public static  void check() throws JspTagException 
{
        if (commonsLangChecked)
        {
            return;
        }
        try
        {
            // Do they have commons lang ?
            Class stringUtils = Class.forName("org.apache.commons.lang.StringUtils"); //$NON-NLS-1$
            try
            {
                // this method is new in commons-lang 2.1
                stringUtils.getMethod("splitPreserveAllTokens", new Class[]{String.class}); //$NON-NLS-1$
            }
            catch (NoSuchMethodException ee)
            {
                throw new JspTagException(Messages.getString("DependencyChecker.lib.incompatible", //$NON-NLS-1$
                    new Object[]{"commons-lang", "2.1", "http://jakarta.apache.org/commons/lang"} //$NON-NLS-1$ //$NON-NLS-1$
                    ));
            }
        }
        catch (ClassNotFoundException e)
        {
            throw new JspTagException(Messages.getString("DependencyChecker.lib.missing", //$NON-NLS-1$
                new Object[]{"commons-lang", "2.1", "http://jakarta.apache.org/commons/lang"} //$NON-NLS-1$ //$NON-NLS-1$
                ));
        }
        commonsLangChecked = true;
}

Displaytag requires commons-lang 2.x or better; it is not compatible with earlier versions.