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#023 Java Serialization

by nobaksan

Lab 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

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects. Click Next.
  • Under Name and Location pane, for the Project Name field, type in SerializeAndDeserializeCurrrentTime as project name.
  • For Create Main Class field, type in SerializeTime.
  • Click Finish.
  • Observe that SerializeAndDeserializeCurrrentTime project appears and IDE generated SerializeTime.java is displayed in the source editor window of NetBeans IDE.

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;
}
}

InputFile-1.12: farrago.txt

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());
}
}

InputFile-1.12: farrago.txt

4. Build and run the Serialization part of the project

  • Right click SerializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-1.13 below)
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

  • Right click DeserializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-1.13 below)
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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javaserialization/samples/SerializeAndDeserializeCurrrentTime.  You can just open it and run it.

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

  • Add another field called myName which is String type to the PersistentTIme class.
  • Modify SerializeTime.java and DeserializeTime.java to display the myName field.

(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

  • Right click SerializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-1.13 below)
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

  • Right click DeserializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-1.13 below)
Previously serialized time: null
Current time: Mon Feb 26 02:07:09 EST 2007

Figure-1.13: Result of running SerializeAndDeserializeCurrrentTime application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javaserialization/samples/SerializeAndDeserializeCurrrentTimeTransient.  You can just open it and run it.

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

  • Make myName field transient.
  • Object that the field is not serialized.

Summary

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

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.

  • Right click SerializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-2.10 below)
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.

  • Right click PersistentTime.java and select Compile File.

4. Build and run the Deserialization part of the project

  • Right click DeserializeTime.java project and select Run File.
  • Observe the java.io.InvalidClassException exception in the Output window. (Figure-2.12 below)
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.

  • Right click PersistentTime.java and select Compile File.

3. Build and run the Serialization part of the project.

  • Right click SerializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-1.13 below)
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{

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.

  • Right click PersistentTime.java and select Compile File.

6. Build and run the Deserialization part of the project

  • Right click DeserializeTime.java project and select Run File.
  • Observe the result in the Output window. (Figure-2.23 below)
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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javaserialization/samples/SerializeAndDeserializeCurrrentTimeVersionControl.  You can just open it and run it.

Summary

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

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

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects. Click Next.
  • Under Name and Location pane, for the Project Name field, type in SerializeAnimationThreadNotStarted as project name.
  • For Create Main Class field, type in SerializeAnimationThreadNotStarted.
  • Click Finish.
  • Observe that SerializeAnimationThreadNotStarted project appears and IDE generated SerializeAnimationThreadNotStarted.java is displayed in the source editor window of NetBeans IDE.

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.  Study the code by paying special attention to the bold fonted parts.

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

  • Right click SerializeAnimationThreadNotStarted project and select Run Project.
  • Observe the result in the Output window. (Figure-3.13 below)  Observe that the thread is not started automatically when the object is deserialized.
PersistentAnimation thread is started

Figure-3.13: Result of running SerializeAnimationThreadNotStarted application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javaserialization/samples/SerializeAnimationThreadNotStarted.  You can just open it and run it.

(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

  • Right click SerializeAnimationThreadStarted project and select Run Project.
  • Observe the result in the Output window. (Figure-3.22 below)  Observe that the thread is now started when the object is deserialized.
PersistentAnimation thread is started
PersistentAnimation thread is started

Figure-3.22: Result of running SerializeAnimationThreadStarted application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javaserialization/samples/SerializeAnimationThreadStarted.  You can just open it and run it.

Summary

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

Homework exercise (for people who are taking Sang Shin’s “Java Programming online course”)

 

1. The homework is to either modify SerializeTime NetBeans project you’ve done in Exercise 8 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.)
  • Write MyClassToBePersisted.java, which contains the following properties
    • Profile field, which has the following fields
      • String name
      • int age
      • String hobby
    • School field, which has the following fields
      • String nameOfSchool
      • int yearStarted (this is transient field)
    • Write SerializeMyClassToBePersisted.java which creates an instance of MyClassToBePersisted class and serialize it into a file in its main method.
    • Write DeserializeMyClassToBePersisted.java which read the serialized file and deserialize it into instance of MyClassToBePersisted class in its main method.
  • Write MyClassToBePersisted.java, which contains the following properties with version control.
2. Send the following files to javaprogramminghomework@sun.com with Subject as JavaIntro-javaserialization.
  • Zip file of the the MySerializationProject NetBeans project.  (Someone else should be able to open and run it as a NetBeans project.)  You can use your favorite zip utility or you can use “jar” utility that comes with JDK as following.
    • cd <parent directory that contains MySerializationProject directory> (assuming you named your project as MySerializationProject)
    • jar cvf MySerializationProject.zip MySerializationProject (MySerializationProject should contain nbproject directory)
  • Captured output screen  – name it as JavaIntro-javaserialization.gif or JavaIntro-javaserialization.jpg (or JavaIntro-javaserialization.<whatver graphics format>)
    • Any screen capture that shows that your program is working is good enough.  No cosmetic polishment is required.
  • If you decide to use different IDE other than NetBeans, the zip file should contain all the files that are needed for rebuilding the project.

 

 

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