Support properties in kotlinx.serialization codecs

This commit adds support for Kotlin properties in Spring WebFlux
controllers, supported for reasons explained in gh-31856, with
kotlinx.serialization codecs.

See gh-34284

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/codec/KotlinSerializationSupport.java

@@ -133,24 +133,25 @@ protected final 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/codec/json/KotlinSerializationJsonEncoderTests.kt

@@ -145,10 +145,24 @@ class KotlinSerializationJsonEncoderTests : AbstractEncoderTests<KotlinSerializa
 		assertThat(encoder.canEncode(ResolvableType.forClass(BigDecimal::class.java), null)).isFalse()
 	}
 
+	@Test
+	fun encodeProperty() {
+		val input = Mono.just(value)
+		val method = this::class.java.getDeclaredMethod("getValue")
+		val methodParameter = MethodParameter.forExecutable(method, -1)
+		testEncode(input, ResolvableType.forMethodParameter(methodParameter), null, null) {
+			it.consumeNextWith(expectString("42"))
+				.verifyComplete()
+		}
+	}
+
 
 	@Serializable
 	data class Pojo(val foo: String, val bar: String, val pojo: Pojo? = null)
 
 	fun handleMapWithNullable(map: Map<String, String?>) = map
 
+	val value: Int
+		get() = 42
+
 }