The easiest way to remove the bidirectional recursive relationships?

Question

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

score 29 · Accepted Answer · edited Oct 14 '15 at 14:01

29

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.

edited Oct 14 '15 at 14:01

approxiblue

6,982
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answered Apr 06 '12 at 16:14

Jesse Wilson

39,078
8
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1

Thanks! This was a lifesaver when working with hibernate. Saved me a bunch of trouble when converting an hibernate entity using gson. – Knubo Oct 02 '12 at 11:21
4

which jar contains this "GraphAdapterBuilder" class ? – pavan Dec 30 '13 at 10:47
The json string is properly builded, but the references don't seem to be working. How can access a property like 0x1.beats.name --> output should be 'SCISSORS'? So jump within the object graph. – Tunguska Mar 31 '14 at 12:00
I was able to solve the problem by myself. If you have a similar problem please check: http://stackoverflow.com/a/22788445/2182503 – Tunguska Apr 01 '14 at 14:14
awesome, saved me lots of time. For future reference, just copy the GraphAdapterBuilder source from: [ https://github.com/google/gson/blob/1d9e86e27c97cd85d898104b4ac42bb487d0d7d0/extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java ] and use it directly in your project – Arthur Feb 15 '17 at 11:39
Someone added gson-extras to maven central in the mean time; For Gradle: `compile 'org.danilopianini:gson-extras:0.2.1'` – Rolf W. Feb 26 '17 at 19:49

score 2 · Answer 2 · answered Apr 06 '12 at 02:25

2

I don't know about with Gson but I'm working with Jackson. Look up an example of using its ObjectMapper class. As for the recursion, use @JsonManagedReference and @JsonBackReference to stop that. Look those up for example usage too.

answered Apr 06 '12 at 02:25

Javateer

21
1

score 2 · Answer 3 · answered Jan 14 '20 at 19:00

I would configure GsonBuilder to work with jackson annotations:

GsonBuilder builder = new GsonBuilder()
    .addSerializationExclusionStrategy(serializationExclusionStrategy);

final ExclusionStrategy serializationExclusionStrategy = new ExclusionStrategy() {
  @Override
  public boolean shouldSkipField(FieldAttributes f) {
    return f.getAnnotation(JsonIgnore.class) != null
           || f.getAnnotation(JsonBackReference.class) != null;
  }

  @Override
  public boolean shouldSkipClass(Class<?> aClass) {
    return false;
  }
};

Thanks a lot, man. This worked perfectly for me. Even though you posted it in 2014 and I used it in 2020. :) — Shafqat Shafi, Jul 24 '20 at 03:57

score 1 · Answer 4 · answered Jan 21 '15 at 10:46

1

You can use @Expose(serialize = false) to avoid the Collection to be serialized.

answered Jan 21 '15 at 10:46

Navin S

5
2

score 1 · Answer 5 · answered Nov 06 '17 at 15:07

1

Use transient for ignoring @OneToMany sets for google.gson

answered Nov 06 '17 at 15:07

Zhurov Konstantin

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if you do so, OneToMany association will be ignored by most of JPA implementations. – Aleks Ben Maza Mar 06 '19 at 11:42

The easiest way to remove the bidirectional recursive relationships?

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