Avoid payload conversion if unnecessary

Closes gh-22386

Author: rstoyanchev

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -221,6 +221,9 @@ protected Object convertFromInternal(Message<?> message, Class<?> targetClass, @
 					return this.objectMapper.readValue((byte[]) payload, javaType);
 				}
 			}
+			else if (targetClass.isInstance(payload)) {
+				return payload;
+			}
 			else {
 				if (view != null) {
 					return this.objectMapper.readerWithView(view).forType(javaType).readValue(payload.toString());

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -110,6 +110,14 @@ public void fromMessageUntyped() {
 		assertEquals("AQI=", actual.get("bytes"));
 	}
 
+	@Test // gh-22386
+	public void fromMessageMatchingInstance() {
+		MyBean myBean = new MyBean();
+		MappingJackson2MessageConverter converter = new MappingJackson2MessageConverter();
+		Message<?> message = MessageBuilder.withPayload(myBean).build();
+		assertSame(myBean, converter.fromMessage(message, MyBean.class));
+	}
+
 	@Test(expected = MessageConversionException.class)
 	public void fromMessageInvalidJson() {
 		MappingJackson2MessageConverter converter = new MappingJackson2MessageConverter();