PayloadMethodArgumentResolver supports Optional

Closes gh-28945

spring-messaging/src/main/java/org/springframework/messaging/handler/annotation/support/PayloadMethodArgumentResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -17,6 +17,7 @@
 package org.springframework.messaging.handler.annotation.support;
 
 import java.lang.annotation.Annotation;
+import java.util.Optional;
 
 import org.springframework.core.MethodParameter;
 import org.springframework.core.annotation.AnnotationUtils;
@@ -27,6 +28,7 @@ import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.converter.SmartMessageConverter;
 import org.springframework.messaging.handler.annotation.Payload;
 import org.springframework.messaging.handler.invocation.HandlerMethodArgumentResolver;
+import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
@@ -113,24 +115,30 @@ public class PayloadMethodArgumentResolver implements HandlerMethodArgumentResol
 			throw new IllegalStateException("@Payload SpEL expressions not supported by this resolver");
 		}
 
+		boolean isOptionalTargetClass = (parameter.getParameterType() == Optional.class);
 		Object payload = message.getPayload();
 		if (isEmptyPayload(payload)) {
-			if (ann == null || ann.required()) {
+			if ((ann == null || ann.required()) && !isOptionalTargetClass) {
 				String paramName = getParameterName(parameter);
 				BindingResult bindingResult = new BeanPropertyBindingResult(payload, paramName);
 				bindingResult.addError(new ObjectError(paramName, "Payload value must not be empty"));
 				throw new MethodArgumentNotValidException(message, parameter, bindingResult);
 			}
 			else {
-				return null;
+				return (isOptionalTargetClass ? Optional.empty() : null);
 			}
 		}
 
+		if (payload instanceof Optional<?> optional) {
+			payload = optional.get();
+			message = MessageBuilder.createMessage(payload, message.getHeaders());
+		}
+
 		Class<?> targetClass = resolveTargetClass(parameter, message);
 		Class<?> payloadClass = payload.getClass();
 		if (ClassUtils.isAssignable(targetClass, payloadClass)) {
 			validate(message, parameter, payload);
-			return payload;
+			return (isOptionalTargetClass ? Optional.of(payload) : payload);
 		}
 		else {
 			if (this.converter instanceof SmartMessageConverter smartConverter) {
@@ -144,7 +152,7 @@ public class PayloadMethodArgumentResolver implements HandlerMethodArgumentResol
 						payloadClass.getName() + "] to [" + targetClass.getName() + "] for " + message);
 			}
 			validate(message, parameter, payload);
-			return payload;
+			return (isOptionalTargetClass ? Optional.of(payload) : payload);
 		}
 	}
 
@@ -161,11 +169,14 @@ public class PayloadMethodArgumentResolver implements HandlerMethodArgumentResol
 		if (payload == null) {
 			return true;
 		}
-		else if (payload instanceof byte[]) {
-			return ((byte[]) payload).length == 0;
+		else if (payload instanceof byte[] bytes) {
+			return bytes.length == 0;
 		}
-		else if (payload instanceof String) {
-			return !StringUtils.hasText((String) payload);
+		else if (payload instanceof String s) {
+			return !StringUtils.hasText(s);
+		}
+		else if (payload instanceof Optional<?> optional) {
+			return optional.isEmpty();
 		}
 		else {
 			return false;
@@ -184,7 +195,7 @@ public class PayloadMethodArgumentResolver implements HandlerMethodArgumentResol
 	 * @since 5.2
 	 */
 	protected Class<?> resolveTargetClass(MethodParameter parameter, Message<?> message) {
-		return parameter.getParameterType();
+		return parameter.nestedIfOptional().getNestedParameterType();
 	}
 
 	/**