Serialization is not for staitics

[Jerry Ragland]

I found this topic in K&B book during study. To understand what will be the behaviour if a static variable is includede I wrote a code with a static variable. I thought the value will not be include and will give default value to my surpirse it returned the value correctly.

import java.io.*;
 
class serial implements Serializable {
 static int i = 10;
 public void display() {
  System.out.println("The integer value "+ ++i);
 }
}
 
class DoSerial {
 
  public static void main(String[] args) {
    serial s = new serial();
 
    s.display();
    try {
     FileOutputStream fs = new FileOutputStream("ser.txt");
     ObjectOutputStream os = new ObjectOutputStream(fs);
     os.writeObject(s);
     os.close();
    }catch(Exception e) {}
    
    try{
     FileInputStream fi = new FileInputStream("ser.txt");
     ObjectInputStream is = new ObjectInputStream(fi);
     s = (serial)is.readObject();
     is.close();
    }catch(Exception e) {}
 
    System.out.println("The integer value with serialized object "+s.i);
  }
}
 
Answer - 
The integer value 11
The integer value with serialized object 11
 

If I get the o/p like this why Serialization can't be applied for staics.

Thanks in advance.
-Jerry.

[Barry Gaunt]

Try it in two different programs, the first to serialize the object and the second to get it back in again...

[Chris Hurst]

or just do another s.display (); between writing to ser.txt and reading back in the results, this does another i++;

... code snippet follows ...

FileOutputStream fs = new FileOutputStream("ser.txt");     
 ObjectOutputStream os = new ObjectOutputStream(fs);     
 os.writeObject(s);     
 os.close();    
}
catch(Exception e) {}   
 
[B]s.display(); [/B]
 
try
{     
FileInputStream fi = new FileInputStream("ser.txt");

... code continues ...

giving this output ...

The integer value 11
The integer value 12
The integer value with serialized object 12

which basically proves the static wasn't written to ser.txt as when you read it back it should have gone back to 11 but its not stored in the file so it doesn't.

[Jerry Ragland]

Thanks Chris, for the explanation. So when a static variable is serailized it will take the current value of the variale and store it. But we can't rely on this stored value as when retrived back it will again show the current value and not the previously recorded value.

-Jerry.

[Barry Gaunt]

I still recommend that you try two different programs.

[Jerry Ragland]

Originally posted by Barry Gaunt:
I still recommend that you try two different programs.

Barry tried with 2 different programs -

import java.io.*;
 
class serial implements Serializable {
 static int i = 10;
 public void display() {
  System.out.println("The integer value "+ ++i);
 }
}
 
class DoSerial {
 
  public static void main(String[] args) {
    serial s = new serial();
 
    s.display();
    try {
     FileOutputStream fs = new FileOutputStream("ser.txt");
     ObjectOutputStream os = new ObjectOutputStream(fs);
     os.writeObject(s);
     os.close();
    }catch(Exception e) {}
 
  }
}
 
class DoSerial2 {
  public static void main(String[] args) {
    serial s = null;
    try{
     FileInputStream fi = new FileInputStream("ser.txt");
     ObjectInputStream is = new ObjectInputStream(fi);
     s = (serial)is.readObject();
     is.close();
    }catch(Exception e) {}
 
    System.out.println("The integer value with serialized object "+s.i);
  }
}
 
C:\JDK5~1\bin>java DoSerial
The integer value 11
 
C:\JDK5~1\bin>java DoSerial2
The integer value with serialized object 10
 

I am finding it hard to conclude. Could you please help me in concluding the behaviour with statics.

Regards,
-Jerry.

[Henry Wong]

So when a static variable is serailized it will take the current value of the variale and store it.

No... as you have previously mentioned, static variables which are part of the serialization process -- they will not be stored.

But we can't rely on this stored value as when retrived back it will again show the current value and not the previously recorded value.

No... since static variables are not serialized, they will also *not* be deserialized, reliablly or otherwise. Remember, there is only *one* copy of a static variable, so in your first example, the static variable is correct, not because it was deserialized correctly, but because you didn't change the value, period.

Henry

[Jerry Ragland]

Henry, I don't understand why it behaves differently when in a same program and in a different program.

import java.io.*;
 
class serial implements Serializable {
 static int i = 10;
 public void display() {
  System.out.println("The integer value "+ ++i);
 }
}
 
class DoSerial {
 
  public static void main(String[] args) {
    serial s = new serial();
 
    s.display();
    try {
     FileOutputStream fs = new FileOutputStream("ser.txt");
     ObjectOutputStream os = new ObjectOutputStream(fs);
     os.writeObject(s);
     os.close();
    }catch(Exception e) {}
 
    s.display();
 
    try{
     FileInputStream fi = new FileInputStream("ser.txt");
     ObjectInputStream is = new ObjectInputStream(fi);
     s = (serial)is.readObject();
     is.close();
    }catch(Exception e) {}
 System.out.println("The integer value with serialized object "+s.i);
  }
}
 
class DoSerial2 {
  public static void main(String[] args) {
    serial s = null;
    try{
     FileInputStream fi = new FileInputStream("ser.txt");
     ObjectInputStream is = new ObjectInputStream(fi);
     s = (serial)is.readObject();
     is.close();
    }catch(Exception e) {}
 
    System.out.println("The integer value with serialized object "+s.i);
  }
}
 
C:\JDK5~1\bin>java DoSerial
The integer value 11
The integer value 12
The integer value with serialized object 12
 
C:\JDK5~1\bin>java DoSerial2
The integer value with serialized object 10

When I deserialize the object in the same program from where I serialized it I get result as 12. When I again deserialize the same file in a seperate class I get a different result (as 10) ... really confusing


-Jerry

[Barry Gaunt]

When you use a single program the class gets loaded once and stays around within the JVM. When you use two programs the class gets loaded freshly by the second program into a new JVM. That's why you see the static variable at its proper initial value.

[Srinivasan thoyyeti]

Hi Jerry,

Static variables have a single copy per class.
Static variable is a class variable. not instance variable.
We serialize the instance of class. not class.

1)In Program posted first> Imagine that there is no serialization, Now see Is there any problem with output(i) 11. No problem!

2)In the second program>
In DoSerial2 class,
2.1)serial s = null;
2.2)s = (serial)is.readObject();
Above steps cause Class "serial" load again.
At the static variable loads with 10.

Hope you got it.

[victor kamat]

It is interesting to note that for the following class

  
class serial implements Serializable
{
static int i = 10;
static { i = 11; }
{ i = 12; }
 
transient int k = 11;
public String display() { return "The integer value "+ ++i ; }
}
 
  

when it is deserialized (after being serialized) that i = 11 and k = 0;