Mapping JSON back to POJO with JSON and Jackson libs

Question

I have a JSON string:

{
    "fruit": {
        "weight":"29.01",
        "texture":null
    },
    "status":"ok"
}

...that I am trying to map back into a POJO:

public class Widget {
    private double weight; // same as the weight item above
    private String texture; // same as the texture item above

    // Getters and setters for both properties
}

The string above (that I am trying to map) is actually contained inside an org.json.JSONObject and can be obtained by calling that object's toString() method.

I would like to use the Jackson JSON object/JSON mapping framework to do this mapping, and so far this is my best attempt:

try {
    // Contains the above string
    JSONObject jsonObj = getJSONObject();

    ObjectMapper mapper = new ObjectMapper();
    Widget w = mapper.readValue(jsonObj.toString(), Widget.class);

    System.out.println("w.weight = " + w.getWeight());
} catch(Throwable throwable) {
    System.out.println(throwable.getMessage());
}

Unfortunately this code throws an exception when the Jackson readValue(...) method gets executed:

Unrecognized field "fruit" (class org.me.myapp.Widget), not marked as ignorable (2 known properties: , "weight", "texture"])
    at [Source: java.io.StringReader@26c623af; line: 1, column: 14] (through reference chain: org.me.myapp.Widget["fruit"])

I need the mapper to:

Ignore the outer curly brackets ("{" and "}") altogether
Change the fruit to a Widget
Ignore the status altogether

If the only way to do this is to call the JSONObject's toString() method, then so be it. But I'm wondering if Jackson comes with anything "out of the box" that already works with the Java JSON library?

Either way, writing the Jackson mapper is my main problem. Can anyone spot where I'm going wrong? Thanks in advance.

You seem to be thinking you have an object that ... you don't. Your JSON represents (and will map to) an object with a "fruit" field (that holds an object containing two other fields) and a "status" field. — Brian Roach
So there's no way to configure the mapper to ignore/alias fields? This is possible in XML-land with Castor and XStream. I guess I just assumed the same would be true in JSON/Jackson-land. After all, isn't that what mapping is supposed to achieve? How many database tables map back to POJOs perfectly? If libraries like Hibernate didn't allow for configuration, they wouldn't be of much use. — IAmYourFaja
But ... it does map properly. You're trying to map to something different than the JSON object. Related is this: stackoverflow.com/a/13873443/302916 which was my answer to someone trying to do the same thing, but with Gson. I haven't mucked with custom serialization/deserialization in Jackson but I'm sure there's a way to do it. The easiest solution is just making an inner class like the last example in that answer. — Brian Roach
Why is weigth a string in your JSON but a double in your POJO? — fge
@fge - I don't have control over the received JSON but the POJO needs to be a double. — IAmYourFaja

Srinivas Srinivas · Accepted Answer · 2012-12-28T05:22:22

You need to have a class PojoClass which contains (has-a) Widget instance called fruit.

Try this in your mapper:

    String str = "{\"fruit\": {\"weight\":\"29.01\", \"texture\":null}, \"status\":\"ok\"}";
    JSONObject jsonObj = JSONObject.fromObject(str);
    try
    {
        // Contains the above string

        ObjectMapper mapper = new ObjectMapper();
        PojoClass p = mapper.readValue(jsonObj.toString(), new TypeReference<PojoClass>()
        {
        });

        System.out.println("w.weight = " + p.getFruit().getWeight());
    }
    catch (Throwable throwable)
    {
        System.out.println(throwable.getMessage());
    }

This is your Widget Class.

public class Widget
{    private double weight;
     private String texture;
    //getter and setters.
}

This is your PojoClass

public class PojoClass
{
    private Widget fruit;
    private String status;
    //getter and setters.
}

Mapping JSON back to POJO with JSON and Jackson libs

1 Answers