title: ArrayList throwing exception on retrieval from google datastore (with gwt, java)
body: <p>I'm using Google Web Toolkit with java and google datastore as database.
  The entity class has arraylist and on trying to retrieve the data from data base I'm getting the exception:</p>
  
  <blockquote>
    <p>Type 'org.datanucleus.sco.backed.ArrayList' 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.</p>
  </blockquote>
  
  <p>I'm using JPA.</p>
  
  <p>Entity code:</p>
  
  <pre><code> package com.ver2.DY.client;
  
   import java.io.Serializable;
   import java.util.ArrayList;
  
   import javax.jdo.annotations.IdGeneratorStrategy;
   import javax.jdo.annotations.PersistenceCapable;
   import javax.jdo.annotations.Persistent;
   import javax.jdo.annotations.PrimaryKey;
  
   import com.google.gwt.user.client.rpc.IsSerializable;
  
   @PersistenceCapable
   public class ChatInfo implements Serializable, IsSerializable{
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Long topicId;
  
    @Persistent
    private String chatTopic;
  
    @Persistent
    private ArrayList&lt;String&gt; messages = new ArrayList&lt;String&gt;();
  
    @Persistent
    private boolean isFirstPost;
  
    public ChatInfo()
    {
  
    }
  
  
    public Long getTopicId() {    
     return topicId;
    }
    public void setTopicId(Long topicId) {
     this.topicId = topicId;
    }
    public String getChatTopic() {
     return chatTopic;
    }
    public void setChatTopic(String chatTopic) {
     this.chatTopic = chatTopic;
    }
    public ArrayList&lt;String&gt; getMessages() {
     return messages;
    }
    public void addMessage(String newMsg) {
      messages.add(newMsg);
    }
  
    public boolean isFirstPost() {
     return isFirstPost;
    }
    public void setFirstPost(boolean isFirstPost) {
     this.isFirstPost = isFirstPost;
    }
  
   }
  </code></pre>
  
  <p>Method in db class:</p>
  
  <pre><code>@Transactional
    public ChatInfo[] getAllChat() {
     PersistenceManager pm = PMF.get().getPersistenceManager();
     List&lt;ChatInfo&gt; chats = null;
     ChatInfo[] infos = null;
     String query = "select from " + ChatInfo.class.getName();
     try{
      chats = (List&lt;ChatInfo&gt;) pm.newQuery(query).execute();
  
  
     infos = new ChatInfo[chats.size()];
     for(int i=0;i&lt;chats.size();i++)
     {
      infos[i] = new ChatInfo();
      infos[i] = (ChatInfo) chats.get(i);
     }
     }
     finally{
      pm.close();
     }
     return infos;
  
    }
  </code></pre>
  
  <p>It is a bit strange because earlier I was able to insert and retrieve the data but it now throwing an exception. On searching the web I could find that I need to convert the Arraylist from some DataNucleus type to java util but not sure how to do that.</p>