I found that someone has a working use for java.net.SocketImplFactory, that is adding SOCKS support to Java sockets. The code is here.

I thought I'd mention this just for the sake of completeness

I just committed unix-domain sockets (AF_UNIX) for GNU Smalltalk.

GNU Smalltalk's socket library is more or less based on the java.net library, but java.net does not have AF_UNIX sockets! Okay, they are not portable to Windows, but you'd expect someone to have written an extension library for that. But no, there is none. Why?

It turns out that there are two problems.