Serialization Problem

[Rahul Sudip Bose]

This is an attempt to make a code snippet from kathy sierra's scjp book into a fully working program. First an overview of the problem :

Dog class implements Serializable and has :
a size - an int
a Collar - an object

Collar class should not implement Serializable.
I want to serialize and then de-serialize a "dog" object inside SerialTransient class. I do not know what code to use in the lines between 48 and 52.

import java.io.*; 
class Collar 
{
private int collarSize; 

Collar(int size)
{collarSize = size;}
public int getSize( ){return collarSize;}
}

class Dog implements Serializable
{
transient Collar myCollar; 
int dogSize;
Dog(Collar c, int size)
{
myCollar = c;
dogSize = size;
}

public Collar getCollar( ){return myCollar;}

private void writeObject(ObjectOutputStream oos) throws Exception 
{
oos.defaultWriteObject( ); 
oos.writeInt(myCollar.getSize( )); 
}

private void readObject(ObjectInputStream ois) throws Exception     
{
ois.defaultReadObject( );
myCollar = new Collar (ois.readInt( )); 
}

}

public class SerialTransient
{
public static void main(String args[ ])
{
System.out.println("\nWelcome to serialization and transient program !");
Collar c = new Collar(3);
Dog d = new Dog(c,1);
System.out.println("\nBefore serialize, dog size is " + d.dogSize + " collar size is " + d.myCollar.getSize( ));

//**********************************WHAT CODE TO PUT HERE ???**********************************************

System.out.println("\nAfter DE-serialize, dog size is " + d.dogSize + " collar size is " + d.myCollar.getSize( ));

}
}

[Rob Spoor]

You need to serialize the Dog object first, then de-serialize it again. This means using an ObjectOutputStream and ObjectInputStream, using either a file (FileOutputStream and FileInputStream) or memory (ByteArrayOutputStream and ByteArrayInputStream) as backing streams:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(d);
oos.close(); // to write footers etc
byte[] content = baos.toByteArray(); // the Dog in serialized form
ByteArrayInputStream bais = new ByteArrayInputStream(content);
ObjectInputStream ois = new ObjectInputStream(bais);
d = (Dog)ois.readObject();

[Rahul Sudip Bose]

Rob Spoor wrote:You need to serialize the Dog object first, then de-serialize it again. This means using an ObjectOutputStream and ObjectInputStream, using either a file (FileOutputStream and FileInputStream) or memory (ByteArrayOutputStream and ByteArrayInputStream) as backing streams:

Your code here

I put your code in a try-catch block and inserted it into my program an got a runtime exception. Can you explain your approach a little ?

Welcome to serialization and transient program !

Before serialize, dog size is 1 collar size is 3
Exception in thread "main" java.lang.NullPointerException
        at SerialTransient.main(SerialTransient.java:79)

[Rob Spoor]

Strange. I tried your code with my code in the middle* and it ran successfully. Then again, I didn't use try-catch but declared the main method to throw Exception (just to be lazy).

Can you show us the code you added? I think the problem is in there somewhere.

[Rahul Sudip Bose]

Rob Spoor wrote:Strange. I tried your code with my code in the middle* and it ran successfully. Then again, I didn't use try-catch but declared the main method to throw Exception (just to be lazy).

Can you show us the code you added? I think the problem is in there somewhere.

This is the line of code which causes an exception at runtime :

System.out.println("\nAfter DEserialize, dog size is " + d.dogSize + " collar size is " + d.myCollar.getSize( ));

This is the exception i get :

Welcome to serialization and transient program !

Before serialize, dog size is 1 collar size is 3
Exception in thread "main" java.lang.NullPointerException
        at Test.main(Test.java:68)

Here is the complete code :

import java.io.*;

class Collar
{
private int collarSize;
Collar(int size)
{
collarSize = size;
}
public int getSize( )
{
return collarSize;
}
}

class Dog implements Serializable
{

transient Collar myCollar;
int dogSize;
Dog(Collar c, int size)
{
myCollar = c;
dogSize = size;
}

public Collar getCollar( )
{
return myCollar;
}

public void writeObject(ObjectOutputStream oos) throws Exception
{

oos.defaultWriteObject( );
oos.writeInt(myCollar.getSize( ));

}

public void readObject(ObjectInputStream ois) throws Exception
{
ois.defaultReadObject( );
myCollar = new Collar (ois.readInt( ));
}


}

public class Test
{
public static void main(String args[ ]) throws Exception
{
System.out.println("\nWelcome to serialization and transient program !");
Collar c = new Collar(3);
Dog d = new Dog(c,1);
System.out.println("\nBefore serialize, dog size is " + d.dogSize + " collar size is " + d.myCollar.getSize( ));

// YOUR CODE :
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(d);
oos.close( ); // to write footers etc
byte[ ] content = baos.toByteArray( ); // the Dog in serialized form
ByteArrayInputStream bais = new ByteArrayInputStream(content);
ObjectInputStream ois = new ObjectInputStream(bais);
d = (Dog)ois.readObject();

System.out.println("\nAfter DEserialize, dog size is " + d.dogSize + " collar size is " + d.myCollar.getSize( ));

}
}

How do i use these methods, its not mentioned in the book :
1. public void writeObject(ObjectOutputStream oos) throws Exception
2. public void readObject(ObjectInputStream ois) throws Exception

Thanks for helping me.

[Rob Spoor]

Do you seriously don't use any indentation in your code?

The problem is that the two methods only work when they are private. For some reason you made them public somewhere, because they were private in your original post.

[Luigi Plinge]

Why does making these methods public stop them from working?

[Rob Spoor]

To be honest, I have no idea. This page says this about it:

Notice that both methods are (and must be) declared private, proving that neither method is inherited and overridden or overloaded. The trick here is that the virtual machine will automatically check to see if either method is declared during the corresponding method call. The virtual machine can call private methods of your class whenever it wants but no other objects can. Thus, the integrity of the class is maintained and the serialization protocol can continue to work as normal. The serialization protocol is always used the same way, by calling either ObjectOutputStream.writeObject() or ObjectInputStream.readObject(). So, even though those specialized private methods are provided, the object serialization works the same way as far as any calling object is concerned.

[Rahul Sudip Bose]

Rob Spoor wrote:You need to serialize the Dog object first, then de-serialize it again. This means using an ObjectOutputStream and ObjectInputStream, using either a file (FileOutputStream and FileInputStream) or memory (ByteArrayOutputStream and ByteArrayInputStream) as backing streams:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(d);
oos.close(); // to write footers etc
byte[] content = baos.toByteArray(); // the Dog in serialized form
ByteArrayInputStream bais = new ByteArrayInputStream(content);
ObjectInputStream ois = new ObjectInputStream(bais);
d = (Dog)ois.readObject();

Can you tell me what is the role of :
line 4 and why do we need it ?
line 5 ? why did we have to create an array ?

[Rob Spoor]

When object are serialized using an ObjectOutputStream, there are more bytes written than just the objects themselves. There is some overhead, definitely at the start and probably also at the end. By closing the ObjectOutputStream you make sure that the overhead at the end is also written.

The byte[] is where the serialized objects are stored. Not in a File, because I didn't use a FileOutputStream / FileInputStream pair, but in memory as a byte[]. I could write the same in almost the same way using a file:

File file = new File("output.ser"); // or any other file
OutputStream os = new FileOutputStream(file);   
ObjectOutputStream oos = new ObjectOutputStream(os);   
oos.writeObject(d);   
oos.close(); // to write footers etc; also closes the FileOutputStream this time
// no more byte[]
InputStream is = new FileInputStream(file);   
ObjectInputStream ois = new ObjectInputStream(is);   
d = (Dog)ois.readObject();

[Rahul Sudip Bose]

Rob Spoor wrote:.....

Thanks a Bazillion !!!
bazillion is a "very big" number !

[Rob Spoor]

You're welcome