Java Tutorial : Java IO (Java serializable vs externalizable | serialization vs externalization -V1)

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Introduction

In Java, converting an object into a byte stream can be achieved using two main interfaces from the java.io package: Serializable and Externalizable. While both allow objects to be saved or sent over a network, they work in fundamentally different ways when it comes to JVM automation, performance, and customization.

Key Differences: Serializable vs Externalizable

1. Default vs Manual Serialization

Serializable: Managed entirely by the JVM's default mechanism. It automatically serializes the object's non-transient state without requiring custom code.

Externalizable: Gives full control to the programmer. You manually define how each field is written to and read from the stream.

2. Marker Interface vs Custom Methods

Serializable: A marker interface with no methods. Simply implementing it marks your class as serializable.

Externalizable: Extends Serializable and provides two mandatory methods:

• writeExternal(ObjectOutput out)
• readExternal(ObjectInput in)

3. Constructor Calls During Deserialization

Serializable: No constructor of the serialized class is invoked during deserialization.

Externalizable: Requires a public no-argument constructor, which is executed first by the JVM before calling readExternal().

4. Impact of the transient Modifier

Serializable: Fields marked with transient are skipped during serialization.

Externalizable: The transient keyword has no effect because you explicitly write and read fields inside the custom methods.

Quick Reference Summary

Feature	Serializable	Externalizable
Control Level	JVM Default (Automatic)	Developer Managed (Manual)
Methods	0 (Marker Interface)	2 (writeExternal, readExternal)
Constructor Needed	None invoked	Public no-arg constructor required
transient Keyword	Supported	Ignored

Related Tutorials from Ram N Java

• What is Externalizable or Java externalization or Custom Serialization in Java?
• Mastering Serialization of Complex Objects in Java | Java IO | Java Tutorial
• Serialization and Deserialization Notes in Java | Java IO | Java Tutorial

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Java Tutorial : Java IO (Java serializable vs externalizable | serialization vs externalization -V1) 

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