Serialization

[Anit Nair]

As per K&B serialization is not for static variables....but i tried out following piece of code and it shows that static variables are also serialized and saved as part of object state..I m a bit confused now. Please find below the code i executed

import java.io.*;
class tempObject
{
int i;
String name;
tempObject(int i,String name)
{
this.i=i;
this.name=name;
}
}
 
class DemoSerial implements Serializable
{
static int i=5;
static tempObject t_obj;
String s1="Manoj";

}
 
class ch6_ex4
{
public static void main(String args[])
{
try
{
DemoSerial obj = new DemoSerial();
DemoSerial obj2 = new DemoSerial();
obj.i=6;
obj.t_obj= new tempObject(12,"Sunil");
obj2.i=7;
obj2.t_obj =  new tempObject(14,"Vinod");
FileOutputStream fos =  new FileOutputStream("testser.ser");
ObjectOutputStream os=  new ObjectOutputStream(fos);
os.writeObject(obj);
os.close();
}catch(Exception ex)
{}
try
{
FileInputStream fis= new FileInputStream("testser.ser");
ObjectInputStream is = new ObjectInputStream(fis);
DemoSerial obj=(DemoSerial)is.readObject();
is.close();
System.out.println("i= "+obj.i);
System.out.println("S1="+obj.s1);
System.out.println("Temp i="+obj.t_obj.i);
System.out.println("Temp name="+obj.t_obj.name);

}catch(Exception ex)
{}


}
}

Also, if i add static to transient member variables the member variables are still serialized..shouldnt adding transient member variables be skipped
while serialization???
[ May 09, 2008: Message edited by: Anit Nair ]
[ May 09, 2008: Message edited by: Anit Nair ]

[sachin verma]

Please always use code tag

see this Code Tag
[ May 09, 2008: Message edited by: sachin verma ]

[sridhar row]

Hi Anit, if you run your program you can see that the object you are serializing is obj and its instance value is 6 but when you de-serialize the object the value of i is changed to 7. So in effect serialization failed for the obj instance. It didn't save the obj's instance value.

[Lucy Smith]

Sridhar,

That's not correct. Since those variables are defined as static, it doesn't matter what the instance is. After de-serialization the name being obj is just shadowing.

I think Anit has a point and will wait for some expert's take on this.

[Anit Nair]

Hi Sridhar, thats exactly what i want to say....static member are not part of objects state....then why is the value for it stored in the first place...
If you refer the K&B pg 456 under topic [Serialization not for statics]
The last line clearly states "Static variables are NEVER SAVED as part of object state...because they do not belong to the object". Then why is 7 saved in this case...

[sridhar row]

Since those variables are defined as static, it doesn't matter what the instance is

Yes thats the whole point, Serialization is saving an objects state and not a class's state. When you have static variables they are part of the class and not any particular instance. Serialization applies only to objects.

After de-serialization the name being obj is just shadowing.

The var name obj is the same name but it is not shadowing. The object is deserialized and casted to DemoSerial.

The last line clearly states "Static variables are NEVER SAVED as part of object state...because they do not belong to the object". Then why is 7 saved in this case

Yes Static variables are NEVER SAVED as part of object state. 7 is saved because you changed the static variable value using an instance of another object (obj2) and you are trying to save the object referred by obj.

Hope this below code helps

  public class TestStaticVar{
 
 static int i = 10;
 
 
 public static void main(String[] args){
         
     TestStaticVar t = new TestStaticVar();
     t.i = 25;
     System.out.println(t.i);//prints 25
     
     TestStaticVar t1 = new TestStaticVar();
     t1.i = 50;
     System.out.println(t.i);//prints 50
 }
 
}  

but if you change int i from static to non-static

  
 
public class TestStaticVar{
 
 int i = 10;
 
 
 public static void main(String[] args){
         
     TestStaticVar t = new TestStaticVar();
     t.i = 25;
     System.out.println(t.i);//prints 25
     
     TestStaticVar t1 = new TestStaticVar();
     t1.i = 50;
     System.out.println(t.i);//prints 25
 }
 
}  

[ May 09, 2008: Message edited by: sridhar row ]

[Anit Nair]

Hi Sridhar, I guess i have understood....whats happening in this case. Actually, the value is never getting stored at all, i guess. Its just that when you de-serialize and cast the object to (DemoSerial), when you execute the following line

  System.out.println("i= "+obj.i);      

Its similar to calling DemoSerial.i hence the value 7. It just prints the current value of i. not the value belonging to any instance of DemoSerial. Thanks for your inputs

[sridhar row]

you are right!!