Fix Kotlin example in `Multipart Content`

Closes gh-36094
Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

framework-docs/modules/ROOT/pages/web/webflux/controller/ann-methods/multipart-forms.adoc

@@ -103,8 +103,8 @@ Kotlin::
 [source,kotlin,indent=0,subs="verbatim,quotes"]
 ----
 	@PostMapping("/")
-	fun handle(@RequestPart("meta-data") Part metadata, // <1>
-			@RequestPart("file-data") FilePart file): String { // <2>
+	fun handle(@RequestPart("meta-data") metadata: Part, // <1>
+		@RequestPart("file-data") file: FilePart): String { // <2>
 		// ...
 	}
 ----
@@ -169,7 +169,7 @@ Kotlin::
 [source,kotlin,indent=0,subs="verbatim,quotes"]
 ----
 	@PostMapping("/")
-	fun handle(@Valid @RequestPart("meta-data") metadata: MetaData): String {
+	fun handle(@Valid @RequestPart("meta-data") metadata: Mono<MetaData>): String {
 		// ...
 	}
 ----
@@ -202,7 +202,7 @@ Kotlin::
 [source,kotlin,indent=0,subs="verbatim,quotes"]
 ----
 	@PostMapping("/")
-	fun handle(@RequestBody parts: MultiValueMap<String, Part>): String { // <1>
+	fun handle(@RequestBody parts: Mono<MultiValueMap<String, Part>>): String { // <1>
 		// ...
 	}
 ----
@@ -227,87 +227,7 @@ when uploading. If the file is large enough to be split across multiple buffers,
 
 For example:
 
-[tabs]
-======
-Java::
-+
-[source,java,indent=0,subs="verbatim,quotes"]
-----
-  @PostMapping("/")
-  public void handle(@RequestBody Flux<PartEvent> allPartsEvents) { <1>
-      allPartsEvents.windowUntil(PartEvent::isLast) <2>
-              .concatMap(p -> p.switchOnFirst((signal, partEvents) -> { <3>
-                  if (signal.hasValue()) {
-                      PartEvent event = signal.get();
-                      if (event instanceof FormPartEvent formEvent) { <4>
-                          String value = formEvent.value();
-                          // handle form field
-                      }
-                      else if (event instanceof FilePartEvent fileEvent) { <5>
-                          String filename = fileEvent.filename();
-                          Flux<DataBuffer> contents = partEvents.map(PartEvent::content); <6>
-                          // handle file upload
-                      }
-                      else {
-                          return Mono.error(new RuntimeException("Unexpected event: " + event));
-                      }
-                  }
-                  else {
-                      return partEvents; // either complete or error signal
-                  }
-              }));
-  }
-----
-<1> Using `@RequestBody`.
-<2> The final `PartEvent` for a particular part will have `isLast()` set to `true`, and can be
-followed by additional events belonging to subsequent parts.
-This makes the `isLast` property suitable as a predicate for the `Flux::windowUntil` operator, to
-split events from all parts into windows that each belong to a single part.
-<3> The `Flux::switchOnFirst` operator allows you to see whether you are handling a form field or
-file upload.
-<4> Handling the form field.
-<5> Handling the file upload.
-<6> The body contents must be completely consumed, relayed, or released to avoid memory leaks.
-
-Kotlin::
-+
-[source,kotlin,indent=0,subs="verbatim,quotes"]
-----
-	@PostMapping("/")
-	fun handle(@RequestBody allPartsEvents: Flux<PartEvent>) = { // <1>
-      allPartsEvents.windowUntil(PartEvent::isLast) <2>
-          .concatMap {
-              it.switchOnFirst { signal, partEvents -> <3>
-                  if (signal.hasValue()) {
-                      val event = signal.get()
-                      if (event is FormPartEvent) { <4>
-                          val value: String = event.value();
-                          // handle form field
-                      } else if (event is FilePartEvent) { <5>
-                          val filename: String = event.filename();
-                          val contents: Flux<DataBuffer> = partEvents.map(PartEvent::content); <6>
-                          // handle file upload
-                      } else {
-                          return Mono.error(RuntimeException("Unexpected event: " + event));
-                      }
-                  } else {
-                      return partEvents; // either complete or error signal
-                  }
-              }
-          }
-}
-----
-<1> Using `@RequestBody`.
-<2> The final `PartEvent` for a particular part will have `isLast()` set to `true`, and can be
-followed by additional events belonging to subsequent parts.
-This makes the `isLast` property suitable as a predicate for the `Flux::windowUntil` operator, to
-split events from all parts into windows that each belong to a single part.
-<3> The `Flux::switchOnFirst` operator allows you to see whether you are handling a form field or
-file upload.
-<4> Handling the form field.
-<5> Handling the file upload.
-<6> The body contents must be completely consumed, relayed, or released to avoid memory leaks.
-======
+include-code::./PartEventController[tag=snippet,indent=0]
 
 Received part events can also be relayed to another service by using the `WebClient`.
 See xref:web/webflux-webclient/client-body.adoc#webflux-client-body-multipart[Multipart Data].

framework-docs/src/main/java/org/springframework/docs/web/webflux/controller/annmethods/partevent/PartEventController.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2026-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.web.webflux.controller.annmethods.partevent;
+
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.http.codec.multipart.FilePartEvent;
+import org.springframework.http.codec.multipart.FormPartEvent;
+import org.springframework.http.codec.multipart.PartEvent;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RestController;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+@RestController
+public class PartEventController {
+
+	// tag::snippet[]
+	@PostMapping("/")
+	public void handle(@RequestBody Flux<PartEvent> allPartsEvents) { // Using @RequestBody.
+
+		//	The final PartEvent for a particular part will have isLast() set to true, and can be
+		//	followed by additional events belonging to subsequent parts.
+		//	This makes the isLast property suitable as a predicate for the Flux::windowUntil operator, to
+		//	split events from all parts into windows that each belong to a single part.
+		allPartsEvents.windowUntil(PartEvent::isLast)
+
+				//	The Flux::switchOnFirst operator allows you to see whether you are handling
+				//	a form field or file upload.
+				.concatMap(p -> p.switchOnFirst((signal, partEvents) -> {
+						if (signal.hasValue()) {
+								PartEvent event = signal.get();
+								if (event instanceof FormPartEvent formEvent) {
+										String value = formEvent.value();
+										// Handling the form field.
+								}
+								else if (event instanceof FilePartEvent fileEvent) {
+										String filename = fileEvent.filename();
+
+										// The body contents must be completely consumed, relayed, or released to avoid memory leaks.
+										Flux<DataBuffer> contents = partEvents.map(PartEvent::content);
+										// Handling the file upload.
+						}
+						else {
+							return Mono.error(new RuntimeException("Unexpected event: " + event));
+						}
+					}
+					else {
+						return partEvents; // either complete or error signal
+					}
+					return Mono.empty();
+				}));
+	}
+	// end::snippet[]
+
+}

framework-docs/src/main/kotlin/org/springframework/docs/web/webflux/controller/annmethods/partevent/PartEventController.kt

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2026-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *	  https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.docs.web.webflux.controller.annmethods.partevent
+
+import org.springframework.core.io.buffer.DataBuffer
+import org.springframework.http.codec.multipart.FilePartEvent
+import org.springframework.http.codec.multipart.FormPartEvent
+import org.springframework.http.codec.multipart.PartEvent
+import org.springframework.web.bind.annotation.PostMapping
+import org.springframework.web.bind.annotation.RequestBody
+import org.springframework.web.bind.annotation.RestController
+import reactor.core.publisher.Flux
+import reactor.core.publisher.Mono
+
+@RestController
+class PartEventController {
+
+	// tag::snippet[]
+	@PostMapping("/")
+	fun handle(@RequestBody allPartsEvents: Flux<PartEvent>) { // Using @RequestBody.
+
+		//	The final PartEvent for a particular part will have isLast() set to true, and can be
+		//	followed by additional events belonging to subsequent parts.
+		//	This makes the isLast property suitable as a predicate for the Flux::windowUntil operator, to
+		//	split events from all parts into windows that each belong to a single part.
+		allPartsEvents.windowUntil(PartEvent::isLast)
+				.concatMap {
+
+					//	The Flux::switchOnFirst operator allows you to see whether you are handling
+					//	a form field or file upload.
+					it.switchOnFirst { signal, partEvents ->
+						if (signal.hasValue()) {
+							val event = signal.get()
+							if (event is FormPartEvent) {
+								val value: String = event.value()
+								// Handling the form field.
+							} else if (event is FilePartEvent) {
+								val filename: String = event.filename()
+
+								// The body contents must be completely consumed, relayed, or released to avoid memory leaks.
+								val contents: Flux<DataBuffer> = partEvents.map(PartEvent::content)
+								// Handling the file upload.
+							} else {
+								return@switchOnFirst Mono.error(RuntimeException("Unexpected event: $event"))
+							}
+						} else {
+							return@switchOnFirst partEvents // either complete or error signal
+						}
+						Mono.empty()
+					}
+				}
+	}
+	// end::snippet[]
+
+}