Regarding static holder singleton pattern

Question

I have developed singleton in many ways depending upon the condition like volatile/lazy singleton, eager singleton, normal singleton and through Enum also, but specifically

Answer 1

This is a way to make a thread-safe lazy singleton by exploiting the way how JVM loads classes. You can read more about why and how to correctly implement it in Bloch's Effective Java book.

Remember, that from the testable code point of view singletons (and global state in general) are not beneficial and should be avoided.