This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The best way to use singleton objects in Java is making the use of Singleton Session Bean (@Singleton). For you to understand how these issues are resolved, study JEE specification of the life cycle of Singleton Session Bean and hows the server providers have settled in clustered environments.