杰克逊与Spring MVC复制嵌套对象不反序列化

Question

我正在尝试将以下POJO转换为JSON @RestController：

@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

}

嵌套的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，该State对象作为一个整体进入了内部city。但是外部的state（“ UserLocation is showing just an id ofState object. I need to have a samestate object as that ofcity
”的属性，而不只是ID。

我对JackSon API比较陌生。请建议我应该采取哪种方法来达到此要求。

谢谢

Answer 1

这就是杰克逊设计JsonIdentityInfo批注逻辑的方式。

 * 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，只有第二次找到该对象时，才会对ID进行序列化。

因此，有两种解决方法：

1）您可以简单地删除@JsonIdentityInfo批注，Jackson将按预期方式序列化对象，但将从响应中删除@id字段。这可能很好，因为您仍然会拥有’id’属性。

2）我觉得您可以简单地重组对象并删除一些引用。我会说还是做这些更改是很好的。首先，您可以从UserLocation删除对State的引用。我要说的是，因为State附加在City上，所以不必在userLocation类中包含State。这样，您将可以从城市访问州，从而解决了您的问题。另外，我将从City类和State类中删除对userLocations列表的引用。

它看起来像：

UserLocation有城市，没有州。

城市有州，没有用户位置

州没有用户位置，也没有城市。

希望这可以帮助