()).append(" ").append(operator).append(" ");

            if (condition.getValue() instanceof String) {
                statementBuilder.append("'").append(condition.getValue()).append("'");
            } else {
                statementBuilder.append(condition.getValue());
            }

            statementBuilder.append(" && ");
        }

        String statement = statementBuilder.toString();
        
        // remove trailing &&
        return statement.substring(0, statement.length() - 4);
    }
}

template.drl

template header

rule
eventType

package org.ossandme;

global org.ossandme.AlertDecision alertDecision;

template "alert"

rule "alert_@{row.rowNumber}"
when
    @{eventType}(@{rule})
then
    alertDecision.setDoAlert(Boolean.TRUE);
end

end template