[jdjlist] AW: problem with serizlization

[Lesden, Kim]

Hello Zlatko,

a simple way would be to store a version attribute before you store anything else. 
That way you can also easily migrate old data on the fly. I have seen this work very 
well in C++. However, if you have lots of data and  complex version migrations 
(v1->v2, v2->v3, ...) this will cost you lots of extra cpu. (see sample below)

Regards
Kim

/**
  * Version 1:
  */
class A {
  private static final int version = 1;
  private ArrayList data = new ArrayList();

    private void writeObject(ObjectOutputStream ous) throws IOException {
        out.writeObject(version);
        out.writeObject(data);
    }

    private void readObject(ObjectInputStream ois) throws IOException,
ClassNotFoundException {
        version = out.readObject();
        if ( 1 == version ) {
          data = out.readObject();
        }
    }
}

/**
  * Version 2:
  */
class A {
    private static final int version = 2;
    private ArrayList data = new ArrayList();
    private String name = "name";

    private void writeObject(ObjectOutputStream ous) throws IOException {
        out.writeObject(version);
        out.writeObject(data);
        out.writeObject(name);
    }

    private void readObject(ObjectInputStream ois) throws IOException,
ClassNotFoundException {
        version = out.readObject();
        data = out.readObject();
        if (version > 1) {
          name = out.readObject();
        }
        else {
           /* convert version 1 data to meaningful version 2 or throw exception */
           name = "none";
        }
    }
}

/**
  * Version 3:
  */
class A {
    private static final int version = 3;
    private ArrayList data = new ArrayList();
    private String name = "name";
    private int age = 0;

    private void writeObject(ObjectOutputStream ous) throws IOException {
        out.writeObject(version);
        out.writeObject(data);
        out.writeObject(name);
        out.writeObject(age);
    }

    private void readObject(ObjectInputStream ois) throws IOException,
ClassNotFoundException {
        version = out.readObject();
         data = out.readObject();
        if (version > 1) {
          name = out.readObject();
        }
        else {
           /* convert version 1 data to meaningful version 2 or throw exception */
           name = "name";
        }
        if (version > 2) {
          age = out.readObject();
        }
        else {
           /* convert version 2 data to meaningful version 3 or throw exception */
           age = 0;
        }
    }
}


-----Ursprungliche Nachricht-----
Von: Zlatko Kostadinov [mailto:[EMAIL PROTECTED]]
Gesendet: Montag, 26. August 2002 13:14
An: JDJList
Betreff: [jdjlist] problem with serizlization


Hi guys

I had serizlized objects in this way:

class A {
    private ArrayList data = new ArrayList();

    private void writeObject(ObjectOutputStream ous) throws IOException {
        out.writeObject(data);
    }

    private void readObject(ObjectInputStream ois) throws IOException,
ClassNotFoundException {
        data = out.readObject();
    }
}

And my program now works with objects of this type. I want to add new field
in the class so the class will be

class A {
    private ArrayList data = new ArrayList();
    private name = "name";

    private void writeObject(ObjectOutputStream ous) throws IOException {
        out.writeObject(data);
        out.writeObject(name);
    }

    private void readObject(ObjectInputStream ois) throws IOException,
ClassNotFoundException {
        data = out.readObject();
        // here is the problem - how to determine is this an instance of the
new version or no.
        // must I read the name or not.
    }
}

Any ideas are wellcome
Regards
Zlatko


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