Since JBoss AS7 you can use the domain mode which control the server instances by an independent process. The server which handle the application requests are restarted if the JVM crash.
If you need more monitoring JON might be an option.
If your server is so unstable it requires constant monitoring, you've got a problem ;-)
You can of course have a simple cron job that checks if the process from the pid file is still alive.
Yes, a crash should be very unusual
But I've learned that cron will not a good idea as I saw system admins which manipulate the system and the job disapear or start JBoss with every run.
So I would prefer JON or JBoss domain, here 'normal' admins and other system users might keep their fingers away.
If your sysadmins lose cron jobs, you also have a problem ;-)