My Obsidian Notes

Transient Keyword

2 min read

In Java, transient is a keyword used in the context of serialization. It prevents a field from being serialized when an object is converted into a byte stream.

  • Used to mark fields that should not be saved
  • Commonly used for sensitive data (e.g., passwords)
  • Ignored by the serialization process

Example: Using transient in Java

import java.io.*;
 
class User implements Serializable {
    private static final long serialVersionUID = 1L;
 
    String username;
    transient String password;  // Will not be serialized
 
    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }
}
 
public class TransientExample {
    public static void main(String[] args) throws Exception {
        User user = new User("Alice", "Secret123");
 
        // Serialize object
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("user.ser"));
        oos.writeObject(user);
        oos.close();
 
        // Deserialize object
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("user.ser"));
        User deserializedUser = (User) ois.readObject();
        ois.close();
 
        System.out.println("Username: " + deserializedUser.username); // Alice
        System.out.println("Password: " + deserializedUser.password); // null (because it's transient)
    }
}

Output:

Username: Alice  
Password: null

Why use it?

  • Security: Prevents sensitive information (passwords, cryptographic keys) from being exposed in serialized data.
  • Efficiency: Excludes large or non-serializable fields (like database connections, threads) from serialization.
  • Avoiding Redundant Data: Fields that can be recalculated (e.g., cache values) don’t need to be stored.

Important Notes

  • transient fields are assigned default values when deserialized:
    • null for objects
    • 0 for numeric types
    • false for booleans
  • static fields are also not serialized, so marking them transient is redundant.
  • Custom serialization (writeObject() and readObject()) can be used to handle transient fields manually.

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