Static variables are never saved during Serialization, yet my code seems to do the opposite

[Sandra Bachan]

Hello,

I just finished Chapter 6 of Sierra/Bates, and now I am on Chapter 7 that begins its discussion with Hashing. I modified the following code, based on Chapter 7:

import java.io.*;

class SaveMe implements Serializable{
	transient int x;
	static int y;
	
	SaveMe(int xVal){
		x = xVal;
		y = 5;
	}
	
	public int hashCode(){
		return (x ^ y);
	}
	
	public boolean equals(Object o){
		SaveMe test = (SaveMe)o;
		if (test.y == y && test.x == x){
			return true;
		}else{
			return false;
		}
	}
	
	public static void main(String[] args){
		SaveMe saveIt = new SaveMe(4);
		saveIt.x = 45;
		saveIt.y = 50;
		System.out.println(saveIt.x + " " + saveIt.y);
		try{
			FileOutputStream fs = new FileOutputStream("testser.txt");
			ObjectOutputStream os = new ObjectOutputStream(fs);
			os.writeObject(saveIt);
			os.close();
		}catch(Exception e){System.out.println("Failed to serialize");}
		
		try{
			FileInputStream fis = new FileInputStream("testser.txt");
			ObjectInputStream ois = new ObjectInputStream(fis);
			saveIt = (SaveMe) ois.readObject();
			ois.close();
		}catch(Exception e){System.out.println("Failed to deserialize");}
		
		System.out.println(saveIt.x + " " + saveIt.y);
	}
}

My output is:

45 50
0 50



I would think that the output is:

45 50
0 5


because in deserialization, the constructor is called, and y gets it's static value of 5. Please explain.

[Jim Hoglund]

The constructor is not called for your 'Serializable' class. If the JVM must reload
the class, however, the statics will revert to their initialized values. Otherwise, if
you serialize and then immediately deserialize, static values are untouched.

Jim ... ...

[Tom Reilly]

When recreating an instance from a serialized object, the constructor with no parameters is used. Because you did not specify this constructor, the compiler creates one for you.

EDIT: I tried my own test (creating a constructor with no parameters) and found I was wrong. Looks like it's back to reading the book for me :-)

[Sandra Bachan]

Where can I find an example program that demonstrates static variables reverting to their original values after class is de-serialized. Or better yet, how can I modify the code I posted to perform this action. Please guide.

[Unmesh Chowdhury]

Hi Sandra, Jim Hoglund is correct. You can decompose your code into three parts as follows:

1st Part:

  
 import java.io.*;  
   
 class SaveMe implements Serializable{  
     transient int x;  
     static int y;  
       
     SaveMe(int xVal){  
         x = xVal;  
         y = 5;  
     }  
       
     public int hashCode(){  
         return (x ^ y);  
     }  
       
     public boolean equals(Object o){  
         SaveMe test = (SaveMe)o;  
         if (test.y == y && test.x == x){  
             return true;  
         }else{  
             return false;  
         }  
     }  
 }
  

2nd Part:
(As separate compilation unit)

  
import java.io.*;

public class TestSaveMe {
     public static void main(String[] args){  
         SaveMe saveIt = new SaveMe(4);  
         saveIt.x = 45;  
         saveIt.y = 50;  
         System.out.println(saveIt.x + " " + saveIt.y);  
         try{  
             FileOutputStream fs = new FileOutputStream("testser.txt");  
             ObjectOutputStream os = new ObjectOutputStream(fs);  
             os.writeObject(saveIt);  
             os.close();  
         }catch(Exception e){System.out.println("Failed to serialize");}  
     }
}
  

3rd Part:
(As individual CU)

    
import java.io.*;

public class TestSaveMe2 {
	public static void main(String[] args) {
		SaveMe saveIt = null;
	     try{  
             FileInputStream fis = new FileInputStream("testser.txt");  
             ObjectInputStream ois = new ObjectInputStream(fis);  
             saveIt = (SaveMe) ois.readObject();  
             ois.close();  
         }catch(Exception e){System.out.println("Failed to deserialize");}  
           
         System.out.println(saveIt.x + " " + saveIt.y);  
     }  
 }

I think, you can experiment now.

[Sandra Bachan]

Thank you for the code to experiment with, now I understand better!