True singleton implementation

Question

Am I correct in believing that an implementation of the Singleton pattern cannot guarantee the existence of at most one instance of a class across an entire JVM if it uses only simple Java constructs? I understand that the ClassLoader framework allows a class to be loaded several times by different ClassLoaders.
By simple constructs I mean the ClassLoader hierarchy and class initialization and avoiding an "external" service (RMI or other).
I ask because I am trying to implement a file locking procedure. I can guarantee lock exclusivity between JVMs using java.nio.FileLock. However, I am worried that I cannot guarantee exclusivity between Threads within the same JVM (using a classic singleton pattern) - only between Threads with "similar" ClassLoaders.
- Andy

Answer 1

Thanks for your replies everyone.
You could test for the class loader and refuse to
load unless it is the system class loader.How would I do this?You need to test for it in a static initializer.
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It there a simple way to guarantee that this class
will be loaded by the system class loader or does
each class that references it have to be adapted
apropriately?
You test it in a static initializer and throw a runtime exception if the system class loader is not being used.
This also means that the Singleton class needs to be
found in the system class path - this seems like
pretty ugly solution - especially if we're in a J2EE
container...If you are loading via the system then you are already outside the container.
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Also, I cannot find anything that clearly states that
there is only one system class loader. Is this the
case?No idea. If this is of concern to you then perhaps you should actually look at why you really need this sort of thing in the first place.