Serialization & Deserialization object in java using externalizable interface.

• Given user defined object or POJO, we would like to serialize object in java.
• We will use Externalizable interface, so that class is marked for Serialization.
• Externalizable interface is sub interface of Serializable interface.
• We have discussed similar posts:
  • Serialize user defined object using Serializable interface.
  • Impact of inheritance on Serialization, when Base class is Serialization.
  • Impact of inheritance on Serialization, when Concrete class is Serialization.

1. Serialization & Deserialization using Externalizable interface in java

The Externalizable interface has following methods.

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

• We will discuss the serialization & deserialization of user defined object with the help of example.
• User defined object shall implements Externalizable interface.
  • We will have a Person class, which we want to serialize using Externalizable interface.

2. Class hierarchy of Person class (implementing Externalizable interface)

 

3. POJO or User defined class (Person)

1. We will mark lastName as static variable
2. We will mark contact data member as transient variable.
3. If we implements Externalizable interface, then we can serialize static and transient variables.
   1. Customize serialization using writeExternal method of Externalizable interface.
   2. Read serialized object using readExternal method of Externalizable interface.
4. The Person class needs to have default constructor.

4. Person class or POJO implementing Externalizable interface.

package org.learn;

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;

public class Person implements Externalizable {

 private static final long serialVersionUID = 1L;
 public String firstName;
 public static String lastName = "Not Set";
 public int age;
 public transient String contact;

 public Person() {

 }

 public Person(String firstName, int age, String contact) {
  this.firstName = firstName;
  this.age = age;
  this.contact = contact;
 }

 public String toString() {
  return "[" + firstName + " " + lastName + " " + age + " " + contact + "]";
 }

 @Override
 public void writeExternal(ObjectOutput out) throws IOException {
  out.writeObject(firstName);
  out.writeObject(lastName);
  out.writeInt(age);
  out.writeObject(contact);
 }

 @Override
 public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
  firstName = (String) in.readObject();
  lastName = (String) in.readObject();
  age = (int) in.readInt();
  contact = (String) in.readObject();
 }
}

 

5. Serialize and Deserialize methods of Person class 

• serialize method will serialize the person object.
  • We will serialize static variable in write
    • Serialize static variable using writeExternal method in Person class.
    • Person.lastName = “harvey”;
      • Set before serialization
• deSerialize method will de-serialize the Person Object.
  • We will get serialized properties of Person object.
    • Static and non static variables will be deserialized.

6. Program – Serialization & deserialization of user defined object in java.

package org.learn;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class SerializeDeserialize {

 public static void serialize() throws IOException {
  Person person = new Person("Mike", 34, "001894536");
  //Set static variable
  Person.lastName = "harvey";  
  FileOutputStream output = new FileOutputStream(new File("savePerson.txt"));
  ObjectOutputStream outputStream = new ObjectOutputStream(output);
  outputStream.writeObject(person);
  outputStream.flush();
  outputStream.close();

  System.out.println("1. Serialized the person object : ");
  System.out.println(person);
  System.out.println("LastName:"+Person.lastName);
 }
 public static void deSerialize() throws IOException, ClassNotFoundException {
  
  FileInputStream inputStream = new FileInputStream(new File("savePerson.txt"));
  ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);
  Person person = (Person) objectInputStream.readObject();
  inputStream.close();

  System.out.println("\n2. Deserialized person object : ");
  System.out.println(person);
  System.out.println("LastName:"+Person.lastName);
 }
 
 public static void main(String[] args) throws IOException, ClassNotFoundException {
  serialize();
  deSerialize();
 }
}

Download example – Serialization object in java externalizable interface