How do I make the default response Content-type: application/json with Scalatra

I'm learning Scalatra and am wondering how I can make the default Content-Type of my responses application/json. The current default appears to be text/html. Neat, but not really useful to my application.

The current default is text/html.

$ curl -i -X GET 'http://localhost:8080/v1/example'
HTTP/1.1 200 OK
Date: Fri, 19 Apr 2019 07:21:21 GMT
Content-Type: text/html;charset=utf-8
Content-Length: 23
Server: Jetty(9.4.8.v20171121)

HelloWorld(hello,world)

I can get application-json explicitly through the Accepted: application/json header.

$ curl -i -X GET 'http://localhost:8080/v1/example' -H 'Accept: application/json'
HTTP/1.1 200 OK
Date: Fri, 19 Apr 2019 07:22:09 GMT
Content-Type: application/json;charset=utf-8
Transfer-Encoding: chunked
Server: Jetty(9.4.8.v20171121)

{"hello":"hello","world":"world"}

How do I set the default to be application/json.

Solution

defaultFormat could be overridden to make application/json the default case. For example,

import java.io.File
import org.scalatra._
import org.scalatra.util.MimeTypes
import org.json4s.{DefaultFormats, Formats}
import org.scalatra.json._

case class HelloWorld(hello: String, world: String)

class MyScalatraServlet extends ScalatraServlet with JacksonJsonSupport {
  protected implicit lazy val jsonFormats: Formats = DefaultFormats

  override def defaultFormat: Symbol = 'json

  get("/v1/example") {
    HelloWorld("hello", "world")
  }

}

without specifying Accept header should respond with

curl -i -X GET 'http://localhost:8080/v1/example'
HTTP/1.1 200 OK
Date: Thu, 25 Apr 2019 22:28:37 GMT
Content-Type: application/json;charset=utf-8
Content-Length: 33
Server: Jetty(9.4.8.v20171121)

{"hello":"hello","world":"world"}

whilst explicitly requesting Accept: text/html also works

curl -i -X GET 'http://localhost:8080/v1/example' -H 'Accept: text/html'
HTTP/1.1 200 OK
Date: Fri, 26 Apr 2019 15:45:22 GMT
Content-Type: text/html;charset=utf-8
Content-Length: 23
Server: Jetty(9.4.8.v20171121)

HelloWorld(hello,world)