Quite simple really.
Play comes with a number of methods that you can use to expose your actions as web services.
For example
render()
renderJSON()
renderXML()
These can all be used to render data in a particular way.
If you had a web service, let's assume a RESTful webservice, that you wanted to return the sum of two numbers, you could do so in the following way
public class Application extends Controller {
public static void sum(Float num1, Float num2) {
Float result = num1 * num2;
render(result);
}
}
if your route is set up to use XML as the format, or the format is set correctly in the request header, you then return the result using a normal groovy template called app/views/Application/sum.xml
To setup the route to format correctly, then add the following line to your route file
GET /webservices/sum Application.sum(format:'xml')
The sum.xml would then look something like
${result}
The same concept works for JSON.
If however you don't want to use groovy templates, you could simply create the XML or JSON using the renderJSON / renderXML methods, but this does mean you are building presentation logic into your controller, which is bad practice.
As a subnote, if you want to consume webservices, then you use the play.libs.WS class. I have written a blog on how to do that
http://playframework.wordpress.com/2010/08/15/web-services-using-play/
