一.Hello Filter

1.编写FirstFilter

public class FirstFilter implements Filter {

@Override
public void destroy() {

}
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
     //doFilter()方法中的req参数的类型是ServletRequest，需要转换为HttpServletRequest类型方便调用某些方法
    String ip = request.getRemoteAddr();
    //获取用户的ip
    String url = request.getRequestURL().toString();
   //获取用户访问的页面地址
    System.out.printf("%s %s 访问了%s%n",date,ip,url);
   //在控制台打印出来
 chain.doFilter(request,response);
   //过滤器放行
}
@Override
public void init(FilterConfig arg0) throws ServletException {
}
//与servlet需要配置自启动才会随着tomcat的启动而执行init()方法不一样，Filter一定会随着tomcat的启动自启动
}

2.配置web.xml

<filter>
   <filter-name>FirstFilter</filter-name>
   <filter-class>filter.FirstFilter</filter-class>
</filter>
<filter-mapping>
  <filter-name>FirstFilter</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

二.中文处理

    创建一个 EncodingFilter
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
        throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

    request.setCharacterEncoding("UTF-8");

    chain.doFilter(request, response);
}
配置web.xml
<filter>
     <filter-name>EncodingFilter</filter-name>
     <filter-class>filter.EncodingFilter</filter-class>
</filter>

<filter-mapping>
     <filter-name>EncodingFilter</filter-name>
     <url-pattern>/*</url-pattern>
</filter-mapping>

三.登陆验证

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
        throws IOException, ServletException {
    HttpServletRequest request = (HttpServletRequest) req;
    HttpServletResponse response = (HttpServletResponse) res;

    String uri = request.getRequestURI();
    if (uri.endsWith("login.html") || uri.endsWith("login")) {
        chain.doFilter(request, response);
        return;
    }

    String userName = (String) request.getSession().getAttribute("userName");
    if (null == userName) {
        response.sendRedirect("login.html");
        return;
    }

    chain.doFilter(request, response);
}