在Jackson中反序列化对象引用

Question

I am trying to deserialize a object ref ($ref) using ObjectMapper.

 public class Foo {
    @JsonProperty("bar")
    private Bar bar;

    @JsonProperty("bar")
    public Bar getBar() {
        return bar;
    }

    @JsonProperty("bar")
    public void setBar(Bar bar) {
        this.bar = bar;
    }
}

test.json This is the json file I am trying to deserialize. Is this is the correct way to refer to a object/json reference?

{  
  "bar": {"$ref": "/bar.json"}
}

Deserializer.java

ObjectMapper objectMapper = new ObjectMapper();
//load class
URL url = Deserializer.class.getClassLoader().getResource("test.json");

//deserialize 
objectMapper.readValue(url, Foo.class);

the result creates a Foo pojo with additional property of "bar": ""$ref": "/bar.json"" rather than deserializing it. Do I need to implement the deserialize interface and manually deserialize the node?

Answer 1

Traditionally in Comp Sc. this problem is solved using what is known as "Pointer Swizzling".

This means that If you have an Object A that contains a reference to B and you want to serialize this structure (and then deserialize it), you would need to "unswizzle" the pointer to B to a "name" (an identifier that uniquely identifies the instance B) , write it to disk. When deserializing, you would then take that name, find the instance that it points to (B) and "swizzle" the name back to a proper pointer to B.

Now, in Java pointers are called references but it's the same.

Here's an example to illustrate:

originalA = { "id":"id_a", "ref_to_b": originalB}
originalB = { "id":"id_b" }

Applying unswizzling:

readyForSerializationA = { "id":"id_a", "ref_to_b": "id_b"}
readyForSerializationB = { "id": "id_b" }

followed by writing to store/reading back from store.

Applying swizzling:

deserializedB = { "id":"id_b" }
deserializedA = { "id": "id_a", "ref_to_b": deserializedB}

One possible way to do it for your case,is to deserialize all objects first, put them into an HashMap and in a second pass, look up the ObjectReference(s) from the various ObjectID(s) that you have in your JSON (swizzling).

Some further reading: https://en.wikipedia.org/wiki/Pointer_swizzling

Answer 2

You need to store {"$ref": "/bar.json"} this as a Map. That's the simplest way to store it.

Example:

public class Foo {
    @JsonProperty("bar")
    private Map<String, Bar> bar;

    @JsonProperty("bar")
    public Map<String, Bar> getBar() {
        return bar.get("$ref");
    }

    @JsonProperty("bar")
    public void setBar(Map<String, Bar> bar) {
        this.bar = bar;
    }
}

Only then it will get the value of $ref in Bar object. Otherwise, the data will be in incorrect format and Bar object will take the entire bar value into it.