Support decoding Mono in Jaxb2XmlDecoder

Issue: SPR-16759

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

@@ -107,6 +107,12 @@ public class Jaxb2XmlDecoder extends AbstractDecoder<Object> {
 		return splitEvents.map(events -> unmarshal(events, outputClass));
 	}
 
+	@Override
+	public Mono<Object> decodeToMono(Publisher<DataBuffer> inputStream, ResolvableType elementType,
+			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
+		return decode(inputStream, elementType, mimeType, hints).singleOrEmpty();
+	}
+
 	private Object unmarshal(List<XMLEvent> events, Class<?> outputClass) {
 		try {
 			Unmarshaller unmarshaller = this.jaxbContexts.createUnmarshaller(outputClass);

spring-web/src/test/java/org/springframework/http/codec/xml/Jaxb2XmlDecoderTests.java

@@ -23,6 +23,7 @@ import javax.xml.stream.events.XMLEvent;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -159,7 +160,7 @@ public class Jaxb2XmlDecoderTests extends AbstractDataBufferAllocatingTestCase {
 	@Test
 	public void decodeSingleXmlRootElement() throws Exception {
 		Flux<DataBuffer> source = Flux.just(stringBuffer(POJO_ROOT));
-		Flux<Object> output = this.decoder.decode(source, ResolvableType.forClass(Pojo.class),
+		Mono<Object> output = this.decoder.decodeToMono(source, ResolvableType.forClass(Pojo.class),
 				null, Collections.emptyMap());
 
 		StepVerifier.create(output)
@@ -171,7 +172,7 @@ public class Jaxb2XmlDecoderTests extends AbstractDataBufferAllocatingTestCase {
 	@Test
 	public void decodeSingleXmlTypeElement() throws Exception {
 		Flux<DataBuffer> source = Flux.just(stringBuffer(POJO_ROOT));
-		Flux<Object> output = this.decoder.decode(source, ResolvableType.forClass(TypePojo.class),
+		Mono<Object> output = this.decoder.decodeToMono(source, ResolvableType.forClass(TypePojo.class),
 				null, Collections.emptyMap());
 
 		StepVerifier.create(output)

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java

@@ -227,6 +227,13 @@ public class RequestMappingMessageConversionIntegrationTests extends AbstractReq
 						JSON, Person.class).getBody());
 	}
 
+	@Test  // SPR-16759
+	public void personTransformWithMonoAndXml() throws Exception {
+		assertEquals(new Person("ROBERT"),
+				performPost("/person-transform/mono", MediaType.APPLICATION_XML, new Person("Robert"),
+						MediaType.APPLICATION_XML, Person.class).getBody());
+	}
+
 	@Test
 	public void personTransformWithSingle() throws Exception {
 		assertEquals(new Person("ROBERT"),