Protected methods to determine payload target type

Closes gh-22035

Author: rstoyanchev

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

@@ -72,7 +72,7 @@ public boolean supportsParameter(MethodParameter parameter) {
 	@Override
 	public Object resolveArgument(MethodParameter parameter, Message<?> message) throws Exception {
 		Class<?> targetMessageType = parameter.getParameterType();
-		Class<?> targetPayloadType = getPayloadType(parameter);
+		Class<?> targetPayloadType = getPayloadType(parameter, message);
 
 		if (!targetMessageType.isAssignableFrom(message.getClass())) {
 			throw new MethodArgumentTypeMismatchException(message, parameter, "Actual message type '" +
@@ -95,7 +95,19 @@ public Object resolveArgument(MethodParameter parameter, Message<?> message) thr
 		return MessageBuilder.createMessage(payload, message.getHeaders());
 	}
 
-	private Class<?> getPayloadType(MethodParameter parameter) {
+	/**
+	 * Resolve the target class to convert the payload to.
+	 * <p>By default this is the generic type declared in the {@code Message}
+	 * method parameter but that can be overridden to select a more specific
+	 * target type after also taking into account the "Content-Type", e.g.
+	 * return {@code String} if target type is {@code Object} and
+	 * {@code "Content-Type:text/**"}.
+	 * @param parameter the target method parameter
+	 * @param message the message bring processed
+	 * @return the target type to use
+	 * @since 5.2
+	 */
+	protected Class<?> getPayloadType(MethodParameter parameter, Message<?> message) {
 		Type genericParamType = parameter.getGenericParameterType();
 		ResolvableType resolvableType = ResolvableType.forType(genericParamType).as(Message.class);
 		return resolvableType.getGeneric().toClass();

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

@@ -126,7 +126,7 @@ public Object resolveArgument(MethodParameter parameter, Message<?> message) thr
 			}
 		}
 
-		Class<?> targetClass = parameter.getParameterType();
+		Class<?> targetClass = resolveTargetClass(parameter, message);
 		Class<?> payloadClass = payload.getClass();
 		if (ClassUtils.isAssignable(targetClass, payloadClass)) {
 			validate(message, parameter, payload);
@@ -173,6 +173,21 @@ else if (payload instanceof String) {
 		}
 	}
 
+	/**
+	 * Resolve the target class to convert the payload to.
+	 * <p>By default this is simply {@link MethodParameter#getParameterType()}
+	 * but that can be overridden to select a more specific target type after
+	 * also taking into account the "Content-Type", e.g. return {@code String}
+	 * if target type is {@code Object} and {@code "Content-Type:text/**"}.
+	 * @param parameter the target method parameter
+	 * @param message the message bring processed
+	 * @return the target type to use
+	 * @since 5.2
+	 */
+	protected Class<?> resolveTargetClass(MethodParameter parameter, Message<?> message) {
+		return parameter.getParameterType();
+	}
+
 	/**
 	 * Validate the payload if applicable.
 	 * <p>The default implementation checks for {@code @javax.validation.Valid},