This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.0.25!

Web

Router DSL

Spring Framework comes with a Kotlin router DSL available in 3 flavors:

WebMvc.fn DSL with router { }
WebFlux.fn Reactive DSL with router { }
WebFlux.fn [Coroutines] DSL with coRouter { }

These DSL let you write clean and idiomatic Kotlin code to build a RouterFunction instance as the following example shows:

@Configuration
class RouterRouterConfiguration {

	@Bean
	fun mainRouter(userHandler: UserHandler) = router {
		accept(TEXT_HTML).nest {
			GET("/") { ok().render("index") }
			GET("/sse") { ok().render("sse") }
			GET("/users", userHandler::findAllView)
		}
		"/api".nest {
			accept(APPLICATION_JSON).nest {
				GET("/users", userHandler::findAll)
			}
			accept(TEXT_EVENT_STREAM).nest {
				GET("/users", userHandler::stream)
			}
		}
		resources("/**", ClassPathResource("static/"))
	}
}

This DSL is programmatic, meaning that it allows custom registration logic of beans through an if expression, a for loop, or any other Kotlin constructs. That can be useful when you need to register routes depending on dynamic data (for example, from a database).

See MiXiT project for a concrete example.

MockMvc DSL

A Kotlin DSL is provided via MockMvc Kotlin extensions in order to provide a more idiomatic Kotlin API and to allow better discoverability (no usage of static methods).

val mockMvc: MockMvc = ...
mockMvc.get("/person/{name}", "Lee") {
	secure = true
	accept = APPLICATION_JSON
	headers {
		contentLanguage = Locale.FRANCE
	}
	principal = Principal { "foo" }
}.andExpect {
	status { isOk }
	content { contentType(APPLICATION_JSON) }
	jsonPath("$.name") { value("Lee") }
	content { json("""{"someBoolean": false}""", false) }
}.andDo {
	print()
}

Kotlin Script Templates

Spring Framework provides a ScriptTemplateView which supports JSR-223 to render templates by using script engines.

By leveraging scripting-jsr223 dependencies, it is possible to use such feature to render Kotlin-based templates with kotlinx.html DSL or Kotlin multiline interpolated String.

build.gradle.kts

dependencies {
        runtime("org.jetbrains.kotlin:kotlin-scripting-jsr223:${kotlinVersion}")
}

Configuration is usually done with ScriptTemplateConfigurer and ScriptTemplateViewResolver beans.

KotlinScriptConfiguration.kt

@Configuration
class KotlinScriptConfiguration {

    @Bean
	fun kotlinScriptConfigurer() = ScriptTemplateConfigurer().apply {
		engineName = "kotlin"
		setScripts("scripts/render.kts")
		renderFunction = "render"
		isSharedEngine = false
	}

    @Bean
    fun kotlinScriptViewResolver() = ScriptTemplateViewResolver().apply {
        setPrefix("templates/")
        setSuffix(".kts")
    }
}

See the kotlin-script-templating example project for more details.

Kotlin multiplatform serialization

As of Spring Framework 5.3, Kotlin multiplatform serialization is supported in Spring MVC, Spring WebFlux and Spring Messaging (RSocket). The builtin support currently targets CBOR, JSON, and ProtoBuf formats.

To enable it, follow those instructions to add the related dependency and plugin. With Spring MVC and WebFlux, both Kotlin serialization and Jackson will be configured by default if they are in the classpath since Kotlin serialization is designed to serialize only Kotlin classes annotated with @Serializable. With Spring Messaging (RSocket), make sure that neither Jackson, GSON or JSONB are in the classpath if you want automatic configuration, if Jackson is needed configure KotlinSerializationJsonMessageConverter manually.