Finding this information saved me a lot of time http://www.intelligrape.com/blog/2012/07/16/rendering-json-with-formatting/

In grails we we often like to render our grails output like so:

render json as JSON

Grails converter will render the json without any formatting, which sometimes (e.glarge json files) can be hard to read.

To render our json in a pretty way we can do the following:

def json = doc as JSON
json.prettyPrint = true
json.render response

the prettyPrint option returning the grails converter json will also apply for e.g when returning it as an outputstream back to the user.

def json = doc as JSON
json.prettyPrint = true
response.setHeader("Content-disposition","attachment;filename=\"" + fileName + "\"");
response.setContentType("application/json");
response.outputStream << json