Jeff Mesnil
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Tomcat management using jmx4r

July 5, 2007

Something which is not obvious with the way jmx4r leverages JMX API and Ruby metaprogramming is that you can write simple scripts to manage a Java application without any dependency on the MBeans exposed by the application.

For simplicity, in my examples I always use MBeans exposed by the JVM but jmx4r works with any MBean even if its interface is unknown from the JVM running the management scripts.

Here is a simple example to manage Tomcat using jmx4r.

Let's assume that we have Tomcat running locally and manageable remotely on port 3000 (without authentication):

$ export CATALINA_OPTS="-Dcom.sun.management.jmxremote \
    -Dcom.sun.management.jmxremote.port=3000 \
    -Dcom.sun.management.jmxremote.ssl=false \
    -Dcom.sun.management.jmxremote.authenticate=false"
$ ./bin/catalina.sh/run

We want to know among all the Web Modules running in Tomcat which ones are privileged and which ones are not. The whole script to do so is:

# tomcat_modules.rb
require "rubygems"
require "jmx4r"

JMX::MBean.establish_connection :host => "localhost", :port => 3000

web_modules = JMX::MBean.find_all_by_name "Catalina:j2eeType=WebModule,*"
privileged, unprivileged = web_modules.partition { |m| m.privileged }

puts "Privileged:\n"     + privileged.map   {|m| m.path }.join("\n  ")
puts "Unprivileged:\n  " + unprivileged.map {|m| m.path }.join("\n  ")

Executing this script gives:

$ jruby tomcat_modules.rb 
Privileged:
  /balancer
  /manager
  /host-manager
Unprivileged:
  /tomcat-docs
  /servlets-examples
  /jsp-examples

  /webdav

That's were using JRuby shines: it combines the simplicity of using directly the MBeans exposed by a Java application without having to bother with classes dependency.

Using Java to write a corresponding script means choosing your "poison":

  • No class dependency but tedious (and unnatural) use of MBeanServerConnection methods to interact with Tomcat MBeans
  • Use directly Tomcat MBeans (thanks to MBeanServerInvocationHandler) but you must take care having all their interfaces (and dependencies) in the class path of the JVM running the script

I can not thank enough both the JMX and JRuby guys which make it so simple to get the best of both worlds in jmx4r:

  • simple to use directly the MBeans API
  • simple to deploy without class dependency issues