如何将不带www的根域名301重定向到带www的主域名

@Override

public void init(FilterConfig filterConfig) throws ServletException {}

@Override

​public void doFilter(ServletRequest request, ServletResponse response,

    FilterChain chain) throws IOException, ServletException {

    HttpServletRequest req = (HttpServletRequest) request;

    HttpServletResponse resp= (HttpServletResponse)response;

    String tempPathString = req.getRequestURI();

    if(req.getRequestURL().indexOf("http://example.com")>=0){ // 不带www，跳转的首页               String queryString = (req.getQueryString() == null ? "" : "?"+req.getQueryString());   

        resp.setStatus(301);

        resp.setHeader( "Location", "http://www.example.com"+tempPathString+queryString);

        resp.setHeader( "Connection", "close" );  

    }

    chain.doFilter(request, response);

}

@Override

public void destroy() {}