用杰克逊解析JSON到深度嵌套的泛型

Question

I've been trying all kinds of solutions to get this working. But without success.

I have a few classes like this

class Level1<T> {

    public Level2<T> l2;
}

class Level2<T> {

    public Level3<T> l3;
}

class Level3<T> {

    public List<T> objectsList;
}

T in this case can be any object , and should be possible to be resolved during runtime .

My JSON looks like this:

{
    "Level1": {
        "Level2": {
            "Level3": {
                "genericObject": [
                    { 
                      "attribute1": ..., 
                      "attribute2": ...
                    },
                    { 
                      "attribute1": ..., 
                      "attribute2": ...
                    } 
                ]
            }
        }
    }
}

The JSON property "genericObject" changes its name and attributes depending of which kind of object I'm receiving.

I've tried defining in my ObjectMapper like this, in which I pass a Class object to my function (genericObjectClass):

JavaType javaType = TypeFactory.defaultInstance().constructParametricType(ArrayList.class, List.class, genericObjectClass);
javaType = TypeFactory.defaultInstance().constructParametricType(Level3.class, javaType);
javaType = TypeFactory.defaultInstance().constructParametricType(Level2.class, javaType);
javaType = TypeFactory.defaultInstance().constructParametricType(Level1.class, javaType);

Level1<genericObject> l1 = objectMapper.readValue(jsonString, javaType);

Tried solutions:

(de)serialising Nested Generics in Jackson

How to use Jackson with Generics

Is Jackson really unable to deserialize json into a generic type?

Answer 1

There are several things that you need to change

1. Level1 object is not the root object, but a field. If you don't want to create one more wrapper object, use UNWRAP_ROOT_VALUE

    mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE); 

2. You don't need to constructParametricType for the whole object hierarchy, just construct it for top class in the chain

    constructParametricType(Level1.class, genericObjectClass); 

3. Your json fields don't map to pojo fields. So you need to annotate your POJO fields with @JsonProperty("Level2") @JsonProperty("genericObject") and so on.

So your deserialization code should look like

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);

JavaType javaType = TypeFactory.defaultInstance()
         .constructParametricType(Level1.class, c);

mapper.readValue(json, javaType);

And classes look like

class Level1<T> {
    @JsonProperty("Level2")
    public Level2<T> l2;
}
class Level2<T> {
    @JsonProperty("Level3")
    public Level3<T> l3;
}
class Level3<T> {
    @JsonProperty("genericObject")
    public List<T> objectsList;
}

And here is full working gist demo