Scala is here


  • Scala sup­port for Vert.x is here!
  • It is based on Scala 2.12, no sup­port for 2.11 planned
  • all Vert.x-​modules are avail­able in a Scala fla­vor
  • It’s awe­some
  • Get started here


The rise of Scala as one of the most im­por­tant lan­guages on the JVM caught many (me in­cluded) by sur­prise. This hy­brid of func­tional and im­per­a­tive par­a­digms struck a chord with many de­vel­op­ers. Thanks to Scala a lot of peo­ple who’d never have touched a lan­guage like Haskell got ex­posed to func­tional pro­gram­ming. This ex­po­sure was one of the dri­ving forces to get streams and lambda into the JVM.

With the re­lease of Vert.x 3.4.0 we fi­nally in­tro­duced Scala to the fam­ily of sup­ported lan­guages: vertx-​lang-scala.

In this post I will in­tro­duce the new stack and how the power of Scala can be used in your fa­vorite re­ac­tive toolkit.


vertx-​lang-scala is based on Scala 2.12. There are no plans to sup­port 2.11.

All mod­ules avail­able for Vert.x are sup­ported (you can check here ).

Future and Promise both need a ExecutionContext

Mod­ules use the fol­low­ing naming-​scheme: io.vertx:<name-​of-vertx-module>-​scala_2.12:<vertx-​major-version>. The Scala ver­sion of io.vertx:vert-​web:3.4.0 would be io.vertx:vertx-​web-scala_2.12:3.4.0.

There is an sbt-​based quickstart-​project avail­able that will be up­dated for each Vert.x-​release.

Please note: Al­though sbt is used in this quick­start it is by no means re­quired. There are no spe­cial plu­g­ins in­volved so vertx-​lang-scala can eas­ily be used with Gra­dle or Maven.

I use sbt as it is the de­fault build sys­tem used for Scala projects.


Let’s get started by cloning the quick­start:

git clone [email protected]:vert-x3/vertx-sbt-starter.git

You just got the fol­low­ing things:

  • An sbt project con­tain­ing de­pen­den­cies to Vert.x-​core and Vert.x-web
  • The abil­ity to cre­ate a fat-​jat via sbt assembly
  • The abil­ity to cre­ate a docker con­tainer via sbt docker
  • A few ex­am­ple ver­ti­cles
  • Unit test ex­am­ples
  • a pre-​configured Scala-​shell in­side sbt

We will now run the ap­pli­ca­tion to get some quick sat­is­fac­tion. Use sbt assembly to pro­duce the fat-​jar fol­lowed by java -jar target/scala-2.12/vertx-scala-sbt-assembly-0.1-SNAPSHOT.jar. Now point your browser to http://lo­cal­host:8666/hello for a clas­sic wel­come mes­sage.

The details

Open your IDE so we can take a look at what’s going on under the hood. We start with the HttpVer­ti­cle.

package io.vertx.scala.sbt

import io.vertx.lang.scala.ScalaVerticle
import io.vertx.scala.ext.web.Router

import scala.concurrent.Future

class HttpVerticle extends ScalaVerticle { // <1>

  override def startFuture(): Future[Unit] = { // <2>
    val router = Router.router(vertx) // <3>
    val route = router

    vertx //<4>
      .listenFuture(8666, "")  // <5>
        .map(_ => ()) // <6>
  1. ScalaVer­ti­cle is the base class for all Scala-​Verticles. It pro­vides all re­quired meth­ods to in­te­grate with the Vert.x-​runtime.
  2. There are two ways to start a Ver­ti­cle. Over­rid­ing start­Fu­ture, like in this ex­am­ple, tells Vert.x to only con­sider the Ver­ti­cle fully started after the re­turned Fu­ture[Unit] has been suc­cess­fully com­pleted. Al­ter­na­tively one can over­ride start and by that sig­nal to Vert.x the in­stant avail­abil­ity of the Ver­ti­cle.
  3. This block cre­ates a Router for in­com­ing HTTP-​requests. It reg­is­ters a han­dler to an­swer with “world” if a re­quest to the URL “/hello” ar­rives. The class is com­ing from the Vert.x-​web-module.
  4. Every Ver­ti­cle has ac­cess to the Vert.x-​instance. Here we use it to cre­ate a web­server and reg­is­ter our router to han­dle in­com­ing re­quests.
  5. We fi­nally reached the rea­son why I use start­Fu­ture in the first place. All op­er­a­tions in Vert.x are asyn­chro­nous. So start­ing the web­server most def­i­nitely means it takes some more time until it bound to the given port (8666 in this case). That’s why lis­ten­Fu­ture is used, which re­turns a Fu­ture which in turn con­tains the ac­tual in­stance of the web­server that just got started. So our Ver­ti­cle will be ready to re­ceive re­quests after the re­turned Fu­ture has been com­pleted.
  6. In most cases we can re­turn the Fu­ture di­rectly. In this case the Fu­ture re­turned by lis­ten­Fu­ture has the wrong type. We get a Fu­ture[HttpServer] but we need a Fu­ture[Unit] as you can see in the sig­na­ture of start­Fu­ture. This call takes care of map­ping the given Fu­ture[HttpServer] to the re­quired re­turn type.


I use Sca­laT­est for all my test­ing needs. It comes with stel­lar sup­port for asyn­chro­nous op­er­a­tions and is a per­fect fit for test­ing Vert.x-​applications.

The fol­low­ing HttpVer­ti­cle­Spec shows how to test an HTTP-​API using only Vert.x-​classes. Per­son­ally I pre­fer REST-​assured with its rich DSL. For this post I wanted to stick with Vert.x-API, so here we go.

package io.vertx.scala.sbt

import org.scalatest.Matchers

import scala.concurrent.Promise

class HttpVerticleSpec extends VerticleTesting[HttpVerticle] with Matchers { // <1>

  "HttpVerticle" should "bind to 8666 and answer with 'world'" in { // <2>
    val promise = Promise[String] // <3>

    vertx.createHttpClient()  // <4>
      .getNow(8666, "", "/hello",
        r => {
          r.bodyHandler(b => promise.success(b.toString))
        }) => res should equal("world")) // <5>

  1. Ver­ti­cleTest­ing is a base class for your tests in­cluded with the quickstart-​project. It’s a small helper that takes care of de­ploy­ing/un-​deploying the Ver­ti­cle to be tested and man­ages a Vert.x-​instance. It ad­di­tion­ally ex­tends AsyncFlat­Spec so we can use Fu­tures as test-​return-types.
  2. Isn’t it nice and read­able?
  3. The promise is re­quired as the whole test will run async
  4. We use the vertx-​instance pro­vided by Ver­ti­cleTest­ing to cre­ate a Netty-​based Http­Client. We in­struct the client to call the spec­i­fied URL and to suc­ceed the Promise with the re­turned body.
  5. This cre­ates the ac­tual as­ser­tion. After get­ting the Fu­ture from the Promise an as­ser­tion is cre­ated: The Re­sult should be equal to the String “world”. Sca­laT­est takes care of eval­u­at­ing the re­turned Fu­ture.

That’s all you need to get started!

Futures in vertx-lang-scala

Now for a more in depth topic I think is worth men­tion­ing. vertx-​lang-scala treats async op­er­a­tions the Scala-​way which is a lit­tle dif­fer­ent from what you might be used from Vert.x. For async op­er­a­tions like sub­scrib­ing to the event­bus or de­ploy­ing a Ver­ti­cle you would call a method like this:

vertx.deployVerticle("", res -> {
  if (res.succeeded()) {
  } else {;

The de­ployVer­ti­cle method takes the Verticle-​name and a Han­dler[Asyn­cRe­sult] as its ar­gu­ments. The Han­dler[Asyn­cRe­sult] is called after Vert.x tried de­ploy­ing the Ver­ti­cle. This style can also be used for Scala (which might ease the tran­si­tion when com­ing from the Java-​world) but their is a way more scalaish way of doing this.

For every method tak­ing a Han­dler[Asyn­cRe­sult] as its ar­gu­ment I cre­ate an al­ter­na­tive method using Scala-​Futures.

vertx.deployVerticleFuture("") // <1>
  .onComplete{  // <2>
    case Success(s) => println(s"Verticle id is: $s") // <3>
    case Failure(t) => t.printStackTrace()
  1. A method pro­vid­ing a Fu­ture based al­ter­na­tive gets Fu­ture ap­pended to its name and re­turns a Fu­ture in­stead of tak­ing a Han­dler as its ar­gu­ment.
  2. We are now free to use Fu­ture the way we want. In this case on­Com­plete is used to react on the com­ple­tion.
  3. Pat­tern match­ing on the re­sult <3

I strongly rec­om­mend using this ap­proach over using Han­dlers as you won’t run into Callback-​hell and you get all the good­ies Scala pro­vides for async op­er­a­tions.

Future and Promise both need a ExecutionContext

The Vertx­Ex­e­cu­tion­Con­text is made im­plic­itly avail­able in­side the ScalaVer­ti­cle. It makes sure all op­er­a­tions are ex­e­cuted on the cor­rect Event Loop. If you are using Vert.x with­out Ver­ti­cles you have to pro­vide it on your own.

Using the console

A great fea­ture of sbt is the em­bed­ded, con­fig­urable Scala-​console. The con­sole avail­able in the quickstart-​project is pre-​configured to pro­vide a fresh Vert.x-​instance and all re­quired im­ports so you can start play­ing around with Vert.x in an in­stant.

Ex­e­cute the fol­low­ing com­mands in the project-​folder to de­ploy the HttpVer­ti­cle:

> console
scala> vertx.deployVerticle(nameForVerticle[HttpVerticle])
scala> vertx.deploymentIDs

After ex­e­cut­ing this se­quence you can now point your browser http://lo­cal­host:8666/hello to see our mes­sage. The last com­mand is­sued shows the Ids under which Ver­ti­cles have been de­ployed.

To get rid of the de­ploy­ment you can now type vertx.undeploy(vertx.deploymentIDs.head).

That’s it!

This was a very quick in­tro­duc­tion to our new Scala-​stack. I hope to have given you a lit­tle taste of the Scala good­ness now avail­able with Vert.x. I rec­om­mend dig­ging a lit­tle more through the quick­start to get a feel­ing for what’s there. In my next blog post I will ex­plain some of the de­ci­sions I made and the ob­sta­cles I faced with the dif­fer­ences be­tween Java and Scala /Hint: They are way big­ger than I was aware of).


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