GWT+JPA+Hibernate: Hierarchical object can't be serialized in the client side

Question

It's a GWT Project. I made a sample to tell my problem: Hierarchical objects can't be serialized. I don't know how can I solve this problem.

I'm using Hibernate 4.3.5.Final My GWT version is 2.6.0

The application is connecting to a MySQL database.

Project structure

enter image description here

The hieararchical classes

This is my Family class. It's contains a list of Person objects. That cause the problem.

package com.myproject.shared;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
public class Family implements Serializable {
    
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id;
    
    private String familyName;
    
    @OneToMany (cascade={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE/*, CascadeType.ALL*/}/*, orphanRemoval = true*/)
    @JoinColumn (name="fk_family") 
    private List<Person> list=new ArrayList<Person>();
    
    public Family(){
        
    }
    
    public void writeToOutput(){
        System.out.println("The Family: ");
        System.out.println("id: "+getId());
        System.out.println("family name: "+getFamily_name());
        System.out.println();       
        System.out.println("lists size: " + list.size());
        
        for(int j=0; j<list.size(); j++){
            System.out.println("first name: "+ list.get(j).getFirstName());
        }
        System.out.println();
        System.out.println();
    }
    
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getFamily_name() {
        return familyName;
    }

    public void setFamily_name(String family_name) {
        this.familyName = family_name;
    }

    public List<Person> getList() {
        return list;
    }

    public void setList(List<Person> list) {
        this.list = list;
    }
}

This is the Person class:

package com.myproject.shared;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Person implements Serializable {
    
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private int id;
    
    private String firstName;

    
    public Person(){
        
    }
    
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    
}

Database

The project can generate the database schema if it's not exist yet. This is working fine.

This is how my tables look like: enter image description here

enter image description here

Error

But I can't read an object from the database.

If I try to gen an object from the database, I get this error:

Starting Jetty on port 8888
   [WARN] Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'org.hibernate.collection.internal.PersistentBag' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = [com.myproject.shared.Person@4a637e58, com.myproject.shared.Person@40dbe769, com.myproject.shared.Person@2535bfc3, com.myproject.shared.Person@6bc0ae4a, com.myproject.shared.Person@36f2b38a]
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serialize(ServerSerializationStreamWriter.java:667)
    at com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamWriter.writeObject(AbstractSerializationStreamWriter.java:130)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter$ValueWriter$8.write(ServerSerializationStreamWriter.java:153)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeValue(ServerSerializationStreamWriter.java:587)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeClass(ServerSerializationStreamWriter.java:757)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeImpl(ServerSerializationStreamWriter.java:796)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serialize(ServerSerializationStreamWriter.java:669)
    at com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamWriter.writeObject(AbstractSerializationStreamWriter.java:130)
    at com.google.gwt.user.client.rpc.core.java.util.Collection_CustomFieldSerializerBase.serialize(Collection_CustomFieldSerializerBase.java:44)
    at com.google.gwt.user.client.rpc.core.java.util.ArrayList_CustomFieldSerializer.serialize(ArrayList_CustomFieldSerializer.java:39)
    at com.google.gwt.user.client.rpc.core.java.util.ArrayList_CustomFieldSerializer.serializeInstance(ArrayList_CustomFieldSerializer.java:51)
    at com.google.gwt.user.client.rpc.core.java.util.ArrayList_CustomFieldSerializer.serializeInstance(ArrayList_CustomFieldSerializer.java:28)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeImpl(ServerSerializationStreamWriter.java:788)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serialize(ServerSerializationStreamWriter.java:669)
    at com.google.gwt.user.client.rpc.impl.AbstractSerializationStreamWriter.writeObject(AbstractSerializationStreamWriter.java:130)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter$ValueWriter$8.write(ServerSerializationStreamWriter.java:153)
    at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serializeValue(ServerSerializationStreamWriter.java:587)
    at com.google.gwt.user.server.rpc.RPC.encodeResponse(RPC.java:605)
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForSuccess(RPC.java:471)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:563)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:265)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:305)
    at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:755)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:686)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:501)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:137)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:557)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:231)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1086)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:428)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:193)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1020)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:135)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
    at org.eclipse.jetty.server.handler.RequestLogHandler.handle(RequestLogHandler.java:68)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:116)
    at org.eclipse.jetty.server.Server.handle(Server.java:370)
    at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:489)
    at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:960)
    at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:1021)
    at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:865)
    at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:240)
    at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:82)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:668)
    at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:52)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:608)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:543)
    at java.lang.Thread.run(Unknown Source)
[ERROR] 500 - POST /myproject/greet (127.0.0.1) 57 bytes
   Request headers
      Host: 127.0.0.1:8888
      Connection: keep-alive
      Content-Length: 141
      X-GWT-Module-Base: http://127.0.0.1:8888/myproject/
      X-GWT-Permutation: HostedMode
      Origin: http://127.0.0.1:8888
      User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.124 Safari/537.36
      Content-Type: text/x-gwt-rpc; charset=UTF-8
      Accept: */*
      Referer: http://127.0.0.1:8888/MyProject.html?gwt.codesvr=127.0.0.1:9997
      Accept-Encoding: gzip,deflate
      Accept-Language: hu-HU,hu;q=0.8,en-US;q=0.6,en;q=0.4
   Response headers
      Content-Type: text/plain

Questions

How can I solve my probleme? Or what other technology should I use?

Look on anser here http://stackoverflow.com/questions/9534484/gwt-rpc-serializationexception — pasha701, Oct 06 '14 at 08:18
Do not return the `Entity` class to the client. Create a new transfer object, map the `Entity` object to this and return this to the client. — Jens, Oct 06 '14 at 08:41
What do you mean about new transfer object? How can I map the Entity to this? What would be different? — Kroy, Oct 06 '14 at 09:46
I know that the probleme it's cause the PersistentBag. When the hibernate make the object persistent or read it from the database, there would be instead of the List<> type a PersistentBag — Kroy, Oct 08 '14 at 01:06