This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.2.3!

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 multiplatform serialization

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 dependencies and plugin. With Spring MVC and WebFlux, Kotlin serialization is configured by default if it is in the classpath and other variants like Jackson are not. If needed, configure the converters or codecs manually.