The <Listener> element (and the more common variants described below) configures the "remoting" software that connects the in-process and out-of-process portions of the SP. For most purposes, you can ignore it as an internal detail that doesn't require your intervention.

This remoting layer is an inter-process communication mechanism, but is not intended to be used between machines. It is accessed too frequently, typically at least once per web request, to enable a fully distributed separation of the components across a network. The best analogy would be the AJP protocol connector used between Apache and Tomcat, but is even less efficient. Used over a domain socket or loopback connection, the speed is not a factor, but across a network, performance will suffer.

Listener Alternatives

The SP includes a pair of socket-based implementations, one recommended for Unix/Linux use and the other for Windows.

All existing SP variants require one (and only one) of the following elements be specified:

• <UnixListener>
  • Listener implementation that relies on a Unix domain socket.

• <TCPListener>
  • Listener implementation that relies on a TCP socket. Can be used across a network, but this is not recommended, as noted above.

• <Listener>
  • Pluggable extension point for new alternatives, requires a type attribute specifying the plugin's unique type. Will contain other content specific to the plugin.