2019
https://vmlens.com/articles/cp/why_volatile/
2011
https://javarevisited.blogspot.com/2011/06/volatile-keyword-java-example-tutorial.html
    https://javarevisited.blogspot.com/2020/01/what-is-happens-before-in-java-concurrency.html
    https://javarevisited.blogspot.com/2014/05/double-checked-locking-on-singleton-in-java.html
    https://javarevisited.blogspot.com/2012/07/why-enum-singleton-are-better-in-java.html
    https://dzone.com/articles/java-singletons-using-enum
    https://javarevisited.blogspot.com/2011/04/synchronization-in-java-synchronized.html