Extensions
Kotlin 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 Spring Framework KDoc API lists and documents all available Kotlin extensions and DSLs.
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.
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For example, Kotlin reified type parameters
provide a workaround for JVM 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.
| 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:
Flux<User> users = client.get().retrieve().bodyToFlux(User.class)With Kotlin and the Spring Framework extensions, you can instead write the following:
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.