最简单的方法来消除双向递归关系？ [英] The easiest way to remove the bidirectional recursive relationships?

问题描述

我使用 Gson 库将Java对象转换为Json响应。 ..
问题是，在JPA请求之后，从DB检索的对象无法转换，因为与其他实体之间存在递归关系（

<$>

p $ p> public class Gps implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@Basic（可选= false）
@Column（name =IMEI，nullable = false，length = 20）
private String imei;
//一些代码在这里...
@OneToMany（cascade = CascadeType.ALL，mappedBy =gpsImei，fetch = FetchType.LAZY）
private List< Coordonnees> coordonneesList;


公共类Coordonnees实现了Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue（strategy = GenerationType.IDENTITY）
@Basic（可选= false）
@Column（name =IDCOORDONNEES，nullable = false）
私人Integer idcoordonnees;
//这里有一些代码...
@JoinColumn（name =GPS_IMEI，referencedColumnName =IMEI，nullable = false）
@ManyToOne（可选= false，fetch = FetchType。 LAZY）
私人Gps gpsImei;


我的源代码：

  EntityManagerFactory emf = Persistence.createEntityManagerFactory（JavaApplication21PU）; 
 GpsJpaController gjc = new GpsJpaController（emf）; 
 
 Gps gps = gjc.findGps（123456789012345）; （int i = 0; i< gps.getCoordonneesList（）。size（）; i ++）{
 gps.getCoordonneesList（）。get（i）.setGpsImei（null）; 
 
。 
} 
 
 Gson gson = new Gson（）; 
 String json = gson.toJson（gps）; //转换为json响应
 
 System.out.println（json）; 
  

正如您在这里所看到的：

  for（int i = 0; i< gps.getCoordonneesList（）。size（）; i ++）{
 gps.getCoordonneesList（）。get（i）.setGpsImei空值）; 
} 
  

只能通过为coordonneesList中的每个GPS对象设置null来消除递归关系..

您认为这是一个很好的解决方案还是另一种更实用的方法？
Thanks

解决方案

有一个名为 GraphAdapterBuilder ，它可以序列化包含循环引用的对象。下面是相应测试用例的一个简单例子：

  Roshambo rock =新Roshambo（ROCK）; 
 Roshambo剪刀=新Roshambo（剪刀）; 
 Roshambo paper =新Roshambo（PAPER）; 
 rock.beats =剪刀; 
 scissors.beats =纸; 
 paper.beats = rock; 
 
 GsonBuilder gsonBuilder = new GsonBuilder（）; 
 new GraphAdapterBuilder（）
 .addType（Roshambo.class）
 .registerOn（gsonBuilder）; 
 Gson gson = gsonBuilder.create（）; 
 System.out.println（gson.toJson（rock））; 
  

打印：

  {
'0x1'：{'name'：'ROCK'，'beats'：'0x2'}，
'0x2'：{'name'：'SCISSORS'， 'beats'：'0x3'}，
'0x3'：{'name'：'PAPER'，'beats'：'0x1'} 
} 
  

请注意，gson.jar中包含GraphAdapterBuilder类 not 。如果你想使用它，你必须手动将它复制到你的项目中。

I use the Gson library to convert Java objects to a Json response... the problem is that after a JPA requests the object retrieved from DB can not be converted because of a recursive relationship with other entities(see my previous question) for example :

public class Gps implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "IMEI", nullable = false, length = 20)
    private String imei;
    //some code here...
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "gpsImei", fetch = FetchType.LAZY)
    private List<Coordonnees> coordonneesList;


public class Coordonnees implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "IDCOORDONNEES", nullable = false)
    private Integer idcoordonnees;
    //some code here...
    @JoinColumn(name = "GPS_IMEI", referencedColumnName = "IMEI", nullable = false)
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    private Gps gpsImei;

My source code:

  EntityManagerFactory emf=Persistence.createEntityManagerFactory("JavaApplication21PU");
  GpsJpaController gjc=new GpsJpaController(emf);

  Gps gps=gjc.findGps("123456789012345");

  for(int i=0;i<gps.getCoordonneesList().size();i++){
   gps.getCoordonneesList().get(i).setGpsImei(null);
  }  

  Gson gson=new Gson();
  String json=gson.toJson(gps);//convert to json response

  System.out.println(json);  

As you can see here i made :

   for(int i=0;i<gps.getCoordonneesList().size();i++){
     gps.getCoordonneesList().get(i).setGpsImei(null);
   }  

only to kill the recursive relationship by setting null for each GPS object in the coordonneesList..

In your opinion this is a good solution or is there another method more practical? Thanks

解决方案

There's a Gson extension called GraphAdapterBuilder that can serialize objects that contain circular references. Here's a very simplified example from the corresponding test case:

Roshambo rock = new Roshambo("ROCK");
Roshambo scissors = new Roshambo("SCISSORS");
Roshambo paper = new Roshambo("PAPER");
rock.beats = scissors;
scissors.beats = paper;
paper.beats = rock;

GsonBuilder gsonBuilder = new GsonBuilder();
new GraphAdapterBuilder()
    .addType(Roshambo.class)
    .registerOn(gsonBuilder);
Gson gson = gsonBuilder.create();
System.out.println(gson.toJson(rock));

This prints:

{
  '0x1': {'name': 'ROCK', 'beats': '0x2'},
  '0x2': {'name': 'SCISSORS', 'beats': '0x3'},
  '0x3': {'name': 'PAPER', 'beats': '0x1'}
}

Note that the GraphAdapterBuilder class is not included in gson.jar. If you want to use it, you'll have to copy it into your project manually.

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