Validator

//Contoller
class DemoUrlMappingController {
    def action1(DemoUrlMappingCommand com) {...}
    def action2(DemoUrlMappingCommand com) {...}
}

//Command Object
import grails.validation.Validateable
@Validateable
class DemoUrlMappingCommand {
    String userName
    String emailId
    String someOtherParameter
    //Mapped to default 'action' keyword in params
    String action
    static constraints = {
        userName validator: {val, obj ->
            if (obj.action == "action1"){
                //validate according to action1
            } else if (obj.action == "action2"){
                //validate according to action2
            }
        }
        emailId validator: {val, obj ->
            if (obj.action == "action1"){
                //validate according to action1
            } else if (obj.action == "action2"){
                //validate according to action2
            }
        }
    }
}