JSON to Java Class

Example output: Jackson + Lombok

Input:

{ "firstName": "Alice", "age": 30, "address": { "city": "Madrid" } }

Output:

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class User {
    @JsonProperty("firstName")
    private String firstName;

    @JsonProperty("age")
    private int age;

    @JsonProperty("address")
    private Address address;
}

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class Address {
    @JsonProperty("city")
    private String city;
}

Type mapping

Choosing boxed vs primitive

• Primitive (int, long, boolean), non-nullable, efficient, no auto-boxing.
• Boxed (Integer, Long, Boolean), nullable, required if the field can be absent or null in JSON.

The generator defaults to boxed for anything seen as nullable, primitive otherwise.

Jackson vs Gson

Common mistakes

• Using primitive types for nullable fields. int cannot be null; Jackson will throw if the JSON has "age": null. Use Integer.
• Missing date modules. Jackson needs jackson-datatype-jsr310 for Instant/LocalDate. Without it, dates fall back to String or epoch longs.
• Sharing wrapper types across unrelated classes. If two JSON shapes both have a nested Address, the generator creates two Address classes. Rename or unify manually.
• Forgetting @JsonIgnoreProperties(ignoreUnknown = true). Strict Jackson throws on unknown properties; add this annotation (or configure globally) for tolerant deserialization.