November 02, 2015

When your machine lies to you, Part 1: Tomcat, JMX, RMI, and classloading

To quote the great debugging genius Dr House:

Everybody lies.

We would like the tools we use to have useful, non-lying debugging mechanisms. Unfortunately, these tools are built by programmers. Programmers, being a subset of Everybody, also lie, and thus the tools they build often have a somewhat fleeting affair with what we think of as truth.

In what I hope (fear?) will become a series, I’d like to recount various occasions when my tools lied to me, or at least tried to make the truth very non-obvious. Since I spend the majority of my working time somewhere in JVM-land, much of this will be about Java.

If you’re coming here to see the solution without the possibly entertaining story of how I got there, skip to the tl;dr.

The case of the disabled classloader

Recently I started to work on a somewhat old J2EE project that had gone through a number of phases. It had some old RMI-based parts, lots of SOAP webservices, a small web UI, a big Oracle database, and other unpleasant things.

After taking some time to get everything set up locally I deployed everything into Tomcat and started it from the command line with $ run. It worked.

But I would like to deploy and run things from my IDE, in this case IDEA. So, set up the Tomcat instance in IDEA, tell it to deploy the appropriate WARs and launch:

Caused by: java.lang.ClassNotFoundException: foo.customer.SomethingSomethingRmiInterface

That’s weird. This class is definitely present.

Caused by: java.lang.ClassNotFoundException: foo.customer.SomethingSomethingRmiInterface
    (no security manager: RMI class loader disabled)

Okay… A classloader being disabled would explain not finding a class. But why is there no security manager? Surely IDEA doesn’t tell Tomcat to not load a security manager.

Well, let’s make IDEA give the -security switch so it will set up a security manager for us.

log4j:WARN No appenders could be found for logger (org.apache.catalina.startup.Catalina).
log4j:WARN Please initialize the log4j system properly.

What? You were logging just fine a minute ago! Okay, let’s edit catalina.policy so you can actually read your own

Class loader creation threw exception
    java.lang.NoClassDefFoundError: Could not initialize class java.util.logging.LogManager

Screw this, you’re supposed to be using log4j, not JDK logging. The -security switch is not getting me anywhere and I don’t feel like messing around with the logging configuration. Launching from the command line works without that switch, so you’d think IDEA could launch an RMI-using Tomcat without it too.

My gut feeling says something else is going wrong here. Time to look at what IDEA is doing differently from what I do on the command line. We both use to start Tomcat, and that is a /bin/sh script. So let’s just add -x to the shebang to see what is really going on.

No screamingly obvious difference to be found, except… Hey. That’s not my Tomcat’s webapp directory IDEA is using, it’s something under ~/.IntellijIDEA15/. I hadn’t used Tomcat’s ability to split its installation directory ($CATALINA_HOME) and its work/deployment directory ($CATALINA_BASE) in a long time, so that surprised me for a moment. Still, could there be a problem with the configuration created by IDEA in its directory?

$ meld ~/opt/tomcat/ ~/.IntelliJIdea15/system/tomcat/my_project/

Hm. No interesting differences - basically the only things that are different between the two directories are the paths referenced in some configuration files, because they are in different directories, after all.

Oh. I overlooked something in the logs earlier. IDEA is passing some extra system properties to Tomcat. 

The purpose of all these is to configure Tomcat’s JMX support so IDEA can query it for the status of the webapp deployments and other things. This looks completely harmless. But nothing else seems to have been the cause, so let’s experiment. If I pass all of these options on the command line using $CATALINA_OPTSKaboom. Finally, I can reproduce the error mentioned earlier. It blows up with the exact same message, the disabled RMI classloader.

So, let’s try removing these parameters one by one to see which one causes the error. After several tries, I removed And the error disappeared. Slowly a realization dawned on me. Really? I mean, really?

Let’s verify this. Using… it works. And changing the JMX port in IDEA’s Tomcat configuration makes it work in IDEA as well. ARGH!

I even re-checked the logs from earlier. There was nothing at all about a port conflict in there. Zero. I didn’t have Tomcat’s debug logging turned on, but seriously, something like this should be a big glaring ERROR if not FATAL. And it especially shouldn’t claim classloading doesn’t work due to a security manager problem. That is just ridiculous.


Tomcat lies, or maybe it’s RMI that does it. When there is a port conflict between Tomcat’s JMX and RMI, you won’t be told about the colliding ports, instead it will much later claim that it’s missing a security manager and thus RMI classloading is disabled. Easy fix: Change the JMX port.