Question

What should one prefer in case of Singleton Design Pattern.

1)make whole getInstance() method synchronized 
OR
2)make only critical section synchronized.

what should be one’s approach and why?

Answer 1

First of all, try to consider whether or not you need lazy instantiation. If not, there’s no synchronization involved since your INSTANCE will be initialized at class loading time.

If you do need to lazily initialize your instance, then do not make getInstance synchronized, as this would cause all your threads to wait for each other for no reason once the instance is initialized.

If you’ll use a synchronized block inside, you need to double-check for null (outside and inside the synchronized block) to make sure you end up with only one instance; also, you need your instance to be volatile.

The best-practice approach is to have a private nested class SingletonHolder that initializes the instance of your singleton at load time (but is only loaded when getInstance() of the container class is called).

However, if you don’t need lazy instantiation, the best-practice is to use an enum with one constant.

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Long story short, I think you’ll find it all here: http://en.wikipedia.org/wiki/Singleton_pattern