20.7. Custom Serialization with Externalizable

The Externalizable interface allows control over serialization logic.

public interface Externalizable extends Serializable {
    void writeExternal(ObjectOutput out) throws IOException;
    void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;
}

The Externalizable interface is a specialized way of handling serialization in Java. Unlike the Serializable interface, which uses default serialization, Externalizable gives you full control over how your object is serialized and deserialized. When you implement this interface, you need to explicitly define two methods: writeExternal and readExternal. The writeExternal method is responsible for defining how the object is converted into a byte stream, while the readExternal method defines how to reconstruct the object from that byte stream.

Think of Externalizable as a recipe for baking a cake, where you have complete control over every ingredient and the baking process. Just like a baker can choose whether to add frosting or nuts based on their preferences, a developer can choose which attributes of a class to serialize and how to do it using the Externalizable interface.

Example

class Employee implements Externalizable {
    int id;
    String name;
    public Employee() {}
    public void writeExternal(ObjectOutput out) throws IOException {
        out.writeInt(id);
        out.writeObject(name);
    }
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        id = in.readInt();
        name = (String) in.readObject();
    }
}

In this example, we have a class Employee that implements the Externalizable interface. The class has two attributes: id (an integer) and name (a string). The parameterless constructor is essential for deserialization because it allows the creation of an object without any initial attribute values. The writeExternal method specifies that during serialization, the id and name should be written to the output stream. Conversely, the readExternal method retrieves those values from the input stream and assigns them back to the object's attributes during deserialization.

Consider the Employee class like an application form. When you apply for a job (serialization), you fill out your ID and name on the form. When the employer wants to review your details (deserialization), they read your form to get your ID and name back. This process requires both the form and a way to read and interpret it.

--