Java.Serialization.What does the transient keyword mean?

What does the transient keyword mean?

📘 Syntax

transient int secretCode;

🎯 Why Use transient?

Use Case	Why transient Helps
Sensitive data	Don’t serialize passwords, tokens, keys
Large or cache-only fields	Don’t waste space storing something you can rebuild
Non-serializable fields	Avoid exceptions for things like threads, sockets, or DB connections
Temporary runtime data	Timestamp, session ID, logs, etc. that don’t need to persist

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🔧 Example

import java.io.*;

public class User implements Serializable {
    private static final long serialVersionUID = 1L;

    String username;
    transient String password; // excluded!

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

➕ Serialize:

ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("user.ser"));
out.writeObject(new User("Stanley", "mySecret123"));
out.close();

➖ Deserialize:

ObjectInputStream in = new ObjectInputStream(new FileInputStream("user.ser"));
User u = (User) in.readObject();
System.out.println(u.username); // "Stanley"
System.out.println(u.password); // null (not serialized) ✅

⚠️ What Value Does a transient Field Have After Deserialization?

Type	Value
String, Object	null
int, long	0
boolean	false

You can re-initialize them in:

• A constructor
• A custom readObject() method

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🧵 TL;DR

Feature	Meaning
transient	Prevents a field from being serialized
Used for	Sensitive, temporary, or non-serializable data
Value after deserialization	Default Java value (null, 0, false)
Applies to	Fields only (not classes or methods)