For jruby 1.7.11 running either on my iMac or Windows box, I get an illegal constant trying to lookup 'AF_INET' when calling Socket.new. Looking at the code, on line 540 of SocketUtils.java -

    addressFamily = AddressFamily.valueOf("AF_" + domainString);

Apparently, that extra AF_ is not needed if it already starts with AF_?

Here is a small text program that shows this problem:
jruby-1.7.11:~ $ cat bug_af_af_inet.rb

require 'socket'
addrinfo = Socket.getaddrinfo('localhost', 22)
family = addrinfo[0][0] # this is the string 'AF_INET' or 'AF_INET6'
socket = Socket.new(family, Socket::SOCK_STREAM, 0)
socket.close

jruby-1.7.11:~ $ ruby bug_af_af_inet.rb
Enum.java:238:in valueOf': java.lang.IllegalArgumentException: No enum constant jnr.constants.platform.AddressFamily.AF_AF_INET from AddressFamily.java:4:invalueOf'
from SocketUtils.java:540:in addressFamilyFromArg' from RubySocket.java:418:ininitDomain'
from RubySocket.java:337:in initFieldsFromArgs' from RubySocket.java:188:ininitialize19'
from RubySocket$INVOKER$i$initialize19.gen:-1:in call' from JavaMethod.java:1056:incall'
from JavaMethod.java:677:in call' from CachingCallSite.java:286:incacheAndCall'
from CachingCallSite.java:81:in callBlock' from CachingCallSite.java:85:incall'
from RubyClass.java:825:in newInstance' from RubyIO.java:852:innewInstance'
from RubyIO$INVOKER$s$0$0$newInstance.gen:-1:in call' from DynamicMethod.java:226:incall'
from DynamicMethod.java:222:in call' from MethodHandle.java:636:ininvokeWithArguments'
from InvocationLinker.java:191:in invocationFallback' from bug_af_af_inet.rb:5:infile'
from bug_af_af_inet.rb:-1:in load' from Ruby.java:811:inrunScript'
from Ruby.java:804:in runScript' from Ruby.java:673:inrunNormally'
from Ruby.java:522:in runFromMain' from Main.java:395:indoRunFromMain'
from Main.java:290:in internalRun' from Main.java:217:inrun'
from Main.java:197:in `main'

Switching back to MRI, you don't see this behavior:
jruby-1.7.11:~ $ rvm use ruby
ruby-2.1.1:~ $ ruby bug_af_af_inet.rb
ruby-2.1.1:~ $

A workaround is to map the constant yourself:

def get_address_family(addrinfo)
  # jruby handles this wrong, so map it by hand
  family = addrinfo[0][0]
  family = Socket::AF_INET if family == 'AF_INET'
  family = Socket::AF_INET6 if family == 'AF_INET6'
  family
end