Question

I have a node tree built out of Node objects. They are more complex than the code I am showing but only have primitive or Serializable instance members.

Assuming each Node can have up to 10 children the code looks a bit like so.

public class Node implements Serializable{      private static final long serialVersionUID = -848926218090221003L;  private Node _parent; private boolean _hasParent; private Node[] _children; private int _childCount = 0;  public Node(Node parent){      _children = new Node[10];     _parent = parent;     _hasParent = (parent != null); }       ...      //Various accessors etc  } 

Now this tree is quite expensive to build but once it is done I write it to a file:

ObjectOutputStream serializedOuput = new ObjectOutputStream(new FileOutputStream(cacheFile)); serializedOuput.writeObject(tree); serializedOuput.close(); 

When I am done caching the tree I make some irreversable changes to like trimming off unneeded branches.

Then when I next need a base tree to work from I create a new tree object by reading in my serialized file.

The problem…

Creating the tree from the file seems to simply create a new object that points to the old one. In other words the modifications made after writing to file have also been made to the new tree.

If I restart the app and try reading in the serialized file it returns null.

So I seem to be serializing object references rather than the object itself, any ideas where I am going wrong?

Answer 1

Works for me, GNU/Linux with gij 4.3.2.

Make sure all of your fields in the Node class are serializable as well.

This is the code I tried (should be equivalent to yours):

import java.io.*;  public class TestCase {     public static void main(String[] args) {         try {             if (args.length > 0 && args[0].equals('read')) {                 ObjectInputStream ois = new ObjectInputStream(                         new FileInputStream('File.log')                     );                 Node root = (Node) ois.readObject();                 ois.close();                  System.out.println('Root value: ' + root.getValue());                 for(Node n : root.children())                     System.out.println('Child: ' + n.getValue());                  return;             }              Node root = new Node(null, 'First');             root.add(                     new Node(root, 'Second.Child.1'),                     new Node(root, 'Second.Child.2')                 );              ObjectOutputStream oos = new ObjectOutputStream(                     new FileOutputStream('File.log')                 );             oos.writeObject(root);             oos.close();         } catch(Exception e) {             System.err.println(e.getMessage());             e.printStackTrace();         }     } }  class Node implements Serializable {     private static final long serialVersionUID = 2304728943L;     private Node _parent;     private Node[] _children;     private String _value;      public Node(Node parent, String value) {         this._parent = parent;         this._value = value;         this._children = new Node[2];     }     public void add(Node child, Node child2) {         this._children[0] = child;         this._children[1] = child2;     }     public String getValue() {         return this._value;     }     public Node[] children() {         return this._children;     }     public Node parent() {         return this._parent;     } }