[FIXED] Jackson JSON key as value in Java

Issue

I'm using Jackson in Spring MVC application. I want to use a String value as key name for Java POJO --> JSON

"record": {
        "<Dynamic record name String>": {
          "value": { 
          ....
          }
      }
 }     

So the dynamic record name String could be "abcd","xyz" or any other string value. How can I define my "record" POJO to have a key like that ?

Solution

Unfortunately, you cannot have dynamic fields in Java classes (unlike some other languages), so you have two choices:

1. Using Maps
2. Using JSON objects (i.e. JsonNode in case of Jackson)

Suppose, you have a data like this:

{
    "record": {
        "jon-skeet": {
            "name": "Jon Skeet",
            "rep": 982706
        },
        "darin-dimitrov": {
            "name": "Darin Dimitrov",
            "rep": 762173
        },
        "novice-user": {
            "name": "Novice User",
            "rep": 766
        }
    }
}

Create two classes to capture it, one for user and another for the object itself:

User.java:

public class User {
    private String name;
    private Long rep;

    public String getName() { return name; }

    public void setName(String name) { this.name = name; }

    public Long getRep() { return rep; }

    public void setRep(Long rep) { this.rep = rep; }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", rep=" + rep +
                '}';
    }
}

Data.java:

public class Data {
    private Map<String, User> record;

    public Map<String, User> getRecord() { return record; }

    public void setRecord(Map<String, User> record) { this.record = record; }

    @Override
    public String toString() {
        return "Data{" +
                "record=" + record +
                '}';
    }
}

Now, parse the JSON (I assume there is a data.json file in the root of your classpath):

public class App {
    public static void main(String[] args) throws Exception {
        final ObjectMapper objectMapper = new ObjectMapper();

        System.out.println(objectMapper.readValue(App.class.getResourceAsStream("/data.json"), Data.class));
        System.out.println(objectMapper.readTree(App.class.getResourceAsStream("/data.json")));
    }
}

This will output:

Data{record={jon-skeet=User{name='Jon Skeet', rep=982706}, darin-dimitrov=User{name='Darin Dimitrov', rep=762173}, novice-user=User{name='Novice User', rep=766}}}
{"record":{"jon-skeet":{"name":"Jon Skeet","rep":982706},"darin-dimitrov":{"name":"Darin Dimitrov","rep":762173},"novice-user":{"name":"Novice User","rep":766}}}

In case of a Map you can use some static classes, like User in this case, or go completely dynamic by using Maps of Maps (Map<String, Map<String, ...>>. However, if you find yourself using too much maps, consider switching to JsonNodes. Basically, they are the same as Map and "invented" specifically for highly dynamic data. Though, you'll have some hard time working with them later...

Take a look at a complete example, I've prepared for you here.

Answered By - madhead
Answer Checked By - Mary Flores (JavaFixing Volunteer)