How to use Jackson's TypeReference with generi

问题:

For json mapping I use the following method:

public static <T> T mapJsonToObject(String json, T dtoClass) throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    return mapper.readValue(json, new TypeReference<RestResponse<UserDto>>() {
    });
}

And UserDto looks like this:

@JsonIgnoreProperties(ignoreUnknown = true)
public class UserDto {

    @JsonProperty("items")
    private List<User> userList;

    public List<User> getUserList() {
        return userList;
    }

    public void setUserList(List<User> userList) {
        this.userList = userList;
    }
}

I want to improve this method of mapping without being attached to a UserDto class, and replacing it with a generic.

Is it possible? And How?

Thanks.

回答1:

TypeReference requires you to specify parameters statically, not dynamically, so it does not work if you need to further parameterize types.

What I think you need is JavaType: you can build instances dynamically by using TypeFactory. You get an instance of TypeFactory via ObjectMapper.getTypeFactory(). You can also construct JavaType instances from simple Class as well as TypeReference.

回答2:

One approach will be to define a Jackson JavaType representing a list of items of type clazz. You still need to have access to the class of the generic parameter at runtime. The usual approach is something like

<T> class XX { XX(Class<T> clazz, ...) ... } 

to pass the class of the generic parameter into the generic class at construction.

Upon access to the Class clazz variable you can construct a Jackson JavaType representing, for example, a list of items of class clazz with the following statement.

JavaType itemType = mapper.getTypeFactory().constructCollectionType(List.class, clazz);

I hope it helped. I am using this approach in my own code.