Gson: How do I parse polymorphic values that can be either lists or strings?

Question

I need to parse a JSON file that contains long list of customers. In the JSON file each customer may have one id as a string:

{
  \"cust_id\": \"87655\",
  ...
}         


        

Answer 1

If you want to handle both scenarios you can use a custom deserializer. Of course, you have to change the "cust_id" variable to be a list or an array.

Main:

String json1 = "{\"cust_id\": \"87655\"}";
String json2 = "{\"cust_id\": [\"12345\", \"45678\"]}";

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Customer.class, new CustomerDeserializer());
Gson gson = gsonBuilder.create();

Customer customer1 = gson.fromJson(json1, Customer.class);
System.out.println(customer1);

Customer customer2 = gson.fromJson(json2, Customer.class);
System.out.println(customer2);

Customer

public class Customer {

    @SerializedName("cust_id")
    private List custId;

    public List getCustId() {
        return custId;
    }

    public void setCustId(List custId) {
        this.custId = custId;
    }
}

CustomerDeserializer

public class CustomerDeserializer implements JsonDeserializer {

@Override
public Customer deserialize(JsonElement jsonElement, Type typeOf, JsonDeserializationContext context) throws JsonParseException {
    Customer result = null;
    Gson gson = new Gson();

    try {
        // try to deserialize by assuming JSON has a list
        result = gson.fromJson(jsonElement, Customer.class);
    } catch (JsonSyntaxException jse) {
        // error here means JSON has a single string instead of a list

        try {
            // get the single ID
            String custId = jsonElement.getAsJsonObject().get("cust_id").getAsString();

            result = new Customer();
            result.setCustId(Arrays.asList(new String[] {custId}));
        } catch (Exception e) {
            // more error handling here
            e.printStackTrace();
        }
    }

    return result;
}

}

Output

Customer [custId=[87655]]
Customer [custId=[12345, 45678]]