servlet HttpServletResponse.sendRedirect()？

不是用response.sendRedirect()的吗？为什么这个程序中非要用HttpServletResponse.sendRedirect()不可呢？package servletproject;
import javax.servlet.Filter;
import javax.servlet.FilterConfig;
import javax.servlet.FilterChain;import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;import java.io.IOException;
import javax.servlet.ServletException;import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;/**
 * <p>Title: </p>
 *
 * <p>Description: </p>
 *
 * <p>Copyright: Copyright (c) 2011</p>
 *
 * <p>Company: </p>
 *
 * @author not attributable
 * @version 1.0
 */public class ServletFilterTest implements Filter {
    String LOGIN_PAGE = "login_signon.jsp";
    protected FilterConfig filterConfig;
    
    
    
    
    public void init(FilterConfig config) throws ServletException {
        this.filterConfig = config;
    }
    
    public void doFilter(final ServletRequest request, final ServletResponse response, FilterChain chain)
            throws IOException, ServletException  {
        HttpServletRequest hreq = (HttpServletRequest)request;
        HttpServletResponse hres = (HttpServletResponse)response;
        HttpSession session = hreq.getSession();
        
        String isLogin = "";
        
        
        try {
            isLogin = (String)session.getAttribute("isLogin");
            
            if(isLogin.equals("true")) {
                System.out.println("在 SignonFilter 中验证通过");
                chain.doFilter(request, response);
            }
            else {
                System.out.println("被 SignonFilter 拦截一个未认证的请求");
                hres.sendRedirect(LOGIN_PAGE);
            }
        } catch(Exception e) {
            e.printStackTrace();
        }
    }
    
    public void destroy() {
        this.filterConfig = null;
    }
    
    
    
    
    public void setFilterConfig(final FilterConfig filterConfig) {
        this.filterConfig = filterConfig;
    }
}
其他的都不行