Do not ignore charset in Jaxb2XmlDecoder

This commit makes sure that the charset, if defined in the mimetype, is
used when decoding XML to JAXB2 objects.

Closes gh-28599

spring-web/src/main/java/org/springframework/http/codec/xml/Jaxb2XmlDecoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.http.codec.xml;
 
+import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
@@ -189,7 +190,7 @@ public class Jaxb2XmlDecoder extends AbstractDecoder<Object> {
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) throws DecodingException {
 
 		try {
-			Iterator eventReader = inputFactory.createXMLEventReader(dataBuffer.asInputStream());
+			Iterator eventReader = inputFactory.createXMLEventReader(dataBuffer.asInputStream(), encoding(mimeType));
 			List<XMLEvent> events = new ArrayList<>();
 			eventReader.forEachRemaining(event -> events.add((XMLEvent) event));
 			return unmarshal(events, targetType.toClass());
@@ -211,6 +212,20 @@ public class Jaxb2XmlDecoder extends AbstractDecoder<Object> {
 		}
 	}
 
+	@Nullable
+	private static String encoding(@Nullable MimeType mimeType) {
+		if (mimeType == null) {
+			return null;
+		}
+		Charset charset = mimeType.getCharset();
+		if (charset == null) {
+			return null;
+		}
+		else {
+			return charset.name();
+		}
+	}
+
 	private Object unmarshal(List<XMLEvent> events, Class<?> outputClass) {
 		try {
 			Unmarshaller unmarshaller = initUnmarshaller(outputClass);