(Quick Reference)
                scope
Purpose
Changes the scope of a controller
Examples
class BookController {    static scope = "session"    // …
}Description
Unless specified by the 
grails.controllers.defaultScope property in 
application.yml, controllers by default are "prototype" scoped which means that a new controller will be created for each request. 
This behaviour can be set for an individual controller by specifying a 
scope attribute with one of the following values:
- prototype(default) - A new controller will be created for each request (recommended for actions as Closure properties)
- session- One controller is created for the scope of a user session
- singleton- Only one instance of the controller ever exists (recommended for actions as methods)
See 
Controllers and Scopes in the user guide for more information.