Java Jackson @JacksonInject Example

1. Introduction

The @JacksonInject annotation in Jackson is used to indicate a property that should be injected during the deserialization process, rather than being read from the JSON. This allows the application to inject values, like configuration settings or services, into deserialized objects without those values being present in the JSON.

2. Example Steps

1. Create a User class with name and country fields.

2. Annotate the country field with @JacksonInject.

3. Deserialize a JSON string that only contains the name field, while injecting a value for the country field.

4. Print the deserialized User object and observe that it contains the injected country value.

3. Java Jackson @JacksonInject Example

import com.fasterxml.jackson.databind.InjectableValues;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JacksonInject;

public class JacksonInjectExample {

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

        // Configure the ObjectMapper to inject a value for the "country" property
        mapper.setInjectableValues(new InjectableValues.Std().addValue("country", "USA"));

        // Deserialize JSON string to User instance
        User user = mapper.readValue("{\"name\":\"Alice\"}", User.class);

        // Print the deserialized User instance
        System.out.println(user);
    }

    static class User {
        private String name;

        @JacksonInject
        private String country;

        // Constructors, getters, setters and toString methods...
        @Override
        public String toString() {
            return "User{name='" + name + "', country='" + country + "'}";
        }
    }
}

Output:

User{name='Alice', country='USA'}

4. Step By Step Explanation

In this example, the User class contains a country field annotated with @JacksonInject. This indicates that the value for this field should be provided (or injected) by the application during deserialization. 

We configure the ObjectMapper to provide the value "USA" for the "country" property using setInjectableValues(). When deserializing a JSON string that only has a name property, the ObjectMapper automatically injects the "USA" value for the country field in the User object. This results in the User object having both a name from the JSON and a country from the application.