Add Kotlin extensions for function Web API

Issue: SPR-15054

spring-web-reactive/src/main/kotlin/org/springframework/web/reactive/function/BodyExtractorsExtension.kt

@@ -0,0 +1,39 @@
+package org.springframework.web.reactive.function
+
+import org.springframework.http.ReactiveHttpInputMessage
+import reactor.core.publisher.Flux
+import reactor.core.publisher.Mono
+import kotlin.reflect.KClass
+
+/**
+ * Extension for [BodyExtactors] providing [KClass] based API and avoiding specifying
+ * a class parameter when possible thanks to Kotlin reified type parameters.
+ *
+ * @since 5.0
+ */
+object BodyExtractorsExtension {
+
+	/**
+	 * @see BodyExtactors.toMono
+	 */
+	inline fun <reified T : Any> toMono() : BodyExtractor<Mono<T>, ReactiveHttpInputMessage> =
+			BodyExtractors.toMono(T::class.java)
+
+	/**
+	 * @see BodyExtactors.toMono
+	 */
+	fun <T : Any> toMono(elementClass: KClass<T>) : BodyExtractor<Mono<T>, ReactiveHttpInputMessage> =
+			BodyExtractors.toMono(elementClass.java)
+
+	/**
+	 * @see BodyExtactors.toFlux
+	 */
+	inline fun <reified T : Any> toFlux() : BodyExtractor<Flux<T>, ReactiveHttpInputMessage> =
+			BodyExtractors.toFlux(T::class.java)
+
+	/**
+	 * @see BodyExtactors.toFlux
+	 */
+	fun <T : Any> toFlux(elementClass: KClass<T>) : BodyExtractor<Flux<T>, ReactiveHttpInputMessage> =
+			BodyExtractors.toFlux(elementClass.java)
+}

spring-web-reactive/src/main/kotlin/org/springframework/web/reactive/function/BodyInsertersExtension.kt

@@ -0,0 +1,26 @@
+package org.springframework.web.reactive.function
+
+import org.reactivestreams.Publisher
+import org.springframework.http.ReactiveHttpOutputMessage
+import org.springframework.http.server.reactive.ServerHttpResponse
+
+/**
+ * Extension for [BodyInserters] providing [KClass] based API and avoiding specifying
+ * a class parameter when possible thanks to Kotlin reified type parameters.
+ *
+ * @since 5.0
+ */
+object BodyInsertersExtension {
+
+	/**
+	 * @see BodyInserters.fromPublisher
+	 */
+	inline fun <reified T : Publisher<S>, reified S : Any> fromPublisher(publisher: T) : BodyInserter<T, ReactiveHttpOutputMessage> =
+			BodyInserters.fromPublisher(publisher, S::class.java)
+
+	/**
+	 * @see BodyInserters.fromServerSentEvents
+	 */
+	inline fun <reified T : Publisher<S>, reified S : Any> fromServerSentEvents(publisher: T) : BodyInserter<T, ServerHttpResponse> =
+			BodyInserters.fromServerSentEvents(publisher, S::class.java)
+}

spring-web-reactive/src/main/kotlin/org/springframework/web/reactive/function/client/ClientResponseExtension.kt

@@ -0,0 +1,23 @@
+package org.springframework.web.reactive.function.client
+
+import reactor.core.publisher.Flux
+import reactor.core.publisher.Mono
+import kotlin.reflect.KClass
+
+/**
+ * Extension for [ClientResponse] providing [KClass] based API.
+ *
+ * @since 5.0
+ */
+object ClientResponseExtension {
+
+	/**
+	 * @see ClientResponse.bodyToFlux
+	 */
+	fun <T : Any> ClientResponse.bodyToFlux(type: KClass<T>) : Flux<T> = bodyToFlux(type.java)
+
+	/**
+	 * @see ClientResponse.bodyToMono
+	 */
+	fun <T : Any> ClientResponse.bodyToMono(type: KClass<T>) : Mono<T> = bodyToMono(type.java)
+}

spring-web-reactive/src/main/kotlin/org/springframework/web/reactive/function/server/ServerRequestExtension.kt

@@ -0,0 +1,21 @@
+package org.springframework.web.reactive.function.server
+
+import kotlin.reflect.KClass
+
+/**
+ * Extension for [ServerRequest] providing [KClass] based API.
+ *
+ * @since 5.0
+ */
+object ServerRequestExtension {
+
+	/**
+	 * @see ServerRequest.bodyToMono
+	 */
+	fun <T : Any> ServerRequest.bodyToMono(type: KClass<T>) = bodyToMono(type.java)
+
+	/**
+	 * @see ServerRequest.bodyToFlux
+	 */
+	fun <T : Any> ServerRequest.bodyToFlux(type: KClass<T>) = bodyToFlux(type.java)
+}