deserialize Json to Java with Polymorphic members

Question

I'm trying to deserialize a json string to java object with genson but could not succeed. My class structure is like this:

public class Condition {
}

public class SimpleCondition extends Condition {
    String feature;
    String op;
    String value;
    int primitive;
}

public class ComplexCondition extends Condition {
    private Condition left;
    private String joint;
    private Condition right;
}

As you can see ComplexCondition may have another ComplexCondition or a SimpleCondition as its member for both left and right. The json that I get is like this:

{
  "left": {
            "feature":"locality",
            "op":"==",
            "value":"Chino"
            "primitive":9,
          },
  "joint":"and",
  "right": {
              "feature":"ch4",
              "op":">=",
              "value":"1.5",
              "primitive":3
           }
}

In this json, a ComplexCondition has both left and right as SimpleConditions. But a general json string that I receive could be anything ranging from a SimpleCondition to any level of nesting for ComplexConditions. I tried setting @class values in the json string but still genson could not deserialize it. I appreciate any help in deserializing this json to java using any library.

Answer 1

You can register some aliases for your classes and then refer to them in your json like this:

Genson genson = new GensonBuilder()
    .addAlias("ComplexCondition", ComplexCondition.class)
    .addAlias("SimpleCondition", SimpleCondition.class)
    .create();

{
  "@class": "ComplexCondition",
  "left": {
            "@class": "SimpleCondition",
            "feature":"locality",
            "op":"==",
            "value":"Chino",
            "primitive":9
          },
  "joint":"and",
  "right": {
            "@class": "SimpleCondition",
              "feature":"ch4",
              "op":">=",
              "value":"1.5",
              "primitive":3
           }
}

You also need to add get and set methods for your ComplexCondition or make its field public or provide a constructor which takes them as arguments or configure genson to use private fields.

And last note, the class metadata attribute must be defined before attributes that are not prefixed by @. If you generate this json with Genson, it will always respect this constraint.