Java – Transient keyword with Serialization

In this article, we will discuss transient keyword or modifier with serialization in detail

Whenever, we talk about Serialization then definitely there will be loads of questions on transient keyword

Also, it’s one of the favorite interview questions in Java

1. Serialization process:

  • During serialization process i.e.; saving state of an Object to File, all instance variables will be participated and persisted to file storage
  • variables with static modifier will be left out

Q) What if we don’t want to serialize specific variable/attributes for security  reasons ?

  • The answer is to declare that member variable with transient modifier
  • Yes, we can stop persisting specific variable during serialization process by declaring transient modifier for that specific variable

2. transient keyword:

  • Transient keyword or modifier is applicable only for variables
  • We can stop persisting specific variable, by declaring transient keyword
  • During serialization, JVM ignores the original value of transient variable and saves default value to file
  • Examples: Customer SSN or password need not to be stored. Hence, it’s a good practice to declare those variables as transient
  • So whenever we encounter transient keyword, it means that not to serialize

3. Demo example on transient keyword

  • For objects to participate in serialization & de-serialization process, corresponding class should implement interface
  • Otherwise, NotSerializableException will be thrown

Step 1: Create POJO which implements interface

  • In Customer POJO, there are 4 member variables with customerSSN declared with transient keyword
  • Which means, during serialization instead of original value, default value will be saved to file
  • This can be proved by de-serializing serialized object

package in.bench.resources.serialization;


public class Customer implements Serializable {

	// member variables
	int customerId;
	String customerName;
	int customerAge;
	transient int customerSSN;

	// 4-arg parameterized constructor
	public Customer(int customerId, String customerName,
			int customerAge, int customerSSN) {
		this.customerId = customerId;
		this.customerName = customerName;
		this.customerAge = customerAge;
		this.customerAge = customerAge;

	// overriding toString() method
	public String toString() {
		return "Customer ["
				+ "customerId=" + customerId
				+ ", customerName=" + customerName
				+ ", customerAge="  + customerAge
				+ ", customerSSN=" + customerSSN
				+ "]";

Step 2: Main program to demonstrate serialization/de-serialization

  • To Serialize any Object, we can use ObjectOutputStream & FileOutputStream to write/save to file in binary format
  • To De-Serialize any Object, we can use ObjectInputStream & FileInputStream to read/restore from file (which is in binary format) into Java heap memory

package in.bench.resources.serialization;


public class TransientDemo {

	public static void main(String[] args) {

		// create an customer instance using 4-arg constructor
		Customer serializeCustomer =
				new Customer(102, "SR", 17, 112569);

		// creating output stream variables
		FileOutputStream fos = null;
		ObjectOutputStream oos = null;

		// creating input stream variables
		FileInputStream fis = null;
		ObjectInputStream ois = null;

		// creating customer object reference
		// to hold values after de-serialization
		Customer deSerializeCustomer = null;

		try {
			// for writing or saving binary data
			fos = new FileOutputStream("Customer.ser");

			// converting java-object to binary-format
			oos = new ObjectOutputStream(fos);

			// writing or saving customer object's value to stream

			System.out.println("Serialization success: Customer "
					+ "object saved to Customer.ser file\n");

			// reading binary data
			fis = new FileInputStream("Customer.ser");

			// converting binary-data to java-object
			ois = new ObjectInputStream(fis);

			// reading object's value and casting to Customer class
			deSerializeCustomer = (Customer) ois.readObject();

			System.out.println("De-Serialization success: Customer "
					+ "object de-serialized from Customer.ser file\n");
		catch (FileNotFoundException fnfex) {
		catch (IOException ioex) {
		catch (ClassNotFoundException ccex) {

		// printing customer object to console using toString() method
		System.out.println("Printing customer values from "
				+ "de-serialized object... \n" + deSerializeCustomer);


Serialization success: Customer object saved to Customer.ser file

De-Serialization success: Customer object de-serialized
from Customer.ser file

Printing customer values from de-serialized object...
Customer [customerId=102, customerName=SR, customerAge=17, customerSSN=0]


  • In above Customer POJO, customerSSN declared as transient
  • So during serialization process, original value of customerSSN won’t be saved to file
  • Instead default value will be saved (i.e.; 0 for int, null for String, etc)
  • 1st half of the program illustrate serialization process
  • And 2nd half deals with de-serialization process, which de-serializes the serialized Object
  • While de-serializing all instance member values are re-stored back perfectly except for customerSSN
  • Reason: because this is marked with transient keyword


  • So, by declaring instance variable with transient keyword, we can restrict to store/save that particular variable into file storage during serialization process
  • And it depends purely on business requirement that, which all instance variables need to be restricted

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Happy Coding !!
Happy Learning !!

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