Merge pull request #33233 from deblockt

* pr/33233:
  Polish "Support JAXBElement in Jaxb2RootElementHttpMessageConverter"
  Support JAXBElement in Jaxb2RootElementHttpMessageConverter

Closes gh-33233

spring-web/src/main/java/org/springframework/http/converter/xml/Jaxb2RootElementHttpMessageConverter.java

@@ -121,7 +121,9 @@ public class Jaxb2RootElementHttpMessageConverter extends AbstractJaxb2HttpMessa
 
 	@Override
 	public boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType) {
-		return (AnnotationUtils.findAnnotation(clazz, XmlRootElement.class) != null && canWrite(mediaType));
+		boolean supportedType = (JAXBElement.class.isAssignableFrom(clazz) ||
+				AnnotationUtils.findAnnotation(clazz, XmlRootElement.class) != null);
+		return (supportedType && canWrite(mediaType));
 	}
 
 	@Override
@@ -190,12 +192,12 @@ public class Jaxb2RootElementHttpMessageConverter extends AbstractJaxb2HttpMessa
 	}
 
 	@Override
-	protected void writeToResult(Object o, HttpHeaders headers, Result result) throws Exception {
+	protected void writeToResult(Object value, HttpHeaders headers, Result result) throws Exception {
 		try {
-			Class<?> clazz = ClassUtils.getUserClass(o);
+			Class<?> clazz = getMarshallerType(value);
 			Marshaller marshaller = createMarshaller(clazz);
 			setCharset(headers.getContentType(), marshaller);
-			marshaller.marshal(o, result);
+			marshaller.marshal(value, result);
 		}
 		catch (MarshalException ex) {
 			throw ex;
@@ -205,6 +207,15 @@ public class Jaxb2RootElementHttpMessageConverter extends AbstractJaxb2HttpMessa
 		}
 	}
 
+	private static Class<?> getMarshallerType(Object value) {
+		if (value instanceof JAXBElement<?> jaxbElement) {
+			return jaxbElement.getDeclaredType();
+		}
+		else {
+			return ClassUtils.getUserClass(value);
+		}
+	}
+
 	private void setCharset(@Nullable MediaType contentType, Marshaller marshaller) throws PropertyException {
 		if (contentType != null && contentType.getCharset() != null) {
 			marshaller.setProperty(Marshaller.JAXB_ENCODING, contentType.getCharset().name());

spring-web/src/test/java/org/springframework/http/converter/xml/Jaxb2RootElementHttpMessageConverterTests.java

@@ -18,6 +18,9 @@ package org.springframework.http.converter.xml;
 
 import java.nio.charset.StandardCharsets;
 
+import javax.xml.namespace.QName;
+
+import jakarta.xml.bind.JAXBElement;
 import jakarta.xml.bind.Marshaller;
 import jakarta.xml.bind.Unmarshaller;
 import jakarta.xml.bind.annotation.XmlAttribute;
@@ -93,6 +96,8 @@ class Jaxb2RootElementHttpMessageConverterTests {
 				.as("Converter does not support writing @XmlRootElement subclass").isTrue();
 		assertThat(converter.canWrite(rootElementCglib.getClass(), null))
 				.as("Converter does not support writing @XmlRootElement subclass").isTrue();
+		assertThat(converter.canWrite(JAXBElement.class, null))
+				.as("Converter does not support writing JAXBElement").isTrue();
 		assertThat(converter.canWrite(Type.class, null))
 				.as("Converter supports writing @XmlType").isFalse();
 	}
@@ -186,6 +191,18 @@ class Jaxb2RootElementHttpMessageConverterTests {
 			.isSimilarTo("<rootElement><type s=\"Hello World\"/></rootElement>", ev);
 	}
 
+	@Test
+	void writeJaxbElementRootElement() throws Exception {
+		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();
+		JAXBElement jaxbElement = new JAXBElement<>(new QName("custom"), MyCustomElement.class, new MyCustomElement("field1", "field2"));
+		converter.write(jaxbElement, null, outputMessage);
+		assertThat(outputMessage.getHeaders().getContentType())
+				.as("Invalid content-type").isEqualTo(MediaType.APPLICATION_XML);
+		DifferenceEvaluator ev = chain(Default, downgradeDifferencesToEqual(XML_STANDALONE));
+		assertThat(XmlContent.of(outputMessage.getBodyAsString(StandardCharsets.UTF_8)))
+				.isSimilarTo("<custom><field1>field1</field1><field2>field2</field2></custom>", ev);
+	}
+
 	@Test
 	void writeXmlRootElementSubclass() throws Exception {
 		MockHttpOutputMessage outputMessage = new MockHttpOutputMessage();