What is serialization and how to do serialization in java?

Introduction to serialization

We know that object is a real time entity, that means object will be created during runtime of application/program, object can be stored in a JVM memory components until ending of program execution if required, or can be deleted in the middle of program execution with the help of Garbage Collector. But if we want store object data in a file or in a database like normal data ( example: age, name etc..), yes that is possible with serialization concept in java.

Serialization is the process to convert object into sequence of bytes, which holds information object such as type of object and data associated with object.

What is deserialization?

Deserialization is the process to convert byte stream into an object.

Key points related to serialization and deserialization

Serialized object data can be deserialized outside of JVM too, that means serialized object data is independent from platform.
Serialized data can be stored in a file or data base ( as BLOB type of data )
We can deserialize byte stream which stored in a file and database
We can trnasfer serialized object through the network to remote business component , for example sending chat message to chat server
Serializable class must implements java.io.Serialize or java.io.Externalizable interface
All variables will be serialized automatically
If you do not want to serialize any variable, that variable must me declared as transient variable
All related other objects are also be serialized apart from current object

Compatible changes allowing in serialization

For better understanding of what is Compatible changes means , let us consider a situation, that we have a class Student.java, this class is serialized and stored byte stream in a file A, after some time Student.java class is altered, then we want to deserialize byte stream from a file A into Student.java ( new one, which is altered from old one ), then in this case an object representing in a file A is different than the object of new Student.java class, then what changes will allow in a Student.java class will tell us by allowing compatible changes.
In the following example first case deals with serialization and deserialization on the same version of a Student.java class, but in the second part of the diagram you can see that newer class of Student.java ( which are coloured with red color )

Compatible changes with versions of class in serialization

Compatible Changes means: what changes in new version of class will be allowed to compatible with older version class

Changing field and method are compatible changes

Detailed description about compatible changes

Changing field access specifier will be allowed in a newer version of Class
Adding new field will be allowed in a newer version of class
Changing static to non static allowed
Changing transient to non-transient allowed
We can change the access modifiers for constructors and methods of the class. For instance a previously private method can now be made public, an instance method can be changed to static, etc.
We cannot change the default signatures for readObject() and writeObject() if we are implementing custom serialization.

The serialization process looks at only instance data, and not the methods of a class.

Incompatible changes with in serialization

We cannot change non-static to static and non-transient to transient
We cannot delete the field
We cannot change field type in a class
We cannot change package and class name
We cannot change the hierarchy of class
We cannot unimplement serializable interface

ObjectOutputStream for serializing object

We can use the class ObjectOutputStream to create byte stream of object. We have wrtieObject() method in ObjectOutputStream, this method will serialize an object and store the data into a file.

Steps to serialize and store byte stream object into a file

First create file output stream object
Create ObjectOutputStream object by passing file output stream object
Invoke writeObject() on ObjectOutputStream object by passing which object to be serialized and stored

public class Account implements Serializable {

 private static final long serialVersionUID = 1L;

 int acountNo;
 String bankName;

 public Account(int acountNo, String bankName) {
  super();
  this.acountNo = acountNo;
  this.bankName = bankName;
 }

 public int getAcountNo() {
  return acountNo;
 }

 public void setAcountNo(int acountNo) {
  this.acountNo = acountNo;
 }

 public String getBankName() {
  return bankName;
 }

 public void setBankName(String bankName) {
  this.bankName = bankName;
 }

}

Account account= new Account(123, "World Bank");
  File as= new File("bank.txt");
  try {
   FileOutputStream fout= new FileOutputStream(as);
   ObjectOutputStream oos= new ObjectOutputStream(fout);
   oos.writeObject(account);
   oos.close();
   fout.close();
   
  } catch (FileNotFoundException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

In the above code after executing we can get a file named as bank.txt with object stored in it

ObjectInputStream for deserializing an object

Now we will see how to deserialize an object stored in a file ( from the above example it is bank.txt ), we can use ObjectInputStream and its readObject() method to read and deserialize an object

Steps to deserialize an object

First create a File object to the file where we stored object ( in our example bank.txt )
Create FileInputStream object for the file
Create ObjectInputStream for the above FileInputStream object
Invoke readObject() of ObjectInputStream which returns Object class object

File as = new File("bank.txt");
  FileInputStream fis = new FileInputStream(as);
  ObjectInputStream ois = new ObjectInputStream(fis);

  Object object = ois.readObject();

  System.out.println(object.getClass().getCanonicalName());

How to store serialized object in Databases with JDBC

We have setObject() method from JDBC-3, then simply we can pass an object to setObject() method to store into database table, but the field type in database should be BLOB type, no need to use any ObjectOutputStream class and its method writeObject() while storing object into database table.

How to store object in database with jdbc