This document implements a simple org.http4s.HttpRoutes and then walks through the results (i.e. org.http4s.Response) of applying inputs (i.e. org.http4s.Request) to it.

After reading this doc, the reader should feel comfortable writing a unit test using his/her favorite Scala testing library.

Now, let's define an org.http4s.HttpRoutes.

import cats.syntax.all._
import io.circe._
import io.circe.syntax._
import io.circe.generic.semiauto._
import cats.effect._
import org.http4s._
import org.http4s.circe._
import org.http4s.dsl.io._
import org.http4s.implicits._

If you're in a REPL, we also need a runtime:

import cats.effect.unsafe.IORuntime
implicit val runtime: IORuntime = cats.effect.unsafe.IORuntime.global
case class User(name: String, age: Int)
implicit val UserEncoder: Encoder[User] = deriveEncoder[User]

trait UserRepo[F[_]] {
  def find(userId: String): F[Option[User]]

def httpRoutes[F[_]](repo: UserRepo[F])(
      implicit F: Async[F]
): HttpRoutes[F] = HttpRoutes.of[F] {
  case GET -> Root / "user" / id =>
    repo.find(id).map {
      case Some(user) => Response(status = Status.Ok).withEntity(user.asJson)
      case None       => Response(status = Status.NotFound)

For testing, let's define a check function:

// Return true if match succeeds; otherwise false
def check[A](actual:        IO[Response[IO]],
            expectedStatus: Status,
            expectedBody:   Option[A])(
    implicit ev: EntityDecoder[IO, A]
): Boolean =  {
   val actualResp         = actual.unsafeRunSync()
   val statusCheck        = actualResp.status == expectedStatus
   val bodyCheck          = expectedBody.fold[Boolean](
       // Verify Response's body is empty.
       expected => actualResp.as[A].unsafeRunSync() == expected
   statusCheck && bodyCheck

Let's define service by passing a UserRepo that returns Ok(user).

val success: UserRepo[IO] = new UserRepo[IO] {
  def find(id: String): IO[Option[User]] = IO.pure(Some(User("johndoe", 42)))

val response: IO[Response[IO]] = httpRoutes[IO](success).orNotFound.run(
  Request(method = Method.GET, uri = uri"/user/not-used" )
val expectedJson = Json.obj(
      "name" := "johndoe",
      "age" := 42
// expectedJson: Json = JObject(value = object[name -> "johndoe",age -> 42])

check[Json](response, Status.Ok, Some(expectedJson))
// res0: Boolean = true

Next, let's define a service with a userRepo that returns None to any input.

val foundNone: UserRepo[IO] = new UserRepo[IO] {
  def find(id: String): IO[Option[User]] = IO.pure(None)

val respFoundNone: IO[Response[IO]] = httpRoutes[IO](foundNone).orNotFound.run(
  Request(method = Method.GET, uri = uri"/user/not-used" )
check[Json](respFoundNone, Status.NotFound, None)
// res1: Boolean = true

Finally, let's pass a Request which our service does not handle.

val doesNotMatter: UserRepo[IO] = new UserRepo[IO] {
  def find(id: String): IO[Option[User]] =
    IO.raiseError(new RuntimeException("Should not get called!"))

val respNotFound: IO[Response[IO]] = httpRoutes[IO](doesNotMatter).orNotFound.run(
  Request(method = Method.GET, uri = uri"/not-a-matching-path" )
check[String](respNotFound, Status.NotFound, Some("Not found"))
// res2: Boolean = true

Using client

Having HttpApp you can build a client for testing purposes. Following the example above we could define our HttpApp like this:

val httpApp: HttpApp[IO] = httpRoutes[IO](success).orNotFound

From this, we can obtain the Client instance using Client.fromHttpApp and then use it to test our sever/app.

import org.http4s.client.Client

val request: Request[IO] = Request(method = Method.GET, uri = uri"/user/not-used")
val client: Client[IO] = Client.fromHttpApp(httpApp)

val resp: IO[Json]     = client.expect[Json](request)
assert(resp.unsafeRunSync() == expectedJson)


The above documentation demonstrated how to define an HttpRoutes[F], pass Request's, and then test the expected Response.

To add unit tests in your chosen Scala Testing Framework, please follow the above examples.