Previously initiated loading for a different type with name "javax/management/MBeanServer"

When I was doing a migration / upgrade to Spring 3.1, I faced a really weird Error.

My App was using Jetty to run deploy my WAR file.


Short story / Long Story

Although I was not using MX4J library in my project, I remembered that:
"Jetty defaults to overriding the normal J2SE class loader delegation model  (which calls for delegating to the parent)."

Therefore, this could be fixed by using this System Property:

-Dorg.mortbay.jetty.webapp.parentLoaderPriority=true

This is how you should do it in your pom:
<project> ... <plugins> ... <plugin> <groupid>org.mortbay.jetty</groupId> <artifactid>maven-jetty-plugin</artifactId> <configuration> ... <systemproperties> <systemproperty> <name>org.mortbay.jetty.webapp.parentLoaderPriority</name> <value>true</value> </systemProperty> ... </systemProperties> </configuration> </plugin> </plugins> </project>

And that error will go away.