@SuppressWarnings({"unchecked"})
    private Object getPrincipalFromClassName() {
        Object principal = null;

        try {
            Class cls = Class.forName(type);
            principal = getSubject().getPrincipals().oneByType(cls);
        } catch (ClassNotFoundException e) {
            if (LOGGER.isErrorEnabled()) {
                LOGGER.error("Unable to find class for name [" + type + "]");
            }
        }
        return principal;
    }
The default value that should be displayed if the user is not authenticated, or no principal is found.
web/src/main/java/org/apache/shiro/web/tags/PrincipalTag.java