<p>I am working on writing a Highly Available agent for JBoss Application Server to run on Solaris Open HA Cluster. As I don't know much of the JBoss AS, can someone please tell me how can I probe the status of the application server?</p>

<p>I want to know the health of the application server, for example whether it is currently running or not.</p>

Out of the box, JBoss has a JMX console that provides information about the modules loaded into the micro-kernel and the services that are running.  This application is usually available at
http://hostname:8080/jmx-console
, and you could conceivably use its presence or absence as an indicator of whether your JBoss server is running.
In the context of an HA cluster, you probably have a load balancing switch or other layer4-7 aware device in front of the nodes.  If you want to detect the status of each node in the cluster, you'll need to make sure your using the node's local IP address.
Of course, most of the processes that run in the micro-kernel are JMX enabled ... if you want to know the status of an individual process, just ask it!