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

Using @JsonAnySetter and @JsonAnyGetter for Dynamic JSON Properties

When a JSON response contains fields that are not known at compile time, Jackson provides the annotations @JsonAnySetter and @JsonAnyGetter to collect and serialize these dynamic properties.

How @JsonAnySetter works

Annotate a method that receives a key and value; Jackson calls it for each unknown property, storing them in a Map<String, Object>.

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

How @JsonAnyGetter works

Annotate a method that returns the map of dynamic properties; during serialization Jackson merges the map entries into the JSON output.

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

Complete Example

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

    // getters and setters omitted

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

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

Deserialization example:

String json = "{\"name\":\"John\",\"age\":30,\"address\":\"123 Street\",\"nickname\":\"Johnny\"}";
ObjectMapper mapper = new ObjectMapper();
Person person = mapper.readValue(json, Person.class);

Serialization produces:

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

This approach is suitable for JSON structures with unpredictable fields such as configuration items, plugin metadata, or extensible APIs.