Externalization : The Externalizable interface

There might be times when you have special requirements for the serialization of an object. For example, you may have some security-sensitive parts of the object, like passwords, which you do not want to keep and transfer somewhere. Or, it may be worthless to save a particular object referenced from the main object because its value will become worthless after restoring.

You can control the process of serialization by implementing the Externalizable interface instead of Serializable. This interface extends the original Serializable interface and adds writeExternal() and readExternal(). So its not a marker interface. These two methods will automatically be called in your object's serialization and deserialization, allowing you to control the whole process.

There is one major difference between serialization and externalization: When you serialize an Externalizable object, a default constructor will be called automatically; only after that will the readExternal() method be called.

How serialization happens?
JVM first checks for the Externalizable interface and if object supports Externalizable interface, then serializes the object using writeExternal method. If the object does not support Externalizable but implement Serializable, then the object is saved using ObjectOutputStream. Now when an Externalizable object is reconstructed, an instance is created first using the public no-arg constructor, then the readExternal method is called. Again if the object does not support Externalizable, then Serializable objects are restored by reading them from an ObjectInputStream.

Following Listing shows how you can use externalization.

import java.io.*;
import java.util.*;

class Data implements Externalizable {
inti;
String s;
public Data() {
System.out.println("Data default constructor");
}
public Data(String x, int a) {
System.out.println("Second constructor");
s = x; i = a;
}
public String toString() {
return s + i;
}
public void writeExternal(ObjectOutput out) 
throws IOException {
out.writeObject(s);
out.writeInt(i);
}
public void readExternal(ObjectInput in) {
s = (String)in.readObject();
i = in.readInt();
}
public static void main(String[] args) 
throws IOException, ClassNotFoundException {
Data d = new Data("String value",1514);
System.out.println(d);
ObjectOutputStream o = new ObjectOutputStream(
New FileOutputStream("data.out"));
o.writeObject(d);
o.close();

// Now deserialize
ObjectInputStream in = new ObjectInputStream(
new FileInputStream("data.out"));
d = (Data)in.readObject();
}
}

If you inherit some class from a class implementing the Externalizable interface, you must call writeExternal() and readExternal() methods when you serialize or deserialize this class in order to correctly save and restore the object.