Serialization and externalization in java

Serialization and externalization are both mechanisms in Java for converting objects into byte streams to be stored or transmitted and later reconstructed. However, they have distinct characteristics and use cases. Here’s a detailed comparison:

Serialization

Serialization is the default mechanism provided by Java for object persistence. It converts an object into a byte stream using the Serializable interface.

Key Characteristics:

1. Interface:
• Serializable: A marker interface with no methods. Classes implementing this interface can be serialized.
2. Automatic Process:
• Java handles the process of converting an object’s state into a byte stream and vice versa automatically.
3. SerialVersionUID:
• serialVersionUID: A unique identifier used for version control. If the class definition changes, it may affect deserialization. Declaring a serialVersionUID helps in maintaining compatibility.
4. Field Serialization:
• All non-transient fields of the object are serialized. Fields marked as transient are not included in the serialization process.
5. Inheritance:
• Serialization is inherited. If a superclass implements Serializable, its fields are included in the serialization of the subclass.
6. Performance:
• Automatic serialization may not be as optimized for performance and control compared to externalization.

Example:

java

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import java.io.Serializable;

 

public class Employee implements Serializable {

    private static final long serialVersionUID = 1L;

    private String name;

    private transient int id; // transient field

 

    // Constructor, getters, and setters

}

Externalization

Externalization provides a more customizable approach for serialization. It uses the Externalizable interface, which requires explicit implementation of serialization logic.

Key Characteristics:

1. Interface:
• Externalizable: Requires implementing writeExternal and readExternal methods for custom serialization and deserialization.
2. Custom Process:
• The developer is responsible for implementing the entire serialization process. This provides more control over which fields are serialized and how they are stored.
3. No Default Serialization:
• The class must explicitly handle the serialization of its state. Java does not automatically handle any part of this process.
4. Default Constructor:
• A no-argument constructor is required for deserialization. It must be public and must be present to allow the deserialization process to create an instance of the object.
5. Performance:
• Potentially more efficient if implemented correctly, as it allows for custom handling of serialization, which can optimize performance and reduce the byte stream size.

Example:

java

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import java.io.Externalizable;

import java.io.ObjectOutput;

import java.io.ObjectInput;

import java.io.IOException;

 

public class Employee implements Externalizable {

    private String name;

    private int id;

 

    // Default constructor is required for Externalizable

    public Employee() {

    }

 

    public Employee(String name, int id) {

        this.name = name;

        this.id = id;

    }

 

    @Override

    public void writeExternal(ObjectOutput out) throws IOException {

        out.writeUTF(name);

        out.writeInt(id);

    }

 

    @Override

    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {

        name = in.readUTF();

        id = in.readInt();

    }

 

    // Getters and setters

}

Comparison Summary

1. Control:
• Serialization: Less control, automatic handling by Java.
• Externalization: Greater control, requires manual implementation.
2. Implementation:
• Serialization: Implement Serializable interface (marker interface).
• Externalization: Implement Externalizable interface and provide custom serialization methods.
3. Performance:
• Serialization: May be less efficient due to automatic handling and potentially larger byte streams.
• Externalization: Potentially more efficient with custom implementation but requires careful management.
4. Constructor Requirement:
• Serialization: No specific requirements for constructors.
• Externalization: Requires a public no-argument constructor.
5. Field Handling:
• Serialization: Automatically handles all fields unless marked transient.
• Externalization: Developer explicitly decides which fields to serialize and how.
6. Version Control:
• Serialization: Manages version compatibility with serialVersionUID.
• Externalization: Version management is up to the developer and may be more complex.

Choosing between serialization and externalization depends on the specific needs of the application, such as performance requirements and the need for custom serialization logic.