Jackson - Processing JSON with multiple types

Question

Say I have a JSON file that looks like this:

{
"response" : [
    {
        "id" : "10",
        "period" : "month",
        "values" : [
             {
                 "value" : 100,
                 "date" : "2013-05-10"
             }
        ],
        "parent" : "1"
    },
    {
        "id" : "10",
        "period" : "day",
        "values" : [
             {
                 "value" : {
                        "foo" : 10,
                        "bar" : 11,
                        "etc" : 4
                 },
                 "date" : "2013-05-10"
             }
        ],
        "parent" : "1"
    },{
        "id" : "13",
        "period" : "year",
        "values" : [
             {
                 "value" : {
                        "info" : 1,
                        "pages" : 10,
                        "etc" : 4
                 },
                 "date" : "2013-05-10"
             }
        ],
        "parent" : "1"
    }
]
}

Notice the 'values' part can either be a single value, or an object (which is unique).

I want to use the Jackson ObjectMapper to easily map this to a POJO.

What I have so far:

public class Response
{
    List<ResponseEntry> response;
    /*getters + setters */

    public static class ResponseEntry
    {
         private String id;
         private String period;
         private String parent;
         private List<Value> values;
         /*setters + getters*/

         public static class Value
         {
             private Object value;
             private String date;
             /*setters+getters*/
         }
    }
}

To map the response, I just specify the file I want and tell ObjectMapper to map to the 'Response' class

ObjectMapper mapper = new ObjectMapper();
Response r = mapper.readValues(json, Response.class);

This works, but is there a better way than just using 'Object' to hold 'value'? Since 'value' can be either a single value or an object, I'm having a bit of difficult figuring out what it should be. I'm certain there is a polymorphic way of handling this, but I've looked and couldn't find anything that worked. I'm pretty stuck and I would appreciate any help.

Answer 1

Unfortunately, with the structure of the JSON you are handling, the only way to deserialize it is to have the value attribute be type Object. However, once the JSON is deserialized, you can easily figure out whether value is an object or a single value.

Notice that JSON only supports five data types: objects (Map in java), arrays, strings, numeric and boolean. It looks like in your case, value would most likely be either a number or a map of numbers; then you have two possibilities to check for. Using a quick instanceof comparison, you should be able to figure out what type of value it is.

ObjectMapper mapper = new ObjectMapper();
Response r = mapper.readValues(json, Response.class);
Value val = r.response.get(0).values.get(0);
if (val.value instanceof Map)
    ; // multiple
else
    ; // single