Hello Kang,
I have done GSoC for XMPP Standards Foundation a few years ago. XSF in previous years was working as an umbrella organization for a number of smaller organizations.
Some XMPP organizations joined previously also alone, without XSF so you can also look for them. As for Java projects go:
Here is Conversations (Android client).
Java projects (Server, Client Library and Desktop Client ) are also done by Ignite Realtime Foundation ( I have done my project for them as part of XSF ):
( If you don't have an account you can join anonymously through a web client
https://www.igniterealtime.org/support/group_chat.jsp ).
I hope I didn't omit any other Java projects participating in GSoC.
Cheers,