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

Handling Dynamic JSON Properties in Java

When a JSON response contains fields that are not known at compile time, Jackson provides two annotations— @JsonAnySetter and @JsonAnyGetter —to manage these dynamic attributes without breaking the normal deserialization and serialization process.

@JsonAnySetter: Collecting Unknown Fields

The @JsonAnySetter annotation marks a method that receives any JSON property that does not have a matching Java field. Typically the method stores the key‑value pair in a Map<String, Object> called additionalProperties (or any name you prefer).

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"
}

If the Java class defines only name and spicy, the fields limited_edition and extra_notes are automatically added to the map during deserialization.

Complete Deserialization 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);
    }

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

ObjectMapper mapper = new ObjectMapper();
Person p = mapper.readValue(jsonString, Person.class);
System.out.println(p.getAdditionalProperties()); // {address=123 Street, nickname=Johnny}

@JsonAnyGetter: Serializing Dynamic Fields

The @JsonAnyGetter annotation marks a method that returns the map of dynamic properties. During serialization, Jackson flattens the map entries into normal JSON fields.

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

Using the same Person object, the following code produces JSON that includes both static and dynamic fields:

ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(person);
System.out.println(json); // {"name":"John","age":30,"address":"123 Street","nickname":"Johnny"}

Key Takeaways

Define a Map<String, Object> in the class to store extra properties.

Annotate a setter method with @JsonAnySetter to capture unknown fields during deserialization.

Annotate a getter method with @JsonAnyGetter to emit those fields during serialization.

This approach is ideal for JSON structures whose schema may change, such as configuration objects, plugin metadata, or loosely defined APIs.