Implementing a custom SocketImpl

[Lundell, Jared]

I have a use case where I need to implement a custom SocketImpl class and I've run into some difficulty caused by the current design of SocketImpl and the related classes in OpenJDK.  My use case is implementing a custom proxy protocol, similar in spirit to what SocksSocketImpl does but using a different wire protocol.  I need to be able to swap this proxy in underneath some existing code that I can't modify, which is why I think a SocketImpl is the appropriate solution rather than a SocketFactory (which would be much easier if I could modify the code in question).
 
SocketImpl is a public class, so I think I'm fairly safe in assuming it's intended as an extension point for application authors, but in practice it seems very difficult to actually extend.  Since I'm trying to accomplish something very similar to what SocksSocketImpl does I'd like to take the same approach that it and HttpConnectSocketImpl use of delegating all of the OS interactions to the base PlainSocketImpl class and just adding the customizations I need on top.  However, I can't extend PlainSocketImpl the way those other classes do because it is package-private and I can't create new classes in java.net.  Encapsulating a PlainSocketImpl doesn't really seem viable either since all of the methods are also protected or package-private.  The alternatives I can see are to either implement a SocketImpl completely from scratch (including all the native code) or to resort to classloader hacks to be able to add a class to java.net which extends PlainSocketImpl.  Neither of those are particularly appealing.
 
I was going to ask whether it would be reasonable to make PlainSocketImpl public, but reading through the net-dev archives[1] I discovered that there's some refactoring[2] of SocketImpl that's happening right now to prepare for Project Loom related work[3].  That change is introducing DelegatingSocketImpl and PlatformSocketImpl classes and changes SocksSocketImpl and HttpConnectSocketImpl to extend DelegatingSocketImpl rather than PlainSocketImpl.  I'd love to be able to extend DelegatingSocketImpl for my use case, but it's package-private too, nor does there appear to be a way to get a PlatformSocketImpl.
 
Is there any way that we could make it easier to implement a custom SocketImpl (or am I missing some other possible approach)?  If so, would it be reasonable to make DelegatingSocketImpl public and introduce a DefaultSocketImplFactory that can be used to construct a PlatformSocketImpl?
 
Thanks,
Jared
 
PS I just joined the net-dev mailing list, so there may be some historical context I've missed.
 
[1] https://mail.openjdk.java.net/pipermail/net-dev/2019-March/012218.html
[2] http://cr.openjdk.java.net/~alanb/8220493/0/webrev/
[3] http://openjdk.java.net/jeps/8218559