implementing singleton pattern

[gaurav abbi]

hi,
if i implement singleton pattern, i will be doing something like this

public class Singleton{
private static Singleton obj;

private Singleton(){}

public Singleton getInstance(){
if(obj == null) obj = new Singleton();
return obj;
}
}

this is fine, but i was asked one question
if i serialize the object and then desrialize it, what will happen and how can i make sure that when after desrializing on a different jvm, my contructor call should not happen whrn i require the instance..

i know in serialization we have to explicitly serialize static objects, but even in that case i'm not sure hw my constructor call will not happen, because at the most, what i ll do is to serialize the variables definig state of the object and then restoring that state.

i'm somewhat stuck .

[Shawn Vader]

Two things,
1) Your implementation of the singleton pattern although correct is not 100% guaranteed to produce one instance, there is a small chance that there could be more than one instance created. This version on the singleton is preferred.

public class Singleton {

public static final Singleton INSTANCE = new Singleton();
//private constructor
private Singleton() {
//do some construction
}

public Singleton getInstance() {
return INSTANCE;
}
}

//you can then access it by using
Singleton.INSTANCE;
or
Singleton.getInstance()
[/LIST]

Secondly if you wanted to serialize this class you would need to implement serialization, if you don't know much about serialization read this short article on Sun website
http://java.sun.com/developer/technicalArticles/Programming/serialization/
There is a lot more to serialization than people assume so make sure you fully understand serialization before just implementing. One example is the the virtual machine will add a public constructor to this singleton class!


If you correctly serialize this object the state of the object will be maintained so if you deserialize it the getInstance() would not call the constructor again.

Shawn

[gaurav abbi]

hi Shawn,
can you tell me the case in which it can go wrong, if you are looking from multithreading prespective, i can synchronize the block of code which checks whether the obj is null or not...
any other flaw, please let me know