学习相关资料：
简单通俗易懂的理解，就是类似于杀毒软件对电脑系统的功能和作用。即程序中的防火墙。
过滤器运行原理：
若有多个过滤器，则按顺序进行执行，直到最后一个过滤器通过后，才可进行下一步的请求。
项目要求和功能：
1.编码过滤器，对乱码进行过滤
2.登录权限过滤器，只有用户登陆后，才可以访问相关页面
项目思路：
1.在Filter中将reques和response进行强制转换
2.将所需要过滤的东西（本体为编码格式，和验证用户登录）的相关代码放入Filter中
3.对Filter设置适用范围，排除不用过滤的界面
项目源码：
对编码格式的过滤：
package com.filter; 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.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; /** * Servlet Filter implementation class LoginFilter */ @WebFilter(filterName="CodeFilter",urlPatterns="/*")//设置适用的范围 public class CodeFilter implements Filter { /** * Default constructor. */ public CodeFilter() { // 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 HttpServletRequest req=(HttpServletRequest) request; HttpServletResponse res=(HttpServletResponse) response; //用户请求编码设置 request.setCharacterEncoding("utf-8"); response.setCharacterEncoding("ytf-8"); response.setContentType("text/html;charset=utf-8"); System.out.println("进入编码过滤器......"); chain.doFilter(request, response); System.out.println("退出编码过滤器......"); } /** * @see Filter#init(FilterConfig) */ public void init(FilterConfig fConfig) throws ServletException { // TODO Auto-generated method stub } }
对用户登录权限的过滤：
package com.filter; 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 LoginServlet */ @WebFilter(filterName="LoginFilter", urlPatterns="/*", initParams= { @WebInitParam(name="noFilterUrl",value="login.jsp,LoginSErvlet") })//设置适用的范围,排除登录权限的过滤 public class LoginFilter implements Filter { 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 //首先强制转换request,response HttpServletRequest req=(HttpServletRequest) request; HttpServletResponse res=(HttpServletResponse) response; //获取sessionz中的数值 HttpSession session=req.getSession(); String username=(String)req.getAttribute("username"); //判断是否登录 if(username!=null) { //已登录继续执行 chain.doFilter(request, response); }else { //未登录，页面跳转 String url=req.getRequestURI();//获取用户地址 for(String temp:noFilter) { if(url.indexOf(temp)!=-1) { chain.doFilter(request, response);//直接放行 return; } } res.sendRedirect("login.jsp"); } } /** * @see Filter#init(FilterConfig) */ public void init(FilterConfig fConfig) throws ServletException { //排除相关几年的过滤，以便正常运行 // TODO Auto-generated method stub noFilter=fConfig.getInitParameter("noFilerUrl").split(",");//将数组noFilerUrl用","进行分割 } }
servlet:
package com.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 FilterServlet */ @WebServlet("/FilterServlet") public class FilterServlet extends HttpServlet { private static final long serialVersionUID = 1L; /** * @see HttpServlet#HttpServlet() */ public FilterServlet() { 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 stub System.out.println("进入servlet......"); //接受用户登录信息 String username=request.getParameter("username"); String password=request.getParameter("password"); //然后输出 response.getWriter().print("用户名："+username); response.getWriter().print("<br/>"); response.getWriter().print("密码："+password); //将用户信息放入到session中 HttpSession session=request.getSession(); session.setAttribute("username", username); //设置session的有效时间为10秒 session.setMaxInactiveInterval(10); 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 stub doGet(request, response); } }
运行项目跟踪的测试顺序截图：
遇到的问题：
原因：当设置用户登录权限的过滤器时，没有排除非不必过滤的代码和访问页面，从而造成了页面的死循环
解决方法：在LoginFilter进行排除不该受权限的内容
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解决时间：由于初次学Filter，所以解决的时间有点长，大约20分钟。通过查阅相关资料说，这种死循环错误在登录用户过滤器中是很常见的问题。特别注意。
