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In Java IO, converting an object into a sequence of bytes is a fundamental capability. Java gives developers two distinct interfaces to achieve this: Serializable and Externalizable. Deciding between them comes down to whether you prefer automated simplicity or complete custom control over your object's serialized format.
Key Differences: Serializable vs Externalizable
1. Default vs Custom Control
Serializable: The entire serialization and deserialization process is handled automatically by the JVM. The virtual machine determines how fields are written and retrieved.
Externalizable: Control is shifted entirely to the developer. You define the explicit logic for writing and restoring each field, preventing unwanted metadata from being persisted.
2. Methods Present in the Interface
Serializable: A marker interface with zero methods. Simply implementing it marks the class as eligible for JVM default serialization.
Externalizable: Extends Serializable and requires two explicit callback methods:
writeExternal(ObjectOutput out)readExternal(ObjectInput in)
3. Constructor Behavior During Deserialization
Serializable: The constructor of the serialized class is never called during deserialization.
Externalizable: Deserialization mandates a public no-argument constructor to instantiate the object before invoking readExternal().
4. Use of the transient Modifier
Serializable: Any field declared with the transient keyword is automatically excluded from default serialization.
Externalizable: The transient keyword has no impact because field persistence is explicitly coded inside writeExternal() and readExternal().
Quick Reference Summary
| Concept | Serializable | Externalizable |
|---|---|---|
| Control Level | Automatic (JVM managed) | Custom (Developer managed) |
| Methods | None (Marker interface) | 2 (writeExternal / readExternal) |
| Constructor Needed | No constructor called | Public no-argument constructor required |
| Transient Keyword | Supported | Not applicable |
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