JSR-356 comprises both: client and server endpoints.
This screencast demonstrates how to connect to a (Java EE 7) server from a plain Java client using the RI (tyrus):
Also checkout: hellsockets.
Web Apps, SPA, PWA with vanilla Java Script (ES 6+), CSS 3 and WebStandards only. As simple as possible, but not simpler. See you at: (Progressive) Web apps, Single Page Apps and WebStandards airhacks workshops at MUC airport, Winter Edition
airhacks.fm the podcast:
Stay in touch: airhacks.news.