Question

I have an object that contains a few unserializable fields that I want to serialize. They are from a separate API that I cannot change, so making them Serializable is not an option. The main problem is the Location class. It contains four things that can be serialized that I’d need, all ints. How can I use read/writeObject to create a custom serialization method that can do something like this:

// writeObject:
List<Integer> loc = new ArrayList<Integer>();
loc.add(location.x);
loc.add(location.y);
loc.add(location.z);
loc.add(location.uid);
// ... serialization code

// readObject:
List<Integer> loc = deserialize(); // Replace with real deserialization
location = new Location(loc.get(0), loc.get(1), loc.get(2), loc.get(3));
// ... more code

How can I do this?

Answer 1

Java supports Custom Serialization. Read the section Customize the Default Protocol.

To quote:

There is, however, a strange yet crafty solution. By using a built-in
feature of the serialization mechanism, developers can enhance the
normal process by providing two methods inside their class files.
Those methods are:

• private void writeObject(ObjectOutputStream out) throws
  IOException;
• private void readObject(ObjectInputStream in) throws
  IOException, ClassNotFoundException;

In this method, what you could do is serialize it into other forms if you need to such as the ArrayList for Location that you illustrated or JSON or other data format/method and reconstruct it back on readObject()

With your example, you add the following code:

private void writeObject(ObjectOutputStream oos)
throws IOException {
    // default serialization 
    oos.defaultWriteObject();
    // write the object
    List loc = new ArrayList();
    loc.add(location.x);
    loc.add(location.y);
    loc.add(location.z);
    loc.add(location.uid);
    oos.writeObject(loc);
}

private void readObject(ObjectInputStream ois)
throws ClassNotFoundException, IOException {
    // default deserialization
    ois.defaultReadObject();
    List loc = (List)ois.readObject(); // Replace with real deserialization
    location = new Location(loc.get(0), loc.get(1), loc.get(2), loc.get(3));
    // ... more code

}