I've learned a lot about JMX over the past couple of years and have built some pretty fancy MBeans for my web applications. However, I'm not sure I have a good answer to a pretty basic question:
Why use JMX over simple HTTP servlets?
My current web applications offer redundant monitoring options: I can access the data that needs to be monitored via JMX with a client like JConsole or I can access that same data in XML format via a servlet. I don't see a strong reason to use one method over another, but the servlet method does have the major advantage of being readable by a simple http client / web browser.
I can see how JMX would be quite useful for Java applications that aren't web apps, but I can't see any advantage to using JMX for a web app.
Why use JMX over simple HTTP servlets?
From my standpoint, JMX is better for 3 reasons:
JMX offers a much easier interface to specific data items. You can certainly write the same functionality in a number of servlets, but it is easier for me to expose those using JMX.
For example, if you are using Spring then you can use the org.springframework.jmx.export annotations (@ManagedResource, @ManagedAttribute, etc.) to mark up your classes. I've also published my SimpleJmx framework so you can easily expose attributes and operations with just a couple annotations independent of Spring. For example:
@JmxResource(domainName = "j256", objectName = "lookupCache")
public class LookupCache {
// this can also be done as @JmxAttributeMethod on the getter/setters
@JmxAttributeField(description = "Number of hits in the cache")
private int hitCount;
...
@JmxOperation(description = "Flush the cache")
public void flushCache() {
...
}
}
I have a fully working example program to see how it works. So all you need to do to expose a value or operation is add an annotation to the class and each attribute and/or method. The code to publish it using SimpleJmx looks like the following. Spring is similar albeit with beans:
// create a new server listening on port 8000
JmxServer jmxServer = new JmxServer(8000);
jmxServer.start();
// register our lookupCache object defined above
jmxServer.register(lookupCache);
To get similar functionality in servlets would take a lot more code than just annotations. That said, there may exist frameworks which provide similar functionality in servlet land that I don't know about.
Some more notes: