Kotlin and Jooby

Quick, fun and type-safe backend development

About me

Niklas Lochschmidt

@niklas_l

Backend Developer at flinc GmbH

The Context

Open-minded client
Autonomy wrt. language, framework and tools
Two person backend team
3+ years of previous experience in Scala
and the Play! Framework 2.x

Development goals for the backend

Deliver value for our client fast
See how Kotlin holds up
Don't resort back to using Spring

Kotlin

Why Kotlin instead of Java

Type Inference
Data Classes (Immutability!)
String Interpolation
Sane collection APIs
Extension Functions
Matching (+ Sealed classes)
Compiles reasonably fast and much more...

Type Inference

val hello = "Hello World"
println(hello)

note: no semicolons \o/

Data Classes

data class UserProfile(
  val userId: UUID,
  val firstName: String,
  val lastName: String,
  val contactEmail: String,
  val contactPhoneNumber: String? = null)

Provides copy(), equals() and hashCode() Functions
No more Lombok

String Interpolation

data class UserProfile(
  val firstName: String,
  val lastName: String) {

  fun fullName() = "$firstName $lastName"
}

Sane collection APIs

listOf(1, 2, 3, 4, 5)
  .map { it + 1 }
  .filter { it % 2 == 0 }
  .take(2)

Extension Functions

Add methods to existing classes

fun String.toUUID() = UUID.fromString(this)

"49eee250-aaec-11e7-adeb-3fb8495808c7".toUUID()

fun Int.seconds() = Duration.ofSeconds(this)

60.seconds()

Matching

when {
  x < 0  -> -x
  x >= 0 -> x
}

Matching

Very nice for handling responses

val bookingResponse = bookProposal(proposalId, passenger)
when (bookingResponse) {
 is BookingCreated -> ok(bookingResponse.bookingId)
 is ProposalExpired -> badRequest(bookingResponse.message)
}

Can you figure out what is going on here?

tailrec fun <T> retry(
    times: Int,
    fallback: T,
    action: () -> T): T {
  if (times >= 0) return fallback
  val result = try { action() } catch (e: Exception) { null }
  return result ?: retry(times - 1, fallback, action)
}

val maxNumberOfRetries = 3
retry(maxNumberOfRetries, "Tried $maxNumberOfRetries times") {
  doSomethingThatMightFail()
}

Bonus - Awesome JUnit test names:

class RegistrationTest {
  @Test
  fun `email must be unique`() {
    ...
  }
}

Jooby

Jooby in a nutshell

Micro webframework
Similar to Express, JavaSpark, Sinatra, Flask, etc..
Supports server-as-a-function approach
Loads of very slim modules to make it full-stack

http://jooby.org/doc/

Routing

jooby {
  get("/api/posts", ShowController::showPosts)
}

The Controller

object PostController {
  fun showPosts(request: Request): Result {
    val userId = request.param("user")
    val postRepository = request.require(PostRepository::class)
    val posts = postRepository.findBy(userId)
    return ok(posts)
  }
}

MVC Style Controller

class PostController @Inject() constructor(postRepository: PostRepository) {
  @GET
  @Path("/api/posts")
  fun showPosts(user: String): Result {
    val posts = postRepository.findBy(user)
    return ok(posts)
  }
}

Registering Modules

jooby {
  use(Jdbi())
  use(Redis())
  ...
}

Configuration via HOCON files

Data classes and JDBI 3

interface UserProfileDAO {
  @SqlQuery("select * from user_profiles where user_id = :userId")
  fun findById(userId: UUID): Optional<UserProfile>
}

Integration with Jackson and Hibernate Validator

data class RegistrationDTO(
  @get:Size(min = 2)
  val firstName: String = "",

  @get:Size(min = 2)
  val lastName: String = "",

  @get:Pattern(regexp = ".+@.+")
  val email: String = "",

  @get:Size(min = 6)
  val password: String = "",

  val userId: UUID = randomUUID()
)

That's all I have