@Builder
@Value
@JsonDeserialize(builder = User.UserBuilder.class)
class User {
    @NotNull
    @JsonProperty("user_name")
    private final string userName;

    @JsonPOJOBuilder(withPrefix = "")
    public static final class UserBuilder {}
}

final User user = MAPPER.readValue("{"user_name":"my-name"}", User.class);

public static final class UserBuilder {
    private String userName;
}