using StringEscapeUtils throw java.lang.ClassNotFoundException

Question

I followed the instructions in http://stackoverflow.com/questions/3091359/install-apache-common-lang-2-5-in-eclipse to install apache common lang 2.5 in Eclipse.

Then we I use the class

import org.apache.commons.lang.StringEscapeUtils;

writer.printf("<firstname>%s</firstname>", StringEscapeUtils.escapeXml(mystring));

I got ClassNotFoundException thrown, any reason why?

java.lang.ClassNotFoundException: org.apache.commons.lang.StringEscapeUtils 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:307) at 
com.google.appengine.tools.development.IsolatedAppClassLoader.loadClass(IsolatedAppClassLoad  er.java:151) at 
java.lang.ClassLoader.loadClass(ClassLoader.java:248)