Java - Jackson nested arrays

Question

Given the following data

{
   "version" : 1,
   "data" : [ [1,2,3], [4.5,6]]
}

I tried the following definitions and used ObjectMapper.readValue(jsonstring, Outer.class)

class Outer {
  public int version;
  public List<Inner> data
}

class Inner {
   public List<Integer> intlist;
}

I got:

Can not deserialize instance of Inner out of START_ARRAY token"

In the Outer class, if I say

List<List<Integer> data;

then deserialization works.

But in my code, the Outer and Inner classes have some business logic related methods and I want to retain the class stucture.

I understand that the issue is that Jackson is unable to map the inner array to the 'Inner' class. Do I have to use the Tree Model in Jackson? Or is there someway I can still use the DataModel here ?

Alexey Gavrilov · Accepted Answer · 2014-12-06 14:49:54Z

Jackson needs to know how to create an Inner instance from an array of ints. The cleanest way is to declare a corresponding constructor and mark it with the @JsonCreator annotation.

Here is an example:

public class JacksonIntArray {
    static final String JSON = "{ \"version\" : 1, \"data\" : [ [1,2,3], [4.5,6]] }";

    static class Outer {
        public int version;
        public List<Inner> data;

        @Override
        public String toString() {
            return "Outer{" +
                    "version=" + version +
                    ", data=" + data +
                    '}';
        }
    }

    static class Inner {
        public List<Integer> intlist;

        @JsonCreator
        public Inner(final List<Integer> intlist) {
            this.intlist = intlist;
        }

        @Override
        public String toString() {
            return "Inner{" +
                    "intlist=" + intlist +
                    '}';
        }
    }

    public static void main(String[] args) throws IOException {
        final ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.readValue(JSON, Outer.class));
    }

Output:

Outer{version=1, data=[Inner{intlist=[1, 2, 3]}, Inner{intlist=[4, 6]}]}

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