Wednesday, December 9, 2009

The Dreadful JMX Tunneling Problem

JMX is notoriously difficult to access remotely. It relies on RMI which means that you need a connection to an RMI registry and a separate connection to an RMI server. The port (and host name) to be used for the second connection are assigned by the RMI registry, and can thus not be "guessed" in advanced.  This makes it almost impossible to setup the necessary port forwarding or ssh tunnels to access JMX on a server which is not directly in your network.

We used to use JBoss, which provides a html wrapper around the MBean server ("jmx-console"), thus allowing easy access to our MBeans. It's not as nice and graphical as JConsole, but at least http is easy enough to tunnel through ssh or firewalls.

After switching to tomcat (for various other reasons), I was looking for a solution to this "access our MBeans through ssh tunnels"-problem. There are all kinds of workarounds available to make RMI "tunnelable', but everything I tried came with its own set of problems (for example having to append stuff on the class path of JConsole).

So if tunneling turned out not to be a viable option, I started looking for html based alternatives like the "jmx-console" from JBoss. I found a couple of them:
JManage looks very nice and feature complete but it has the downside of being a standalone application running an embedded jetty rather than something I can simply deploy in tomcat.
• On some mailing list I found somebody who once created an application called "jmx-console" which does what I want. I tried it and it seemed to work OK, but it's an old and unmaintained project.
Coldbeans has an application written entirely in JSP. When testing it, it found it somewhat cumbersome to work with though, and I couldn't seem to access all my MBean operations (as if some of them where "read only").

During my search, I learned that the original reference implementation for JMX (or now in contains a class named HtmlAdaptorServer, which didn't make it into JDK 5 when JMX was first introduced. The reference implementation is still usable in itself though (in maven2 repository as com.sun.jdmk:jmxtools:1.2.1). This HtmlAdaptorServer does exactly what I need: expose MBeans in a straightforward html view. I simply instantiate it in a ServletContextListener like this:

final HtmlAdaptorServer adapter = new HtmlAdaptorServer(port);
ManagementFactory.getPlatformMBeanServer().registerMBean(adapter, new ObjectName("my.domain:name=htmladapter,port=8000"));

Although this is a good enough solution for me right now, I still think some configuration options to make JMX/RMI easily 'tunnelable' (being able to fix the port and host name would probably be enough) are something we should consider adding to the JDK...