If you open a connection to a server with the new ws protocol (unlike AJAX, where clients poll the server for updates), WebSocket allows the server to send messages to the clients to push data instead of pulling it.
Polling, long polling, streaming, and WebSocket
In polling, the client polls the server at regular intervals asking for updates; the server replies with any updated content if available. In long polling, the client makes a connection to the server, which is kept alive until the server has an update. After the update is sent to the client, the connection is re-established and held open until another update is obtained from the server.
In streaming, the client creates a connection to the server and this connection is never ended; updates are sent to the clients indefinitely. Now with WebSockets, the client makes a new connection with the ws protocol; the server registers the client and then pushes data to it when an update is available.
The WebSocket object attribute websocketobjext.readystate shows the different stages of connection establishment; the possible values and meanings are: 1) The connection is not yet established; 2) The connection is established and communication is possible; 3) The connection is at the stage of closing the handshake; and 4) The connection is closed. The websocketobject.buffered amount attribute shows the number of bytes queued to send.
WebSocket events include onopen(), triggered when a connection is opened; onclose(), when it is closed; onerror(), when any error occurs; and onmessage(), when a new message is received from the server.
WebSocket methods include send() to send a message to the server, and close() to close an established connection.
You need a server-side implementation for WebSocket to work, of course, and for this you use WebSocketServlet, a new type of servlet available in Tomcat 7.0.32; see the API documentation at http://bit.ly/W6sR16.
WebSocketServlet methods include verifyOrigin(String origin), which helps check the origin of the message (the domain name of the server to which the client is connected) letting you allow or deny messages from a list of servers. Then there’s createWebSocketInbound(String subProtocol, HttpServletRequest request), invoked when a client makes a new WebSocket connection request; it returns an object of type streaminbound, representing a connection. The API doc is at http://bit.ly/V7Lt66.
Its methods include onopen(wsoutbound outbound), invoked when a new connection is established; onclose(int status), if an existing connection is closed; ontextmessage(charbuffer message), when a client sends a text message to the server; onbinarymessage(bytebuffer message), likewise, but message is binary.
The requirements to be able to use this are Tomcat 7.0.32, JDK (a recent version), and Firefox/Chrome. An example of a simple chat application is available at http://www.opensourceforu.com/article_source_code/feb13/