Unwrap MappingJacksonValue before selecting ObjectMapper

See gh-28045

spring-web/src/main/java/org/springframework/http/codec/json/AbstractJackson2Encoder.java

@@ -196,22 +196,26 @@ public abstract class AbstractJackson2Encoder extends Jackson2CodecSupport imple
 	public DataBuffer encodeValue(Object value, DataBufferFactory bufferFactory,
 			ResolvableType valueType, @Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {
 
-		ObjectMapper mapper = selectObjectMapper(valueType, mimeType);
-		if (mapper == null) {
-			throw new IllegalStateException("No ObjectMapper for " + valueType);
-		}
 		Class<?> jsonView = null;
 		FilterProvider filters = null;
 		if (value instanceof MappingJacksonValue) {
 			MappingJacksonValue container = (MappingJacksonValue) value;
 			value = container.getValue();
+			valueType = ResolvableType.forInstance(value);
 			jsonView = container.getSerializationView();
 			filters = container.getFilters();
 		}
+
+		ObjectMapper mapper = selectObjectMapper(valueType, mimeType);
+		if (mapper == null) {
+			throw new IllegalStateException("No ObjectMapper for " + valueType);
+		}
+
 		ObjectWriter writer = createObjectWriter(mapper, valueType, mimeType, jsonView, hints);
 		if (filters != null) {
 			writer = writer.with(filters);
 		}
+
 		ByteArrayBuilder byteBuilder = new ByteArrayBuilder(writer.getFactory()._getBufferRecycler());
 		try {
 			JsonEncoding encoding = getJsonEncoding(mimeType);

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonEncoderTests.java

@@ -231,6 +231,29 @@ public class Jackson2JsonEncoderTests extends AbstractEncoderTests<Jackson2JsonE
 		);
 	}
 
+	@Test // gh-28045
+	public void jacksonValueUnwrappedBeforeObjectMapperSelection() {
+
+		JacksonViewBean bean = new JacksonViewBean();
+		bean.setWithView1("with");
+		bean.setWithView2("with");
+		bean.setWithoutView("without");
+
+		MappingJacksonValue jacksonValue = new MappingJacksonValue(bean);
+		jacksonValue.setSerializationView(MyJacksonView1.class);
+
+		ResolvableType type = ResolvableType.forClass(MappingJacksonValue.class);
+
+		MediaType halMediaType = MediaType.parseMediaType("application/hal+json");
+		ObjectMapper mapper = new ObjectMapper().configure(SerializationFeature.INDENT_OUTPUT, true);
+		this.encoder.registerObjectMappersForType(JacksonViewBean.class, map -> map.put(halMediaType, mapper));
+
+		testEncode(Mono.just(jacksonValue), type, halMediaType, Collections.emptyMap(), step -> step
+				.consumeNextWith(expectString("{\n  \"withView1\" : \"with\"\n}").andThen(DataBufferUtils::release))
+				.verifyComplete()
+		);
+	}
+
 	@Test // gh-22771
 	public void encodeWithFlushAfterWriteOff() {
 		ObjectMapper mapper = new ObjectMapper();