[[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 {docs-spring-framework}/kdoc-api/[Spring Framework KDoc API] lists
and documents all available 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.