Deploying a Kotlin App to Heroku – Latest News Web Dev

Since its earliest launch, Java has touted itself as a “write once, run everywhere” programming language. The thought was {that a} programmer may develop an app in Java, have it compiled right down to bytecode, and change into an executable that may run on any platform, no matter working system or platform. It was ready to take action partly by a runtime often known as the Java Digital Machine, or JVM

To Java’s credit score, the JVM was (and nonetheless is!) an extremely fine-tuned runtime that abstracted away a pc’s underlying {hardware}. Whereas Java as a programming language survives to this present day, it’s usually considered as cumbersome, stodgy, and consultant of an outdated strategy to implementing options. 

Within the final 10 years, increasingly languages that run on the JVM have developed, however appear and feel nothing like Java. One such language is Kotlin. Due to the JVM, it has no actual efficiency benefits over common Java. Nonetheless, its energy lies in the truth that it prioritizes legibility in a method that Java doesn’t. Think about, for instance, printing a substring in Java:



// Java
String enter = "What's the reply to the Final Query of Life, the Universe, and All the things? 42";
String reply = enter.substring(enter.indexOf("?") + 1);

It’s essential to first get the index of the character you wish to be within the substring, add one (since strings are zero-indexed), and name System.out.println to put in writing to stdout.

In Kotlin, that is a lot shorter:



// Kotlin
val enter = "What's the reply to the Final Query of Life, the Universe, and All the things? 42"
val reply = enter.substringAfter("?")

Kotlin has garnered a lot curiosity that Google even recommends it over Java for developing Android apps

On this submit, we’ll take a fast have a look at the best way to develop an app in Kotlin. We’ll construct a easy API with a PostgreSQL database and deploy it to Heroku to see it stay.


Earlier than we start, you will have to be sure to’ve bought the next software program put in in your machine: 

  1. An account on Heroku. That is fully free and would not require any cost data.
  2. The Heroku CLI. As soon as your software is on Heroku, this may make managing it a lot simpler.
  3. You may have to have Kotlin put in (>= 1.4).
  4. You may additionally want Gradle put in (>= 7.0).

Additionally, you will have to be slightly accustomed to Git and have it put in in your machine.

We’re going to be utilizing the IntelliJ IDE for this Kotlin app. Their documentation gives some steering on the best way to create a brand new mission. Ensure you choose the next choices:

  • We wish to create a Kotlin software that makes use of Gradle as a construct system
  • Set the title of the mission to kotlin-api
  • Set the JDK model to 16. In the event you don’t have this model put in, you’ll be able to choose Obtain JDK… from the dropdown, then select Oracle Open JDK model 16

After the IDE units the whole lot up, you must have a listing construction that appears roughly like this:

├── construct.gradle.kts
└── src
├── predominant
    │   ├── kotlin

Our Kotlin recordsdata can be saved in src/predominant/kotlin, and our construct logic can be in construct.gradle.kts.

Getting began

Gradle is a construct device for a wide range of languages. It additionally acts as a dependency administration device, much like Maven. You’ll have already got some traces in your construct.gradle.kts file, which the IDE robotically added to be useful. You’ll be able to delete all of that, and paste in these traces as an alternative:



plugins {
    id("org.jetbrains.kotlin.jvm") model "1.5.10"
    id("org.springframework.boot") model "2.4.3"

    id("io.spring.dependency-management") model "1.0.11.RELEASE"

group "com.instance"
model "0.0.1"

repositories {

dependencies {



These traces specify our mission’s dependencies and the place to search out them. For instance, we wish to use org.springframework.boot at model 2.4.3, which is why it is outlined throughout the plugins block. We level out the repositories the place the packages may be discovered—at mavenCentral()—and which uncovered lessons we wish to use (implementation( "org.springframework.boot:spring-boot-starter-web")).

Let’s create two small recordsdata to check our setup. Create a file known as Utility.kt within the src/predominant/kotlin folder and paste within the following:



package deal com.instance

import org.springframework.boot.SpringApplication
import org.springframework.boot.autoconfigure.SpringBootApplication

open class Utility

enjoyable predominant(args: Array<String>) {, *args)

This begins a default app utilizing the Spring framework. The true magic occurs on this subsequent file, Controller.kt, which you must create alongside Utility.kt in src/predominant/kotlin:



package deal com.instance

import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RestController

class GreetingController {

    enjoyable get(@PathVariable title: String) = "Hey, $title"

Right here, we outline a route (@GetMapping("/{title}")), the place {title} is a dynamic worth. By putting this decorator over a Kotlin technique (enjoyable get, or “a operate named get”), we’re in a position to match the path to no matter habits we would like—on this case, returning a greeting with the trail title for a GET request. 

To ensure that the IDE to know the best way to launch our software, we have to create a run configuration. On the prime of the IDE menu, click on the button that claims Add Configuration…. Choose Add new run configuration, then select Gradle from the listing. For the Gradle mission title, enter kotlin-api. Within the Duties discipline, kind bootRun. bootRun is a Gradle activity offered by the Spring framework which is able to compile our code and begin the server. Click on Okay; you must now have a inexperienced Play button in your IDE menu bar. Whenever you click on on this, the IDE will execute  gradle bootRun to construct this Kotlin app and begin the server. When that finishes, navigate to http://localhost:8080/world. You need to see a pleasant greeting.

Interacting with the database

Now, let’s get to the (considerably) critical stuff. Suppose we needed to connect a database to this mission. In a Maven/Java world, we would have to replace an XML file and add a reference to a JAR file. In Gradle, we will get by with simply including just a few traces to our construct.gradle.kts file:



dependencies {
    # ...


    # ...

Right here, we have included HikariCP in our mission, which is a well-liked database connection driver. We additionally point out that we wish to “load” the org.postgresql library throughout runtime. With simply these two traces, we have let our configuration know that we wish to work together with a PostgreSQL database. If you have already got a PostgreSQL database operating regionally, that is nice. You can proceed the remainder of this information regionally and see the outcomes when looking localhost. If you do not have PostgreSQL, do not fret—we’ll present you simply how simple it’s to deploy this app on Heroku, which is able to handle the infrastructure for you. 

Head again to Controller.kt, and exchange it with the contents under. This takes a few of what we had from earlier than however provides to it. We’ll go over the adjustments shortly.



package deal com.instance
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RestController
import org.springframework.web.bind.annotation.PostMapping
import org.springframework.web.bind.annotation.RequestBody
import org.springframework.http.MediaType
import com.zaxxer.hikari.HikariConfig
import com.zaxxer.hikari.HikariDataSource
import java.internet.URI
import javax.sql.DataSource

class GreetingController {

   val dataSource = dataSource()
   val connection = dataSource.connection


   enjoyable get(@PathVariable title: String) = "Hey, $title"

   @PostMapping(worth = ["/add-name"], consumes = [MediaType.TEXT_PLAIN_VALUE])

   enjoyable submit(@RequestBody requestBody: String) : String {
       val stmt = connection.createStatement()
       stmt.executeUpdate("INSERT INTO names values('$requestBody')")
       return "Added $requestBody"


   enjoyable getAll() : String {
       val stmt = connection.createStatement()
       val rs = stmt.executeQuery("SELECT title FROM names")
       val output = ArrayList<String>()
       whereas (rs.subsequent()) {
       val names = output.joinToString(", ")
       return "Listed below are the names: $names"

   inside enjoyable initDb() {
       val stmt = connection.createStatement()
       stmt.executeUpdate("CREATE TABLE IF NOT EXISTS names (title textual content)")

   inside enjoyable dataSource(): HikariDataSource {
       val config = HikariConfig()
       var dbUri = URI(System.getenv("DATABASE_URL") ?: "postgresql://localhost:5432/")
       var dbUserInfo =  dbUri.getUserInfo()
       var username: String?; var password: String?;
       if (dbUserInfo != null) {
           username = dbUserInfo.break up(":").get(0)
           password = dbUserInfo.break up(":").get(1)
       } else {
           username = System.getenv("DATABASE_USERNAME") ?: null
           password = System.getenv("DATABASE_PASSWORD") ?: null
       if (username != null) {
       if (password != null) {
       val dbUrl = "jdbc:postgresql://${dbUri.getHost()}:${dbUri.getPort()}${dbUri.getPath()}"
       return HikariDataSource(config)

There’s quite a bit happening right here! Let’s begin from the underside. We outline a operate known as dataSource which gives a connection to our database. As a result of we’re constructing a 12-Factor app, our database credentials are saved in an setting variable known as DATABASE_URL. We fetch that URL and pull out the username and password from it if one exists. If not, we examine one other two setting variables for that data—DATABASE_USERNAME and DATABASE_PASSWORD. We then put all that data collectively right into a format that the database connector wants. The initDb operate creates a desk known as names, with a single textual content column known as title. The /everybody endpoint has a @GetMapping decorator identical to earlier than. This defines a GET /everybody route, which will get all of the names from the database.

Lastly, we have added one thing moderately new: a @PostMapping decorator. Right here, we have to outline what sorts of content material this POST route can settle for. On this case, it consumes a TEXT_PLAIN_VALUE media kind (in different phrases, "Content material-Kind: textual content/plain"). No matter string of knowledge we put within the request physique can be added to the database. In just some traces, we have constructed a small API that we will add to and question.

In the event you begin this server now—and if in case you have PostgreSQL operating regionally—you must have the ability to work together with it. Strive making the next request:

$ curl -H “Content material-Kind: textual content/plain” -X POST http://localhost:8080/add-name -d ‘Frank’

In the event you navigate to http://localhost:8080/everybody, you will see that Frank was included.

Deploying to Heroku

It is time to see simply how simple it’s to get Kotlin operating on Heroku. First, we have to create a file that is particular to Heroku: the Procfile. This textual content file defines how our software ought to boot and run. 

Create a file named Procfile within the root stage listing, proper subsequent to your construct.gradle.kts file. Copy-paste the next traces into it:

web: java -jar construct/libs/kotlin-api.jar –server.port=$PORT

Right here, we’re saying that we would like Heroku to run java -jar construct/libs/kotlin-api.jar. That JAR is packaged and constructed in the course of the deployment course of; Heroku will create it robotically for us as a result of it is aware of the best way to execute the Gradle activity to take action. We’re additionally binding the $PORT setting variable in order that Heroku is aware of which port the server is listening to.

Subsequent, run the next Git instructions:

$ git init
$ git add .
$ git commit -m “Getting ready my first Kotlin app”

Since we have now the Heroku CLI put in, we will call heroku create on the command line to generate an app:

$ heroku create
Creating app… executed, ⬢ desolate-plains-67007
Created | [email protected]:desolate-plains-67007.git

Your app can be assigned a random title—on this instance, it is desolate-plains-67007—in addition to a publicly accessible URL.

With a purpose to provision a database, we use the heroku addons command. Calling it with out arguments will tell us if any exist:

$ heroku addons
No add-ons for app desolate-plains-67007.

No add-ons exist for our app, which is smart—we simply created it! So as to add a PostgreSQL database, we will use the addons:create command like this:

$ heroku addons:create heroku-postgresql:hobby-dev

Heroku presents a number of tiers of PostgreSQL databases. hobby-dev is the free tier, so we will mess around with this with out paying a dime.

Your code is prepared, your Heroku app is configured—you’re able to deploy. That is the straightforward half! Simply kind out the next command:

$ git push heroku grasp

Your code can be pushed to Heroku. From that time on, Heroku will take over. You may see your construct logs scrolling via your terminal. This can present you what Heroku is putting in in your behalf and the place you might be within the construct course of. After it’s full, you’ll be able to go to your particular URL within the browser (on this case, and work together with the API on the web!

Studying extra

Kotlin’s performant design and legible syntax—mixed with the benefit of Gradle—make it ideally suited for enterprises that have to depend on battle-tested Java packages. Due to its interoperability with Java, Kotlin is good as a transitional language; huge swaths of Java code may be transformed into Kotlin whereas nonetheless sustaining a practical app. Deploying to Heroku is easy, and I did not even benefit from the other ways to fine-tune Java and JVM-based apps for deployment.

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