A request handler can return any kind of object. If no response transformer is registered this object will be transformed by calling it's toString() method.

For API's you usually want to return json so we included a JsonResponseTransformer. It's enabled by default (config: http.server.json) in Web-Framework. To register it manually you can use the responseTransformer method.

server.responseTransformer(new JsonResponseTransformer().ignoreStrings());

As you've seen it also has an optional method ignoreStrings() that prevents strings from getting transformed. The constructor also optionally accepts a custom instance of AbstractMapper

To create your own transformer you need to implement the ResponseTransformer interface. You need to implement the transform(Object object) and you can optionally override the default method transformBytes(Object object)

public class YamlResponseTransformer implements ResponseTransformer {

    public String transform(Object object) {
        return new AbstractMapper().toAbstract(object).toYaml(true);
    }

}

If the transformer can't transform the given object it should return null so the next transformer will be tried. If no transformer accepts the object it will fall back to toString()