Object Serialization: Using Transient and serialVersionUID

This topic is part of Java Serialization and explains two important concepts used when serializing objects:

1. transient keyword
2. serialVersionUID (Version UID)

Both help control how objects are stored and restored during serialization.

transient Keyword

The transient keyword is used to skip a field during serialization.

Prevents sensitive or unnecessary data from being saved in a serialized file.

Example

import java.io.Serializable;

class User implements Serializable {
    String username;
    transient String password;   // will NOT be serialized
}

When serialized, password will be stored as null (default value).

Why Do We Use transient?

To prevent serialization of:

• Sensitive data (passwords, OTP, tokens)
• Derived values (calculated fields)
• Temporary or cache-like data
• Fields not needed after deserialization

What Happens Without transient?

Without transient, every non-static field is serialized, which may cause:

• Security issues (saving passwords)
• Performance issues (saving unnecessary fields)
• Exceptions (if the field type is not serializable)

Key Points About transient

• Used to exclude fields from serialization
• Static fields are automatically not serialized
• transient + static = still NOT serialized
• Useful for security and performance

serialVersionUID (Object Version Control)

serialVersionUID is a unique identifier for a class that is used during serialization and deserialization.

It ensures that the sender and receiver of a serialized object have the same class structure.

If not, it throws:

InvalidClassException

To avoid this, we define a fixed serialVersionUID.

Example

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

    int id;
    String name;
}

Why Manually Declare serialVersionUID?

If not declared manually:

• JVM auto-generates it based on class structure
• Small changes break compatibility
• Old serialized files become unreadable

Declaring your own ID solves this.

When to Change the serialVersionUID?

Change only when:

• The class structure changes in a way that breaks compatibility
• Old serialized objects should no longer work

Don't change for minor edits like comments, formatting, etc.

Putting It All Together

class Employee implements Serializable {
    private static final long serialVersionUID = 100L;

    int id;
    String name;

    transient double salary;   // not serialized
}

• id, name → serialized
• salary → skipped

Difference Between transient and serialVersionUID