我是后端方面的新手，我想在我的项目中使用modelmapper。我知道如何在基本级别上使用modelmapper。
我的实体类有：

public class Content {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", nullable = false, unique = true)
    private int id;

    @ManyToOne
    @JsonIgnore
    @JoinColumn(name = "userId",
            referencedColumnName = "userId",
            foreignKey = @ForeignKey(name = "fk_content_userId"))
    private User user;

    @Column(name = "title")
    private String title;

    @Column(name = "createdDate")
    private Double createdDate;
}

public class User {
    @Id
    @Column(name = "userId", nullable = false, unique = true)
    private String userId;

    @Column(name = "name")
    private String name;

    @Column(name = "phoneNumber")
    private String phoneNumber;

    @Column(name = "email")
    private String email;
}

还有我的React课：

public class Response {
    private String title;
    private Double createdDate;
    private String userId;
}

那我想要什么？
有api的主体：

response: {
    "title": "title",
    "createdDate": "1616758604",
    "userId": "101010"
}

我想将此响应转换为以下内容：

Content: {
    "title": "title",
    "createdDate": "1616758604",
    "User" : {
        "userId" : "101010"
        "name" : "",
        "phoneNumber" : "",
        "email" : ""
    }
}

注意：最后我用json格式写了“content”。但我需要java的。