Manual serialization and versioning

Question

Hi all
I have a class with n fields that is serialized in this way:
private void writeObject(ObjectOutputStream out) throws IOException {
for (int i = 0; i < n; i ++){
out.writeXXX(field_n);
private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
for (int i = 0; i < n; i ++){
in.readXXX(field_n);
I have serialized versions of the class and I am reading them in my program.
Now I have to add new field in the class. I intend to implement the write method in this way:
private void writeObject(ObjectOutputStream out) throws IOException {
for (int i = 0; i < n; i ++){
out.writeXXX(field_n);
out.writeXXX(newfield);
But I have no idea how to implement the read method to work with the two versions.
Does anyone know how to deal with this?

Answer 1

I found a way to deal with the problem. Now my code looks like this:
private void writeObject(ObjectOutputStream out) throws IOException {
final ObjectOutputStream.PutField field = out.putFields();
field.put(FIELD_VERSION, version);
field.put(FIELD_DATA, toShow);
out.writeFields();
private void readObject(ObjectInputStream in)
throws IOException, ClassNotFoundException {
try {
final ObjectInputStream.GetField field = in.readFields();
int oldversion = field.get(FIELD_VERSION, 0);
if (oldversion > 0) {
toShow = (ArrayList) field.get(FIELD_TO_SHOW, toShow);
} else {
final ObjectStreamClass streamClass = field.getObjectStreamClass();
ObjectStreamField [] o = streamClass.getFields();
toShow = (ArrayList)field.get(o[1].getName(), new ArrayList());
Looks like the data stored in the old way (directly with outputStream.writeObject()) can be taken from the GetField. So I am using PutField and GetField to write/read the data, and when I see the data was not written in the new way (
int oldversion = field.get(FIELD_VERSION, 0);
if (oldversion > 0) {*old data*/}
) I am reading the old objects wrote trough the writeObject method using the names of the variables I had wrote. I am not sure what exactly is specified in the serialization spec., but this works now.
Hope this will be useful for someone
Best Regards
Zlatko