Do Java web-server apps have any way to PUSH?

Web-servers work in response to incoming HTTP requests... process the request and return an HTTP response. Are there any common ways that a server can PUSH data to clients in this architecture... e.g a request comes in from开发者_如何学JAVA client1 and the server wants to notify client2? It can obviously be done by a non-web server, using sockets, but what about a web-server app which has to support page requests AND allow PUSHing data..?

what about a web-server app which has to support page requests AND allow PUSHing data..?

Servlet 3.0 introduces Async support allowing to write Comet style applications (i.e. applications using Long-lived HTTP connections and either long polling or streaming).

If you can't wait for Servlet 3.0 Async support and don't want to use proprietary Comet or WebSocket support from containers (like GlassFish, Jetty), then have a look at Atmosphere.

See also

• JavaOne 2008: Comet (AJAX, Grizzly and Cometd)
• Asynchronous processing support in Servlet 3.0
• Servlet 3.0 Async API or Atmosphere? A Simple Comparison

You can use web app containers like Jetty which support Web Sockets if you don't mind waiting for the web world to catch up to this up-and-coming standard. Then you'll have real bi-directional communications instead of HTTP + Polling or special plug-ins or the like.

No, not without some client side tech (Flash, Silverlight, Applets, etc.)

You could have the page poll the server with AJAX though.

Another possibility would be to abuse HTTP Keep Alive to achieve this. See http://en.wikipedia.org/wiki/HTTP_persistent_connection for some background. In your scenario you would have client2 initiate a connection to the server that then would stay open listening for notifications.

This is not a great solution, first off you need to keep lots of long lived TCP connections around, and if a connection is lost there is no way for the server to reconnect. It must wait for the client to come back.