Support JAXBElement subtypes in Jaxb2XmlEncoder

Closes gh-33258

Author: Dawid Olbiński

spring-web/src/main/java/org/springframework/http/codec/xml/Jaxb2XmlEncoder.java

@@ -93,7 +93,7 @@ public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType
 			Class<?> outputClass = elementType.toClass();
 			return (outputClass.isAnnotationPresent(XmlRootElement.class) ||
 					outputClass.isAnnotationPresent(XmlType.class) ||
-					elementType.isAssignableFrom(JAXBElement.class));
+					JAXBElement.class.isAssignableFrom(outputClass));
 		}
 		else {
 			return false;

spring-web/src/test/java/org/springframework/http/codec/xml/Jaxb2XmlEncoderTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.http.codec.xml;
 
+import java.io.Serial;
 import java.util.Arrays;
 import java.util.List;
 import java.util.function.Consumer;
@@ -43,6 +44,7 @@
 import static org.springframework.core.io.buffer.DataBufferUtils.release;
 
 /**
+ * Tests for {@link Jaxb2XmlEncoder}.
  * @author Sebastien Deleuze
  * @author Arjen Poutsma
  */
@@ -64,6 +66,7 @@ protected void canEncode() {
 		assertThat(this.encoder.canEncode(forClass(getClass()), MediaType.APPLICATION_XML)).isFalse();
 
 		assertThat(this.encoder.canEncode(forClass(JAXBElement.class), MediaType.APPLICATION_XML)).isTrue();
+		assertThat(this.encoder.canEncode(forClass(JAXBElementSubclass.class), MediaType.APPLICATION_XML)).isTrue();
 
 		// SPR-15464
 		assertThat(this.encoder.canEncode(ResolvableType.NONE, null)).isFalse();
@@ -122,6 +125,17 @@ protected Consumer<DataBuffer> expectXml(String expected) {
 		};
 	}
 
+	public static class JAXBElementSubclass extends JAXBElement<Pojo> {
+		@Serial
+		private static final long serialVersionUID = 1L;
+
+		protected static final QName NAME = new QName("http://foo/schema/common/1.0", "Pojo");
+
+		public JAXBElementSubclass() {
+			super(NAME, Pojo.class, null, null);
+		}
+	}
+
 	public static class Model {}
 
 	public static class Foo extends Model {