Today there are still a number of standard Ruby libraries that we don't support, usually due to their dependence on C extensions or libraries not easily called from a JVM language. I propose that instead of omitting these libraries entirely, we should have stub files in our stdlib that present a more informative error. Something like this:

$ jruby -e 'require "gdbm"'
LoadError: gdbm is not supported on JRuby.
Suggested alternatives: JavaDB, HSQL, HypersonicDB

   <main> at /Users/headius/projects/jruby/lib/ruby/stdlib/gdbm.rb:1
  require at org/jruby/RubyKernel.java:956
   (root) at /Users/headius/projects/jruby/lib/ruby/stdlib/rubygems/core_ext/kernel_require.rb:1
   <main> at -e:1

The alternatives could be hardcoded or somehow sourced from https://github.com/jruby/jruby/wiki/C-Extension-Alternatives

Here's a short list of the stdlib exts we don't support: dbm, gdbm, objspace, sdbm, tk. There may be others, and there may be subcomponents of mostly-Ruby libraries that are native and not supported in JRuby right now.