Java Jackson @JsonAnySetter Example

1. Introduction

The @JsonAnySetter annotation in Jackson allows for the deserialization of properties for which there are no direct matches in the target class. This is useful for handling dynamic properties in JSON data, where the exact structure might not be known in advance. With this annotation, instead of causing a failure, these properties can be collected and processed.

2. Example Steps

1. Create a UserProfile class with a name field and a properties map.

2. Annotate a method to set values to the properties map with @JsonAnySetter.

3. Deserialize a JSON string that contains both matched and unmatched fields.

4. Print the deserialized UserProfile object and observe that unmatched fields are captured in the properties map.

3. Java Jackson @JsonAnySetter Example

import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.HashMap;
import java.util.Map;

public class JsonAnySetterExample {

    public static void main(String[] args) throws Exception {
        // Sample JSON input
        String json = "{\"name\":\"John\",\"age\":30,\"email\":\"john@example.com\"}";

        // Use Jackson to deserialize the JSON input
        ObjectMapper objectMapper = new ObjectMapper();
        UserProfile profile = objectMapper.readValue(json, UserProfile.class);

        // Print the resulting object
        System.out.println(profile);
    }

    public static class UserProfile {
        private String name;
        private Map<String, Object> properties = new HashMap<>();

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

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

        @Override
        public String toString() {
            return "UserProfile{name='" + name + "', properties=" + properties + '}';
        }
    }
}

Output:

UserProfile{name='John', properties={age=30, email=john@example.com}}

4. Step By Step Explanation

In this example, the UserProfile class contains a name field and a properties map to capture additional fields. The method setProperty is annotated with @JsonAnySetter which indicates that this method should be used to handle any unmatched fields in the incoming JSON. The method accepts a key and a value, and simply adds them to the properties map.

When deserializing the provided JSON string, the name field is directly matched to the name property in the UserProfile class. The age and email fields, however, do not have direct matches. Instead of causing an error, they are captured by the setProperty method and added to the properties map. As a result, the final UserProfile object contains both the matched name field and the unmatched fields inside the properties map.