Saturday, March 8, 2014
Serialization is the process of converting the current state of an object into a byte stream. Deserialization is the process of converting the serialized form of an object back into a copy of the object.  A Java object is Serializable if a class implements java.io.Serializable interface. java.io.Serializable is a marker interface which tells that object is Serializable.

A quick example. In this example Employee class implements the marker interface Serializable and can be serialized. 
package com.sourcetricks.java;

import java.io.Serializable;

public class Employee implements Serializable {

 private static final long serialVersionUID = 2L;
 private int id;
 private String name;
 private int deptId;
 Employee() {   
 }
 Employee(int id, String name, int deptId) {
  this.id = id;
  this.name = name;
  this.deptId = deptId;
 }
 public int getId() {
  return id;
 }
 public String getName() {
  return name;
 }
 public int getDeptId() {
  return deptId;
 }
 public void setId(int id) {
  this.id = id;
 }
 public void setName(String name) {
  this.name = name;
 }
 public void setDeptId(int deptId) {
  this.deptId = deptId;
 }
 public void print() {
  System.out.println(id);
  System.out.println(name);
  System.out.println(deptId);
 }
}

In the test program we create an instance of Employee and using writeObject on ObjectOutputStream we persist the serialized object to the file system. During deserialization we do the reverse using readObject on ObjectInputStream to create a copy of the object back.
package com.sourcetricks.java;

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

public class JavaSerializationTest {
 
 public static void main (String[] args) {
  
  Employee e = new Employee(100, "John", 14);
  e.print();
  
  // Serialize and save the object to file
  FileOutputStream fout = null;
  ObjectOutputStream out = null;
  try {
   fout = new FileOutputStream("data.out");
   out = new ObjectOutputStream(fout);
   out.writeObject(e);
   out.close();
   fout.close();
  } catch (IOException e1 ) {
   e1.printStackTrace();
  }
  
  // Read object from file
  FileInputStream fin = null;
  ObjectInputStream in = null;
  try {
   fin = new FileInputStream("data.out");
   in = new ObjectInputStream(fin);
   Employee e1 = (Employee) in.readObject();
   e1.print();
   in.close();
   fin.close();
  } catch (IOException e1 ) {
   e1.printStackTrace();
  } catch (ClassNotFoundException e2) {
   e2.printStackTrace();
  }
 }
}
Output:
100
John
14
100
John
14

Significance of serialVersionUID

serialVersionUID is important for classes to be serialized. It allows to specify a version to the object. Lets say object version is 1 when the object was written to the file system. At a later point of time there is a type change in the object and the version is upgraded to 2. If we try to deserialize the original object written to file system then it results in an InvalidClassException. 

java.io.InvalidClassException: com.sourcetricks.java.Employee; local class incompatible: stream classdesc serialVersionUID = 1, local class serialVersionUID = 2

Significance of transient

Sometimes it is necessary that one of the class members should not be included as part of the serialization process. Either it has sensitive information or which is not relevant once the object is de-serialized. (e.g. current time). In such scenarios associating transient keyword prevents the class member from being included as part of writeObject.

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