Exception in thread "main" java.lang.NoClassDefFoundError: org/codehaus/plexus/classworlds/launcher/Launcher Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.classworlds.launcher.Launcher

After installing maven 3.0.4. I got this error for every maven command.

wm720l17884a:~ glondhe$ mvn clean install
Exception in thread "main" java.lang.NoClassDefFoundError: org/codehaus/plexus/classworlds/launcher/Launcher
Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.classworlds.launcher.Launcher
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

wm720l17884a:bin glondhe$ mvn -version
Exception in thread "main" java.lang.NoClassDefFoundError: org/codehaus/plexus/classworlds/launcher/Launcher
Caused by: java.lang.ClassNotFoundException: org.codehaus.plexus.classworlds.launcher.Launcher
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

After troubleshooting for a while, i found the fix. The M2_HOME  environment variable is the culprit. M2 looks for mvn2 directory structure. Theoretically the directory structure and files will be different but here we are executing M3 commands and env variable is looking for M2.  Removing M2_HOME Fix the issue.

wm720l17884a:local glondhe$ mvn -version
Apache Maven 3.0.4 (r1232337; 2012-01-17 00:44:56-0800)
Maven home: /usr/share/maven
Java version: 1.6.0_37, vendor: Apple Inc.
Java home: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
Default locale: en_US, platform encoding: MacRoman
OS name: "mac os x", version: "10.6.8", arch: "x86_64", family: "mac"