What is serialization? How I have to serialize java objects?

Serializable is a marker interface. It does not have any methods. When an object has to be transferred over a network (typically through rmi or EJB) or persist the state of an object to a file, the object Class needs to implement Serializable interface

What happens if an object is Serializable but it includes a reference to a non-serializable object – NotSerializableException at runtime

1. Transient - These variables are not included in the process of serialization.

2. The static variables belong to the class and not to an object they are not the part of the state of the object so they are not saved as the part of serialized object.

**To serialize an array or a collection all the members of it must be serializable

The Serializable interface relies on the Java runtime default mechanism to save an object's state. Writing an object is done via the writeObject() method in the ObjectOutputStream class (or the ObjectOutput interface). Writing a primitive value may be done through the appropriate write<datatype>() method. Reading the serialized object is accomplished using the readObject() method of the ObjectInputStream class, and primitives may be read using the various read<datatype>() methods.

Example

import java.io.*;

public class SerializationDemo {

public static void main(String args[]) {

// Object serialization

try {

MyClass object1 = new MyClass("Hello", -7, 2.7e10);

System.out.println("object1: " + object1);

FileOutputStream fos = new FileOutputStream("serial");

ObjectOutputStream oos = new ObjectOutputStream(fos);

oos.writeObject(object1);

oos.flush();

oos.close();

}

catch(Exception e) {

System.out.println("Exception during serialization: " + e);

System.exit(0);

}

// Object deserialization

try {

MyClass object2;

FileInputStream fis = new FileInputStream("serial");

ObjectInputStream ois = new ObjectInputStream(fis);

object2 = (MyClass)ois.readObject();

ois.close();

System.out.println("object2: " + object2);

}

catch(Exception e) {

System.out.println("Exception during deserialization: " +

e);

System.exit(0);

}

}

}

class MyClass implements Serializable {

String s;

int i;

double d;

public MyClass(String s, int i, double d) {

this.s = s;

this.i = i;

this.d = d;

}

public String toString() {

return "s=" + s + "; i=" + i + "; d=" + d;

}

}

 Can you Customize Serialization process or can you override default Serialization process in Java?

The answer is yes you can. We all know that for serializing an object ObjectOutputStream.writeObject (saveThisobject) is invoked and for reading object ObjectInputStream.readObject() is invoked but there is one more thing which Java Virtual Machine provides you is to define these two method in your class. If you define these two methods in your class then JVM will invoke these two methods instead of applying default serialization mechanism. You can customize behavior of object serialization and deserialization here by doing any kind of pre or post processing task. Important point to note is making these methods private to avoid being inherited, overridden or overloaded. Since only Java Virtual Machine can call private method integrity of your class will remain and Java Serialization will work as normal.

 Suppose super class of a new class implement Serializable interface, how can you avoid new class to being serialized?

One of the tricky interview question in Serialization in Java. If Super Class of a Class already implements Serializable interface in Java then its already Serializable in Java, since you can not unimplemented an interface its not really possible to make it Non Serializable class but yes there is a way to avoid serialization of new class. To avoid java serialization you need to implement writeObject() and readObject() method in your Class and need to throw NotSerializableException from those method. This is another benefit of customizing java serialization process as described in above Serialization interview question and normally it asked as follow-up question as interview progresses

What is externalizer in java 1.5?

We can control the process of serialization by implementing the Externalizable interface instead of Serializable. This interface extends the original Serializable interface and adds writeExternal() and readExternal(). These two methods will automatically be called in your object's serialization and deserialization, allowing you to control the whole process.

There might be times when you have special requirements for the serialization of an object. For example, you may have some security-sensitive parts of the object, like passwords, which you do not want to keep and transfer somewhere. Or, it may be worthless to save a particular object referenced from the main object because its value will become worthless after restoring.

public interface Externalizable extends Serializable

What is serialVersionUID? What would happen if you don't define this?

SerialVersionUID is an ID which is stamped on object when it get serialized usually hashcode of object, you can use tool serialver to see serialVersionUID of a serialized object .  SerialVersionUID is used for version control of object. you can specify serialVersionUID in your class file also.  Consequence of not  specifying  serialVersionUID is that when you add or modify any field in class then already serialized class will not be able to recover because serialVersionUID generated for new class and for old serialized object will be different. Java serialization process relies on correct serialVersionUID for recovering state of serialized object and throws java.io.InvalidClassException in case of serialVersionUID mismatch.

One example of How Serialization can put constraints on your ability to change class is SerialVersionUID. If you don't explicitly declare SerialVersionUID then JVM generates its based upon structure of class which depends upon interfaces a class implements and several other factors which is subject to change. Suppose you implement another interface than JVM will generate a different SerialVersionUID for new version of class files and when you try to load old object object serialized by old version of your program you will get InvalidClassException.

Ex:

public final class EmailAddress implements Validator, Serializable {

    private static final long serialVersionUID = 1L;

………