Java Serialization

Object serialization is the process of saving an object's state to a sequence of bytes, as well as the process of rebuilding those bytes into a live object at some future time. The Java Serialization API provides a standard mechanism for developers to handle object serialization. The API is small and easy to use, provided the classes and methods are understood.  This hands-on lab takes you through the basics of using Java serialization.


Resources

Exercises

Exercise 1: Serialize and Deserialize an object


In this exercise, you will learn how to do serialization and deserialization of the an object.  You will also learn how to use transient keyword.
  1. Serialize the current time
  2. Use transient keyword

(1.1) Serialize the current time


0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

2. Modify the IDE generated SerializeTime.java as shown in Code-1.11 below.  Study the code by paying special attention to the bold fonted parts.

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

public class SerializeTime{
   
    public static void main(String [] args){
       
        String filename = "time.ser";
        if(args.length > 0) {
            filename = args[0];
        }
       
        // Create an object
        PersistentTime time = new PersistentTime();
       
        // Serialize the object instance and save it in
        // a file.
        FileOutputStream fos = null;
        ObjectOutputStream out = null;
        try {
            fos = new FileOutputStream(filename);
            out = new ObjectOutputStream(fos);
            out.writeObject(time);
            out.close();
        } catch(IOException ex) {
            ex.printStackTrace();
        }

        System.out.println("Current time is saved into " + filename);
    }
}
Code-1.11: SerializeTime.java



3. Write PersistentTime.java.

import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;

public class PersistentTime implements Serializable{
    private Date time;
   
    public PersistentTime() {
        time = Calendar.getInstance().getTime();
    }
   
    public Date getTime() {
        return time;
    }
}



3. Write DeserializeTime.java.

import java.io.ObjectInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.Calendar;

public class DeserializeTime {
   
    public static void main(String [] args) {
       
        String filename = "time.ser";
        if(args.length > 0) {
            filename = args[0];
        }
       
        // Deserialize the previously saved
        // PersistentTime object instance.
        PersistentTime time = null;
        FileInputStream fis = null;
        ObjectInputStream in = null;
        try {
            fis = new FileInputStream(filename);
            in = new ObjectInputStream(fis);
            time = (PersistentTime)in.readObject();
            in.close();
        } catch(IOException ex) {
            ex.printStackTrace();
        } catch(ClassNotFoundException ex) {
            ex.printStackTrace();
        }
       
        // print out restored time
        System.out.println("Previously serialized time: " + time.getTime());
       
        // print out the current time
        System.out.println("Current time: " + Calendar.getInstance().getTime());
    }
}



4. Build and run the Serialization part of the project

Current time is saved into time.ser
Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application



5. Build and run the Deserialization part of the project

Previously serialized time: Mon Feb 26 01:57:16 EST 2007
Current time: Mon Feb 26 01:58:58 EST 2007
Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application




5. For your own exercise, do the following tasks. Build and run the application.
                                                                                                              return to top of the exercise


(1.2) Use transient keyword


1. Modify PersistentTime.java as shown in Code-1.11 below. The code fragment that needs to be modified is highlighted in bold and blue-colored font. The change is to make the time field to be transient.

import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;

public class PersistentTime implements Serializable{
    transient private Date time;
   
    public PersistentTime() {
        time = Calendar.getInstance().getTime();
    }
   
    public Date getTime() {
        return time;
    }
}
Code-1.11: SerializeTime.java



2. Build and run the Serialization part of the project
Current time is saved into time.ser
Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application

3. Build and run the Deserialization part of the project
Previously serialized time: null
Current time: Mon Feb 26 02:07:09 EST 2007
Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application




4. For your own exercise, do the following tasks. Build and run the application.

Summary


In this exercise, you learned how to do serialization and deserialization of the an object.  You also learned how to use transient keyword.


                                                                                                                  return to the top


Exercise 2: Version Control

In this exercise, you will learn how to do version control.

    1. Modify the class after serialization
    2. Use unique version id

(2.1) Modify the class after serialization


1. Build and run the Serialization part of the project.
Current time is saved into time.ser
Figure-2.10: Result

2. Modify the PersistentTim.java as shown in Code-2.11 below.  The change is to add another field to the PersistentTime class after the serialization.

import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;

public class PersistentTime implements Serializable{
   
    private Date time;
    private String aNewField;
   
    public PersistentTime() {
        time = Calendar.getInstance().getTime();
    }
   
    public Date getTime() {
        return time;
    }
}
Code-2.11: FileReaderWriter.java



3. Compile the PersistentTime.java.


4. Build and run the Deserialization part of the project
java.io.InvalidClassException: PersistentTime; local class incompatible: stream classdesc serialVersionUID = -3126998878902358585, local class serialVersionUID = -5560460247034149373
        at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:519)
        at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1546)
        at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1460)
        at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1693)
        at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1299)
        at java.io.ObjectInputStream.readObject(ObjectInputStream.java:339)
        at DeserializeTime.main(DeserializeTime.java:23)
Exception in thread "main" java.lang.NullPointerException
Figure-2.12: java.io.InvalidClassException exception




(2.2) Use a version id


1. Modify the PersistentTim.java as shown in Code-2.11 below.  The change is to add unique version id to the class.

import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;

public class PersistentTime implements Serializable{
   
    static final long serialVersionUID = -3126998878902358585L;
   
    private Date time;
    private String aNewField;
   
    public PersistentTime() {
        time = Calendar.getInstance().getTime();
    }
   
    public Date getTime() {
        return time;
    }
}
Code-2.21: Assign a unique version id to the class file



2. Compile the PersistentTime.java.
3. Build and run the Serialization part of the project.
Current time is saved into time.ser
Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application

4. Modify the PersistentTim.java as shown in Code-2.11 below.  The change is to add another field to the PersistentTime class after the serialization.

import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;

public class PersistentTime implements Serializable{

    static final long serialVersionUID = -3126998878902358585L;
   
    private Date time;
    private String aNewField;
    private String aNewNewField;

    public PersistentTime() {
        time = Calendar.getInstance().getTime();
    }

    public Date getTime() {
        return time;
    }
}
Code-2.11: FileReaderWriter.java



5. Compile the PersistentTime.java.
6. Build and run the Deserialization part of the project
Previously serialized time: Mon Feb 26 03:04:59 EST 2007
Current time: Mon Feb 26 03:05:13 EST 2007
Figure-2.23: Result





                                                                                                              return to top of the exercise

Summary

In this exercise,  you have learned how to do version control for serialization and deserialization of an object.

                                                                                                                        return to the top



Exercise 3: Customizing default protocol

In this exercise, you will learn how to provide your own readObject() and writeObject() methods, thus changing the behavior of the default protocol.

    1. Using the default readObject() and writeObject()
    2. Using your own readObject() and writeObject()

(3.1) Using the default readObject() and writeObject()


1. Create a new NetBeans project

2. Modify the IDE generated SerializeAnimationThreadNotStarted.java as shown in Code-3.11 below.  Study the code by paying special attention to the bold fonted parts.

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

public class SerializeAnimationThreadNotStarted {
   
    public static void main(String[] args) {
       
        // Create an object instance
        PersistentAnimation a = new PersistentAnimation(1);
       
        // Serialize the object
        FileOutputStream fos = null;
        ObjectOutputStream out = null;
        try {
            fos = new FileOutputStream("serializedfile");
            out = new ObjectOutputStream(fos);
            out.writeObject(a);
            out.close();
        } catch(IOException ex) {
            ex.printStackTrace();
        }
       
        // Deserialize the object.  The problem is that the
        // PersistentAnimation thread does not get started
        // automatically after the deserialization since its
        // constructor method does not get called when the
        // serialized object is deserialized.  This is why
        // the PersistentAnimation class has to have its own
        // readObject() method in which the thread is
        // explicitly started.
       
        PersistentAnimation b = null;
        FileInputStream fis = null;
        ObjectInputStream in = null;
        try {
            fis = new FileInputStream("serializedfile");
            in = new ObjectInputStream(fis);
            b = (PersistentAnimation)in.readObject();
            in.close();
        } catch(IOException ex) {
            ex.printStackTrace();
        } catch(ClassNotFoundException ex) {
            ex.printStackTrace();
        }      
             
    }
   
}
Code-3.11: SerializeAnimationThreadNotStarted.java



3. Write PersistentAnimation.java as shown in Code-3.12 below. 

import java.io.Serializable;

public class PersistentAnimation implements Serializable, Runnable {
   
    transient private Thread animator;
    private int animationSpeed;
   
    public PersistentAnimation(int animationSpeed) {
        this.animationSpeed = animationSpeed;
        animator = new Thread(this);
        animator.start();
    }
   
    public void run() {
        System.out.println("PersistentAnimation thread is started");
    }
}
Code-3.12: PersistentAnimation.java



4. Build and run the project

PersistentAnimation thread is started
Figure-3.13: Result of running SerializeAnimationThreadNotStarted application





                                                                                                              return to top of the exercise

(3.2) Using your own readObject() and writeObject()


1. Modify PersistentAnimation.java as shown in Code-3.21 below.  The code fragments that need to be changed are highlighted in bold and blue-colored font.

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class PersistentAnimation implements Serializable, Runnable {
   
    transient private Thread animator;
    private int animationSpeed;
   
    public PersistentAnimation(int animationSpeed) {
        this.animationSpeed = animationSpeed;
        startAnimation();
    }
   
    public void run() {
        System.out.println("PersistentAnimation thread is started");
    }
   
    // Provide your own writeObject method
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
    }
   
    // Provide your own readObject method
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        // our "pseudo-constructor"
        in.defaultReadObject();
        // now we are a "live" object again, so let's run rebuild and start
        startAnimation();
       
    }
   
    private void startAnimation() {
        animator = new Thread(this);
        animator.start();
    }
}
Code-3.21: PersistentAnimation.java



2. Build and run the project
PersistentAnimation thread is started
PersistentAnimation thread is started
Figure-3.22: Result of running SerializeAnimationThreadStarted application




                                                                                                              return to top of the exercise


Summary


In this exercise, you learned how to provide your own readObject() and writeObject() methods, thus changing the behavior of the default protocol.


                                                                                                                    return to the top



Homework


1. The homework is to either modify SerializeTime NetBeans project you've done in Exercise 1 above or create a new project as following.  (You might want to create a new project by copying the SerializeTime project.  You can name the homework project in any way you want but here I am going to call it MySerializationProject.)