Serialization and Deserialization in Java

Serialization is a process of converting an object into a sequence of bytes which can be persisted to a disk or database or can be sent through streams. The reverse process of creating object from sequence of bytes is called deserialization.

Serialization is usually used when there is need to send your data over network or to store in files. By data I mean objects and not text.

Now the problem is your Network infrastructure and your Hard disk are hardware components that understand bits and bytes but not Java objects.

Serialization is the translation of Java object’s values/states to bytes to send it over network or to save it. On other hand, Deserialization is conversion of byte code to corresponding java objects.

A class must implement Serializable interface present in java.io package in order to serialize its object successfully. Serializable is a marker interface that adds serializable behaviour to the class implementing it.

Java provides Serializable API encapsulated under java.io package for serializing and deserializing objects which include,

• java.io.serializable
• java.io.Externalizable
• ObjectInputStream
• and ObjectOutputStream etc.

Marker interface

Marker Interface is a special interface in Java without any field and method. Marker interface is used to inform compiler that the class implementing it has some special behaviour or meaning. Some example of Marker interface are,

• java.io.Serializable
• java.lang.Cloneable
• java.rmi.Remote
• java.util.RandomAccess

All these interfaces do not have any method and field. They only add special behavior to the classes implementing them. However, marker interfaces have been deprecated since Java 5, they were replaced by Annotations. Annotations are used in place of Marker Interface that play the exact same role as marker interfaces did before.

Concept of serialVersionUID :

serialVersionUID is used to ensure that same class(That was used during Serialization) is loaded during Deserialization.serialVersionUID is used for version control of object.

Signature of writeObject() and readObject()

writeObject() method of ObjectOutputStream class serializes an object and send it to the output stream.

public final void writeObject(object x) throws IOException

readObject() method of ObjectInputStream class references object out of stream and deserialize it.

public final Object readObject() throws IOException,ClassNotFoundException

while serializing if you do not want any field to be part of object state then declare it either static or transient based on your need and it will not be included during java serialization process.

Serializing an Object

import java.io.*;

class studentinfo implements Serializable

{

 String name;

 int rid;

 static String contact;

 studentinfo(string n, int r, string c)

 {

  this.name = n;

  this.rid = r;

  this.contact = c;

 }

}

class Test

{

 public static void main(String[] args)

 {

 try

 {

  Studentinfo si = new studentinfo("Abhi", 104, "110044");

  FileOutputStream fos = new FileOutputStream("student.ser");

  Objectoutputstream oos = new ObjectOutputStream(fos);

  oos.writeObject(si);

  oos.close();

  fos.close();

  }

  catch (Exception e)

  { e. printStackTrace(); }

 }

}

Object of Studentinfo class is serialized using writeObject() method and written to student.ser file.

Deserialization of Object

import java.io * ;

class DeserializationTest

{

 public static void main(String[] args)

 {

  studentinfo si=null ;

  try 

  {

   FileInputStream fis = new FileInputStream("student.ser");

   ObjectOutputStream ois = new ObjectOutputStream(fis);

   si = (studentinfo)ois.readObject();

  }

  catch (Exception e)

   { e.printStackTrace(); }

  System.out.println(si.name);

  System.out. println(si.rid);

  System.out.println(si.contact);

 }

}

Output :

Abhi

104

null

Contact field is null because, it was marked as static and as we have discussed earlier static fields does not get serialized.

NOTE: Static members are never serialized because they are connected to class not object of class.

transient Keyword

While serializing an object, if we don't want certain data member of the object to be serialized we can mention it transient. transient keyword will prevent that data member from being serialized.

class studentinfo implements Serializable

{

 String name;

 transient int rid;

 static String contact;

}

• Making a data member transient will prevent its serialization.
• In this example rid will not be serialized because it is transient, and contact will also remain unserialized because it is static.

What if superclass is Serializable?

If superclass is serialzable then all its subclasses are automatically serializable.

What if superclass is not Serializable?

If super class is not serializable then we have to handle it quite differently.

If superclass is not serializable then it must have no argument constructor.

If superclass is not Serializable then all values of the instance variables inherited from super class will be initialized by calling constructor of Non-Serializable Super class during deserialization process. so here name is inherited from person so during deserialization,name is initialized to default.

What if superclass is Serializable but you don’t want subclass to be Serializable
If you don’t want subclass to serializable then you need to implement writeObject() and readObject() method and need to throw NotSerializableException from this methods.