The Java API for WebSocket (JSR 356) is one of the key parts of Java EE 7. It allows Java developers to write Java HTML5/WebSocket applications with a simple, fluent, high-level API, both on the client and the server-side. With the official Java EE 7 launch looming there's already a flurry of great articles on topics like JSR 356.
In a recent OTN article, Johan Vos provides an overview of the Java API for WebSocket. He covers WebSockets, the JSR, the programming model, API details through code examples and advanced features. The article is a great starting point to learning WebSocket. You could also take a look at my slide deck below.
You are of course encouraged to take a look at the official specification (the JSR recently had it's final release). At fifty some pages, it is pretty digestible.