Hibernate and JSON - is there a definitive solution to circular dependencies?

Question

I'm struggling with Hibernate entities and JSON in these days and, although there is a lot of questions regarding the object, I'm yet not capable to serialize in presence of circular dependencies. I tried with both Gson and jackson but I didn't get a lot of progresses. Here is an excerpt from my objects. This is the "parent" class.

@Entity
public class User extends RecognizedServerEntities implements java.io.Serializable
{
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "user", orphanRemoval = false)
    @Cascade({CascadeType.SAVE_UPDATE})
    private Set<Thread> threads = new HashSet<Thread>(0);
    //...other attributes, getters and setters
}

and this is the "children" class

@Entity
@Table(name = "thread")
public class Thread extends RecognizedServerEntities implements java.io.Serializable
{
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author", nullable = true)
    private User user;
    //...other attributes, getters and setters
}

I've written a simple class to test both gson and jackson features; as said, they both raise an exception.

public class MyJsonsTest
{
    private static User u;
    public static void main(String[] args)
    {
        u = new User("mail", "password", "nickname", new Date());
        u.setId(1); // Added with EDIT 1
    //  testGson();
        testJackson();
    }

    private static void testJackson()
    {
        Thread t = new Thread("Test", u, new Date(), new Date());
        t.setId(1); // Added with EDIT 1
        u.getThreads().add(t);

        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        try
        {
            mapper.writeValue(new File("result.json"), u);
        }
        catch {/[various exceptions catched, but a JsonMappingException was thrown]}
    }

    private static void testGson()
    {
        Gson gson = new Gson();
        System.out.println(u.toString());
        System.out.println(gson.toJson(u, User.class));

        Thread t = new Thread("Test", u, new Date(), new Date());
        u.getThreads().add(t);

        //This raise an exception overflow
        System.out.println(gson.toJson(u, User.class));
    }
}

To solve the problem, on jackson side, I tried to use this annotation

@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")

on both User and Thread class. However, it doesn't solve the problem. On gson side, I read about the GraphAdapterBuilder class, but I wasn't able to properly use it. I don't find any jar, so I copy/pasted the source code from here. However, there is a compile time error at this line

 private final ConstructorConstructor constructorConstructor = new ConstructorConstructor();

because the ConstructorConstructor() is undefined; the right syntax should be

ConstructorConstructor(Map<Type>, InstanceCreator<?> instanceCreators)

So, is there a definitive solution to this problem? Obviously, I can't use transient variables.

EDIT 1

I finally found the issue with jackson. In the test class, I forgot to initialize the id field (in real scenarios it is initialized by the database) and this is the reason of the exception. When I finally set the id, all works. This is the output

{
  "id" : 1,
  "email" : "mail",
  "password" : "password",
  "nick" : "nickname",
  "registeredDate" : 1414703168409,
  "threads" : [ {
    "id" : 1,
    "thread" : null,
    "user" : 1,
    "title" : "Test",
    "lastModifiedDate" : 1414703168410,
    "createdDate" : 1414703168410,
    "messages" : [ ],
    "threads" : [ ]
  } ],
  "messages" : [ ]
}

Answer 1

When dealing with circular dependencies you need to build a parent-children JSON hierarchy, because the marshalling must be cascaded from root to the inner-most child.

For bi-directional associations, when the Parent has a one-to-many children collection and the child has a many-to-one reference to Parent, you need to annotate the many-to-one side with @JsonIgnore:

@Entity
@Table(name = "thread")
public class Thread extends RecognizedServerEntities implements java.io.Serializable
{
    @Id
    @GeneratedValue(strategy = IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private Integer id;

    @org.codehaus.jackson.annotate.JsonIgnore
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author", nullable = true)
    private User user;
    //...other attributes, getters and setters
}

This way you will no longer have a Json serializing-time circular dependency.

Answer 2

---

Jackson

As said, I was able to solve the problem using

@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id", scope=MyEntity.class)` 

for each entity as suggested here. The scope attribute was necessary to make sure that the name "id" is unique within the scope. Actually, without the scope attribute, as you can see here, it throws an exception saying

com.fasterxml.jackson.databind.JsonMappingException: Already had POJO for id java.lang.String) [com.fasterxml.jackson.annotation.ObjectIdGenerator$IdKey@3372bb3f] (through reference chain: ParentEntity["children"]->java.util.ArrayList[0]->ChildEntity["id"])
...stacktrace...
Caused by: java.lang.IllegalStateException: Already had POJO for id (java.lang.String) [com.fasterxml.jackson.annotation.ObjectIdGenerator$IdKey@3372bb3f]
...stacktrace...

---

Gson

I still haven't found a clean way to serialize circular dependencies.