🥇 Spring Boot: Top 5 Server-Side Frameworks for Kotlin in 2022

This is a demo inspired by @antonarhipov's Top 5 Server-Side Frameworks for Kotlin in 2022 @ Kotlin by JetBrains where, spoiler alert, the author shares this top 5 list:

🥇 Spring Boot
🥈 Quarkus
🥉 Micronaut
🏅 Ktor
🏅 http4k

I have a lot of experience in Spring Boot, so I wanted to take a look at the other ones 😜

To do so we will create a simple application with each one of these frameworks, implementing the following scenario:

rogervinas / top-5-server-side-kotlin-frameworks-2022

⭐ Top 5 Server-Side Frameworks for Kotlin in 2022

This post will describe the step-by-step Spring Boot implementation, you can check the other ones in this series too.

We can create a project using Spring Initialzr and download it locally.

A lot of documentation guides at spring.io/spring-boot.

Implementation

YAML Configuration

By default, Spring Initialzr creates a template using application.properties file. We can just rename it to application.yaml and it will work the same.

We can put there our first configuration property:

greeting:
  name: "Bitelchus"

More documentation about configuration sources at Externalized Configuration and Profiles.

GreetingRepository

We will create a GreetingRepository:

interface GreetingRepository {
  fun getGreeting(): String
}

@Repository
class GreetingJdbcRepository(
  private val jdbcTemplate: JdbcTemplate
): GreetingRepository {
  fun getGreeting(): String = jdbcTemplate
    .queryForObject(
      """
        SELECT greeting FROM greetings
        ORDER BY random() LIMIT 1
      """.trimIndent(), 
      String::class.java
    )!!
}

• The @Repository annotation will make Spring Boot to create a singleton instance at startup.
• We inject a JdbcTemplate (provided by the spring-boot-starter-jdbc autoconfiguration) to execute queries to the database.
• We use queryForObject and that SQL to retrieve one random greeting from the greetings table.

Additional to spring-boot-starter-jdbc we will need to add these extra dependencies:

implementation("org.postgresql:postgresql")
implementation("org.flywaydb:flyway-core")

And the following configuration in application.yaml:

spring:
  datasource:
    url: "jdbc:postgresql://${DB_HOST:localhost}:5432/mydb"
    username: "myuser"
    password: "mypassword"
    driver-class-name: "org.postgresql.Driver"
  flyway:
    enabled: true

And Flyway migrations under src/main/resources/db/migration to create and populate greetings table.

GreetingController

We will create a GreetingController serving /hello endpoint:

@RestController
@RequestMapping("/hello")
class GreetingController(
  private val repository: GreetingRepository, 
  @Value("\${greeting.name}")
  private val name: String,
  @Value("\${greeting.secret:unknown}")
  private val secret: String
) {
  @GetMapping(produces = [MediaType.TEXT_PLAIN_VALUE])
  fun hello() 
    = "${repository.getGreeting()} my name is $name" +
        " and my secret is $secret"
}

• @RestController annotation will make Spring Boot to create an instance on startup and wire it properly as a REST endpoint on /hello path, scanning its annotated methods.
• @GetMapping will map hello function answering to GET /hello requests.
• The controller expects a GreetingRepository to be injected as well as two configuration properties, no matter what property source they come from (environment variables, system properties, configuration files, Vault, ...).
• We expect to get greeting.secret from Vault, that is why we configure unknown as its default value, so it does not fail until we configure Vault properly.

GreetingApplication

As a Spring Boot requirement, we need to create a main application:

@SpringBootApplication
class GreetingApplication

fun main(args: Array<String>) { 
  runApplication<GreetingApplication>(*args)
}

By convention, all classes under the same package of the main application will be scanned for annotations.

Vault Configuration

We just add the dependency org.springframework.cloud:spring-cloud-starter-vault-config and we add the following configuration in application.yaml:

spring:
  cloud:
    vault:
      enabled: true
      uri: "http://${VAULT_HOST:localhost}:8200"
      authentication: "TOKEN"
      token: "mytoken"
      kv:
        enabled: true
        backend: "secret"
        default-context: "myapp"
        application-name: "myapp"
  config:
    import: optional:vault://

Then we can access the configuration property greeting.secret stored in Vault.

You can check the documentation at Spring Vault.

Testing the endpoint

We can test the endpoint with a "slice test", meaning only the parts needed by the controller will be started:

@WebFluxTest
@TestPropertySource(properties = [
  "spring.cloud.vault.enabled=false",
  "greeting.secret=apple"
])
class GreetingControllerTest {

  @MockBean
  private lateinit var repository: GreetingRepository

  @Autowired
  private lateinit var client: WebTestClient

  @Test
  fun `should say hello`() {
    doReturn("Hello").`when`(repository).getGreeting()

    client
      .get().uri("/hello")
      .exchange()
      .expectStatus().isOk
      .expectBody<String>()
      .isEqualTo(
        "Hello my name is Bitelchus and my secret is apple"
      )
    }
}

• We use WebTestClient to execute requests to the endpoint.
• We mock the repository with @MockBean.
• We can use a @TestPropertySource to configure the greeting.secret property and spring.cloud.vault.enabled=false to disable Vault.

Testing the application

To test the whole application we will use Testcontainers and the docker compose file:

@SpringBootTest(webEnvironment = RANDOM_PORT)
@Testcontainers
class GreetingApplicationTest {

  companion object {
    @Container
    private val container = DockerComposeContainer(File("../docker-compose.yaml"))
      .withServices("db", "vault", "vault-cli")
      .withLocalCompose(true)
      .waitingFor("db", forLogMessage(".*database system is ready to accept connections.*", 1))
      .waitingFor("vault", forLogMessage(".*Development mode.*", 1))
  }

  @Autowired
  private lateinit var client: WebTestClient

  @Test
  fun `should say hello`() { 
    client
      .get().uri("/hello")
      .exchange()
      .expectStatus().isOk
      .expectBody<String>().consumeWith {
        assertThat(it.responseBody!!)
         .matches(
           ".+ my name is Bitelchus and my secret is watermelon"
         )
      }
  }
}

• We use the shared docker compose to start the required three containers.
• We use WebTestClient again to test the endpoint.
• We use pattern matching to check the greeting, as it is random.
• As Vault is now enabled, the secret should be watermelon.

Test

./gradlew test

Run

# Start Vault and Database
docker compose up -d vault vault-cli db

# Start Application
./gradlew bootRun

# Make requests
curl http://localhost:8080/hello

# Stop Application with control-c

# Stop all containers
docker compose down

Build a fatjar and run it

# Build fatjar
./gradlew bootJar

# Start Vault and Database
docker compose up -d vault vault-cli db

# Start Application
java -jar build/libs/springboot-app-0.0.1-SNAPSHOT.jar

# Make requests
curl http://localhost:8080/hello

# Stop Application with control-c

# Stop all containers
docker compose down

Build a docker image and run it

# Build docker image
./gradlew bootBuildImage

# Start Vault and Database
docker compose up -d vault vault-cli db

# Start Application
docker compose --profile springboot up -d

# Make requests
curl http://localhost:8080/hello

# Stop all containers
docker compose --profile springboot down
docker compose down

That's it! Happy coding! 💙