Skip to main content

7.4 Serialization Fundamentals & the serialVersionUID Contract

← Back to Chapter 7: Garbage Collection & Serialization

Serialization transforms an in-memory Java object graph into a flat sequence of binary bytes for disk persistence or transmission over a network socket. Deserialization reconstructs that live object graph from the byte stream.

What is Serialization?​

Serialization means converting a Java object into a format that can be stored or transferred.

Think of it like this:

Java Object
    ↓
Serialization
    ↓
Bytes
    ↓
Store / Send over Network

Later, we can convert those bytes back into the original Java object.

Bytes
   ↓
Deserialization
   ↓
Java Object

Why do we need Serialization?​

There are mainly two common reasons.

1. Store an object​

For example, you want to save an object into a file.

Java Object → Bytes → File

2. Transfer an object​

For example, one Java application needs to send object data to another system.

Application A
     ↓
 Object
     ↓
 Serialization
     ↓
 Network
     ↓
 Deserialization
     ↓
Application B

How do we serialize an object?​

Java provides: Serializable

It's a marker interface, meaning it doesn't contain any methods and thus it doesn't require you to implement any methods.

Example:

import java.io.Serializable;

class Employee implements Serializable {
    // Serializable tells Java that objects of this class
    // are allowed to be serialized.

    private String name;
    private int age;

    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Then Java's serialization APIs can serialize the object.​

Writing and Reading Streams​

// Serialization
try (var oos = new ObjectOutputStream(new FileOutputStream("session.ser"))) {
    oos.writeObject(new UserSession("user_101", "ADMIN"));
}

// Deserialization
try (var ois = new ObjectInputStream(new FileInputStream("session.ser"))) {
    UserSession session = (UserSession) ois.readObject();
}

What is serialVersionUID?​

You can define:

private static final long serialVersionUID = 1L;

Example:

class Employee implements Serializable {

    // Used to verify that the serialized object
    // and current class version are compatible.
    private static final long serialVersionUID = 1L;

    private String name;
    private int age;
}

Think of it as a version number for the serialized class.

Object saved with version 1
          ↓
Class changed
          ↓
Still compatible?
          ↓
serialVersionUID helps JVM check this

If the versions don't match, you can get:

InvalidClassException