Json, JPA, ManytoOne and OneToMany => recursive

Question

I've tried to read all the Qs about this topic and still there is something I'm missing.

My problem is: that the two tables KIDS and PARENT are calling eachother recursivly

I've got an EJB asking for a Parent with an id, - I've (I think) used @OneToMany(mappedBy="parent") where needed. The result could be better: When asking for the parent (with an id) it that sees there is a kid that take the parent that ask for the kid... (in an almost never ending story :)

JSON OUTPUT

{
  "parentsid": 1,
  ...
  "parentsusername": null,
  "calendars": [],
  "kids": [
    {
      "kidsid": 2,
        ...
      "parent": {
        "parentsid": 1,
        ...
        "calendars": [],
        "kids": [
          {
            "kidsid": 2,               
            "calendars": [],
            "checkins": [],
            "parent": {
              "parentsid": 1,                 
              "calendars": [],
              "kids": [
                {
                  "kidsid": 2,                     
                  "calendars": [],    
                 .....

The Classes I've got a Stateless Bean calling:

@Stateless
@LocalBean
@Path("/checkin")
public class CheckinService {

 @PersistenceContext(unitName="chkin",type=PersistenceContextType.TRANSACTION)
 EntityManager entityManager;

@GET
@Produces(MediaType.APPLICATION_JSON)
@Path("/{id}")    
public Parent read(@PathParam("id") int id) {
    Parent parent = entityManager.find(Parent.class, id);        
    return parent;
}

The Entity classes are made the following way with

/**
 * The persistent class for the KIDS database table.
 */
@Entity
@Table(name="KIDS")
@NamedQuery(name="Kid.findAll", query="SELECT k FROM Kid k")
public class Kid implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="KIDSID")
    private int kidsid;

    //bi-directional many-to-one association to Calendar
    @OneToMany(mappedBy="kid")
    private List<Calendar> calendars;

    //bi-directional many-to-one association to Checkin
    @OneToMany(mappedBy="kid")
    private List<Checkin> checkins;

    //bi-directional many-to-one association to Parent
    @ManyToOne             **// <=============**
    @JoinColumn(name="parentsId")
    private Parent parent;
}

Entity call for Parents:

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

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="PARENTSID")
    private int parentsid;

    ...

    //bi-directional many-to-one association to Calendar
    @OneToMany(mappedBy="parent")
    private List<Calendar> calendars;

    //bi-directional many-to-one association to Kid
    @OneToMany(mappedBy="parent") **// <=============**
    private List<Kid> kids;

The Database look like this: Two database Tables one named PARENTS and one KIDS there is a onetomany from PARENTS to KIDS with a foreng key called FK-KIDS_parentsID

PARENTS ==========FK_KIDS_parentsId==========< KIDS
(PK) PARENTSID                                 (PK) KIDSID
                                               (FK) parentsId

Stacktrace

[error occurred during error reporting (printing native stack), id 0xc0000005]

Java frames: (J=compiled Java code, j=interpreted, Vv=VM code)
j  sun.nio.ch.WindowsSelectorImpl$SubSelector.poll0(JI[I[I[IJ)I+0
j  sun.nio.ch.WindowsSelectorImpl$SubSelector.poll()I+43
j  sun.nio.ch.WindowsSelectorImpl$SubSelector.access$400(Lsun/nio/ch/WindowsSelectorImpl$SubSelector;)I+1
j  sun.nio.ch.WindowsSelectorImpl.doSelect(J)I+63
j  sun.nio.ch.SelectorImpl.lockAndDoSelect(J)I+37
j  sun.nio.ch.SelectorImpl.select(J)I+30
j  sun.nio.ch.SelectorImpl.select()I+2
j  org.xnio.nio.SelectorUtils.await(Lorg/xnio/nio/NioXnio;Ljava/nio/channels/SelectableChannel;I)V+23
j  org.xnio.nio.NioSocketConduit.awaitWritable()V+26
j  org.xnio.conduits.AbstractSinkConduit.awaitWritable()V+7
j  io.undertow.conduits.ChunkedStreamSinkConduit.awaitWritable()V+7
j  org.xnio.conduits.ConduitStreamSinkChannel.awaitWritable()V+4
j  io.undertow.channels.DetachableStreamSinkChannel.awaitWritable()V+20
j  io.undertow.server.HttpServerExchange$WriteDispatchChannel.awaitWritable()V+20
j  org.xnio.channels.Channels.writeBlocking(Ljava/nio/channels/GatheringByteChannel;[Ljava/nio/ByteBuffer;II)J+34
j  io.undertow.servlet.spec.ServletOutputStreamImpl.write([BII)V+311
j  org.jboss.resteasy.plugins.server.servlet.HttpServletResponseWrapper$DeferredOutputStream.write([BII)V+15
j  org.jboss.resteasy.util.CommitHeaderOutputStream.write([BII)V+11
j  com.fasterxml.jackson.core.json.UTF8JsonGenerator._flushBuffer()V+24
j  com.fasterxml.jackson.core.json.UTF8JsonGenerator._writeBytes([B)V+17
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979c78 [0x00000000029796c0+0x5b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979b78 [0x00000000029796c0+0x4b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41
j  com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(Ljava/util/List;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+163
j  com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+7
j  com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+34
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979b78 [0x00000000029796c0+0x4b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979b78 [0x00000000029796c0+0x4b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41
j  com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(Ljava/util/List;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+163
j  com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+7
j  com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+34
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979b78 [0x00000000029796c0+0x4b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41
J 1041 C2 com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V (185 bytes) @ 0x0000000002979b78 [0x00000000029796c0+0x4b8]
j  com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(Ljava/lang/Object;Lcom/fasterxml/jackson/core/JsonGenerator;Lcom/fasterxml/jackson/databind/SerializerProvider;)V+41

Answer 1

As the comments indicate, this doesn't seem to be so much a JPA issue as it is a JSON serialization issue. Fortunately, Jackson provides several annotations to deal with this sort of situation.

Use @JsonManagedReference on the parent and @JsonBackReference on the child, and you may want to try playing with @JsonIdentityInfo.

Here's some more reading you may find helpful.

Answer 2

This is a problem with Circular dependencies and JSON serialization, an easy way to solve this would be to use @JsonIdentityInfo on both classes, you should have something like this:

@Entity
@Table(name="KIDS")
@NamedQuery(name="Kid.findAll", query="SELECT k FROM Kid k")
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Kid implements Serializable {

and

@Entity
@Table(name="PARENTS")
@NamedQuery(name="Parent.findAll", query="SELECT p FROM Parent p")
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Parent implements Serializable {

This way you ensure that in each kid serialization the parent won't be serialized but you'll get only his id instead.