How do I give type hints to the Jackson deserializer?

Question

I'm using Jackson as a tool to declare some objects whose classes I can't annotate (or modify at all). One of the classes has a setter and getter for an untyped list. Here's a sanitized version: ``` public class Family { private List members; public List getMembers() { return members; } public void setMembers(List members) { this.members = members; } //...many, many other properties } public class Member { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } ``` Here's the JSON I'm trying to deserialize: ``` { "members" : [ { "name" : "Mark" } ] } ``` The naive code I would use is this: ``` ObjectMapper mapper = new ObjectMapper(); Family family = mapper.readValue(json, Family.class); Member member = (Member) family.getMembers().get(0); System.out.println(member.getName()); ``` But of course this fails, as Jackson did not know to create a list of `Members` instead of its fallback, a list of `LinkedHashMap`s. What's the…

Accepted Answer

Mix-in annotations were the critical piece of the puzzle I was missing. Here's a much cleaner way of solving this problem:

```
ObjectMapper mapper = new ObjectMapper();
mapper.getDeserializationConfig().addMixInAnnotations(Family.class, FamilyMixin.class);

Family family = mapper.readValue(json, Family.class);
Member member = (Member) family.getMembers().get(0);

//...

interface FamilyMixin {
    @JsonDeserialize(contentAs = Member.class)
    void setMembers(List members);
}
```

What mix-in annotations let you do is annotate a proxy that is under your control. When that mix-in class is applied to the real class, Jackson behaves as if those annotations annotated the real class's members.

In my case, I use `JsonDeserialize.contentAs()` to specify the container's content type. But I believe most annotations should be available using this method.

How do I give type hints to the Jackson deserializer?

Solution

Problem