Why serialVersionUID Is Required??

  

serialVersionUID is used to identify the class during deserialization process.if you don't mention serialVersionUID in your class then  by default JVM will generate it .

JVM generated  serialVersionUID will be a 64 bit hash value of class name ,method and fields.But there is a problem in not declaring serialVersionUID manually.

Let me explain it with the example ,

 This is my class which i want to serialize

public class Employee implements Serializable {

   
    /**
     *
     */
    public int getEmpId() {
        return empId;
    }
    public void setEmpId(int empId) {
        this.empId = empId;
    }
    public String getEmpName() {
        return empName;
    }
    public void setEmpName(String empName) {
        this.empName = empName;
    }
    private int empId;
    transient private String empName;

}

In the above code i didn't  declare serialVersionUID.

Below code used to serialize the Employee object

public class SerializationTest {

    /**
     * @param args
     * @throws IOException
     */
    public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
       
        try {
            Employee emp = new Employee();
            emp.setEmpId(1);
            emp.setEmpName("saradha");
            FileOutputStream fout = new FileOutputStream("test.text");
            ObjectOutputStream out= new ObjectOutputStream(fout);
            out.writeObject(emp);
            fout.close();
            out.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}
 

out put file will be generated with serialized Employee object .

now i am changing the empId field name to empId1,then  i am going to deserialize it  using the following  class

public class deserializationTest {

    /**
     * @param args
     * @throws IOException
     * @throws ClassNotFoundException
     */
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        // TODO Auto-generated method stub
        FileInputStream fin = null;
        try {
            fin = new FileInputStream("test.text");
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        ObjectInputStream out= new ObjectInputStream(fin);
        Employee e=(Employee)out.readObject();
        fin.close();
        out.close();
        System.out.println("id-"+e.getEmpId());
        System.out.println("name-"+e.getEmpName());
    }

}
 

Now the following exception will be thrown in console:

Exception in thread "main" java.io.InvalidClassException: com.saradha.serialize.test.Employee; local class incompatible: stream classdesc serialVersionUID = 4984626804719984326, local class serialVersionUID = -4174571632522634906
    at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:562)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1582)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1495)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1731)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1328)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:350)
    at com.saradha.serialize.test.deserializationTest.main(deserializationTest.java:26)

This is because after changing the field name in Employee class it's  serialVersionUID  will get changed . so it is a good practice to   declare serialVersionUID always.