JSON to Java Mapping Parametric / Generic types

Question

I am looking for advice on the best practice for handling the following scenario.

Given a Java based Web API that accepts JSON requests we need to map the JSON object to Java. This question is about the case where the Java object is a parametric type. For my particular case I am using Spring and the MappingJacksonHttpMessageConverter to do the mapping.

Example REST Service

@RequestMapping(method = RequestMethod.POST) 
public void updateContainer(@RequestBody Container<?> container) {
    realService.updateContainer(container);
}

Example Parametric Type

public class Container<T extends ContainerItem> {
      private T containerItem;
      ...
}

When I call this method I will get a 400 error (bad request) because the type of Container is erased at runtime and Jackson doesn't know how to map my containerItem (since it has no idea what type to map to).

I have a very basic solution implemented, in which I add a field to the JSON data which specifies the Java classname of containerItem, and then added a custom HttpMessageConverter which uses this classname to construct a parametric JavaType which the Jackson ObjectMapper can then use to do the mapping, but I am wondering if anyone has a better way of handling this?

I'd avoid exposing constructs that have no equivalent whatsoever in JSON (like generics, collections etc notwithstanding) in services. It makes it difficult to figure out how exactly that service is meant to be consumed. — millimoose
– millimoose, Commented Sep 30, 2013 at 1:05
Hi millimoose, it's a fair point but I guess I am hoping that this would be a common enough problem that there might be some best practice pattern or language features of Jackson to help deal with it. As an example JSON doesn't have an equivalent to polymorphic types, and yet Jackson is able to handle those through annotations. — Josh
– Josh, Commented Sep 30, 2013 at 5:42

Community · Accepted Answer · 2017-05-23 12:29:31Z

4

A solution would be to declare a parametric type and use this as the target of @RequestBody this way:

public class MyCustomTypeContainer extends Container<SubContainerItem>  {
..
}

public void updateContainer(@RequestBody MyCustomTypeContainer container) {
    realService.updateContainer(container);
}

Note, that the MyCustomTypeContainer has to be a parametric type(generic types fully realized). This potentially means that you may have to create a subtype for every containerItem type.

See here for another question along the same lines.

edited May 23, 2017 at 12:29

CommunityBot

11 silver badge

answered Sep 30, 2013 at 1:00

Biju Kunjummen

50.1k14 gold badges117 silver badges126 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Josh Over a year ago

Hi Biju, this solution would work if I want to only updateContainer with one concrete type of sub container (SubContainerItem) - but what if I also want to be able to update a container full of SubContainerItem2?

Biju Kunjummen Over a year ago

Yes true Josh, actually your approach does sound good also. One change that I can think of is not having the class name in xml, that does not look clean, instead you an try put this information in a custom annotation and use that annotation along with a HandlerMethodArgumentResolver to create the appropriate JavaType and do the unmarshalling there.

Collectives™ on Stack Overflow

JSON to Java Mapping Parametric / Generic types

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related