Java serialization of non serializable third party class

I am currently developing a web application and I would like to make java objects persistent at the server so that they can be retrieved at any time. Since a database is an overkill for my application, I choose the easiest way of persisting java objects: serialization to xml or to bytes. Unfortunately a big part of the code I use are java classes which I cannot modify and these classes do not implement the interface 'serializable'. What are my options regarding to serializing objects of these classes, as well as other interacting objects of my own classes?

As I said in my comments, I'd go for a SerializationService which would find the proper Serializer<T> for every object you want to save.

Something like :

public interface Serializer<T> {

Serializable toSerializable(T objectToSerialize);

//to build a factory/service around it
boolean canDeserialize(Serializable serializedObject);

T fromSerializable(Serializable serializedObject);

}

And if you want a basic, concrete example : with the quite-common Path :

public class PathSerializer implements Serializer<Path> {

@Override
public Serializable toSerializable(Path objectToSerialize) {
    return objectToSerialize.toString();
}

@Override
public Path fromSerializable(Serializable serializedObject) {
    if(!canDeserialize(serializedObject)){
        throw new IllegalArgumentException("Cannot deserialize this");
    }
    return Paths.get((String)serializedObject);
}

@Override
public boolean canDeserialize(Serializable serializedObject) {
    return serializedObject != null && serializedObject instanceof String;
}

}

You could also very well store POJO containing the name your original object class and the list of parameters needed in its constructor an/or a map of its fields to be able to regenerate your objects by reflection.

It's all up to you and the complexity of your application.