杰克逊与 Spring MVC 重复嵌套对象不反序列化

Question

我正在尝试将以下 POJO 转换为 @RestController 中的 JSON：

@Entity
@Table(name="user_location")
@NamedQuery(name="UserLocation.findAll", query="SELECT u FROM UserLocation u")
public class UserLocation implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    private String addr1;

    private String addr2;

    private String landmark;

    private BigDecimal lat;

    private BigDecimal lng;

    private String zipcode;

    //bi-directional many-to-one association to City
    @ManyToOne
    private City city;

    //bi-directional many-to-one association to State
    @ManyToOne
    private State state;

    public UserLocation() {
    }

    //Getter - Setters

}

嵌套City.java如下：

@Entity
@NamedQuery(name="City.findAll", query="SELECT c FROM City c")
@JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property="@id", scope = City.class)
public class City implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    private String name;

    //bi-directional many-to-one association to State
    @ManyToOne
    @JsonIgnore
    private State state;

    //bi-directional many-to-one association to UserLocation
    @OneToMany(mappedBy="city")
    @JsonIgnore
    private List<UserLocation> userLocations;

    public City() {
    }

    public int getId() {
        return this.id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @JsonProperty("state")
    public State getState() {
        return this.state;
    }

    public void setState(State state) {
        this.state = state;
    }


    public List<UserLocation> getUserLocations() {
        return this.userLocations;
    }

    public void setUserLocations(List<UserLocation> userLocations) {
        this.userLocations = userLocations;
    }

    public UserLocation addUserLocation(UserLocation userLocation) {
        getUserLocations().add(userLocation);
        userLocation.setCity(this);

        return userLocation;
    }

    public UserLocation removeUserLocation(UserLocation userLocation) {
        getUserLocations().remove(userLocation);
        userLocation.setCity(null);

        return userLocation;
    }

}

另一个嵌套类State.java如下：

@Entity
@NamedQuery(name="State.findAll", query="SELECT s FROM State s")
@JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property="@id", scope = State.class)
public class State implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    private String name;

    //bi-directional many-to-one association to City
    @OneToMany(mappedBy="state")
    @JsonIgnore
    private List<City> cities;

    //bi-directional many-to-one association to UserLocation
    @OneToMany(mappedBy="state")
    @JsonIgnore
    private List<UserLocation> userLocations;

    public State() {
    }

    public int getId() {
        return this.id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<City> getCities() {
        return this.cities;
    }

    public void setCities(List<City> cities) {
        this.cities = cities;
    }

    public City addCity(City city) {
        getCities().add(city);
        city.setState(this);

        return city;
    }

    public City removeCity(City city) {
        getCities().remove(city);
        city.setState(null);

        return city;
    }

    public List<UserLocation> getUserLocations() {
        return this.userLocations;
    }

    public void setUserLocations(List<UserLocation> userLocations) {
        this.userLocations = userLocations;
    }

    public UserLocation addUserLocation(UserLocation userLocation) {
        getUserLocations().add(userLocation);
        userLocation.setState(this);

        return userLocation;
    }

    public UserLocation removeUserLocation(UserLocation userLocation) {
        getUserLocations().remove(userLocation);
        userLocation.setState(null);

        return userLocation;
    }

}

从 UserLocation.java 转换的 JSON 如下：

{
    id: 1,
    addr1: "11905 Technology",
    addr2: "Eden Prairie",
    landmark: null,
    lat: null,
    lng: null,
    zipcode: "55344",
    city: {
        @id: 1,
        id: 2,
        name: "Westborough",
        state: {
            @id: 1,
            id: 2,
            name: "MA"
        }
    },
    state: 1
}

如您所见，State 对象作为一个整体对象进入city。但是外部state（'UserLocationis showing just an id ofStateobject. I need to have a samestateobject as that ofcity`的属性，而不仅仅是id。

我对 JackSon api 比较陌生。请建议我应该采用哪种方法来实现这一要求。

谢谢

Answer 1

这就是杰克逊设计 JsonIdentityInfo 注释逻辑的方式。

 * Annotation used for indicating that values of annotated type
 * or property should be serializing so that instances either
 * contain additional object identifier (in addition actual object
 * properties), or as a reference that consists of an object id
 * that refers to a full serialization. In practice this is done
 * by serializing the first instance as full object and object
 * identity, and other references to the object as reference values.

Jackson 将第一次运行完整的序列化，第二次找到该对象时只会序列化 id。

所以，有两种方法可以解决它：

1) 您可以简单地删除 @JsonIdentityInfo 注释，Jackson 将按照您的预期序列化对象，但它会从响应中删除 @id 字段。这可能没问题，因为您仍然会有 'id' 属性。

2) 我觉得你可以简单地重组你的对象并删除一些引用。我会说无论如何做这些改变是好的。首先，您可以从 UserLocation 中删除对 State 的引用。我想说没有必要在 userLocation 类中包含 State，因为 State 附属于 City。 通过这样做，您将从 City 访问 State，您的问题就解决了。 我还会从 City 类和 State 类中删除对 userLocations 列表的引用。

它看起来像：

UserLocation 有 City 而没有 State。

City 有 State 而没有 userLocations

州没有 userLocations 和城市。

希望对你有帮助

Answer 2

首先从您的 State.java 和 City.java 中删除该注释

@JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property="@id", scope = State.class)

@JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class, property="@id", scope = City.class)

不需要这些注释，在 RestController 中添加返回类型为 @ResponseBody UserLocation 。它会给你那个类的json。