Serialization

[Arpan Ghoshal]

So I was studying serialization for OCJP exam from the book written by Jeanne Boyarsky and Scott Selikoff when on page number 448  I found a question :

Why shouldn't every class be marked Serializable ?

One of the answers said : The compiler will throw an exception if certain classes are marked Serializable
I thought this was a correct answer but according to the book this is not a correct I don't understand why, check this code:

public class MyClass implements Serializable
{
    Object ob=new Object();
    
    public static void main(String args[]) {
       
    }
}

This code throws a java.io.NotSerializableException when executed. Here the MyClass class if marked Serializable will throw an exception so the option should be correct.


Next although I understood serializability I did not understand one thing the book said. What does this mean ?

"Java will call the constructor for the first non-serializable no-arguments parent class during deserialization, skipping any constructors and default initializations for serializable classes in between"

Can someone explain this sentence possibly with an example because it seems too difficult to understand for me.

By the way a big thank you to this forum and all the awesome people in it. It has proven to be life saviour for me in clearing all my doubts....

[Stephan van Hulst]

That code doesn't throw any exceptions when run.

Arpan Ghoshal wrote:Can someone explain this sentence possibly with an example because it seems too difficult to understand for me.

public class A {
  public A() {
    System.out.println("A");
  }
}

public class B extends A {
  public B() {
    System.out.println("B");
  }
}

public class C extends B implements Serializable {
  public C() {
    System.out.println("C");
  }
}

public class D extends C {
  public D() {
    System.out.println("D");
  }
}

final class Example throws Exception {
  public static void main(String... args) {
    Path path = Paths.get("D.bin");

    try (
      InputStream inputStream = new BufferedInputStream(Files.newInputStream(path));
      ObjectInputStream objectStream = new ObjectInputStream(inputStream);
    ) {
      D d = (D) objectStream.readObject();
    }
  }
}

For this you must first have written an object of type D to a file named D.bin.

The expected output is:
A B

[Arpan Ghoshal]

Stephan van Hulst wrote:That code doesn't throw any exceptions when run.

Yes I don't know how I got a NotSerializableException before but I want to clarify something the book says :

"The requirement for properly marking an object serializable may involve nested objects.For example if a Cat class is marked serializable and contains a reference to a Tail object,then the class definition for the tail object must also be marked as Serializable."

Consider this..

import java.io.Serializable;

public class Cat implements Serializable 
{
    Tail tail=new Tail();
    public static void main(String args[]) 
    { }
}

class Tail //Not serializable code compiles without any errors or exception
{}

Here the Cat class contains an instance variable containing an object of class Tail but the Tail class is not marked Serializable then how does this code work ?
Also does a class need to marked serializable to be written to a file using writeObject() ? I think the answer is no then I can't understand what is actually the need for Serialization ?

Stephan van Hulst wrote:The expected output is:

Why doesn't it print D also ?

[Arpan Ghoshal]

Arpan Ghoshal wrote:
Yes I don't know how I got a NotSerializableException before

Okay I found out why I got the exception before. If a field is not serializable like Object ob=new Object(); and the class containing the field is marked serializable then the code works and there is no exception but the problem arises if we try to write an object of the class in a file using writeObject() then this will throw a java.io.NotSerializableException.

[Stephan van Hulst]

Arpan Ghoshal wrote:Here the Cat class contains an instance variable containing an object of class Tail but the Tail class is not marked Serializable then how does this code work ?

You didn't try to write an object of type Cat.

Also does a class need to marked serializable to be written to a file using writeObject() ? I think the answer is no then I can't understand what is actually the need for Serialization ?

Yes. If you want to write an object to an ObjectOutputStream, it needs to be Serializable and all its instance fields that are not transient also need to be Serializable.

Why doesn't it print D also ?

Because D is Serializable.

Why shouldn't every class be marked Serializable ?

One of the answers said : The compiler will throw an exception if certain classes are marked Serializable

You've already seen that the compiler doesn't throw an exception when you mark a class as Serializable. It's only when you try to write an instance at runtime, and not all non-transient instance fields are also Serializable.