As you know, HTTP is a stateless protocol. For most use cases in the enterprise, the statelessness of HTTP is a huge boon for scalability - a benefit that is also extended to REST. However, there is a certain class of arguably emerging use cases for which the stateless nature of HTTP is actually a scalability challenge. These use cases require the bidirectional, full-duplex, asynchronous characteristics of stateful protocols such as TCP.
Examples include chat-like online collaboration, stock-ticker like command-control/monitoring, online gaming and the like. WebSocket (and the Java API for WebSocket newly included in Java EE 7 :-)) is ideal for these cases.
Between the seemingly diametric opposites of plain HTTP and WebSocket, there's also Server Sent Events (SSE). SSE is aimed at yet another scenario - a steam of events generated by the server and consumed passively by the client. Although not standardized in JAX-RS 2/Java EE 7,
Jersey/GlassFish includes support for SSE.
In an very detailed blog post, Rahman Usta of the Istanbul JUG explains SSE and demonstrates a non-trivial use of the Jersey/GlassFish support for SSE.