Things to consider while making a class singleton?

Question

sometimes i need to share the state of object across application.I get confused should i declare it singleton or declare state vaiables as static.
As per mythoughts, if intention is just to share the state of object across application i should go for static variables. Is this right?
My understanding about when we should go about singleton is below
We should declare the class as singleton when we need only one instance across jvm. But i am not unable to find any practical scenario
when we where we may need singleton . Any help here will be appreciated. Everywhere on different sites i get to see the example of logger
class where the reason is generally given as so that single log file is created not multiple log files.
But again this could have been achieved with declaring the file variable as static in logger file?
So actual reason is for declaring the logger as singleton is becuase we need the single log file with issues avoiding concurrent writing
which wont be possible if we make the separate instance of logger for each write..
Is the above reasoning correct so that i can proceed in right direction?

Answer 1

How will declaring its state as static accomplish that objective?With declaring variables as class variable instead of instance variables, there will be a single copy of each variable. In each instance (in this case logger if we dont declare it singleton) we can check if file is already created or not. I mean it will be visible across all instances.
No, because the file name isn't the only state. There is also the output stream/writer, its current position, its charset, the log level, the logger name, its filters, its formatters, ...Agreed. I just wanted to convey the point. As you said there will be other parameters,in that case we can declare all of them as static.
A configuration file holder is a good example: there is only one configuration file so there should only be one holder instance.Thanks for pointing it out. Configuration file is used mainly to read the properties. we usually dont update the values there.So even if we dont make it singleton it may be correct. The advantage i can think of making it singleton is that if configuration file is used frequently then we dont have create the object again and again.
So actual reason is for declaring the logger as singleton is becuase we need the single log file with issues avoiding concurrent writing
No it isn't, and that doesn't follow from anything you said previously so the 'so' part is meaningless.I want to say here is that t to have single log file should not be the only reason behind making the logger file as singleton, other reasons can be handling of concurrent writing too.
Have a look at the Wikipedia article on the Singleton pattern, or buy the booki have gone through the singleton pattern in headfirst book and some of the articles on net. But they mainly describe how the make the class as singleton and the reason that We should declare the class as singleton when we need only one instance. But looking for actual scenarios where we need singleton. So i took the examplle of logger file which is used in many project and trying to understand it is constructed as singleton so that i can use in my project if required.
Edited by: JavaFunda on Aug 28, 2011 3:51 AM
Edited by: JavaFunda on Aug 28, 2011 3:56 AM