ConversionService detects generic type declaration on target class behind proxy as well

Issue: SPR-14822

spring-context/src/main/java/org/springframework/format/support/FormattingConversionService.java

@@ -24,6 +24,7 @@ import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.context.EmbeddedValueResolverAware;
 import org.springframework.context.i18n.LocaleContextHolder;
+import org.springframework.core.DecoratingProxy;
 import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
@@ -97,9 +98,13 @@ public class FormattingConversionService extends GenericConversionService
 
 	static Class<?> getFieldType(Formatter<?> formatter) {
 		Class<?> fieldType = GenericTypeResolver.resolveTypeArgument(formatter.getClass(), Formatter.class);
+		if (fieldType == null && formatter instanceof DecoratingProxy) {
+			fieldType = GenericTypeResolver.resolveTypeArgument(
+					((DecoratingProxy) formatter).getDecoratedClass(), Formatter.class);
+		}
 		if (fieldType == null) {
-			throw new IllegalArgumentException("Unable to extract parameterized field type argument from Formatter [" +
-					formatter.getClass().getName() + "]; does the formatter parameterize the <T> generic type?");
+			throw new IllegalArgumentException("Unable to extract the parameterized field type from Formatter [" +
+					formatter.getClass().getName() + "]; does the class parameterize the <T> generic type?");
 		}
 		return fieldType;
 	}