What is the difference between Serializable and Externalizable?

Table of Contents

• Introduction
• Differences Between Serializable and Externalizable
  • 1. Interface Definition
  • 2. Serialization Mechanism
  • 3. Performance
  • 4. Default Constructor Requirement
  • 5. Version Control
• Practical Examples
  • Example of Serializable
  • Example of Externalizable
• Conclusion

Introduction

In Java, serialization is the process of converting an object into a byte stream for storage or transmission. Two main interfaces facilitate this process: Serializable and Externalizable. While both are used for object serialization, they have distinct characteristics and use cases. This guide will outline the key differences between Serializable and Externalizable, helping you choose the right approach for your serialization needs.

Differences Between Serializable and Externalizable

1. Interface Definition

• Serializable: This is a marker interface, meaning it does not contain any methods. By implementing the Serializable interface, you indicate that the class can be serialized automatically using the default serialization mechanism provided by Java.
• Externalizable: This interface extends Serializable and requires you to implement two methods: writeExternal(ObjectOutput out) and readExternal(ObjectInput in). This allows you to control the serialization process explicitly.

2. Serialization Mechanism

• Serializable: The serialization process is automatic, meaning Java's default serialization mechanism takes care of writing and reading the object's state. All non-transient fields are serialized by default.
• Externalizable: With Externalizable, you have complete control over the serialization process. You decide which fields to serialize, how to serialize them, and in what order. This can lead to more efficient serialization, especially for large objects.

3. Performance

• Serializable: The default serialization can be less efficient, especially for large object graphs with many references, as it serializes all non-transient fields without optimization.
• Externalizable: Since you can customize the serialization process, Externalizable is generally more performant than Serializable. You can choose to serialize only the necessary fields, potentially reducing the serialized object's size.

4. Default Constructor Requirement

• Serializable: There is no requirement for a default constructor. Java will automatically handle the instantiation of the object during deserialization.
• Externalizable: A class implementing Externalizable must have a public no-argument (default) constructor. This is necessary for the deserialization process to create an object of the class.

5. Version Control

• Serializable: It uses a serialVersionUID for version control. If the class definition changes, it can lead to InvalidClassException during deserialization if the serialVersionUID is not maintained.
• Externalizable: Since you manually control the serialization process, you also manage versioning. You need to ensure that the serialization logic in writeExternal and readExternal remains compatible across versions.

Practical Examples

Example of Serializable

Example of Externalizable

Conclusion

Understanding the differences between Serializable and Externalizable is crucial for effective object serialization in Java. While Serializable offers a straightforward approach with automatic handling of the serialization process, Externalizable provides fine-grained control over what and how data is serialized, leading to potential performance improvements. Choose the right approach based on your application needs, object complexity, and performance requirements.