I was seeing this error after installing ant on CentOS 6.2 via yum:

$ ant
Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/tools/ant/launch/Launcher
Caused by: java.lang.ClassNotFoundException: org.apache.tools.ant.launch.Launcher
	at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: org.apache.tools.ant.launch.Launcher. Program will exit.

Odd, generally yum packages work out of the box. The problem ends up being that ant can’t find ant-launcher.jar The solution is to add the following to your .bashrc

export ANT_HOME=/usr/share/ant
export JAVA_HOME=/usr/lib/jvm/java

Once you do that it should find the necessary jar and work just fine.

Hopefully that saves someone else the hassle of figuring this out.