Difference Between Serialization and Externalization in Java

Serialization and externalization both are the processes of converting an object to stream byte and storing byte stream in database or memory. The class that implements java.io.Serializable interface can be serialized. 

What is Serialization?

Java provides a mechanism called object serialization where an object can be converted into a byte stream that includes the object's data and details about the object's type.

Example

The following is an example of Serialization in Java:

import java.io.Serializable;

class SerializableExample implements Serializable {
    private static final long serialVersionUID = 5081877L;
    String name;
}

public class Main {
    public static void main(String[] args) {
        SerializableExample obj = new SerializableExample();
        obj.name = "Tutorialspoint";
        System.out.println("Object name: " + obj.name);
    }
}

The output of the above Java program is:

Object name: Tutorialspoint

What is Externalization?

Externalization is used for custom serialization based on the requirement in the application. If a class implements an Externalizable interface then, object serialization will be done using the writeExternal() method. Externalization extends java.io.Serializable.

Example

The following is an example of Externalization in Java:

import java.io.*;

class ExternalizableExample implements Externalizable {
    Integer id;

    // Public no-argument constructor is required
    public ExternalizableExample() {
        // Optional initialization can be done here if needed
    }

    @Override
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeInt(id);
    }

    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        this.id = in.readInt();
    }
}

public class Main {
    public static void main(String[] args) {
        // Create an instance and assign a value
        ExternalizableExample example = new ExternalizableExample();
        example.id = 123;

        // Serialization: write the object to a file
        try (ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("data.ser"))) {
            out.writeObject(example);
            System.out.println("Serialized successfully.");
        } catch (IOException e) {
            e.printStackTrace();
        }

        // Deserialization: read the object back from the file
        try (ObjectInputStream in = new ObjectInputStream(new FileInputStream("data.ser"))) {
            ExternalizableExample newExample = (ExternalizableExample) in.readObject();
            System.out.println("Deserialized ID: " + newExample.id);
        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

The output of the above Java program is:

Serialized successfully.
Deserialized ID: 123

Difference between Serialization and Externalization

The following are the differences between Serialization and Externalization: