/**
 * 设置字符集、过滤未登录的非法请求
 */
public class UserLoginFilter implements Filter {
    protected String encoding = null;

    protected FilterConfig filterConfig = null;

    protected boolean ignore = false;

    protected String forwardPath = null;

    public void destroy() {
        this.encoding = null;
        this.filterConfig = null;
    }

    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        // 设置编码方式，web.xml里面有filter参数的初始化设置
        if (ignore || (request.getCharacterEncoding() == null)) {
            String encoding = selectEncoding(request);
            if (encoding != null)
                request.setCharacterEncoding(encoding);
        }
        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        HttpServletResponse httpServletResponse = (HttpServletResponse) response;
        String requesturi = httpServletRequest.getRequestURI();
        // 通过检查session中的变量，过虑请求
        HttpSession session = httpServletRequest.getSession();
        Object currentUser = session.getAttribute(AppConstants.LOGIN_USER);
        // 当前会话用户为空而且不是请求登录，退出登录，欢迎页面和根目录则退回到应用的根目录
        if (currentUser == null
                && !requesturi.endsWith("/processLogin.do")
                && !requesturi.endsWith("/logout.do")
                && !requesturi.endsWith("/index.jsp")
                && !requesturi.endsWith(httpServletRequest.getContextPath()
                        + "/")) {
            httpServletResponse.sendRedirect(httpServletRequest
                    .getContextPath()
                    + "/");
            return;
        }
        chain.doFilter(request, response);
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        this.encoding = filterConfig.getInitParameter("encoding");
        this.forwardPath = filterConfig.getInitParameter("forwardpath");
        String value = filterConfig.getInitParameter("ignore");
        if (value == null)
            this.ignore = true;
        else if (value.equalsIgnoreCase("true"))
            this.ignore = true;
        else if (value.equalsIgnoreCase("yes"))
            this.ignore = true;
        else
            this.ignore = false;
    }

    protected String selectEncoding(ServletRequest request) {
        return (this.encoding);
    }
}

------解决方案--------------------
多谢楼上的，我先看看。感觉基本实现了我需要的功能。不过需要改动的地方还很多。赫赫。得看看把这个filter类放在哪个包里面先。
-----------------------------
filter类与普通类不同 它的作用域是在web.xml中进行配置
   <filter>
       <filter-name>Login Filter</filter-name>
       <filter-class>com.bluedn.common.control.UserLoginFilter</filter-class>
   </filter>
   <filter-mapping>
       <filter-name>Login Filter</filter-name>