Serialization in java — Inheritance

Serialization is a mechanism of converting the state of an object into a byte stream. Deserialization is the reverse process where the byte stream is used to recreate the actual Java object in memory. This mechanism is used to persist the object.

There are some cases of Serialization with respect to inheritance :

  1. Case 1: If superclass is serializable then subclass is automatically serializable : If superclass is Serializable, then by default every subclass is serializable. Hence, even though subclass doesn’t implement Serializable interface( and if it’s superclass implements Serializable), then we can serialize subclass object.

// Java program to demonstrate
// that if superclass is
// serializable then subclass
// is automatically serializable

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

// superclass A
// implementing Serializable interface
class A implements Serializable
{
int i;

// parameterized constructor
public A(int i)
{
this.i = i;
}

}

// subclass B
// B class doesn’t implement Serializable
// interface.
class B extends A
{
int j;

// parameterized constructor
public B(int i, int j)
{
super(i);
this.j = j;
}
}

// Driver class
public class Test
{
public static void main(String[] args)
throws Exception
{
B b1 = new B(10,20);

System.out.println(“i = “ + b1.i);
System.out.println(“j = “ + b1.j);

/* Serializing B’s(subclass) object */

//Saving of object in a file
FileOutputStream fos = new FileOutputStream(“abc.ser”);
ObjectOutputStream oos = new ObjectOutputStream(fos);

// Method for serialization of B’s class object
oos.writeObject(b1);

// closing streams
oos.close();
fos.close();

System.out.println(“Object has been serialized”);

/* De-Serializing B’s(subclass) object */

// Reading the object from a file
FileInputStream fis = new FileInputStream(“abc.ser”);
ObjectInputStream ois = new ObjectInputStream(fis);

// Method for de-serialization of B’s class object
B b2 = (B)ois.readObject();

// closing streams
ois.close();
fis.close();

System.out.println(“Object has been deserialized”);

System.out.println(“i = “ + b2.i);
System.out.println(“j = “ + b2.j);
}
}

Output:

i = 10
j = 20
Object has been serialized
Object has been deserialized
i = 10
j = 20

If a superclass is not serializable then subclass can still be serialized : Even though superclass doesn’t implements Serializable interface, we can serialize subclass object if subclass itself implements Serializable interface. So we can say that to serialize subclass object, superclass need not to be serializable. But what happens with the instances of superclass during serialization in this case. The following procedure explain this.

What happens when a class is serializable but its superclass is not ?

  • Serialization: At the time of serialization, if any instance variable is inheriting from non-serializable superclass, then JVM ignores original value of that instance variable and save default value to the file.
  • De- Serialization: At the time of de-serialization, if any non-serializable superclass is present, then JVM will execute instance control flow in the superclass. To execute instance control flow in a class, JVM will always invoke default(no-arg) constructor of that class. So every non-serializable superclass must necessarily contain default constructor, otherwise we will get runtime-exception.

// Java program to demonstrate

// the case if superclass need

// not to be serializable

// while serializing subclass

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

import java.io.Serializable;;

// superclass A

// A class doesn't implement Serializable

// interface.

class A

{

int i;

// parameterized constructor

public A(int i)

{

this.i = i;

}

// default constructor

// this constructor must be present

// otherwise we will get runtime exception

public A()

{

i = 50;

System.out.println("A's class constructor called");

}

}

// subclass B

// implementing Serializable interface

class B extends A implements Serializable

{

int j;

// parameterized constructor

public B(int i,int j)

{

super(i);

this.j = j;

}

}

// Driver class

public class Test

{

public static void main(String[] args)

throws Exception

{

B b1 = new B(10,20);

System.out.println("i = " + b1.i);

System.out.println("j = " + b1.j);

// Serializing B's(subclass) object

//Saving of object in a file

FileOutputStream fos = new FileOutputStream("abc.ser");

ObjectOutputStream oos = new ObjectOutputStream(fos);

// Method for serialization of B's class object

oos.writeObject(b1);

// closing streams

oos.close();

fos.close();

System.out.println("Object has been serialized");

// De-Serializing B's(subclass) object

// Reading the object from a file

FileInputStream fis = new FileInputStream("abc.ser");

ObjectInputStream ois = new ObjectInputStream(fis);

// Method for de-serialization of B's class object

B b2 = (B)ois.readObject();

// closing streams

ois.close();

fis.close();

System.out.println("Object has been deserialized");

System.out.println("i = " + b2.i);

System.out.println("j = " + b2.j);

}

}

Output:

i = 10
j = 20
Object has been serialized
A's class constructor called
Object has been deserialized
i = 50
j = 20

If the superclass is serializable but we don’t want the subclass to be serialized : There is no direct way to prevent subclass from serialization in java. One possible way by which a programmer can achieve this is by implementing the writeObject() and readObject() methods in the subclass and needs to throw NotSerializableException from these methods. These methods are executed during serialization and de-serialization respectively. By overriding these methods, we are just implementing our own custom serialization.

// Java program to demonstrate

// how to prevent

// subclass from serialization

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.NotSerializableException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

import java.io.Serializable;

// superclass A

// implementing Serializable interface

class A implements Serializable

{

int i;

// parameterized constructor

public A(int i)

{

this.i = i;

}

}

// subclass B

// B class doesn't implement Serializable

// interface.

class B extends A

{

int j;

// parameterized constructor

public B(int i,int j)

{

super(i);

this.j = j;

}

// By implementing writeObject method,

// we can prevent

// subclass from serialization

private void writeObject(ObjectOutputStream out) throws IOException

{

throw new NotSerializableException();

}

// By implementing readObject method,

// we can prevent

// subclass from de-serialization

private void readObject(ObjectInputStream in) throws IOException

{

throw new NotSerializableException();

}

}

// Driver class

public class Test

{

public static void main(String[] args)

throws Exception

{

B b1 = new B(10, 20);

System.out.println("i = " + b1.i);

System.out.println("j = " + b1.j);

// Serializing B's(subclass) object

//Saving of object in a file

FileOutputStream fos = new FileOutputStream("abc.ser");

ObjectOutputStream oos = new ObjectOutputStream(fos);

// Method for serialization of B's class object

oos.writeObject(b1);

// closing streams

oos.close();

fos.close();

System.out.println("Object has been serialized");

// De-Serializing B's(subclass) object

// Reading the object from a file

FileInputStream fis = new FileInputStream("abc.ser");

ObjectInputStream ois = new ObjectInputStream(fis);

// Method for de-serialization of B's class object

B b2 = (B)ois.readObject();

// closing streams

ois.close();

fis.close();

System.out.println("Object has been deserialized");

System.out.println("i = " + b2.i);

System.out.println("j = " + b2.j);

}

}