If I'm given two Java Libraries in Jar format, 1 having no bells and whistles, and the other having lots of them that will mostly go unused.... my question is:
How will the larger, mostly unused library affect the application's performance? Does the JVM do lazy loading of classes, does having a larger class library necessarily mean a larger memory footprint?
A quote from here:
When is a Type Loaded?
This is a surprisingly tricky question to answer. This is due in large part to the significant flexibility afforded, by the JVM spec, to JVM implementations.
Loading must be performed before linking and linking must be performed before initialization. The VM spec does stipulate the timing of initialization. It strictly requires that a type be initialized on its first active use (see Appendix A for a list of what constitutes an "active use"). This means that loading (and linking) of a type MUST be performed at or before that type's first active use.
The short answer is that classes are loaded whenever they are first needed. Note that "needed" also means "referenced by any other class that is being loaded".
So if you have a whole pile of classes that are never touched by any active code, it will not be loaded.
If you would like to see exactly which classes the JVM is loading and when, you can invoke the java command/process with the -verbose:class option.