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
