How to get the Generic type for the Jackson ObjectMapper

Question

Java erases normally the Generics data on compilation, but there is a possibility to get that information (the Jackson ObjectMapper does that pretty well).

My Problem: I have a Class with a property of List:

public class User {
    public List<Long> listProp;//it is public only to keep the example simple
}

How can I get the correct TypeReference (or JavaType ?) so that I can map programatically a JSON String to the correct List Type, having the instance of the Class class (User.class) and the property name (listProp)? What I mean is this:

TypeReference typeReference = ...;//how to get the typeReference?
List<Long> correctList = om.readValue(jsonEntry.getValue(), typeReference);//this should return a List<Long> and not eg. a List<Integer>

Answer 1

Have you tried the mappers constructType method?

Type genericType = User.class.getField("listProp").getGenericType();
List<Long> correctList = om.readValue(jsonEntry.getValue(), om.constructType(genericType));

Answer 2

jackson use TypeReference to construct generic type

TypeReference typeReference =new TypeReference<List<Long>>(){}

jackson use JavaType to construct generic type

JavaType jt = om.getTypeFactory().constructArrayType(Long.class);

jackson support three types

1. Class
2. JavaType
3. TypeReference

i like use JavaType, it is more clear for generic type, for normal object use Class

Answer 3

Perhaps a less exotic approach to deserializing a generic type is to wrap it inside a concrete type:

class ListLongWrapper extends ArrayList<Long> {} // package scope
... or ...
static class ListLongWrapper extends ArrayList<Long> {} // class scope

then

String jsonStr = objMapper.writeValueAsString(user1.listProp); // serialize
user2.listProp = objMapper.readValue(jsonStr,ListLongWrapper.class); // deserialize

Notice that extends requires a class type (here I used ArrayList) instead of the interface List.

---

This suggests an even more direct approach for the given example -- User is already a wrapper (and listProp is public):

public class User {
    public List<Long> listProp;
}

then

String jsonStr = objMapper.writeValueAsString(user1); // serialize
var user2 = objMapper.readValue(jsonStr,User.class); // deserialize

In this case, you can use the interface List as-is as a field type within the wrapping class, but this means you have no control over the concrete type that Jackson will use.