Add Kotlin body advices

This commit introduces KotlinRequestBodyAdvice and
KotlinResponseBodyAdvice in order to set a KType hint when relevant.

Closes gh-34923

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

@@ -17,27 +17,20 @@
 package org.springframework.http.converter;
 
 import java.io.IOException;
-import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Map;
 
-import kotlin.reflect.KFunction;
 import kotlin.reflect.KType;
-import kotlin.reflect.full.KCallables;
-import kotlin.reflect.jvm.ReflectJvmMapping;
 import kotlinx.serialization.KSerializer;
 import kotlinx.serialization.SerialFormat;
 import kotlinx.serialization.SerializersKt;
 import org.jspecify.annotations.Nullable;
 
-import org.springframework.core.KotlinDetector;
-import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.http.HttpInputMessage;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
-import org.springframework.util.Assert;
 import org.springframework.util.ConcurrentReferenceHashMap;
 
 
@@ -84,12 +77,12 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 
 	@Override
 	protected boolean supports(Class<?> clazz) {
-		return serializer(ResolvableType.forClass(clazz)) != null;
+		return serializer(ResolvableType.forClass(clazz), null) != null;
 	}
 
 	@Override
 	public boolean canRead(ResolvableType type, @Nullable MediaType mediaType) {
-		if (!ResolvableType.NONE.equals(type) && serializer(type) != null) {
+		if (!ResolvableType.NONE.equals(type) && serializer(type, null) != null) {
 			return canRead(mediaType);
 		}
 		else {
@@ -99,7 +92,7 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 
 	@Override
 	public boolean canWrite(ResolvableType type, Class<?> clazz, @Nullable MediaType mediaType) {
-		if (!ResolvableType.NONE.equals(type) && serializer(type) != null) {
+		if (!ResolvableType.NONE.equals(type) && serializer(type, null) != null) {
 			return canWrite(mediaType);
 		}
 		else {
@@ -111,7 +104,7 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 	public final Object read(ResolvableType type, HttpInputMessage inputMessage, @Nullable Map<String, Object> hints)
 			throws IOException, HttpMessageNotReadableException {
 
-		KSerializer<Object> serializer = serializer(type);
+		KSerializer<Object> serializer = serializer(type, hints);
 		if (serializer == null) {
 			throw new HttpMessageNotReadableException("Could not find KSerializer for " + type, inputMessage);
 		}
@@ -129,7 +122,7 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 			@Nullable Map<String, Object> hints) throws IOException, HttpMessageNotWritableException {
 
 		ResolvableType resolvableType = (ResolvableType.NONE.equals(type) ? ResolvableType.forInstance(object) : type);
-		KSerializer<Object> serializer = serializer(resolvableType);
+		KSerializer<Object> serializer = serializer(resolvableType, hints);
 		if (serializer == null) {
 			throw new HttpMessageNotWritableException("Could not find KSerializer for " + resolvableType);
 		}
@@ -149,29 +142,21 @@ public abstract class AbstractKotlinSerializationHttpMessageConverter<T extends
 	 * @param resolvableType the type to find a serializer for
 	 * @return a resolved serializer for the given type, or {@code null}
 	 */
-	private @Nullable KSerializer<Object> serializer(ResolvableType resolvableType) {
-		if (resolvableType.getSource() instanceof MethodParameter parameter) {
-			Method method = parameter.getMethod();
-			Assert.notNull(method, "Method must not be null");
-			if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
-				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-				if (function != null) {
-					KType type = (parameter.getParameterIndex() == -1 ? function.getReturnType() :
-							KCallables.getValueParameters(function).get(parameter.getParameterIndex()).getType());
-					KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
-					if (serializer == null) {
-						try {
-							serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
-						}
-						catch (IllegalArgumentException ignored) {
-						}
-						if (serializer != null) {
-							this.kTypeSerializerCache.put(type, serializer);
-						}
-					}
-					return serializer;
+	private @Nullable KSerializer<Object> serializer(ResolvableType resolvableType, @Nullable Map<String, Object> hints) {
+		if (hints != null && hints.containsKey(KType.class.getName())) {
+			KType type = (KType) hints.get(KType.class.getName());
+			KSerializer<Object> serializer = this.kTypeSerializerCache.get(type);
+			if (serializer == null) {
+				try {
+					serializer = SerializersKt.serializerOrNull(this.format.getSerializersModule(), type);
+				}
+				catch (IllegalArgumentException ignored) {
+				}
+				if (serializer != null) {
+					this.kTypeSerializerCache.put(type, serializer);
 				}
 			}
+			return serializer;
 		}
 		Type type = resolvableType.getType();
 		KSerializer<Object> serializer = this.typeSerializerCache.get(type);

spring-web/src/test/kotlin/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverterTests.kt

@@ -33,8 +33,10 @@ import org.springframework.web.testfixture.http.MockHttpOutputMessage
 import java.lang.reflect.ParameterizedType
 import java.math.BigDecimal
 import java.nio.charset.StandardCharsets
+import kotlin.reflect.KType
 import kotlin.reflect.javaType
 import kotlin.reflect.jvm.javaMethod
+import kotlin.reflect.jvm.jvmName
 import kotlin.reflect.typeOf
 
 /**
@@ -246,7 +248,10 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		val inputMessage = MockHttpInputMessage(body.toByteArray(StandardCharsets.UTF_8))
 		inputMessage.headers.contentType = MediaType.APPLICATION_JSON
 		val methodParameter = MethodParameter.forExecutable(::handleMapWithNullable::javaMethod.get()!!, 0)
-		val result = converter.read(ResolvableType.forMethodParameter(methodParameter), inputMessage, null) as Map<String, String?>
+		val hints = mapOf(KType::class.jvmName to typeOf<Map<String, String?>>())
+
+		val result = converter.read(ResolvableType.forMethodParameter(methodParameter), inputMessage,
+			hints) as Map<String, String?>
 
 		assertThat(result).containsExactlyEntriesOf(mapOf("value" to null))
 	}
@@ -400,9 +405,10 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		val serializableBean = mapOf<String, String?>("value" to null)
 		val expectedJson = """{"value":null}"""
 		val methodParameter = MethodParameter.forExecutable(::handleMapWithNullable::javaMethod.get()!!, -1)
+		val hints = mapOf(KType::class.jvmName to typeOf<Map<String, String?>>())
 
 		this.converter.write(serializableBean, ResolvableType.forMethodParameter(methodParameter), null,
-			outputMessage, null)
+			outputMessage, hints)
 
 		val result = outputMessage.getBodyAsString(StandardCharsets.UTF_8)
 

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupport.java

@@ -17,7 +17,6 @@
 package org.springframework.web.servlet.config.annotation;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
@@ -106,8 +105,12 @@ import org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionRes
 import org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver;
 import org.springframework.web.servlet.mvc.method.annotation.JsonViewRequestBodyAdvice;
 import org.springframework.web.servlet.mvc.method.annotation.JsonViewResponseBodyAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.KotlinRequestBodyAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.KotlinResponseBodyAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.RequestBodyAdvice;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping;
+import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;
 import org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver;
 import org.springframework.web.servlet.resource.ResourceUrlProvider;
 import org.springframework.web.servlet.resource.ResourceUrlProviderExposingInterceptor;
@@ -225,6 +228,8 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 
 	private static final boolean jsonbPresent;
 
+	private static final boolean kotlinSerializationPresent;
+
 	private static final boolean kotlinSerializationCborPresent;
 
 	private static final boolean kotlinSerializationJsonPresent;
@@ -248,9 +253,10 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 		jackson2YamlPresent = jackson2Present && ClassUtils.isPresent("com.fasterxml.jackson.dataformat.yaml.YAMLFactory", classLoader);
 		gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", classLoader);
 		jsonbPresent = ClassUtils.isPresent("jakarta.json.bind.Jsonb", classLoader);
-		kotlinSerializationCborPresent = ClassUtils.isPresent("kotlinx.serialization.cbor.Cbor", classLoader);
-		kotlinSerializationJsonPresent = ClassUtils.isPresent("kotlinx.serialization.json.Json", classLoader);
-		kotlinSerializationProtobufPresent = ClassUtils.isPresent("kotlinx.serialization.protobuf.ProtoBuf", classLoader);
+		kotlinSerializationPresent = ClassUtils.isPresent("kotlinx.serialization.Serializable", classLoader);
+		kotlinSerializationCborPresent = kotlinSerializationPresent && ClassUtils.isPresent("kotlinx.serialization.cbor.Cbor", classLoader);
+		kotlinSerializationJsonPresent = kotlinSerializationPresent && ClassUtils.isPresent("kotlinx.serialization.json.Json", classLoader);
+		kotlinSerializationProtobufPresent = kotlinSerializationPresent && ClassUtils.isPresent("kotlinx.serialization.protobuf.ProtoBuf", classLoader);
 	}
 
 
@@ -699,9 +705,19 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 		adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
 		adapter.setErrorResponseInterceptors(getErrorResponseInterceptors());
 
-		if (jacksonPresent || jackson2Present) {
-			adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
-			adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
+		if (jacksonPresent || jackson2Present || kotlinSerializationPresent) {
+			List<RequestBodyAdvice> requestBodyAdvices = new ArrayList<>(2);
+			List<ResponseBodyAdvice<?>> responseBodyAdvices = new ArrayList<>(2);
+			if (jacksonPresent || jackson2Present) {
+				requestBodyAdvices.add(new JsonViewRequestBodyAdvice());
+				responseBodyAdvices.add(new JsonViewResponseBodyAdvice());
+			}
+			if (kotlinSerializationPresent) {
+				requestBodyAdvices.add(new KotlinRequestBodyAdvice());
+				responseBodyAdvices.add(new KotlinResponseBodyAdvice());
+			}
+			adapter.setRequestBodyAdvice(requestBodyAdvices);
+			adapter.setResponseBodyAdvice(responseBodyAdvices);
 		}
 
 		AsyncSupportConfigurer configurer = getAsyncSupportConfigurer();
@@ -1122,9 +1138,15 @@ public class WebMvcConfigurationSupport implements ApplicationContextAware, Serv
 		exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());
 		exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());
 		exceptionHandlerResolver.setErrorResponseInterceptors(getErrorResponseInterceptors());
-		if (jacksonPresent || jackson2Present) {
-			exceptionHandlerResolver.setResponseBodyAdvice(
-					Collections.singletonList(new JsonViewResponseBodyAdvice()));
+		if (jacksonPresent || jackson2Present || kotlinSerializationPresent) {
+			List<ResponseBodyAdvice<?>> responseBodyAdvices = new ArrayList<>(2);
+			if (jacksonPresent || jackson2Present) {
+				responseBodyAdvices.add(new JsonViewResponseBodyAdvice());
+			}
+			if (kotlinSerializationPresent) {
+				responseBodyAdvices.add(new KotlinResponseBodyAdvice());
+			}
+			exceptionHandlerResolver.setResponseBodyAdvice(responseBodyAdvices);
 		}
 		if (this.applicationContext != null) {
 			exceptionHandlerResolver.setApplicationContext(this.applicationContext);

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/KotlinRequestBodyAdvice.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2002-2025 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 java.lang.reflect.Type;
+import java.util.Collections;
+import java.util.Map;
+import java.util.Objects;
+
+import kotlin.reflect.KFunction;
+import kotlin.reflect.KParameter;
+import kotlin.reflect.KType;
+import kotlin.reflect.jvm.ReflectJvmMapping;
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.http.converter.AbstractKotlinSerializationHttpMessageConverter;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.SmartHttpMessageConverter;
+
+/**
+ * A {@link RequestBodyAdvice} implementation that adds support for resolving
+ * Kotlin {@link KType} from the parameter and providing it as a hint with a
+ * {@code "kotlin.reflect.KType"} key.
+ *
+ * @author Sebastien Deleuze
+ * @since 7.0
+ * @see AbstractKotlinSerializationHttpMessageConverter
+ */
+@SuppressWarnings("removal")
+public class KotlinRequestBodyAdvice extends RequestBodyAdviceAdapter {
+
+	@Override
+	public boolean supports(MethodParameter methodParameter, Type targetType,
+			Class<? extends HttpMessageConverter<?>> converterType) {
+
+		return AbstractKotlinSerializationHttpMessageConverter.class.isAssignableFrom(converterType);
+	}
+
+	@Override
+	public @Nullable Map<String, Object> determineReadHints(MethodParameter parameter, Type targetType,
+			Class<? extends SmartHttpMessageConverter<?>> converterType) {
+
+		KFunction<?> function = ReflectJvmMapping.getKotlinFunction(Objects.requireNonNull(parameter.getMethod()));
+		int i = 0;
+		int index = parameter.getParameterIndex();
+		for (KParameter p : Objects.requireNonNull(function).getParameters()) {
+			if (KParameter.Kind.VALUE.equals(p.getKind())) {
+				if (index == i++) {
+					return Collections.singletonMap(KType.class.getName(), p.getType());
+				}
+			}
+		}
+		return null;
+	}
+}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/KotlinResponseBodyAdvice.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2002-2025 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 java.util.Collections;
+import java.util.Map;
+import java.util.Objects;
+
+import kotlin.reflect.KFunction;
+import kotlin.reflect.KType;
+import kotlin.reflect.jvm.ReflectJvmMapping;
+import org.jspecify.annotations.Nullable;
+
+import org.springframework.core.MethodParameter;
+import org.springframework.http.MediaType;
+import org.springframework.http.converter.AbstractKotlinSerializationHttpMessageConverter;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.server.ServerHttpRequest;
+import org.springframework.http.server.ServerHttpResponse;
+
+/**
+ * A {@link ResponseBodyAdvice} implementation that adds support for resolving
+ * Kotlin {@link KType} from the return type and providing it as a hint with a
+ * {@code "kotlin.reflect.KType"} key.
+ *
+ * @author Sebastien Deleuze
+ * @since 7.0
+ */
+@SuppressWarnings("removal")
+public class KotlinResponseBodyAdvice implements ResponseBodyAdvice<Object> {
+
+	@Override
+	public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
+		return AbstractKotlinSerializationHttpMessageConverter.class.isAssignableFrom(converterType);
+	}
+
+	@Override
+	public @Nullable Object beforeBodyWrite(@Nullable Object body, MethodParameter returnType, MediaType selectedContentType,
+			Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
+
+		return body;
+	}
+
+	@Override
+	public @Nullable Map<String, Object> determineWriteHints(@Nullable Object body, MethodParameter returnType,
+			MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType) {
+
+		KFunction<?> function = ReflectJvmMapping.getKotlinFunction(Objects.requireNonNull(returnType.getMethod()));
+		KType type = Objects.requireNonNull(function).getReturnType();
+		return Collections.singletonMap(KType.class.getName(), type);
+	}
+
+}

spring-webmvc/src/test/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupportTests.java

@@ -80,6 +80,8 @@ import org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionRes
 import org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver;
 import org.springframework.web.servlet.mvc.method.annotation.JsonViewRequestBodyAdvice;
 import org.springframework.web.servlet.mvc.method.annotation.JsonViewResponseBodyAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.KotlinRequestBodyAdvice;
+import org.springframework.web.servlet.mvc.method.annotation.KotlinResponseBodyAdvice;
 import org.springframework.web.servlet.mvc.method.annotation.MvcUriComponentsBuilder;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
 import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping;
@@ -204,9 +206,11 @@ class WebMvcConfigurationSupportTests {
 		DirectFieldAccessor fieldAccessor = new DirectFieldAccessor(adapter);
 		@SuppressWarnings("unchecked")
 		List<Object> bodyAdvice = (List<Object>) fieldAccessor.getPropertyValue("requestResponseBodyAdvice");
-		assertThat(bodyAdvice).hasSize(2);
+		assertThat(bodyAdvice).hasSize(4);
 		assertThat(bodyAdvice.get(0).getClass()).isEqualTo(JsonViewRequestBodyAdvice.class);
-		assertThat(bodyAdvice.get(1).getClass()).isEqualTo(JsonViewResponseBodyAdvice.class);
+		assertThat(bodyAdvice.get(1).getClass()).isEqualTo(KotlinRequestBodyAdvice.class);
+		assertThat(bodyAdvice.get(2).getClass()).isEqualTo(JsonViewResponseBodyAdvice.class);
+		assertThat(bodyAdvice.get(3).getClass()).isEqualTo(KotlinResponseBodyAdvice.class);
 	}
 
 	@Test
@@ -238,8 +242,9 @@ class WebMvcConfigurationSupportTests {
 
 		DirectFieldAccessor fieldAccessor = new DirectFieldAccessor(eher);
 		List<Object> interceptors = (List<Object>) fieldAccessor.getPropertyValue("responseBodyAdvice");
-		assertThat(interceptors).hasSize(1);
+		assertThat(interceptors).hasSize(2);
 		assertThat(interceptors.get(0).getClass()).isEqualTo(JsonViewResponseBodyAdvice.class);
+		assertThat(interceptors.get(1).getClass()).isEqualTo(KotlinResponseBodyAdvice.class);
 
 		LocaleContextHolder.setLocale(Locale.ENGLISH);
 		try {

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

@@ -34,6 +34,7 @@ import org.springframework.web.method.support.ModelAndViewContainer
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest
 import org.springframework.web.testfixture.servlet.MockHttpServletResponse
 import java.nio.charset.StandardCharsets
+import kotlin.collections.mapOf
 import kotlin.reflect.jvm.javaMethod
 
 /**
@@ -84,6 +85,22 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 			.contains("\"value\":\"bar\"")
 	}
 
+	@Test
+	fun writeNullableMapWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::writeNullableMap::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, listOf(KotlinResponseBodyAdvice()))
+
+		val returnValue: Any = SampleController().writeNullableMap()
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString)
+			.isEqualTo("""{"value":null}""")
+	}
+
 	@Test
 	fun readWithKotlinSerializationJsonMessageConverter() {
 		val content = "{\"value\" : \"foo\"}"
@@ -119,6 +136,24 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 		Assertions.assertThat(result).containsExactly(Message("foo"), Message("bar"))
 	}
 
+	@Suppress("UNCHECKED_CAST")
+	@Test
+	fun readNullableMapWithKotlinSerializationJsonMessageConverter() {
+		val content = "{\"value\" : null}"
+		this.servletRequest.setContent(content.toByteArray(StandardCharsets.UTF_8))
+		this.servletRequest.setContentType("application/json")
+
+		val converters = listOf(StringHttpMessageConverter(), KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, listOf(KotlinRequestBodyAdvice()))
+
+		val method = SampleController::readNullableMap::javaMethod.get()!!
+		val methodParameter = MethodParameter(method, 0)
+
+		val result = processor.resolveArgument(methodParameter, container, request, factory) as Map<String, String ?>
+
+		Assertions.assertThat(result).isEqualTo(mapOf("value" to null))
+	}
+
 
 	private class SampleController {
 
@@ -135,7 +170,20 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 		fun readMessage(message: Message) = message.value
 
 		@RequestMapping
-		@ResponseBody fun readMessages(messages: List<Message>) = messages.map { it.value }.reduce { acc, string -> "$acc $string" }
+		@ResponseBody
+		fun readMessages(messages: List<Message>) = messages.map { it.value }.reduce { acc, string -> "$acc $string" }
+
+		@RequestMapping
+		@ResponseBody
+		fun writeNullableMap(): Map<String, String?> {
+			return mapOf("value" to null)
+		}
+
+		@RequestMapping
+		@ResponseBody
+		fun readNullableMap(map: Map<String, String?>): String {
+			return map.toString()
+		}
 
 	}