Make @ResponseBody method return type available for message converters

This commit adds canWrite() and write() methods to the
GenericHttpMessageConverter interface. These are type aware variants
of the methods available in HttpMessageConverter, in order to keep
parametrized type information when serializing objects.

AbstractMessageConverterMethodProcessor now calls those type aware
methods when the message converter implements GenericHttpMessageConverter.

AbstractJackson2HttpMessageConverter and GsonHttpMessageConverter uses
these new methods to make @ResponseBody method return type available
for type resolution instead of just letting the JSON serializer trying
to guess the type to use from the object to serialize.

Issue: SPR-12811

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/AbstractMessageConverterMethodProcessor.java

@@ -17,6 +17,7 @@
 package org.springframework.web.servlet.mvc.method.annotation;
 
 import java.io.IOException;
+import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.LinkedHashSet;
@@ -27,8 +28,10 @@ import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
+import org.springframework.http.converter.GenericHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.http.server.ServletServerHttpResponse;
@@ -158,7 +161,20 @@ public abstract class AbstractMessageConverterMethodProcessor extends AbstractMe
 							(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
 							inputMessage, outputMessage);
 					if (returnValue != null) {
-						((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);
+						if (messageConverter instanceof GenericHttpMessageConverter) {
+							Type type;
+							if (HttpEntity.class.isAssignableFrom(returnType.getParameterType())) {
+								returnType.increaseNestingLevel();
+								type = returnType.getNestedGenericParameterType();
+							}
+							else {
+								type = returnType.getGenericParameterType();
+							}
+							((GenericHttpMessageConverter<T>) messageConverter).write(returnValue, type, selectedMediaType, outputMessage);
+						}
+						else {
+							((HttpMessageConverter<T>) messageConverter).write(returnValue, selectedMediaType, outputMessage);
+						}
 						if (logger.isDebugEnabled()) {
 							logger.debug("Written [" + returnValue + "] as \"" +
 									selectedMediaType + "\" using [" + messageConverter + "]");