Grails rest render src/groovy classes from rest controller as JSON and create custom JSON renderer

Question

I am working with grails 2.5.0. I want to render src/groovy class as JSON and custom JSON. If I perform same with domain classes, its working fine for me. I want same thing with src/groovy classes. For rendering data with json and custom json I followed this link.

I tried to these thing. Create pogo class in src/groovy.

class UserDTO {
    String username
    String lastName
    String firstName
    String email
}

Created a controller with restful feature.

import grails.converters.JSON
import grails.rest.RestfulController

class RenderDTOController extends RestfulController {

    static responseFormats = ['json','pogoshort','pogodetails']
    RenderDTOController() {
        super(UserDTO)
    }


    def index() {
        respond new UserDTO(username: "vivek", firstName: "Vivek", lastName: "Yadav")
    }
}

Made an entry on urlMapping.groovy.

"/vivek"(resources:'renderDTO')

Registered new mime type for custom renderer in resources.groovy.

userPogoDetailsRenderer(JsonRenderer, UserDTO) {

    mimeTypes = [new MimeType('application/vnd.grails.rest.sample.pogodetails+json', 'pogodetails')]

    namedConfiguration = 'pogodetails'
}
output.
userPogoShortRenderer(JsonRenderer, UserDTO) {
    mimeTypes = [new MimeType('application/vnd.grails.rest.sample.pogoshort+json', 'pogoshort')]

    namedConfiguration = 'pogoshort'
}

userPogoRenderer(JsonRenderer, UserDTO) {
    mimeTypes = [new MimeType('application/json', 'json')]
} 

Added new mime type in mime type.

grails.mime.types = [ // the first one is the default format
                      all          : '*/*', // 'all' maps to '*' or the first available format in withFormat
                      atom         : 'application/atom+xml',
                      css          : 'text/css',
                      csv          : 'text/csv',
                      form         : 'application/x-www-form-urlencoded',
                      html         : ['text/html', 'application/xhtml+xml'],
                      js           : 'text/javascript',
                      json         : ['application/json', 'text/json'],
                      multipartForm: 'multipart/form-data',
                      rss          : 'application/rss+xml',
                      text         : 'text/plain',
                      hal          : ['application/hal+json', 'application/hal+xml'],
                      xml          : ['text/xml', 'application/xml'],
                     pogodetails      : ['application/vnd.grails.rest.sample.pogodetails+json', 'application/json'],
                      pogoshort     : ['application/vnd.grails.rest.sample.pogoshort+json', 'application/json']
] 

Added custom marshaller in Bootstrap.groovy.

JSON.createNamedConfig("pogodetails") {
        println "pogodetails renderer in bootstrap"
        it.registerObjectMarshaller(UserDTO) { UserDTO user ->
            final String fullname = [user.firstName, user.lastName].join(' ')
            final userMap = [
                    lastName      : user.lastName,
                    username: user.username,
                    email   : user.email,
            ]
            if (fullname) {
                userMap.fullname = fullname
            }
            userMap
        }
    }

    JSON.createNamedConfig('pogoshort') {
        println "pogoshort renderer in bootstrap"
        it.registerObjectMarshaller(UserDTO) { UserDTO user ->
            println "Coming here in pogoshort renderer"
            final userMap = [
                    lastName      : user.lastName,
                    username: user.username
            ]
            userMap
        }
    }

While I am running, I am getting exception.

URI
/grails-rest-2.5.0/vivek
Class
org.codehaus.groovy.runtime.typehandling.GroovyCastException
Message
Cannot cast object 'com.vivek.UserDTO@12c671a2' with class 'com.vivek.UserDTO' to class 'grails.converters.JSON'

If I change in controller like this.

respond new UserDTO(username: "vivek", firstName: "Vivek", lastName: "Yadav").encodeAsJSON()

I am able to convert it into JSON format. Also, If user this syntax.

JSON.use("pogodetails")
respond new UserDTO(username: "vivek", firstName: "Vivek", lastName: "Yadav").encodeAsJSON()

I am also able to render pogo in custom format. but I need it to do as rest full controller is doing for doamin.

Is there any way to do same thing with simple POGO defined in src/groovy directory?

Thanks in advance.