We have distributed web application developed few years back on JDK 5.
How JMX will help this application?
JMX is a standardized way of getting information out of a running system and to invoke operations on it. The JVM gives you a set of MBeans through which you can access runtime data like memory consumption, GC stats and some more data. You can also invoke a number of operations. Your app server will also give you a number of MBeans which you can use to control the server and installed applications.
Take a look at jconsole (included in the JDK) to see what JMX offers for you!