简单实现一个登陆过滤器Filter

public class LoginFilter implements Filter{
    private String redirectUrl;
    private String uncheckedUrls;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        ServletContext servletContext = filterConfig.getServletContext();
        //获取XML文件中配置参数
        redirectUrl = servletContext.getInitParameter("redirectPage");
       System.out.println("redirectPage======" + redirectUrl);
        uncheckedUrls = servletContext.getInitParameter("uncheckedUrls");
        System.out.println("uncheckedUrls=====" + uncheckedUrls);
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        // 获得在下面代码中要用的request,response,session对象
        HttpServletRequest httpRequest = (HttpServletRequest) servletRequest;
        HttpServletResponse httpResponse = (HttpServletResponse) servletResponse;
        //1.获取请求URL
        String servletPath = httpRequest.getServletPath();    

        //2.检测1中获取的servletPath是否为不需要检测的URl中的一个.若是,放行
        List<String> urls = Arrays.asList(uncheckedUrls.split(","));
        for (String url : urls) {
			if(servletPath.contains(url)) {
				filterChain.doFilter(httpRequest, httpResponse);
	            return;
			}
		}

        //3.从session中获取SessionKey对应值,若值不存在,则重定向到redirectUrl
        Object user = httpRequest.getSession().getAttribute(GlobalUtil.SESSION_USER_KEY);
        if (user == null) {
           httpResponse.sendRedirect(httpRequest.getContextPath() + redirectUrl);            
           return;
        }

        //4.若存在,则放行
        filterChain.doFilter(httpRequest, httpResponse);
    }

    @Override
    public void destroy() {
    }
}

 <!--若未登录，需要重定向的页面-->
    <context-param>
        <param-name>redirectPage</param-name>
        <param-value>/login</param-value>
    </context-param>
    <!--不需要拦截的URL列表-->
    <context-param>
        <param-name>uncheckedUrls</param-name>
        <param-value>/login,/resources,/bootstrap,/img,favicon.ico</param-value>
    </context-param>

    <filter>
        <filter-name>LoginFilter</filter-name>
        <filter-class>com.weixin.filter.LoginFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>