Fix regression in MappingJackson2MessageConverter

As of 4.3.13 MappingJackson2MessageConverter uses the MethodParameter
hint to obtain generic type information but it needs to be careful, and
nest one level, if the target parameter type has a Message wrapper.

Issue: SPR-16486

spring-messaging/src/main/java/org/springframework/messaging/converter/MappingJackson2MessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -237,6 +237,9 @@ public class MappingJackson2MessageConverter extends AbstractMessageConverter {
 		if (conversionHint instanceof MethodParameter) {
 			MethodParameter param = (MethodParameter) conversionHint;
 			param = param.nestedIfOptional();
+			if (Message.class.isAssignableFrom(param.getParameterType())) {
+				param = param.nested();
+			}
 			Type genericParameterType = param.getNestedGenericParameterType();
 			Class<?> contextClass = param.getContainingClass();
 			Type type = getJavaType(genericParameterType, contextClass);

spring-messaging/src/test/java/org/springframework/messaging/converter/MappingJackson2MessageConverterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -39,7 +39,7 @@ import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 
 /**
- * Test fixture for {@link org.springframework.messaging.converter.MappingJackson2MessageConverter}.
+ * Test fixture for {@link MappingJackson2MessageConverter}.
  *
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
@@ -136,6 +136,20 @@ public class MappingJackson2MessageConverterTests {
 		assertEquals(Arrays.asList(1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L), actual);
 	}
 
+	@Test // SPR-16486
+	public void fromMessageToMessageWithPojo() throws Exception {
+		MappingJackson2MessageConverter converter = new MappingJackson2MessageConverter();
+		String payload = "{\"string\":\"foo\"}";
+		Message<?> message = MessageBuilder.withPayload(payload.getBytes(StandardCharsets.UTF_8)).build();
+
+		Method method = getClass().getDeclaredMethod("handleMessage", Message.class);
+		MethodParameter param = new MethodParameter(method, 0);
+		Object actual = converter.fromMessage(message, MyBean.class, param);
+
+		assertTrue(actual instanceof MyBean);
+		assertEquals("foo", ((MyBean) actual).getString());
+	}
+
 	@Test
 	public void toMessage() throws Exception {
 		MappingJackson2MessageConverter converter = new MappingJackson2MessageConverter();
@@ -229,6 +243,8 @@ public class MappingJackson2MessageConverterTests {
 
 	void handleList(List<Long> payload) {}
 
+	void handleMessage(Message<MyBean> message) {}
+
 
 	public static class MyBean {