public void init(FilterConfig filterConfig) throws ServletException;//初始化//filter对象只会创建一次，init方法也只会执行一次和我们编写的Servlet程序一样，Filter的创建和销毁由WEB容器负责。 web 应用程序启动时，web 容器读取web.xml配置文件，将创建Filter 的实例对象，并调用其init方法（这个init方法中可以读取web.xml文件中过滤器的参数）完成对象的初始化功能，从而为后续的用户请求作好拦截的准备工作。开发人员可以通过init方法的参数FilterConfig对象，获得当前filter配置信息。public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;//拦截请求//可执行n次这个方法完成实际的过滤操作。当客户请求访问与过滤器关联的URL的时候执行该方法。FilterChain参数的doFilter方法用于访问后续过滤器或者调用目标资源。public void destroy();//销毁//该方法在Filter的生命周期中仅执行一次。Filter对象创建后会驻留在内存，当web应用移除或服务器停止时才销毁。在Web容器卸载 Filter 对象之前被调用。在这个方法中，可以释放过滤器使用的资源。

<form action="doLogin" method="post">
用户名：<input type="text" name="userName"/><br/>
密码：<input type="password" name="password"/><br/>
<input type="submit" value="登录"/>
<input type="reset" value="重置"/>
</form>

package com.servlet;import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;@SuppressWarnings("serial")
public class DoLoginServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String userName=req.getParameter("userName");String password=req.getParameter("password");System.out.println(userName);if("张三".equals(userName)&&"123".equals(password)){//校验通过HttpSession session = req.getSession();session.setAttribute("userName", userName);　　　　　　　//这里必须使用重定向，不能使用请求转发　　　　　　　//请求转发：request.getRequestDispatcher().forward()是属于FORWARD类型的过滤器，需要在配置过滤器时，写明FORWARD类型的<dispatcher>resp.sendRedirect("success.jsp");}else{resp.sendRedirect("login.jsp");}}
}

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.http.HttpServletRequest;public class SetEconding implements Filter {@Overridepublic void destroy() {// TODO Auto-generated method stub}@Overridepublic void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)throws IOException, ServletException {HttpServletRequest req=(HttpServletRequest)arg0;//解决乱码问题req.setCharacterEncoding("UTF-8");//别忘了放行arg2.doFilter(arg0, arg1);}@Overridepublic void init(FilterConfig arg0) throws ServletException {// TODO Auto-generated method stub}}

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.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;public class LoginFilter implements Filter {
　　//定义一个FilterConfig，用于传递出初始化方法init()中的FilterConfig对象private FilterConfig config=null;@Overridepublic void destroy() {// TODO Auto-generated method stub}@Overridepublic void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)throws IOException, ServletException {HttpServletRequest req=(HttpServletRequest)arg0;HttpServletResponse resp=(HttpServletResponse)arg1;HttpSession session=req.getSession();//获取配置文件中配置的初始化参数，在这里是获取不过滤页面名单String initParam=config.getInitParameter("NotFilter");String[] notFilterPages=initParam.split(";");　　　　　//不过滤的页面直接调用FilterChain的doFilter()方法跳转到下个过滤器或者目标资源if(notFilterPages!=null){for(int i=0;i<notFilterPages.length;++i){//跳过字段为空和空字符串的情况if(notFilterPages[i]==null||"".equals(notFilterPages[i]))continue;//注意这里是URI路径中的是否含有某个初始值，对于servlet来说，是指配置servlet时指定的url-pattern的值，而不是servlet类名if(req.getRequestURI().indexOf(notFilterPages[i])!=-1){arg2.doFilter(arg0, arg1);return;}}}　　　　　//当登录成功（即session中存在“userName”的值），则跳转到下一个过滤器或者目标资源//只要未登录成功，所有的页面跳转请求（免除过滤的页面和servlet处理类除外）都将跳转到登录页面if(session.getAttribute("userName")!=null){arg2.doFilter(arg0, arg1);return;}else{resp.sendRedirect("login.jsp");return;}}@Overridepublic void init(FilterConfig arg0) throws ServletException {config=arg0;}}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1"><display-name>javaEE_filter</display-name><welcome-file-list><welcome-file>index.html</welcome-file></welcome-file-list><!-- 第1个过滤器 --><!-- 配置解决乱码过滤器 --><filter><filter-name>SetEcondingFilter</filter-name><filter-class>com.filter.SetEconding</filter-class></filter><!-- 配置解决乱码过滤器映射 --><filter-mapping><filter-name>SetEcondingFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping><!-- 第2个过滤器 --><!-- 配置登录过滤器 --><filter><filter-name>LoginFilter</filter-name><filter-class>com.filter.LoginFilter</filter-class><init-param><param-name>NotFilter</param-name><param-value>login;doLogin</param-value></init-param></filter><!-- 配置登录过滤器映射 --><filter-mapping><filter-name>LoginFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping><!-- 配置处理登录的Servlet --><servlet><servlet-name>LoginServlet</servlet-name><servlet-class>com.servlet.DoLoginServlet</servlet-class></servlet><!-- 配置处理登录Servlet的映射 --><servlet-mapping><servlet-name>LoginServlet</servlet-name><url-pattern>/doLogin</url-pattern></servlet-mapping>
</web-app>

req.setCharacterEncoding("UTF-8");

userName=new String(userName.getBytes("ISO-8859-1"),"UTF-8");

req.setCharacterEncoding("UTF-8");