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

When a JSON API returns fields that are not known at compile time, Jackson’s @JsonAnySetter and @JsonAnyGetter annotations allow you to collect and serialize these dynamic properties without defining them as explicit class members.

Using @JsonAnySetter

The annotation marks a method that receives unknown JSON fields during deserialization. The method typically stores the key‑value pair in a Map<String, Object>.

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

Example JSON with extra fields:

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

Even though the class only defines name and spicy, the fields limited_edition and extra_notes are captured in additionalProperties.

Using @JsonAnyGetter

The annotation marks a method that returns a map of dynamic properties during serialization, causing its entries to be emitted as regular JSON fields.

@JsonAnyGetter
public Map<String, Object> getOtherProps() {
    return otherProps;
}

Combined usage enables a class to both accept unknown fields when reading JSON and emit them when writing JSON, making it ideal for flexible data structures such as configuration objects, plugin metadata, or any payload where fields may vary.

Full 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;
    }
}

Serializing an instance with extra properties produces JSON like:

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

This approach is especially suitable for JSON structures with non‑fixed fields that may need dynamic extension.