一、filter过滤器的作用
如果想要获取中文字符，或者是显示提交的中文，就需要添加以下代码，来防止乱码的情况发生。
request.setCharacterEncoding("utf-8");response.setCharacterEncoding("utf-8");response.setContentType("text/html;charset=utf-8");
如果只是设置一个页面的情况，上述方法是可以实现的。但是，要是很多个页面要显示中文的话，上述方法就过于繁琐。这种情况下要实现显示中文的功能，就要用到filter过滤器。这样就实现了，一处编码，到处执行的用途。
二、实现简单用户登录验证
（1）登录界面：login.jsp文件
<%@ page language="java" contentType="text/html; charset=utf-8"pageEncoding="utf-8"%><!DOCTYPE html><html><head><meta charset="utf-8"><title>Insert title here</title></head><body><form action="LoginServlet" method="post"> 用户名：<input type="text" name="username"/><br/>密码：<input type="password" name="pwd"/><br/><input type="submit" value="提交"/></form><a href="result1.jsp">验证登录</a></body></html>
（2）servlet交互文件：LoginServlet.java
package com.filter.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;/*** Servlet implementation class LoginServlet*/@WebServlet("/LoginServlet")public class LoginServlet extends HttpServlet {private static final long serialVersionUID = 1L;/*** @see HttpServlet#HttpServlet()*/public LoginServlet() {super();// TODO Auto-generated constructor stub}/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubSystem.out.println("进入servlet......");String username = request.getParameter("username");String pwd = request.getParameter("pwd");//获取SessionHttpSession session = request.getSession();//验证用户名和密码是否有效//将用户名放入session 并且设置名称为uname(在过滤器中获取时，直接获取uname)session.setAttribute("uname",username);//设定session的有效期session.setMaxInactiveInterval(10);//将获取的用户名和密码打印在页面上response.getWriter().print("用户名："+username);response.getWriter().print("<br/>");response.getWriter().print("密码："+pwd);System.out.println("退出servlet......");}/*** @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)*/protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}
（3）filter过滤器文件：LoginFilter.java
package com.filter.servlet;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.annotation.WebFilter;import javax.servlet.annotation.WebInitParam;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;/*** Servlet Filter implementation class LoginFilter*///设置不被过滤的信息@WebFilter(filterName="LoginFilter",urlPatterns = "/*",initParams = {@WebInitParam(name="noFilterUrl",value="login.jsp,LoginServlet") })public class LoginFilter implements Filter {//定义一个字符串数组private String[] noFilter;/*** Default constructor. */public LoginFilter() {// TODO Auto-generated constructor stub}/*** @see Filter#destroy()*/public void destroy() {// TODO Auto-generated method stub}/*** @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)*/public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {// TODO Auto-generated method stub// place your code hereHttpServletRequest req = (HttpServletRequest)request;HttpServletResponse resp = (HttpServletResponse) response;//获取用户的sessionHttpSession session = req.getSession();//session是用来保存服务端的用户信息String username =(String) session.getAttribute("uname");if(username != null) {chain.doFilter(request, response);}else {//用户没有登录String url = req.getRequestURI();//获取用户请求地址for(String temp : noFilter) {if(url.indexOf(temp)!= -1){chain.doFilter(request, response);return ;}}resp.sendRedirect("login.jsp");}}/*** @see Filter#init(FilterConfig)*/public void init(FilterConfig fConfig) throws ServletException {// TODO Auto-generated method stub//使用字符分割函数，对noFilter进行赋值noFilter = fConfig.getInitParameter("noFilterUrl").split(",");}}
（4）验证结果文件：result.jsp文件
<%@ page language="java" contentType="text/html; charset=utf-8"pageEncoding="utf-8"%><!DOCTYPE html><html><head><meta charset="utf-8"><title>Insert title here</title></head><body>用户名：${uname}</body></html>
