Treat Kotlin nullable as non-required

Where `isOptional` is used, also check for `isNullable` i.e.
values are not considered required if they are Kotlin nullables:
- spring-messaging: named value method arguments
- spring-web: named value method arguments
- spring-webmvc: request parts

This means that Kotlin client code no longer has to explicity specify
"required=false" for Kotlin nullables -- this information is inferred
automatically by the framework.

Issue: SPR-14165

spring-core/src/main/java/org/springframework/core/MethodParameter.java

@@ -30,6 +30,7 @@ import java.util.Map;
 import java.util.Optional;
 
 import org.springframework.util.Assert;
+import org.springframework.util.KotlinUtils;
 
 /**
  * Helper class that encapsulates the specification of a method parameter, i.e. a {@link Method}
@@ -329,6 +330,16 @@ public class MethodParameter {
 		return (isOptional() ? nested() : this);
 	}
 
+	/**
+	 * Return whether this method parameter is declared as a "nullable" value, if supported by
+	 * the underlying language. Currently the only supported language is Kotlin.
+	 * @since 5.0
+	 */
+	public boolean isNullable() {
+		return KotlinUtils.isKotlinPresent() &&
+				KotlinUtils.isKotlinClass(getContainingClass()) &&
+				KotlinUtils.isNullable(this.parameterIndex, this.method, this.constructor);
+	}
 
 	/**
 	 * Set a containing class to resolve the parameter type against.

spring-core/src/main/java/org/springframework/util/KotlinUtils.java

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2002-2016 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
+ *
+ *      http://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.util;
+
+import kotlin.Metadata;
+import kotlin.reflect.KFunction;
+import kotlin.reflect.KParameter;
+import kotlin.reflect.jvm.ReflectJvmMapping;
+import org.springframework.core.MethodParameter;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Miscellaneous Kotlin utility methods.
+ *
+ * @author Raman Gupta
+ * @since 5.0
+ */
+public class KotlinUtils {
+
+	private static final boolean kotlinPresent;
+
+	static {
+		kotlinPresent = ClassUtils.isPresent("kotlin.Unit", MethodParameter.class.getClassLoader());
+	}
+
+	public static boolean isKotlinPresent() {
+		return kotlinPresent;
+	}
+
+	public static boolean isKotlinClass(Class<?> type) {
+		return type != null && type.getDeclaredAnnotation(Metadata.class) != null;
+	}
+
+	public static boolean isNullable(int parameterIndex, Method method, Constructor<?> constructor) {
+		if(parameterIndex < 0) {
+			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+			return function != null && function.getReturnType().isMarkedNullable();
+		} else {
+			KFunction<?> function = method != null ?
+				ReflectJvmMapping.getKotlinFunction(method) :
+				ReflectJvmMapping.getKotlinFunction(constructor);
+			if(function != null) {
+				@SuppressWarnings("unchecked")
+				List<KParameter> parameters = function.getParameters();
+				return parameters
+						.stream()
+						.filter(p -> KParameter.Kind.VALUE.equals(p.getKind()))
+						.collect(Collectors.toList())
+						.get(parameterIndex)
+						.getType()
+						.isMarkedNullable();
+			}
+			return false;
+		}
+	}
+}