Add native support for Kotlin default values in controllers

Closes gh-33384

Author: sdeleuze

spring-web/src/main/java/org/springframework/web/bind/annotation/ControllerMappingReflectiveProcessor.java

@@ -25,11 +25,13 @@
 import org.springframework.aot.hint.ExecutableMode;
 import org.springframework.aot.hint.ReflectionHints;
 import org.springframework.aot.hint.annotation.ReflectiveProcessor;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.http.HttpEntity;
 import org.springframework.lang.Nullable;
 import org.springframework.stereotype.Controller;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * {@link ReflectiveProcessor} implementation for {@link Controller} and
@@ -71,6 +73,11 @@ protected void registerTypeHints(ReflectionHints hints, Class<?> type) {
 
 	protected void registerMethodHints(ReflectionHints hints, Method method) {
 		hints.registerMethod(method, ExecutableMode.INVOKE);
+		Class<?> declaringClass = method.getDeclaringClass();
+		if (KotlinDetector.isKotlinType(declaringClass)) {
+			ReflectionUtils.doWithMethods(declaringClass, m -> hints.registerMethod(m, ExecutableMode.INVOKE),
+					m -> m.getName().equals(method.getName() + "$default"));
+		}
 		for (Parameter parameter : method.getParameters()) {
 			registerParameterTypeHints(hints, MethodParameter.forParameter(parameter));
 		}

spring-web/src/test/kotlin/org/springframework/web/bind/annotation/ControllerMappingReflectiveProcessorKotlinTests.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2002-2024 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.bind.annotation
+
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.Test
+import org.springframework.aot.hint.*
+
+/**
+ * Kotlin tests for {@link ControllerMappingReflectiveProcessor}.
+ *
+ * @author Sebastien Deleuze
+ */
+class ControllerMappingReflectiveProcessorKotlinTests {
+
+	private val processor = ControllerMappingReflectiveProcessor()
+
+	private val hints = ReflectionHints()
+
+	@Test
+	fun registerReflectiveHintsForFunctionWithDefaultArgumentValue() {
+		val method = SampleController::class.java.getDeclaredMethod("defaultValue", Boolean::class.javaObjectType)
+		processor.registerReflectionHints(hints, method)
+		Assertions.assertThat(hints.typeHints()).satisfiesExactlyInAnyOrder(
+			{
+				Assertions.assertThat(it.type).isEqualTo(TypeReference.of(SampleController::class.java))
+				Assertions.assertThat(it.methods()).extracting<String> { executableHint: ExecutableHint -> executableHint.name }
+					.containsExactlyInAnyOrder("defaultValue", "defaultValue\$default")
+			}
+		)
+	}
+
+	class SampleController {
+
+		@GetMapping("/defaultValue")
+		fun defaultValue(@RequestParam(required = false) argument: Boolean? = false) = argument
+	}
+
+}