Java partial (de)serialization of objects

Question

Let's say we have 3 Classes:

class foo { // not a singleton
    String s;
}
class bar {
    foo f;
    int i;
}
class baz {
    foo sameF;
}

Now we create instances

foo onlyFoo = new foo("the only one");
bar theBar = new bar(onlyFoo, 123);
baz theBaz = new baz(onlyFoo);

After that we want to store them serilazed in a file. If we deserialze theBaz, modify onlyFoo and deserialize theBaz to the file again, theBar still contains the original onlyFoo, so there are two different versions of onlyFoo.

What I want instead is that I store theBaz and theBar without onlyFoo, store the three objects separately and once someone deserialize theBaz I want to give him onlyFoo, too. If he deserializes the changed onlyFoo again, theBaz and theBar will have the same modified onlyFoo, so if someone requests an object (for example theBar) he gets the full serialized object with all the referenced objects (onlyFoo) like the normal serialization process would return.

I know that I have to store the objects and keep the references manually and separately because the default serialization cannot handle this problem. What I don't know is how do I partially serialize/deserialize Java objects? I need to separate the primitives and their wrappers from the 'higher' objects and store this objects separately.

Update

1. I cannot modify the classes.
2. I don't know all classes. It should work with all serializable objects I maybe never heard about (they may or may not be final)

Answer 1

If you want more controll you could overwrite writeObject() and readObject() and serialize yourself.

class bar {
    ...

    private void writeObject(ObjectOutputStream stream) throws IOException {
      // let version 1, later when you need to have versioning. 
      stream.writeInt(version);
      stream.writeInt(i);
      // leave out 
      // stream.writeObject(foo);

    }
}
// read object the analog, see 

http://docs.oracle.com/javase/6/docs/platform/serialization/spec/output.html#861

Answer 2

Mark references you don't want serialized with transient keyword.

Answer 3

You can make foo transient as below.

class bar {
    transient foo f;
    int i;
}