ImplementadodoFilter(). Como cobrir adequadamente o filtro com jUnit?

public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
        throws java.io.IOException, javax.servlet.ServletException
{
    HttpServletRequest request = (HttpServletRequest) servletRequest;
    HttpServletResponse response = (HttpServletResponse) servletResponse;
    String currentURL = request.getRequestURI();

    if (!currentURL.equals("/maintenance.jsp") && modeService.getOnline())
    {
        response.sendRedirect("/maintenance.jsp");
    }
    filterChain.doFilter(servletRequest, servletResponse);
}