[[kotlin]]
= Kotlin

https://kotlinlang.org[Kotlin] is a statically typed language that targets the JVM
(and other platforms) which allows writing concise and elegant code while providing
very good https://kotlinlang.org/docs/reference/java-interop.html[interoperability]
with existing libraries written in Java.

The Spring Framework provides first-class support for Kotlin and lets developers write
Kotlin applications almost as if the Spring Framework was a native Kotlin framework.

The easiest way to build a Spring application with Kotlin is to leverage Spring Boot and
its https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-kotlin.html[dedicated Kotlin support].
https://spring.io/guides/tutorials/spring-boot-kotlin/[This comprehensive tutorial]
will teach you how to build Spring Boot applications with Kotlin using https://start.spring.io/#!language=kotlin&type=gradle-project[start.spring.io].

Feel free to join the #spring channel of https://slack.kotlinlang.org/[Kotlin Slack]
or ask a question with `spring` and `kotlin` as tags on
https://stackoverflow.com/questions/tagged/spring+kotlin[Stackoverflow] if you need support.

[[kotlin-requirements]]
== Requirements

Spring Framework supports Kotlin 1.3 and requires
https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib[`kotlin-stdlib`]
(or one of its variants, such as https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-stdlib-jdk8[`kotlin-stdlib-jdk8`])
and https://bintray.com/bintray/jcenter/org.jetbrains.kotlin%3Akotlin-reflect[`kotlin-reflect`]
to be present on the classpath. They are provided by default if you bootstrap a Kotlin project on
https://start.spring.io/#!language=kotlin&type=gradle-project[start.spring.io].

[[kotlin-extensions]]
== Extensions

Kotlin https://kotlinlang.org/docs/reference/extensions.html[extensions] provide the ability
to extend existing classes with additional functionality. The Spring Framework Kotlin APIs
use these extensions to add new Kotlin-specific conveniences to existing Spring APIs.

The {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/[Spring Framework KDoc API] lists
and documents all available the Kotlin extensions and DSLs.

NOTE: Keep in mind that Kotlin extensions need to be imported to be used. This means,
for example, that the `GenericApplicationContext.registerBean` Kotlin extension
is available only if `org.springframework.context.support.registerBean` is imported.
That said, similar to static imports, an IDE should automatically suggest the import in most cases.

For example, https://kotlinlang.org/docs/reference/inline-functions.html#reified-type-parameters[Kotlin reified type parameters]
provide a workaround for JVM https://docs.oracle.com/javase/tutorial/java/generics/erasure.html[generics type erasure],
and the Spring Framework provides some extensions to take advantage of this feature.
This allows for a better Kotlin API `RestTemplate`, for the new `WebClient` from Spring
WebFlux, and for various other APIs.

NOTE: Other libraries, such as Reactor and Spring Data, also provide Kotlin extensions
for their APIs, thus giving a better Kotlin development experience overall.

To retrieve a list of `User` objects in Java, you would normally write the following:

[source,java,indent=0]
----
	Flux<User> users  = client.get().retrieve().bodyToFlux(User.class)
----

With Kotlin and the Spring Framework extensions, you can instead write the following:

[source,kotlin,indent=0]
----
	val users = client.get().retrieve().bodyToFlux<User>()
	// or (both are equivalent)
	val users : Flux<User> = client.get().retrieve().bodyToFlux()
----

As in Java, `users` in Kotlin is strongly typed, but Kotlin's clever type inference allows
for shorter syntax.

[[kotlin-null-safety]]
== Null-safety

One of Kotlin's key features is https://kotlinlang.org/docs/reference/null-safety.html[null-safety],
which cleanly deals with `null` values at compile time rather than bumping into the famous
`NullPointerException` at runtime. This makes applications safer through nullability
declarations and expressing "`value or no value`" semantics without paying the cost of wrappers, such as `Optional`.
(Kotlin allows using functional constructs with nullable values. See this
https://www.baeldung.com/kotlin-null-safety[comprehensive guide to Kotlin null-safety].)

Although Java does not let you express null-safety in its type-system, the Spring Framework
provides <<core#null-safety, null-safety of the whole Spring Framework API>>
via tooling-friendly annotations declared in the `org.springframework.lang` package.
By default, types from Java APIs used in Kotlin are recognized as
https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types[platform types],
for which null-checks are relaxed.
https://kotlinlang.org/docs/reference/java-interop.html#jsr-305-support[Kotlin support for JSR-305 annotations]
and Spring nullability annotations provide null-safety for the whole Spring Framework API to Kotlin developers,
with the advantage of dealing with `null`-related issues at compile time.

NOTE: Libraries such as Reactor or Spring Data provide null-safe APIs to leverage this feature.

You can configure JSR-305 checks by adding the `-Xjsr305` compiler flag with the following
options: `-Xjsr305={strict|warn|ignore}`.

For kotlin versions 1.1+, the default behavior is the same as `-Xjsr305=warn`.
The `strict` value is required to have Spring Framework API null-safety taken into account
in Kotlin types inferred from Spring API but should be used with the knowledge that Spring
API nullability declaration could evolve even between minor releases and that more checks may
be added in the future).

NOTE: Generic type arguments, varargs, and array elements nullability are not supported yet,
but should be in an upcoming release. See https://github.com/Kotlin/KEEP/issues/79[this discussion]
for up-to-date information.

[[kotlin-classes-interfaces]]
== Classes and Interfaces

The Spring Framework supports various Kotlin constructs, such as instantiating Kotlin classes
through primary constructors, immutable classes data binding, and function optional parameters
with default values.

Kotlin parameter names are recognized through a dedicated `KotlinReflectionParameterNameDiscoverer`,
which allows finding interface method parameter names without requiring the Java 8 `-parameters`
compiler flag to be enabled during compilation.

The https://github.com/FasterXML/jackson-module-kotlin[Jackson Kotlin module], which is required
for serializing or deserializing JSON data, is automatically registered when
found in the classpath, and a warning message is logged if Jackson and Kotlin are
detected without the Jackson Kotlin module being present.

You can declare configuration classes as
https://kotlinlang.org/docs/reference/nested-classes.html[top level or nested but not inner],
since the later requires a reference to the outer class.

[[kotlin-annotations]]
== Annotations

The Spring Framework also takes advantage of https://kotlinlang.org/docs/reference/null-safety.html[Kotlin null-safety]
to determine if a HTTP parameter is required without having to explicitly
define the `required` attribute. That means `@RequestParam name: String?` is treated
as not required and, conversely, `@RequestParam name: String` is treated as being required.
This feature is also supported on the Spring Messaging `@Header` annotation.

In a similar fashion, Spring bean injection with `@Autowired`, `@Bean`, or `@Inject` uses
this information to determine if a bean is required or not.

For example, `@Autowired lateinit var thing: Thing` implies that a bean
of type `Thing` must be registered in the application context, while `@Autowired lateinit var thing: Thing?`
does not raise an error if such a bean does not exist.

Following the same principle, `@Bean fun play(toy: Toy, car: Car?) = Baz(toy, Car)` implies
that a bean of type `Toy` must be registered in the application context, while a bean of
type `Car` may or may not exist. The same behavior applies to autowired constructor parameters.

NOTE: If you use bean validation on classes with properties or a primary constructor
parameters, you may need to use
https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets[annotation use-site targets],
such as `@field:NotNull` or `@get:Size(min=5, max=15)`, as described in
https://stackoverflow.com/a/35853200/1092077[this Stack Overflow response].

[[kotlin-bean-definition-dsl]]
== Bean Definition DSL

Spring Framework supports registering beans in a functional way by using lambdas
as an alternative to XML or Java configuration (`@Configuration` and `@Bean`). In a nutshell,
it lets you register beans with a lambda that acts as a `FactoryBean`.
This mechanism is very efficient, as it does not require any reflection or CGLIB proxies.

In Java, you can, for example, write the following:

[source,java,indent=0]
----
	class Foo {}

	class Bar {
		private final Foo foo;
		public Bar(Foo foo) {
			this.foo = foo;
		}
	}

	GenericApplicationContext context = new GenericApplicationContext();
	context.registerBean(Foo.class);
	context.registerBean(Bar.class, () -> new Bar(context.getBean(Foo.class)));
----

In Kotlin, with reified type parameters and `GenericApplicationContext` Kotlin extensions,
you can instead write the following:

[source,kotlin,indent=0]
----
	class Foo

	class Bar(private val foo: Foo)

	val context = GenericApplicationContext().apply {
		registerBean<Foo>()
		registerBean { Bar(it.getBean()) }
	}
----
====

When the class `Bar` has a single constructor, you can even just specify the bean class,
the constructor parameters will be autowired by type:

====
[source,kotlin,indent=0]
----
	val context = GenericApplicationContext().apply {
		registerBean<Foo>()
		registerBean<Bar>()
	}
----

In order to allow a more declarative approach and cleaner syntax, Spring Framework provides
a {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.context.support/-bean-definition-dsl/[Kotlin bean definition DSL]
It declares an `ApplicationContextInitializer` through a clean declarative API,
which lets you deal with profiles and `Environment` for customizing
how beans are registered.

In the following example notice that:

* Type inference usually allows to avoid specifying the type for bean references like `ref("bazBean")`
* It is possible to use Kotlin top level functions to declare beans using callable references like `bean(::myRouter)` in this example
* When specifying `bean<Bar>()` or `bean(::myRouter)`, parameters are autowired by type
* The `FooBar` bean will be registered only if the `foobar` profile is active

[source,kotlin,indent=0]
----
	class Foo
	class Bar(private val foo: Foo)
	class Baz(var message: String = "")
	class FooBar(private val baz: Baz)

	val myBeans = beans {
		bean<Foo>()
		bean<Bar>()
		bean("bazBean") {
			Baz().apply {
				message = "Hello world"
			}
		}
		profile("foobar") {
			bean { FooBar(ref("bazBean")) }
		}
		bean(::myRouter)
	}

	fun myRouter(foo: Foo, bar: Bar, baz: Baz) = router {
		// ...
	}
----

NOTE: This DSL is programmatic, meaning it allows custom registration logic of beans
through an `if` expression, a `for` loop, or any other Kotlin constructs.

You can then use this `beans()` function to register beans on the application context,
as the following example shows:

[source,kotlin,indent=0]
----
	val context = GenericApplicationContext().apply {
		myBeans.initialize(this)
		refresh()
	}
----


NOTE: Spring Boot is based on JavaConfig and
https://github.com/spring-projects/spring-boot/issues/8115[does not yet provide specific support for functional bean definition],
but you can experimentally use functional bean definitions through Spring Boot's `ApplicationContextInitializer` support.
See https://stackoverflow.com/questions/45935931/how-to-use-functional-bean-definition-kotlin-dsl-with-spring-boot-and-spring-w/46033685#46033685[this Stack Overflow answer]
for more details and up-to-date information. See also the experimental Kofu DSL developed in https://github.com/spring-projects/spring-fu[Spring Fu incubator].

[[kotlin-web]]
== Web

=== Router DSL

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

* WebMvc.fn DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.servlet.function/router.html[router { }]
* WebFlux.fn <<web-reactive#webflux-fn, Reactive>> DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/router.html[router { }]
* WebFlux.fn <<Coroutines>> DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }]

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

[source,kotlin,indent=0]
----
@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/"))
	}
}
----

NOTE: 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 https://github.com/mixitconf/mixit/[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).

[source,kotlin,indent=0]
----
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
https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/servlet/view/script/ScriptTemplateView.html[`ScriptTemplateView`]
which supports https://www.jcp.org/en/jsr/detail?id=223[JSR-223] to render templates by using script engines.

By leveraging `kotlin-script-runtime` and `scripting-jsr223-embeddable` dependencies, it
is possible to use such feature to render Kotlin-based templates with
https://github.com/Kotlin/kotlinx.html[kotlinx.html] DSL or Kotlin multiline interpolated `String`.

`build.gradle.kts`
[source,kotlin,indent=0]
----
dependencies {
	compile("org.jetbrains.kotlin:kotlin-script-runtime:${kotlinVersion}")
	runtime("org.jetbrains.kotlin:kotlin-scripting-jsr223-embeddable:${kotlinVersion}")
}
----

Configuration is usually done with `ScriptTemplateConfigurer` and `ScriptTemplateViewResolver`
beans.


`KotlinScriptConfiguration.kt`
[source,kotlin,indent=0]
----
@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 https://github.com/sdeleuze/kotlin-script-templating[kotlin-script-templating] example
project for more details.

== Coroutines

Kotlin https://kotlinlang.org/docs/reference/coroutines-overview.html[Coroutines] are Kotlin
lightweight threads allowing to write non-blocking code in an imperative way. On language side,
suspending functions provides an abstraction for asynchronous operations while on library side
https://github.com/Kotlin/kotlinx.coroutines[kotlinx.coroutines] provides functions likes
https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html[`async { }`]
and types like https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[`Flow`].

Spring Framework provides support for Coroutines on the following scope:

* https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-deferred/index.html[Deferred] and https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[Flow] return values support in Spring WebFlux annotated `@Controller`
* Suspending function support in Spring WebFlux annotated `@Controller`
* Extensions for WebFlux {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.client/index.html[client] and {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/index.html[server] functional API.
* WebFlux.fn {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }] DSL
* Suspending function and `Flow` support in RSocket `@MessageMapping` annotated methods
* Extensions for {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.messaging.rsocket/index.html[`RSocketRequester`]

=== Dependencies

Coroutines support is enabled when `kotlinx-coroutines-core` and `kotlinx-coroutines-reactor`
dependencies are in the classpath:

`build.gradle.kts`
[source,kotlin,indent=0]
----
dependencies {

	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:${coroutinesVersion}")
	implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:${coroutinesVersion}")
}
----

Version `1.3.0` and above are supported.

=== How Reactive translates to Coroutines?

For return values, the translation from Reactive to Coroutines APIs is the following:

* `fun handler(): Mono<Void>` becomes `suspend fun handler()`
* `fun handler(): Mono<T>` becomes `suspend fun handler(): T` or `suspend fun handler(): T?` depending on if the `Mono` can be empty or not (with the advantage of beeing more statically typed)
* `fun handler(): Flux<T>` becomes `fun handler(): Flow<T>`

For input parameters:

* If laziness is not needed, `fun handler(mono: Mono<T>)` becomes `fun handler(value: T)` since a suspending functions can be invoked to get the value parameter.
* If laziness is needed, `fun handler(mono: Mono<T>)` becomes `fun handler(supplier: suspend () -> T)` or `fun handler(supplier: suspend () -> T?)`

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/index.html[`Flow`] is `Flux` equivalent in Coroutines world, suitable for hot or cold stream, finite or infinite streams, with the following main differences:

* `Flow` is push-based while `Flux` is push-pull hybrid
* Backpressure is implemented via suspending functions
* `Flow` has only a https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/collect.html[single suspending `collect` method] and operators are implemented as https://kotlinlang.org/docs/reference/extensions.html[extensions]
* https://github.com/Kotlin/kotlinx.coroutines/tree/master/kotlinx-coroutines-core/common/src/flow/operators[Operators are easy to implement] thanks to Coroutines
* Extensions allow to add custom operators to `Flow`
* Collect operations are suspending functions
* https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/map.html[`map` operator] supports asynchronous operation (no need for `flatMap`) since it takes a suspending function parameter

Read this blog post about https://spring.io/blog/2019/04/12/going-reactive-with-spring-coroutines-and-kotlin-flow[Going Reactive with Spring, Coroutines and Kotlin Flow]
for more details, including how to run code concurrently with Coroutines.

=== Controllers

Here is an example of a Coroutines `@RestController`.

[source,kotlin,indent=0]
----
@RestController
class CoroutinesRestController(client: WebClient, banner: Banner) {

	@GetMapping("/suspend")
	suspend fun suspendingEndpoint(): Banner {
		delay(10)
		return banner
	}

	@GetMapping("/flow")
	fun flowEndpoint() = flow {
		delay(10)
		emit(banner)
		delay(10)
		emit(banner)
	}

	@GetMapping("/deferred")
	fun deferredEndpoint() = GlobalScope.async {
		delay(10)
		banner
	}

	@GetMapping("/sequential")
	suspend fun sequential(): List<Banner> {
		val banner1 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		val banner2 = client
				.get()
				.uri("/suspend")
				.accept(MediaType.APPLICATION_JSON)
				.awaitExchange()
				.awaitBody<Banner>()
		return listOf(banner1, banner2)
	}

	@GetMapping("/parallel")
	suspend fun parallel(): List<Banner> = coroutineScope {
		val deferredBanner1: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		val deferredBanner2: Deferred<Banner> = async {
			client
					.get()
					.uri("/suspend")
					.accept(MediaType.APPLICATION_JSON)
					.awaitExchange()
					.awaitBody<Banner>()
		}
		listOf(deferredBanner1.await(), deferredBanner2.await())
	}

	@GetMapping("/error")
	suspend fun error() {
		throw IllegalStateException()
	}

	@GetMapping("/cancel")
	suspend fun cancel() {
		throw CancellationException()
	}

}
----

View rendering with a `@Controller` is also supported.

[source,kotlin,indent=0]
----
@Controller
class CoroutinesViewController(banner: Banner) {

	@GetMapping("/")
	suspend fun render(model: Model): String {
		delay(10)
		model["banner"] = banner
		return "index"
	}
}
----

=== WebFlux.fn

Here is an example of Coroutines router defined via the {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }] DSL and related handlers.

[source,kotlin,indent=0]
----
@Configuration
class RouterConfiguration {

	@Bean
	fun mainRouter(userHandler: UserHandler) = coRouter {
		GET("/", userHandler::listView)
		GET("/api/user", userHandler::listApi)
	}
}
----

[source,kotlin,indent=0]
----
class UserHandler(builder: WebClient.Builder) {

	private val client = builder.baseUrl("...").build()

	suspend fun listView(request: ServerRequest): ServerResponse =
			ServerResponse.ok().renderAndAwait("users", mapOf("users" to
			client.get().uri("...").awaitExchange().awaitBody<User>()))

	suspend fun listApi(request: ServerRequest): ServerResponse =
				ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).bodyAndAwait(
				client.get().uri("...").awaitExchange().awaitBody<User>())
}
----

[[kotlin-spring-projects-in-kotlin]]
== Spring Projects in Kotlin

This section provides some specific hints and recommendations worth for developing Spring projects
in Kotlin.



=== Final by Default

By default, https://discuss.kotlinlang.org/t/classes-final-by-default/166[all classes in Kotlin are `final`].
The `open` modifier on a class is the opposite of Java's `final`: It allows others to inherit from this
class. This also applies to member functions, in that they need to be marked as `open` to be overridden.

While Kotlin's JVM-friendly design is generally frictionless with Spring, this specific Kotlin feature
can prevent the application from starting, if this fact is not taken into consideration. This is because
Spring beans (such as `@Configuration` annotated classes which by default need to be inherited at runtime for technical
reasons) are normally proxied by CGLIB. The workaround was to add an `open` keyword on each class and
member function of Spring beans that are proxied by CGLIB, which can
quickly become painful and is against the Kotlin principle of keeping code concise and predictable.

NOTE: It is also possible to avoid CGLIB proxies on configurations by using `@Configuration(proxyBeanMethods = false)`,
see {api-spring-framework}/context/annotation/Configuration.html#proxyBeanMethods--[`proxyBeanMethods` Javadoc] for more details.

Fortunately, Kotlin now provides a
https://kotlinlang.org/docs/reference/compiler-plugins.html#kotlin-spring-compiler-plugin[`kotlin-spring`]
plugin (a preconfigured version of the `kotlin-allopen` plugin) that automatically opens classes
and their member functions for types that are annotated or meta-annotated with one of the following
annotations:

* `@Component`
* `@Async`
* `@Transactional`
* `@Cacheable`

Meta-annotations support means that types annotated with `@Configuration`, `@Controller`,
`@RestController`, `@Service`, or `@Repository` are automatically opened since these
annotations are meta-annotated with `@Component`.

https://start.spring.io/#!language=kotlin&type=gradle-project[start.spring.io] enables it by default, so, in practice,
you can write your Kotlin beans without any additional `open` keyword, as in Java.

=== Using Immutable Class Instances for Persistence

In Kotlin, it is convenient and considered to be a best practice to declare read-only properties
within the primary constructor, as in the following example:

[source,kotlin,indent=0]
----
	class Person(val name: String, val age: Int)
----

You can optionally add https://kotlinlang.org/docs/reference/data-classes.html[the `data` keyword]
to make the compiler automatically derive the following members from all properties declared
in the primary constructor:

* `equals()` and `hashCode()`
* `toString()` of the form `"User(name=John, age=42)"`
* `componentN()` functions that correspond to the properties in their order of declaration
* `copy()` function

As the following example shows, this allows for easy changes to individual properties, even if `Person` properties are read-only:

[source,kotlin,indent=0]
----
	data class Person(val name: String, val age: Int)

	val jack = Person(name = "Jack", age = 1)
	val olderJack = jack.copy(age = 2)
----

Common persistence technologies (such as JPA) require a default constructor, preventing this
kind of design. Fortunately, there is now a workaround for this
https://stackoverflow.com/questions/32038177/kotlin-with-jpa-default-constructor-hell["`default constructor hell`"],
since Kotlin provides a https://kotlinlang.org/docs/reference/compiler-plugins.html#kotlin-jpa-compiler-plugin[`kotlin-jpa`]
plugin that generates synthetic no-arg constructor for classes annotated with JPA annotations.

If you need to leverage this kind of mechanism for other persistence technologies, you can configure
the https://kotlinlang.org/docs/reference/compiler-plugins.html#how-to-use-no-arg-plugin[`kotlin-noarg`]
plugin.

NOTE: As of the Kay release train, Spring Data supports Kotlin immutable class instances and
does not require the `kotlin-noarg` plugin if the module uses Spring Data object mappings
(such as MongoDB, Redis, Cassandra, and others).

=== Injecting Dependencies

Our recommendation is to try and favor constructor injection with `val` read-only (and
non-nullable when possible) https://kotlinlang.org/docs/reference/properties.html[properties],
as the following example shows:

[source,kotlin,indent=0]
----
	@Component
	class YourBean(
		private val mongoTemplate: MongoTemplate,
		private val solrClient: SolrClient
	)
----

NOTE: Classes with a single constructor have their parameters automatically autowired,
that's why there is no need for an explicit `@Autowired constructor` in the example shown
above.

If you really need to use field injection, you can use the `lateinit var` construct,
as the following example shows:

[source,kotlin,indent=0]
----
	@Component
	class YourBean {

		@Autowired
		lateinit var mongoTemplate: MongoTemplate

		@Autowired
		lateinit var solrClient: SolrClient
	}
----

=== Injecting Configuration Properties

In Java, you can inject configuration properties by using annotations (such as `@Value("${property}")`).
However, in Kotlin, `$` is a reserved character that is used for
https://kotlinlang.org/docs/reference/idioms.html#string-interpolation[string interpolation].

Therefore, if you wish to use the `@Value` annotation in Kotlin, you need to escape the `$`
character by writing `@Value("\${property}")`.

NOTE: If you use Spring Boot, you should probably use
https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html#boot-features-external-config-typesafe-configuration-properties[`@ConfigurationProperties`]
instead of `@Value` annotations.

As an alternative, you can customize the properties placeholder prefix by declaring the
following configuration beans:

[source,kotlin,indent=0]
----
	@Bean
	fun propertyConfigurer() = PropertySourcesPlaceholderConfigurer().apply {
		setPlaceholderPrefix("%{")
	}
----

You can customize existing code (such as Spring Boot actuators or `@LocalServerPort`)
that uses the `${...}` syntax, with configuration beans, as the following example shows:

[source,kotlin,indent=0]
----
	@Bean
	fun kotlinPropertyConfigurer() = PropertySourcesPlaceholderConfigurer().apply {
		setPlaceholderPrefix("%{")
		setIgnoreUnresolvablePlaceholders(true)
	}

	@Bean
	fun defaultPropertyConfigurer() = PropertySourcesPlaceholderConfigurer()
----


=== Checked Exceptions

Java and https://kotlinlang.org/docs/reference/exceptions.html[Kotlin exception handling]
are pretty close, with the main difference being that Kotlin treats all exceptions as
unchecked exceptions. However, when using proxied objects (for example classes or methods
annotated with `@Transactional`), checked exceptions thrown will be wrapped by default in
an `UndeclaredThrowableException`.

To get the original exception thrown like in Java, methods should be annotated with
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-throws/index.html[`@Throws`]
to specify explicitly the checked exceptions thrown (for example `@Throws(IOException::class)`).

=== Annotation Array Attributes

Kotlin annotations are mostly similar to Java annotations, but array attributes (which are
extensively used in Spring) behave differently. As explained in
https://kotlinlang.org/docs/reference/annotations.html[Kotlin documentation] you can omit
the `value` attribute name, unlike other attributes, and specify it as a `vararg` parameter.

To understand what that means, consider `@RequestMapping` (which is one of the most widely
used Spring annotations) as an example. This Java annotation is declared as follows:

[source,java,indent=0]
----
	public @interface RequestMapping {

		@AliasFor("path")
		String[] value() default {};

		@AliasFor("value")
		String[] path() default {};

		RequestMethod[] method() default {};

		// ...
	}
----

The typical use case for `@RequestMapping` is to map a handler method to a specific path
and method. In Java, you can specify a single value for the annotation array attribute,
and it is automatically converted to an array.

That is why one can write
`@RequestMapping(value = "/toys", method = RequestMethod.GET)` or
`@RequestMapping(path = "/toys", method = RequestMethod.GET)`.

However, in Kotlin, you must write `@RequestMapping("/toys", method = [RequestMethod.GET])`
or `@RequestMapping(path = ["/toys"], method = [RequestMethod.GET])` (square brackets need
to be specified with named array attributes).

An alternative for this specific `method` attribute (the most common one) is to
use a shortcut annotation, such as `@GetMapping`, `@PostMapping`, and others.

NOTE: Reminder: If the `@RequestMapping` `method` attribute is not specified,
all HTTP methods will be matched, not only the `GET` one.

=== Testing

This section addresses testing with the combination of Kotlin and Spring Framework.
The recommended testing framework is https://junit.org/junit5/[JUnit 5], as well as
https://mockk.io/[Mockk] for mocking.

NOTE: If you are using Spring Boot, see
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-kotlin-testing[this related documentation].

==== Constructor injection

As described in the <<testing#testcontext-junit-jupiter-di#spring-web-reactive, dedicated section>>,
JUnit 5 allows constructor injection of beans which is pretty useful with Kotlin
in order to use `val` instead of `lateinit var`. You can use
{api-spring-framework}/test/context/TestConstructor.html[`@TestConstructor(autowire = true)`]
to enable autowiring for all parameters.

====
[source,kotlin,indent=0]
----
@SpringJUnitConfig(TestConfig::class)
@TestConstructor(autowire = true)
class OrderServiceIntegrationTests(val orderService: OrderService,
                                   val customerService: CustomerService) {

    // tests that use the injected OrderService and CustomerService
}
----
====

==== `PER_CLASS` Lifecycle

Kotlin lets you specify meaningful test function names between backticks (```).
As of JUnit 5, Kotlin test classes can use the `@TestInstance(TestInstance.Lifecycle.PER_CLASS)`
annotation to enable a single instantiation of test classes, which allows the use of `@BeforeAll`
and `@AfterAll` annotations on non-static methods, which is a good fit for Kotlin.

You can also change the default behavior to `PER_CLASS` thanks to a `junit-platform.properties`
file with a `junit.jupiter.testinstance.lifecycle.default = per_class` property.

The following example demonstrates `@BeforeAll` and `@AfterAll` annotations on non-static methods:

[source,kotlin,indent=0]
----
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class IntegrationTests {

  val application = Application(8181)
  val client = WebClient.create("http://localhost:8181")

  @BeforeAll
  fun beforeAll() {
    application.start()
  }

  @Test
  fun `Find all users on HTML page`() {
    client.get().uri("/users")
        .accept(TEXT_HTML)
        .retrieve()
        .bodyToMono<String>()
        .test()
        .expectNextMatches { it.contains("Foo") }
        .verifyComplete()
  }

  @AfterAll
  fun afterAll() {
    application.stop()
  }
}
----

==== Specification-like Tests

You can create specification-like tests with JUnit 5 and Kotlin.
The following example shows how to do so:

[source,kotlin,indent=0]
----
class SpecificationLikeTests {

  @Nested
  @DisplayName("a calculator")
  inner class Calculator {
     val calculator = SampleCalculator()

     @Test
     fun `should return the result of adding the first number to the second number`() {
        val sum = calculator.sum(2, 4)
        assertEquals(6, sum)
     }

     @Test
     fun `should return the result of subtracting the second number from the first number`() {
        val subtract = calculator.subtract(4, 2)
        assertEquals(2, subtract)
     }
  }
}
----

[[kotlin-webtestclient-issue]]
==== `WebTestClient` Type Inference Issue in Kotlin

Due to a https://youtrack.jetbrains.com/issue/KT-5464[type inference issue], you must
use the Kotlin `expectBody` extension (such as `.expectBody<String>().isEqualTo("toys")`),
since it provides a workaround for the Kotlin issue with the Java API.

See also the related https://jira.spring.io/browse/SPR-16057[SPR-16057] issue.

[[kotlin-getting-started]]
== Getting Started

The easiest way to learn how to build a Spring application with Kotlin is to follow
https://spring.io/guides/tutorials/spring-boot-kotlin/[the dedicated tutorial].

=== `start.spring.io`

The easiest way to start a new Spring Framework project in Kotlin is to create a new Spring
Boot 2 project on https://start.spring.io/#!language=kotlin&type=gradle-project[start.spring.io].

=== Choosing the Web Flavor

Spring Framework now comes with two different web stacks: <<web#mvc, Spring MVC>> and
<<web-reactive#spring-web-reactive, Spring WebFlux>>.

Spring WebFlux is recommended if you want to create applications that will deal with latency,
long-lived connections, streaming scenarios or if you want to use the web functional
Kotlin DSL.

For other use cases, especially if you are using blocking technologies such as JPA, Spring
MVC and its annotation-based programming model is the recommended choice.

[[kotlin-resources]]
== Resources

We recommend the following resources for people learning how to build applications with
Kotlin and the Spring Framework:

* https://kotlinlang.org/docs/reference/[Kotlin language reference]
* https://slack.kotlinlang.org/[Kotlin Slack] (with a dedicated #spring channel)
* https://stackoverflow.com/questions/tagged/spring+kotlin[Stackoverflow, with `spring` and `kotlin` tags]
* https://try.kotlinlang.org/[Try Kotlin in your browser]
* https://blog.jetbrains.com/kotlin/[Kotlin blog]
* https://kotlin.link/[Awesome Kotlin]

=== Examples

The following Github projects offer examples that you can learn from and possibly even extend:

* https://github.com/sdeleuze/spring-boot-kotlin-demo[spring-boot-kotlin-demo]: Regular Spring Boot and Spring Data JPA project
* https://github.com/mixitconf/mixit[mixit]: Spring Boot 2, WebFlux, and Reactive Spring Data MongoDB
* https://github.com/sdeleuze/spring-kotlin-functional[spring-kotlin-functional]: Standalone WebFlux and functional bean definition DSL
* https://github.com/sdeleuze/spring-kotlin-fullstack[spring-kotlin-fullstack]: WebFlux Kotlin fullstack example with Kotlin2js for frontend instead of JavaScript or TypeScript
* https://github.com/spring-petclinic/spring-petclinic-kotlin[spring-petclinic-kotlin]: Kotlin version of the Spring PetClinic Sample Application
* https://github.com/sdeleuze/spring-kotlin-deepdive[spring-kotlin-deepdive]: A step-by-step migration guide for Boot 1.0 and Java to Boot 2.0 and Kotlin
* https://github.com/spring-cloud/spring-cloud-gcp/tree/master/spring-cloud-gcp-kotlin-samples/spring-cloud-gcp-kotlin-app-sample[spring-cloud-gcp-kotlin-app-sample]: Spring Boot with Google Cloud Platform Integrations

=== Issues

The following list categorizes the pending issues related to Spring and Kotlin support:

* Spring Framework
** https://github.com/spring-projects/spring-framework/issues/20606[Unable to use WebTestClient with mock server in Kotlin]
** https://github.com/spring-projects/spring-framework/issues/20496[Support null-safety at generics, varargs and array elements level]
* Kotlin
** https://youtrack.jetbrains.com/issue/KT-6380[Parent issue for Spring Framework support]
** https://youtrack.jetbrains.com/issue/KT-5464[Kotlin requires type inference where Java doesn't]
** https://youtrack.jetbrains.com/issue/KT-20283[Smart cast regression with open classes]
** https://youtrack.jetbrains.com/issue/KT-14984[Impossible to pass not all SAM argument as function]
** https://youtrack.jetbrains.com/issue/KT-15125[Support JSR 223 bindings directly via script variables]
** https://youtrack.jetbrains.com/issue/KT-6653[Kotlin properties do not override Java-style getters and setters]
