Java Singleton Design Pattern : Questions

Question

I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can i

Answer 1

I disagree with @irreputable.

The scope of a Singleton is its node in the Classloader tree. Its containing classloader, and any child classloaders can see the Singleton.

It's important to understand this concept of scope, especially in the application servers which have intricate Classloader hierarchies.

For example, if you have a library in a jar file on the system classpath of an app server, and that library uses a Singleton, that Singleton is going to (likely) be the same for every "app" deployed in to the app server. That may or may not be a good thing (depends on the library).

Classloaders are, IMHO, one of the most important concepts in Java and the JVM, and Singletons play right in to that, so I think it is important for a Java programmer to "care".