@RestResource(urlMapping='/mypath/*')
global without sharing class MyRest {

    @HttpGet
    global static void get() {
        RestResponse res = RestContext.response;
        if (res == null) {
            res = new RestResponse();
            RestContext.response = res;
        }
        try {
            res.responseBody = Blob.valueOf(JSON.serialize(doGet()));
            res.statusCode = 200;
        } catch (EndUserMessageException e) {
            res.responseBody = Blob.valueOf(e.getMessage());
            res.statusCode = 400;
        } catch (Exception e) {
            res.responseBody = Blob.valueOf(
                    String.valueOf(e) + 'nn' + e.getStackTraceString()
                    );
            res.statusCode = 500;
        }
    }

    private static Object doGet() {
        ...
    }
}