There is a great command line JMX client available at http://crawler.archive.org/cmdline-jmxclient/index.html which allows you to query everything JMX. This is especially useful for a quick insight into the health of your Java server. It can also be used for monitoring, but beware that every time it is run a new JVM instance is created. This new instance only lives long enough for the query to run, but depending on how much you are trying to monitor, and how often you want to check, this can cause some serious performance issues. An alternative would be to use SNMP, but there is much more information available from JMX.
First, JMX needs to be enabled using command line options similar to the following on your Java server (adjust as needed)
-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=1616 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.ssl=false
Then, here is a quick way to list everything JMX can tell you using the command line client.
java -jar cmdline-jmxclient.jar - localhost:1616