Java Transient Keyword

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In Java the transient keyword indicates to the serialization mechanism to leave a specific variable out of the serialization process.

Syntax

 transient dataType variableName;
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Transient fields are not included in the byte stream when an object gets serialized.

Why Use transient?

  • Transient should be used to prevent sensitive data such as passwords and PINs from being stored.
  • The transient keyword allows developers to bypass serializing temporary or computed values.
  • You can decrease the size of a serialized object by using transient

Example

import java.io.*;

class User implements Serializable {
    String username;
    transient String password; // will not be saved

    User(String username, String password) {
        this.username = username;
        this.password = password;
    }
}

Example

public class TransientDemo {
    public static void main(String[] args) throws Exception {
        User user = new User("john_doe", "secret123");

        // Serialize
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user.ser"));
        oos.writeObject(user);
        oos.close();

        // Deserialize
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user.ser"));
        User u = (User) ois.readObject();
        ois.close();

        System.out.println("Username: " + u.username);
        System.out.println("Password: " + u.password); // null
    }
}

output

Username: john_doe
Password: null
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The password was not serialized, so it's null after deserialization.

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