Providing Custom Serialization for Map Keys in Jackson

If you're working with Java and are serialising/deserialising content to or from JSON, you will likely be using Jackson.

If you have a Map<String, T>, you may want to tweak the way that the keys are then represented in i.e. JSON serialisation, because Jackson will by default use camel case.

In my case, I wanted to replace keys in a camel case format (such as likeOf) with a kebab case (such as like-of), which I used Guava's CaseFormat functionality for, but the logic you want can be plumbed in as needed.

Because these are String keys, we can extend the existing StringKeySerializer class that Jackson provides, and write our own implementation for how to serialise the key:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdKeySerializers.StringKeySerializer;

public class KebabCaseKeySerialiser extends StringKeySerializer {

  @Override
  public void serialize(Object value, JsonGenerator g, SerializerProvider provider)
      throws IOException {
    // unnecessary assignment for readability
    String newKeyValue = CaseFormat.LOWER_CAMEL.to(CaseFormat.LOWER_HYPHEN, (String) value);
    g.writeFieldName(newKeyValue);
  }
}

Then, on the POJO that is being serialised:

import com.fasterxml.jackson.databind.annotation.JsonSerialize;

@JsonSerialize(keyUsing = KebabCaseKeySerialiser.class)
public class Properties extends HashMap<String, Object> {
  // ...
}

This then instructs Jackson to serialise the keys using our custom serialiser - awesome!

This was tested with Jackson v2.10.2.