Externalizable Interface

java.io.Externalizable interface

There are two methods readExternal() and writeExternal() in Externalizable interface those can be used in serialization process.

@Override

public void readExternal(ObjectInput arg0) throws IOException, ClassNotFoundException { }

@Override

public void writeExternal(ObjectOutput arg0) throws IOException { }

Externalizable interface extends Serializable interface.

Need of Externalizable

1.If you are not happy with the way java writes/reads objects from stream.

2.Special handling for super types on object construction during serialization.

import java.io.Externalizable;

import java.io.IOException;

import java.io.ObjectInput;

import java.io.ObjectOutput;

public class Person implements Externalizable {

       private int id;

       private String name;

       private String gender;

       @Override

       public void writeExternal(ObjectOutput out) throws IOException {

              out.writeInt(id);

              out.writeObject(name+"xyz");

              out.writeObject("abc"+gender);

       }

       @Override

       public void readExternal(ObjectInput in)throws IOException, 

                                            ClassNotFoundException{

              id=in.readInt();

              //read in the same order as written

              name=(String) in.readObject();

              if(!name.endsWith("xyz")) {

                     throw new IOException("corrupted data");

              } else {

                     name=name.substring(0, name.length()-3);

              }

              gender=(String) in.readObject();

              if(!gender.startsWith("abc")) {

                     throw new IOException("corrupted data");

              } else {

                     gender=gender.substring(3);

              }

       }

       @Override

       public String toString(){

              return "Person{id="+id+",name="+name+",gender="+gender+"}";

       }

       // Setters and Getters

}

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.ObjectInputStream;

import java.io.ObjectOutputStream;

public class ExternalizationTest {

       public static void main(String[] args) {

              String fileName = "person.ser";

              Person person = new Person();

              person.setId(1);

              person.setName("Pankaj");

              person.setGender("Male");

              try {

                     FileOutputStream fos = new FileOutputStream(fileName);

                     ObjectOutputStream oos = new ObjectOutputStream(fos);

                     oos.writeObject(person);

                     oos.close();

              } catch (IOException e) {

                     e.printStackTrace();

              }

              FileInputStream fis;

              try {

                     fis = new FileInputStream(fileName);

                     ObjectInputStream ois = new ObjectInputStream(fis);

                     Person p = (Person)ois.readObject();

                     ois.close();

                     System.out.println("Person Object Read= "+p);

              } catch (IOException | ClassNotFoundException e) {

                     e.printStackTrace();

              }

       }

}