How to rename root key in JSON serialization with Jackson

Question

I am using Jackson for JSON serialization of a list of objects.

Here is what I get:

{"ArrayList":[{"id":1,"name":"test name"}]}

But I want this :

{"rootname":[{"id":1,"name":"test name"}]} // ie showing the string I want as the root name.

Below is my approach to this:

Interface:

public interface MyInterface {
    public long getId();
    public String getName();
}

Implementation class:

@JsonRootName(value = "rootname")
public class MyImpl implements MyInterface {
    private final long id;
    private String name;

    public MyImpl(final long id,final name) {
        this.id = id;
        this.name = name;
    }

   // getters     
}

JSon serialization:

public class MySerializer {
    public static String serializeList(final List<MyInterface> lists) {
        //check for null value.Throw Exception
        final ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationConfig.Feature.WRAP_ROOT_VALUE, true);
        return mapper.writeValueAsString(lists);
    }
}

Test:

final List<MyInterface> list = new ArrayList<MyImpl>();
MyImpl item = new MyImpl(1L,"test name");
list.add(item);
final String json = MySerializer.serializeList(list);
System.out.println(json);

Here is what I get:

{"ArrayList":[{"id":1,"name":"test name"}]}

But I want this :

{"rootname":[{"id":1,"name":"test name"}]} // ie showing the string I want as the root     name.

I have tried all suggested solutions I could find but failed to achieve my goal. I have looked at:

• Jackson : custom collection serialization to JSON
• How do I rename the root key of a JSON with Java Jackson?
• Jackson : custom collection serialization to JSON

Or am I missing something? I am using jackson 1.9.12 for this. Any help in this regard is welcome.

Answer 1

Well, by default Jackson uses one of two annotations when trying to determine the root name to be displayed for wrapped values - @XmlRootElement or @JsonRootName. It expects this annotation to be on the type being serialized, else it will use the simple name of the type as the root name.

In your case, you are serializing a list, which is why the root name is 'ArrayList' (simple name of the type being serialized). Each element in the list may be of a type annotated with @JsonRootName, but the list itself is not.

When the root value you are trying to wrap is a collection then you need some way of defining the wrap name:

Holder/Wrapper Class

You can create a wrapper class to hold the list, with an annotation to define the desired property name (you only need to use this method when you do not have direct control of the ObjectMapper/JSON transformation process):

class MyInterfaceList {
    @JsonProperty("rootname")
    private List<MyInterface> list;

    public List<MyInterface> getList() {
        return list;
    }

    public void setList(List<MyInterface> list) {
        this.list = list;
    }
}

final List<MyInterface> lists = new ArrayList<MyInterface>(4);
lists.add(new MyImpl(1L, "test name"));
MyInterfaceList listHolder = new MyInterfaceList();
listHolder.setList(lists);
final String json = mapper.writeValueAsString(listHolder);

Object Writer

This is the preferable option. Use a configured ObjectWriter instance to generate the JSON. In particular, we are interested in the withRootName method:

final List<MyInterface> lists = new ArrayList<MyInterface>(4);
lists.add(new MyImpl(1L, "test name"));
final ObjectWriter writer = mapper.writer().withRootName("rootName");
final String json = writer.writeValueAsString(lists);