Box Kotlin value class parameters in web endpoint

In order to avoid "java.lang.IllegalArgumentException:
object is not an instance of declaring class" errors.

Closes gh-31698

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -18,6 +18,7 @@ package org.springframework.web.reactive.result.method;
 
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
@@ -28,6 +29,8 @@ import java.util.stream.Stream;
 
 import kotlin.Unit;
 import kotlin.coroutines.CoroutineContext;
+import kotlin.jvm.JvmClassMappingKt;
+import kotlin.reflect.KClass;
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
 import kotlin.reflect.jvm.KCallablesJvm;
@@ -44,8 +47,10 @@ import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.ObjectUtils;
+import org.springframework.util.ReflectionUtils;
 import org.springframework.validation.method.MethodValidator;
 import org.springframework.web.method.HandlerMethod;
 import org.springframework.web.reactive.BindingContext;
@@ -70,6 +75,9 @@ public class InvocableHandlerMethod extends HandlerMethod {
 
 	private static final Object NO_ARG_VALUE = new Object();
 
+	private static final ReflectionUtils.MethodFilter boxImplFilter =
+			(method -> method.isSynthetic() && Modifier.isStatic(method.getModifiers()) && method.getName().equals("box-impl"));
+
 
 	private final HandlerMethodArgumentResolverComposite resolvers = new HandlerMethodArgumentResolverComposite();
 
@@ -321,7 +329,15 @@ public class InvocableHandlerMethod extends HandlerMethod {
 						case INSTANCE -> argMap.put(parameter, target);
 						case VALUE -> {
 							if (!parameter.isOptional() || args[index] != null) {
-								argMap.put(parameter, args[index]);
+								if (parameter.getType().getClassifier() instanceof KClass<?> kClass && kClass.isValue()) {
+									Class<?> javaClass = JvmClassMappingKt.getJavaClass(kClass);
+									Method[] methods = ReflectionUtils.getUniqueDeclaredMethods(javaClass, boxImplFilter);
+									Assert.state(methods.length == 1, "Unable to find a single box-impl synthetic static method in " + javaClass.getName());
+									argMap.put(parameter, ReflectionUtils.invokeMethod(methods[0], null, args[index]));
+								}
+								else {
+									argMap.put(parameter, args[index]);
+								}
 							}
 							index++;
 						}

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/InvocableHandlerMethodKotlinTests.kt

@@ -178,6 +178,22 @@ class InvocableHandlerMethodKotlinTests {
 		assertHandlerResultValue(result, null)
 	}
 
+	@Test
+	fun valueClass() {
+		this.resolvers.add(stubResolver(1L))
+		val method = ValueClassController::valueClass.javaMethod!!
+		val result = invoke(ValueClassController(), method,1L)
+		assertHandlerResultValue(result, "1")
+	}
+
+	@Test
+	fun valueClassDefaultValue() {
+		this.resolvers.add(stubResolver(Mono.empty()))
+		val method = ValueClassController::valueClassWithDefault.javaMethod!!
+		val result = invoke(ValueClassController(), method)
+		assertHandlerResultValue(result, "3.1")
+	}
+
 
 	private fun invokeForResult(handler: Any, method: Method, vararg providedArgs: Any): HandlerResult? {
 		return invoke(handler, method, *providedArgs).block(Duration.ofSeconds(5))
@@ -266,4 +282,20 @@ class InvocableHandlerMethodKotlinTests {
 			return null
 		}
 	}
+
+	class ValueClassController {
+
+		fun valueClass(limit: LongValueClass) =
+			"${limit.value}"
+
+		fun valueClassWithDefault(limit: DoubleValueClass = DoubleValueClass(3.1)) =
+			"${limit.value}"
+
+	}
+
+	@JvmInline
+	value class LongValueClass(val value: Long)
+
+	@JvmInline
+	value class DoubleValueClass(val value: Double)
 }