Static values don't get serialized, yet the code seems to show the contrary

[Sandra Bachan]

Sierra/Bates, Chapter 6, Question 8

import java.io.*;

public class TestSer {
  public static void main(String[] args) {
    SpecialSerial s = new SpecialSerial();
    try {
      ObjectOutputStream os = new ObjectOutputStream(
         new FileOutputStream("myFile"));
      os.writeObject(s);  os.close();
      System.out.print(++s.z + " ");

      ObjectInputStream is = new ObjectInputStream(
         new FileInputStream("myFile"));
      SpecialSerial s2 = (SpecialSerial)is.readObject();
      is.close();
      System.out.println(s2.y + " " + s2.z);
    } catch (Exception x) {System.out.println("exc"); }
  }
}
class SpecialSerial implements Serializable {
  transient int y = 7;
  static int z = 9;
}

Choices (select all that apply)
1. Compilation fails

2. The output is 10 0 9

3. The output is 10 0 10

4. The output is 10 7 9

5. The output is 10 7 10

6. In order to alter the standard deserialization process you would implement the readObject() method in SpecialSerial

7. In order to alter the standard deserialization process you would implement the defaultReadObject() method in SpecialSerial


According to the book, 3. and 6 are correct. 3 is correct because static and transient variables are not serialized when an object is serialized. 6 is a valid statement.

To me, it seems that static IS serialized because the original value of 10 is restored. On line 22 if I remove static from int z = 9, I get the following output:

10 0 9


Now, z is NOT serialized.

I'm puzzled....

[David Newton]

But... static variables belong to the class definition itself, right?

Change it to something *crazy* after you write it, like 1776. Then see what's printed out after it's read back in... If what you're saying is true, then it should be read in as the same value that was written: but what's actually there is the "crazy" value the variable was changed to after writing. And that's the point of the example--what's "being written" (in your view, it isn't really) is the value 9: *before* the increment. What's "being read" (but isn't really) is 10, which is the original value (what you believe was written) +1.

In a nutshell, the original value is *9*, not 10. It's only 10 after it's incremented, which is happening *after* the write.

[Sandra Bachan]

David Newton wrote:But... static variables belong to the class definition itself, right?

Change it to something *crazy* after you write it, like 1776. Then see what's printed out after it's read back in... If what you're saying is true, then it should be read in as the same value that was written: but what's actually there is the "crazy" value the variable was changed to after writing. And that's the point of the example--what's "being written" (in your view, it isn't really) is the value 9: *before* the increment. What's "being read" (but isn't really) is 10, which is the original value (what you believe was written) +1.

In a nutshell, the original value is *9*, not 10. It's only 10 after it's incremented, which is happening *after* the write.

I think I understand, especially when you said static variables belong to class definition.

Just to help others, I wrote two sets of code, along with output:

Serialization of a static variable:

import java.io.*;

public class TestSer {
  public static void main(String[] args) {
    SpecialSerial s = new SpecialSerial();
    try {
      System.out.println("Before serialization");
      System.out.println("z = " + s.z);
      ObjectOutputStream os = new ObjectOutputStream(
         new FileOutputStream("myFile"));
      os.writeObject(s);  os.close();
      s.z=1776;
      System.out.println("After serialization");
      System.out.println("z = " + s.z);

     	
      ObjectInputStream is = new ObjectInputStream(
         new FileInputStream("myFile"));
      SpecialSerial s2 = (SpecialSerial)is.readObject();
      is.close();
      System.out.println("After deserialization");
      System.out.println("z = " + s2.z);
    } catch (Exception x) {System.out.println("exc"); }
  }
}
class SpecialSerial implements Serializable {
  transient int y = 7;
  static int z = 9;
}

Output:

Before serialization
z = 9
After serialization
z = 1776
After deserialization
z = 1776


Serialization of a non-static variable

import java.io.*;

public class TestSer {
  public static void main(String[] args) {
    SpecialSerial s = new SpecialSerial();
    try {
      System.out.println("Before serialization");
      System.out.println("z = " + s.z);
      ObjectOutputStream os = new ObjectOutputStream(
         new FileOutputStream("myFile"));
      os.writeObject(s);  os.close();
      s.z=1776;
      System.out.println("After serialization");
      System.out.println("z = " + s.z);

     	
      ObjectInputStream is = new ObjectInputStream(
         new FileInputStream("myFile"));
      SpecialSerial s2 = (SpecialSerial)is.readObject();
      is.close();
      System.out.println("After deserialization");
      System.out.println("z = " + s2.z);
    } catch (Exception x) {System.out.println("exc"); }
  }
}
class SpecialSerial implements Serializable {
  transient int y = 7;
  int z = 9;
}

Before serialization
z = 9
After serialization
z = 1776
After deserialization
z = 9

[David Newton]

   I think you've got it!