public <T> T getLibraryInstance(String className) throws Exception {
    return getLibraryInstance(null, className);
  }
Loads an instance of the class from the stack classloader, if available.

@param className
         the name of the class to get an instance
@return
         the instance of the class
@throws Exception
         when the class cannot be loaded or instantiated
ambari-server/src/main/java/org/apache/ambari/server/state/StackInfo.java