Master Dynamic JSON Fields in Java with @JsonAnyGetter and @JsonAnySetter

When a JSON API returns fields that are not known at compile time, you cannot define static Java properties for them. Jackson provides two annotations, @JsonAnySetter and @JsonAnyGetter, to gracefully handle such dynamic attributes.

Using @JsonAnySetter

The annotation marks a method that receives any JSON property not mapped to a regular field. The method typically stores the key‑value pair in a Map<String, Object> (often called additionalProperties).

public void addAdditionalProperty(String key, Object value) {
    additionalProperties.put(key, value);
}

Example JSON:

{
  "name": "豆瓣酱",
  "spicy": true,
  "limited_edition": "yes",
  "extra_notes": "只在冬天卖"
}

Even though the Java class only defines name and spicy, the unknown fields limited_edition and extra_notes are automatically added to the map.

Using @JsonAnyGetter

The annotation marks a method that returns a Map containing dynamic properties. During serialization, Jackson flattens this map into regular JSON fields.

@JsonAnyGetter
public Map<String, Object> getAdditionalProperties() {
    return additionalProperties;
}

Combined usage example:

public class Person {
    private String name;
    private int age;
    private Map<String, Object> additionalProperties = new HashMap<>();

    @JsonAnySetter
    public void addAdditionalProperty(String key, Object value) {
        additionalProperties.put(key, value);
    }

    @JsonAnyGetter
    public Map<String, Object> getAdditionalProperties() {
        return additionalProperties;
    }
}

public class Main {
    public static void main(String[] args) throws Exception {
        String json = "{\"name\":\"John\",\"age\":30,\"address\":\"123 Street\",\"nickname\":\"Johnny\"}";
        ObjectMapper mapper = new ObjectMapper();
        Person person = mapper.readValue(json, Person.class);
        System.out.println(mapper.writeValueAsString(person));
    }
}

The output is

{"name":"John","age":30,"address":"123 Street","nickname":"Johnny"}

, showing that both static and dynamic fields are serialized together.

Key Takeaways

Use @JsonAnySetter to collect unknown JSON properties into a map during deserialization.

Use @JsonAnyGetter to expose the map’s entries as regular JSON fields during serialization.

This approach is ideal for flexible data structures such as configuration objects, plugin metadata, or any API where fields may vary.