Java Jackson @JsonAnyGetter Example

1. Introduction

Jackson's @JsonAnyGetter annotation allows the flexibility to serialize Java Map properties as normal fields. Without this annotation, if you serialize an object that has a Map field, the output JSON will include the field name. However, using @JsonAnyGetter, you can flatten the Map, so its keys and values become fields in the JSON object.

2. Example Steps

1. Create a User class with a Map field to store dynamic properties.

2. Annotate the getter method of this Map with @JsonAnyGetter.

3. Serialize an instance of the User class and observe the output.

3. Java Jackson @JsonAnyGetter Example

import com.fasterxml.jackson.annotation.JsonAnyGetter;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.HashMap;
import java.util.Map;

public class JacksonJsonAnyGetterExample {

    public static void main(String[] args) throws Exception {
        // Initialize ObjectMapper
        ObjectMapper mapper = new ObjectMapper();

        // Create a User instance
        User user = new User("John", "Doe");
        user.add("age", 30);
        user.add("city", "New York");

        // Serialize the User instance
        String serializedUser = mapper.writeValueAsString(user);

        // Print the serialized user
        System.out.println(serializedUser);
    }

    static class User {
        private String firstName;
        private String lastName;
        private Map<String, Object> properties = new HashMap<>();

        public User(String firstName, String lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
        }

        // This method allows dynamic properties to be added to the map
        public void add(String key, Object value) {
            properties.put(key, value);
        }

        // Annotated with @JsonAnyGetter to flatten the properties map
        @JsonAnyGetter
        public Map<String, Object> getProperties() {
            return properties;
        }

        // Other getters and setters omitted for brevity
    }
}

Output:

{"firstName":"John","lastName":"Doe","age":30,"city":"New York"}

4. Step By Step Explanation

The User class contains a properties Map to store the dynamic attributes of the user. Instead of serializing this map as a nested object, the @JsonAnyGetter annotation flattens it. This means the keys of the map become top-level fields in the serialized JSON. As seen in the output, the map's keys "age" and "city" are serialized alongside the "firstName" and "lastName" fields, providing a flat representation of the User object in the JSON format.