JSON to Java: Object Mapping with Jackson and Gson

Jackson: The Standard Choice

Jackson is the most widely used JSON library for Java and is bundled with Spring Boot:

// Maven dependency
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.17.0</version>
</dependency>

import com.fasterxml.jackson.databind.ObjectMapper;

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

        // Deserialize JSON to Java object
        String json = "{\"name\":\"Alice\",\"age\":30}";
        User user = mapper.readValue(json, User.class);
        System.out.println(user.getName()); // Alice

        // Serialize Java object to JSON
        String output = mapper.writeValueAsString(user);
        // or pretty print:
        String pretty = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(user);
    }
}

Jackson Annotations

import com.fasterxml.jackson.annotation.*;

public class User {
    @JsonProperty("user_id")
    private int id;

    private String name;

    @JsonIgnore
    private String password;

    @JsonInclude(JsonInclude.Include.NON_NULL)
    private String email;

    @JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
    private LocalDate createdAt;

    // getters and setters...
}

Handling Generic Types

// List of objects
List<User> users = mapper.readValue(json, new TypeReference<List<User>>() {});

// Map
Map<String, User> userMap = mapper.readValue(json, new TypeReference<Map<String, User>>() {});

Gson: Google's Alternative

// Gradle
implementation 'com.google.code.gson:gson:2.10.1'

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

Gson gson = new GsonBuilder().setPrettyPrinting().create();

// Deserialize
User user = gson.fromJson(json, User.class);

// Serialize
String json = gson.toJson(user);

// Generic types
Type listType = new TypeToken<List<User>>(){}.getType();
List<User> users = gson.fromJson(json, listType);

Jackson vs Gson

FAQ