Fix collection support in AbstractKotlinSerializationHttpMessageConverter

AbstractKotlinSerializationHttpMessageConverter#getSupportedMediaTypes(Class<?>)
currently invokes transitively supports(Class<?>) which always return false
with generic types.

This commit adds an override that just invokes getSupportedMediaTypes().

Closes gh-34992

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/converter/AbstractKotlinSerializationHttpMessageConverter.java

@@ -20,6 +20,7 @@
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -76,6 +77,11 @@ protected AbstractKotlinSerializationHttpMessageConverter(T format, MediaType...
 		this.format = format;
 	}
 
+	@Override
+	public List<MediaType> getSupportedMediaTypes(Class<?> clazz) {
+		return getSupportedMediaTypes();
+	}
+
 	@Override
 	protected boolean supports(Class<?> clazz) {
 		return serializer(ResolvableType.forClass(clazz)) != null;

spring-webmvc/spring-webmvc.gradle

@@ -1,6 +1,7 @@
 description = "Spring Web MVC"
 
 apply plugin: "kotlin"
+apply plugin: "kotlinx-serialization"
 
 dependencies {
 	api(project(":spring-aop"))

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/mvc/method/annotation/RequestResponseBodyMethodProcessorKotlinTests.kt

@@ -0,0 +1,133 @@
+/*
+ * Copyright 2025 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.web.servlet.mvc.method.annotation
+
+import kotlinx.serialization.Serializable
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.Test
+import org.springframework.http.converter.StringHttpMessageConverter
+import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter
+import org.springframework.web.bind.annotation.RequestMapping
+import org.springframework.web.bind.annotation.ResponseBody
+import org.springframework.web.context.request.NativeWebRequest
+import org.springframework.web.context.request.ServletWebRequest
+import org.springframework.web.method.HandlerMethod
+import org.springframework.web.method.support.ModelAndViewContainer
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest
+import org.springframework.web.testfixture.servlet.MockHttpServletResponse
+import kotlin.reflect.jvm.javaMethod
+
+/**
+ * Kotlin tests for [RequestResponseBodyMethodProcessor].
+ */
+class RequestResponseBodyMethodProcessorKotlinTests {
+
+	private val container = ModelAndViewContainer()
+
+	private val servletRequest = MockHttpServletRequest()
+
+	private val servletResponse = MockHttpServletResponse()
+
+	private val request: NativeWebRequest = ServletWebRequest(servletRequest, servletResponse)
+
+	@Test
+	fun writeWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::writeMessage::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, null)
+
+		val returnValue: Any = SampleController().writeMessage()
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString)
+			.contains("\"value\":\"foo\"")
+	}
+
+	@Test
+	fun writeGenericTypeWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::writeMessages::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, null)
+
+		val returnValue: Any = SampleController().writeMessages()
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString)
+			.contains("\"value\":\"foo\"")
+			.contains("\"value\":\"bar\"")
+	}
+
+	@Test
+	fun readWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::readMessage::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(StringHttpMessageConverter(), KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, null)
+
+		val returnValue: Any = SampleController().readMessage(Message("foo"))
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString).isEqualTo("foo")
+	}
+
+	@Test
+	fun readGenericTypeWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::readMessages::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(StringHttpMessageConverter(), KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, null)
+
+		val returnValue: Any = SampleController().readMessages(listOf(Message("foo"), Message("bar")))
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString)
+			.isEqualTo("foo bar")
+	}
+
+
+	private class SampleController {
+
+		@RequestMapping
+		@ResponseBody
+		fun writeMessage() = Message("foo")
+
+		@RequestMapping
+		@ResponseBody
+		fun writeMessages() = listOf(Message("foo"), Message("bar"))
+
+		@RequestMapping
+		@ResponseBody
+		fun readMessage(message: Message) = message.value
+
+		@RequestMapping
+		@ResponseBody fun readMessages(messages: List<Message>) = messages.map { it.value }.reduce { acc, string -> "$acc $string" }
+
+	}
+
+	@Serializable
+	data class Message(val value: String)
+}