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.