10.11.2 Implementing a Custom Renderer

If you want even more control of the rendering or prefer to use your own marshalling techniques then you can implement your own Renderer instance. For example below is a simple implementation that customizes the rendering of the Book class:

package myapp
import grails.rest.render.*
import grails.web.mime.MimeType
class BookXmlRenderer extends AbstractRenderer<Book> {
    BookXmlRenderer() {
        super(Book, [MimeType.XML,MimeType.TEXT_XML] as MimeType[])
    }
    void render(Book object, RenderContext context) {
        context.contentType = MimeType.XML.name
        def xml = new groovy.xml.MarkupBuilder(context.writer)
        xml.book(id: object.id, title:object.title)
    }
}

The AbstractRenderer super class has a constructor that takes the class that it renders and the MimeType(s) that are accepted (via the ACCEPT header or file extension) for the renderer.

To configure this renderer, simply add it is a bean to grails-app/conf/spring/resources.groovy:

beans = {
    bookRenderer(myapp.BookXmlRenderer)
}

The result will be that all Book instances will be rendered in the following format:

<book id="1" title="The Stand"/>

Container Renderers

A grails.rest.render.ContainerRenderer is a renderer that renders responses for containers of objects (lists, maps, collections etc.). The interface is largely the same as the Renderer interface except for the addition of the getComponentType() method, which should return the "contained" type. For example:

class BookListRenderer implements ContainerRenderer<List, Book> {
    Class<List> getTargetType() { List }
    Class<Book> getComponentType() { Book }
    MimeType[] getMimeTypes() { [ MimeType.XML] as MimeType[] }
    void render(List object, RenderContext context) {
        ....
    }
}