16.9. Customizing Serialization with Externalizable

Use java.io.Externalizable for more control over serialization.

The Externalizable interface allows developers to gain more control over the serialization process compared to the Serializable interface. When a class implements Externalizable, it must define how its fields are serialized and deserialized by providing custom methods.

Think of Externalizable like a recipe book where you customize your dish exactly how you want it, as opposed to using a pre-defined recipe. You decide what ingredients (fields) to include or exclude in your dish when preparing it for serving (serialization).

Methods to Implement: • writeExternal(ObjectOutput out) • readExternal(ObjectInput in)

When implementing Externalizable, you need to provide two specific methods: 'writeExternal' and 'readExternal'. The 'writeExternal' method is used to define what happens during serialization; here, you specify how the object’s state is converted into a byte stream. The 'readExternal' method is used during deserialization to define how the byte stream is converted back into the object’s state.

Imagine you're packing your suitcase (serialization) for a trip. The 'writeExternal' method is when you decide what clothes (fields) to put in and how to arrange them. When you get back (deserialization), 'readExternal' is like unpacking your suitcase and figuring out exactly how you packed it to restore everything (the object) to how it was.

Example:

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

In this example, we have an 'Employee' class that implements Externalizable. Inside the class, the 'writeExternal' method is defined to write the employee’s name and salary to the output stream. Similarly, the 'readExternal' method reads the name and salary back from the input stream to reconstruct the Employee object. This complete control over what gets serialized and how it is structured ensures that only the necessary data is transferred.

Think of the Employee class as a form you fill out when starting a new job. The 'writeExternal' method is like filling in the fields of your name and salary—only the necessary information is provided. The 'readExternal' method represents the HR department taking this form and entering all the details into their database.

--