Question

I’m serializing an object to a blob in a SQLiteOpenHelper like so:

final ByteArrayOutputStream bos = new ByteArrayOutputStream();
final ObjectOutputStream out = new ObjectOutputStream(bos);
out.writeObject(value);
out.close();

Each class define its own serialVersionUID that never change.

How does the serialization/deserialization actually work? Does it remember the name of the field, and the value associated with it? If I keep adding/removing field, is there a chance that an existing field have their value modified?

For example, let’s say I serialize this class:

public class Foo implements Serializable {
     private static final long serialVersionUID = 4068188167994381987L;
     String x;
     int y;
}

Where x is "A", and y is 1;

Then I modify the class like so:

public class Foo implements Serializable {
     private static final long serialVersionUID = 4068188167994381987L;
     float f;
     String x;
}

If I keep adding/removing field, will each time that I deserialize Foo x still be “A”?

Answer 1

You cannot delete a field. See here at Oracle for compatible changes to your Serializable class.

Yes after the change in class Foo a deserialization of a older Foo would set x to ‘A’, but because you removed a field this does not work.

I suggest keeping the old field y and marking it @deprecated:

public class Foo implements Serializable {
    private static final long serialVersionUID = 4068188167994381987L;
    String x;
    float f;
    @deprecated int y;
}