Deserialise a JavaBean using a modified version of its class

Question

Hi,
When I serialize an object, then modify its class (add a new field), then unserialize this object. It takes into account the new field and set it to null even if I haven't serialized this object again using it's modified class.
How does the java.io.ObjectInputStream class know the class has been changed?
For example I have this class Person:
import java.io.Serializable;
public class Personimplements Serializable {
  private static final long serialVersionUID = -7463768285847981313L;
     private String lastName;
     private String firstName;
     public Person(String firstname, String lastname) {
          lastName = lastname;
          firstName = firstname;
     public String toString(){
          return "Person " + firstName + " " + lastName;
}Then I create an instance of this class and serialize it, and store it to text file, with the following class:
public class WriteInstance {
     public static void main(String[] args) {
          Person me = new Person("John", "Smith");
          try{
               FileOutputStream fos = new FileOutputStream(args[0]);
               ObjectOutputStream oos = new ObjectOutputStream(fos);
               oos.writeObject(me);
          }catch (FileNotFoundException e){
               System.out.println("Error: " + e);
          }catch (IOException e){
               System.out.println("Error: " + e);
}If I deserialize this object using the following class:
public class ReadInstance {
     public static void main(String[] args) {
          try{
               FileInputStream fis = new FileInputStream(args[0]);
               ObjectInputStream ois = new ObjectInputStream(fis);
               Person me = (Person)ois.readObject();
               // print out the instance of Person created in writeInstance.java
               System.out.println(me.toString());
               // print out the result returned from Humanoid.getHeadCount()
               System.out.println(me.getHeadCount());
          }catch (FileNotFoundException e){
               System.out.println("Error: " + e);
          }catch (IOException e){
               System.out.println("Error: " + e);
          }catch (ClassNotFoundException e){
               System.out.println("Error: " + e);
}I got printed out: Person John Smith
Now I modify the class Person and I add a new field private String middleName and add this field to show in my toString() function like this:
public String toString(){
     return "Person " + firstName + " " + middleName + " " + lastName;
}If I deserialize this object again without serializing it first, I got printed out:
Person Charles null Bernard.
Why is that? How does it know the class Person has been modified?
Thanks.
Edited by: Charles60 on Apr 8, 2008 2:56 AM

Answer 1

How does the java.io.ObjectInputStream class know the class has been changed?It doesn't, unless you've altered the serialVersionUID.
So any new fields are set to their default values: zero, boolean, or null.