filter用法。

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首先：配置文件，声明一个拦截器，两个servlet，一个登录一个退出

web.xml

filter:过滤器

<filter>
<fiasfdlter-name>LogFilter</filter-name> //过滤器名字
<filter-class>com.ee.filter.LogFilter</filter-class> //所在包
</filter>
<filter-mapping>
<filter-name>LogFilter</filter-name> //过滤器名字和上面的一致
<url-pattern>/</url-pattern> //拦截 / 例如/aa 如果/* 则会拦截所有（因为做的登录所以不能拦截所有）
</filter-mapping>
<servlet>
<servlet-name>LoginServlet</servlet-name> //servlet名字
<servlet-class>com.ee.filter.LoginServlet</servlet-class> //servlet所在包
</servlet>
<servlet>
<servlet-name>InvalidateServlet</servlet-name> //servlet名字
<servlet-class>com.ee.filter.InvalidateServlet</servlet-class> //servlet所在包
</servlet>

<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>   //servlet所在包
<url-pattern>/loginServlet</url-pattern> //路径
</servlet-mapping>
<servlet-mapping>
<servlet-name>InvalidateServlet</servlet-name> //servlet所在包
<url-pattern>/jsp/aa</url-pattern> //路径
</servlet-mapping>

LogFilter：过滤器类实现Filter接口

package com.ee.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.HttpSession;

/**
* @author Administrator
*
*/
public class LogFilter implements Filter {
public void destroy() {
}

public void doFilter(ServletRequest servletRequest,
ServletResponse servletResponse, FilterChain filterChain)
throws IOException, ServletException {

HttpServletRequest req = (HttpServletRequest) servletRequest;
HttpSession session = req.getSession();
System.out.println("lanjie");
String username = (String)session.getAttribute("username");

if (username != null&&username!="") {
// 如果现在存在了session，则请求向下继续传递
filterChain.doFilter(servletRequest, servletResponse);
} else {
// 跳转到提示登陆页面
System.out.println("login");
servletRequest.getRequestDispatcher("/login.jsp").forward(servletRequest, servletResponse);
}
}

public void init(FilterConfig filterConfig) throws ServletException {
}
}

LoginServlet:继承HttpServlet

package com.ee.filter;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginServlet extends HttpServlet {


/**
*
*/
private static final long serialVersionUID = -1193203808034931858L;

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=UTF-8");
request.setCharacterEncoding("UTF-8");
PrintWriter out = response.getWriter();

String username = request.getParameter("username");
HttpSession session = request.getSession();
session.setAttribute("username", username); //用户登录加入到session中

response.sendRedirect("jsp/success.jsp"); //登录成功跳入success.jsp

//测试
System.out.println("username: "+username);

out.flush();
out.close();
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

this.doGet(request, response);
}

}

InvalidateServlet:

package com.ee.filter;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class InvalidateServlet extends HttpServlet {


/**
*
*/
private static final long serialVersionUID = 3701729463942281614L;

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=gbk");
request.setCharacterEncoding("gbk");
PrintWriter out = response.getWriter();
System.out.println("tuichu");
String aa=request.getContextPath();
HttpSession session =request.getSession(); //得到session对象
session.invalidate(); //注销session 使其失效
//然后跳转到登录页面
System.out.println(aa);
response.sendRedirect(aa+"/login.jsp");
// request.getRequestDispatcher("/login.jsp").forward(request, response);
out.flush();
out.close();
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("tuichu");
doGet(request, response);

}

}

<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<script type="text/javascript">
//检查是否输入用户名否则不予提交
function check(){
var username = document.getElementById("username").value;
if(username==null||""==username){
alert("请输入用户名");
return false;
}
return true;
}
</script>
<body>

<body>
<center>
<form action="loginServlet" method="post" onsubmit="return check()">
<table>
<caption>用户登录</caption>
<tr>
<td>用户名</td><td><input type="text" id="username" name="username" /></td>
</tr>
<tr>
<td>密码</td><td><input type="text" name="password"/></td>
</tr>
<tr>
<td align="right" colspan="2"><input type="submit" value="登录"></td>
</tr>
</table>
</form>
</center>
</body>
</body>
</html>

success.jsp:成功界面

/ ，/* 区别

<url-pattern>/</url-pattern>
会匹配到/springmvc这样的路径型url，不会匹配到模式为*.jsp这样的后缀型url。
<url-pattern>/*</url-pattern>
会匹配所有的url：路径型的和后缀型的url(包括/springmvc，.jsp，.js和*.html等)

我觉得可能有点乱，个人理解吧，主要是给自己留个印象。

sission：：同一个回话不同界面可以传值

filter：根据你的请求判断是否拦截，主要是dofilter方法，判断sission里是否有东西，有则放行登陆成功，没有东西拦截

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