Support properties in kotlinx.serialization converters

This commit adds support for Kotlin properties in Spring WebMVC
controllers, supported for reasons explained in spring-projectsgh-31856, with
kotlinx.serialization converters.

Closes spring-projectsgh-34284

Author: sdeleuze

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

@@ -150,24 +150,25 @@ private KSerializer<Object> serializer(ResolvableType resolvableType) {
 			Assert.notNull(method, "Method must not be null");
 			if (KotlinDetector.isKotlinType(method.getDeclaringClass())) {
 				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-				Assert.notNull(function, "Kotlin function must not be 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) {
-						if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
-							return null;
+				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) {
+							if (hasPolymorphism(serializer.getDescriptor(), new HashSet<>())) {
+								return null;
+							}
+							this.kTypeSerializerCache.put(type, serializer);
 						}
-						this.kTypeSerializerCache.put(type, serializer);
 					}
+					return serializer;
 				}
-				return serializer;
 			}
 		}
 		Type type = resolvableType.getType();

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

@@ -16,27 +16,24 @@
 
 package org.springframework.http.converter.json
 
-import java.lang.reflect.ParameterizedType
-import java.lang.reflect.Type
-import java.nio.charset.StandardCharsets
-
 import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatExceptionOfType
 import org.junit.jupiter.api.Test
 import org.springframework.core.MethodParameter
-import kotlin.reflect.javaType
-import kotlin.reflect.typeOf
-
 import org.springframework.core.Ordered
 import org.springframework.core.ResolvableType
 import org.springframework.http.MediaType
 import org.springframework.http.converter.HttpMessageNotReadableException
 import org.springframework.http.customJson
 import org.springframework.web.testfixture.http.MockHttpInputMessage
 import org.springframework.web.testfixture.http.MockHttpOutputMessage
+import java.lang.reflect.ParameterizedType
 import java.math.BigDecimal
+import java.nio.charset.StandardCharsets
+import kotlin.reflect.javaType
 import kotlin.reflect.jvm.javaMethod
+import kotlin.reflect.typeOf
 
 /**
  * Tests for the JSON conversion using kotlinx.serialization.
@@ -388,6 +385,19 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 		assertThat(result).isEqualTo(expectedJson)
 	}
 
+	@Test
+	fun writeProperty() {
+		val outputMessage = MockHttpOutputMessage()
+		val method = this::class.java.getDeclaredMethod("getValue")
+		val methodParameter = MethodParameter.forExecutable(method, -1)
+
+		this.converter.write(value, ResolvableType.forMethodParameter(methodParameter), null, outputMessage, null)
+		val result = outputMessage.getBodyAsString(StandardCharsets.UTF_8)
+
+		assertThat(outputMessage.headers).containsEntry("Content-Type", listOf("application/json"))
+		assertThat(result).isEqualTo("42")
+	}
+
 
 	@Serializable
 	@Suppress("ArrayInDataClass")
@@ -413,4 +423,7 @@ class KotlinSerializationJsonHttpMessageConverterTests {
 
 	fun handleMapWithNullable(map: Map<String, String?>) = map
 
+	val value: Int
+		get() = 42
+
 }