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. 
The byte stream created is platform independent. So, the object serialized on one platform can be deserialized on a different platform. To make a Java object serializable we implement the java.io.Serializable interface. The ObjectOutputStream class contains writeObject() method for serializing an Object.
public final void writeObject(Object obj)
throws IOException
The ObjectInputStream class contains readObject() method for deserializing an object.
public final Object readObject()
throws IOException,
ClassNotFoundException
Advantages of Serialization
- To save/persist state of an object.
- To travel an object across a network.
Only the objects of those classes can be serialized which are implementing java.io.Serializable interface. Serializable is a marker interface (has no data member and method). It is used to “mark” java classes so that objects of these classes may get certain capability. Other examples of marker interfaces are:- Cloneable and Remote.
Points to remember
1. If a parent class has implemented Serializable interface then child class doesn’t need to implement it but vice-versa is not true. 2. Only non-static data members are saved via Serialization process.
3. Static data members and transient data members are not saved via Serialization process. So, if you don’t want to save value of a non-static data member then make it transient.
4. Constructor of object is never called when an object is deserialized.
5. Associated objects must be implementing Serializable interface. Example :
class A implements Serializable{
// B also implements Serializable
// interface.
B ob=new B();
}
SerialVersionUID The Serialization runtime associates a version number with each Serializable class called a SerialVersionUID, which is used during Deserialization to verify that sender and receiver of a serialized object have loaded classes for that object which are compatible with respect to serialization. If the receiver has loaded a class for the object that has different UID than that of corresponding sender’s class, the Deserialization will result in an InvalidClassException.
A Serializable class can declare its own UID explicitly by declaring a field name. It must be static, final and of type long. i.e- ANY-ACCESS-MODIFIER static final long serialVersionUID=42L; If a serializable class doesn’t explicitly declare a serialVersionUID, then the serialization runtime will calculate a default one for that class based on various aspects of class, as described in Java Object Serialization Specification. However it is strongly recommended that all serializable classes explicitly declare serialVersionUID value, since its computation is highly sensitive to class details that may vary depending on compiler implementations, any change in class or using different id may affect the serialized data. It is also recommended to use private modifier for UID since it is not useful as inherited member. serialver The serialver is a tool that comes with JDK. It is used to get serialVersionUID number for Java classes.
You can run the following command to get serialVersionUID serialver [-classpath classpath] [-show] [classname…] 
Example 1:
Java
// Java code for serialization and deserialization// of a Java objectimport java.io.*;class Demo implements java.io.Serializable{ public int a; public String b; // Default constructor public Demo(int a, String b) { this.a = a; this.b = b; }}class Test{ public static void main(String[] args) { Demo object = new Demo(1, "neveropen"); String filename = "file.ser"; // Serialization try { //Saving of object in a file FileOutputStream file = new FileOutputStream(filename); ObjectOutputStream out = new ObjectOutputStream(file); // Method for serialization of object out.writeObject(object); out.close(); file.close(); System.out.println("Object has been serialized"); } catch(IOException ex) { System.out.println("IOException is caught"); } Demo object1 = null; // Deserialization try { // Reading the object from a file FileInputStream file = new FileInputStream(filename); ObjectInputStream in = new ObjectInputStream(file); // Method for deserialization of object object1 = (Demo)in.readObject(); in.close(); file.close(); System.out.println("Object has been deserialized "); System.out.println("a = " + object1.a); System.out.println("b = " + object1.b); } catch(IOException ex) { System.out.println("IOException is caught"); } catch(ClassNotFoundException ex) { System.out.println("ClassNotFoundException is caught"); } }} |
Output :
Object has been serialized Object has been deserialized a = 1 b = neveropen
Example 2:
Java
// Java code for serialization and deserialization// of a Java objectimport java.io.*;class Emp implements Serializable {private static final long serialversionUID = 129348938L; transient int a; static int b; String name; int age; // Default constructorpublic Emp(String name, int age, int a, int b) { this.name = name; this.age = age; this.a = a; this.b = b; }}public class SerialExample {public static void printdata(Emp object1) { System.out.println("name = " + object1.name); System.out.println("age = " + object1.age); System.out.println("a = " + object1.a); System.out.println("b = " + object1.b); }public static void main(String[] args) { Emp object = new Emp("ab", 20, 2, 1000); String filename = "shubham.txt"; // Serialization try { // Saving of object in a file FileOutputStream file = new FileOutputStream (filename); ObjectOutputStream out = new ObjectOutputStream (file); // Method for serialization of object out.writeObject(object); out.close(); file.close(); System.out.println("Object has been serialized\n" + "Data before Deserialization."); printdata(object); // value of static variable changed object.b = 2000; } catch (IOException ex) { System.out.println("IOException is caught"); } object = null; // Deserialization try { // Reading the object from a file FileInputStream file = new FileInputStream (filename); ObjectInputStream in = new ObjectInputStream (file); // Method for deserialization of object object = (Emp)in.readObject(); in.close(); file.close(); System.out.println("Object has been deserialized\n" + "Data after Deserialization."); printdata(object); // System.out.println("z = " + object1.z); } catch (IOException ex) { System.out.println("IOException is caught"); } catch (ClassNotFoundException ex) { System.out.println("ClassNotFoundException" + " is caught"); } }} |
Output:
Object has been serialized Data before Deserialization. name = ab age = 20 a = 2 b = 1000 Object has been deserialized Data after Deserialization. name = ab age = 20 a = 0 b = 2000
Description for Output: You have seen while deserializing the object the values of a and b has changed. The reason being a was marked as transient and b was static.
In case of transient variables:- A variable defined with transient keyword is not serialized during serialization process.This variable will be initialized with default value during deserialization. (e.g: for objects it is null, for int it is 0).
In case of static Variables:- A variable defined with static keyword is not serialized during serialization process.This variable will be loaded with current value defined in the class during deserialization.
Transient Vs Final:
final variables will be participated into serialization directly by their values.
Hence declaring a final variable as transient there is no use.
//the compiler assign the value to final variable
example:
final int x= 10; int y = 20; System.out.println(x);// compiler will replace this as System.out.println(10)->10 because x is final. System.out.println(y);//20
Example 3:
Java
//java code for final with transientimport java.io.*;class Dog implements Serializable{ int i=10; transient final int j=20;}class GFG { public static void main (String[] args)throws IOException,ClassNotFoundException { Dog d1=new Dog(); //Serialization started System.out.println("serialization started"); FileOutputStream fos= new FileOutputStream("abc.ser"); ObjectOutputStream oos=new ObjectOutputStream(fos); oos.writeObject(d1); System.out.println("Serialization ended"); //Deserialization started System.out.println("Deserialization started"); FileInputStream fis=new FileInputStream("abc.ser"); ObjectInputStream ois=new ObjectInputStream(fis); Dog d2=(Dog) ois.readObject(); System.out.println("Deserialization ended"); System.out.println("Dog object data"); //final result System.out.println(d2.i+"\t" +d2.j); }} |
serialization started Serialization ended Deserialization started Deserialization ended Dog object data 10 20
