《Java Web开发技术应用——过滤器》
过滤器是一个程序，它先于与之相关的servlet或JSP页面运行在服务器上。过滤器可附加到一个或多个servlet或JSP页面上，并且可以检查进入这些资源的请求信息。在这之后，过滤器可以作如下的选择：
①以常规的方式调用资源（即，调用servlet或JSP页面）。
②利用修改过的请求信息调用资源。
③调用资源，但在发送响应到客户机前对其进行修改。
④阻止该资源调用，代之以转到其他的资源，返回一个特定的状态代码或生成替换输出。
用户请求——>过滤器——>WEB资源——>过滤器——>用户
过滤器的生命周期
1.实例化 web.xml 在web容器启动时依据web.xml实例化
2.初始化 init()
3.过滤 doFilter()
4.销毁 destroy()
配置：
可以通过Design界面快速配置
过滤器需要实现接口javax.servlet.Filter，开始以为是类……找了好久……
需要实现三个方法
public void destroy() {}
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException {}
public void init(FilterConfig arg0) throws ServletException {}
在doFilter中实现逻辑
如果多个过滤器对应一个路径
那么按照在web.xml的中定义的顺序执行过滤器
请求——>过滤器1——>过滤器2——>Servlet——>过滤器2——>过滤器1——>用户
过滤器的分类：（默认是request
ASYNC：Servlet中异步执行过滤器和业务逻辑内容。
ERROR：处理error-page
FORWARD：通过request.getRequestDispatcher(“url”).forward(request, response);或者<jsp:forward page=”…”></jsp:forward>
INCLUDE：通过request.getRequestDispatcher(“url”).include(request, response);或者<jsp:include page=”…”></jsp:include>
REQUEST：通过链接直接访问，或者通过response.sendRedirect(“url”);
在web.xml里面配置error-page
<error-page>
<error-code>404</error-code>
<location>/error.jsp</location>
</error-page>
在类上面通过注解配置过滤器
@WebFilter(filterName=”…”, value={“/….jsp”}, dispatcherTypes={DispatcherType.REQUEST, DispatcherType.ASYNC})
public class FilterName implements Filter {…}
案例：登录校验，如果没有登录，不能直接通过url访问登录后才能访问的页面
（突然发现右键有Servlet的选项，内心是崩溃的……窝每次都是新建java类……
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>" rel="external nofollow" > <title>My JSP 'login.jsp' starting page</title> <meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css" rel="external nofollow" >
--> </head> <%
request.setCharacterEncoding("utf-8");
%> <body>
<form action="<%=request.getContextPath() %>/servlet/LoginServlet" method="post">
用户名：<input type="text" name="username">
密码：<input type="password" name="password">
<input type="submit" value="提交">
</form> </body>
</html>
login.jsp
package com.imooc.servlet;import java.io.IOException;
import java.io.PrintWriter;import javax.servlet.ServletConfig;
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 { /**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String username = request.getParameter("username");
String password = request.getParameter("password"); System.out.println(username); if("admin".equals(username) && "admin".equals(password)){
//校验通过
HttpSession session = request.getSession();
session.setAttribute("username", username);
response.sendRedirect(request.getContextPath()+"/success.jsp");
return ;
} else{
//校验失败
response.sendRedirect(request.getContextPath()+"/fail.jsp");
return ;
}
}}
LoginServlet.java
package com.imooc.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 { private FilterConfig config; @Override
public void destroy() {
// TODO Auto-generated method stub } @Override
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) arg0;
HttpServletResponse response = (HttpServletResponse) arg1; String charset = config.getInitParameter("charset");
if (charset == null) {
charset = "utf-8";
} request.setCharacterEncoding("utf-8"); String noLoginPaths = config.getInitParameter("noLoginPaths");
if (noLoginPaths != null) {
String[] strArray = noLoginPaths.split(";");
for (String s: strArray) {
if (s != null && !s.equals("") && request.getRequestURL().indexOf(s) != -1) {
arg2.doFilter(arg0, arg1);
return;
}
}
} HttpSession session = request.getSession();
if (session.getAttribute("username") != null) {
arg2.doFilter(arg0, arg1);
} else {
response.sendRedirect("login.jsp");
}
} @Override
public void init(FilterConfig arg0) throws ServletException {
// TODO Auto-gegnerated method stub
config = arg0;
}}
LoginFilter.java
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<display-name>LoginFilter</display-name>
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>com.imooc.servlet.LoginServlet</servlet-class>
</servlet> <servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/servlet/LoginServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>index.jsp</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
<filter>
<filter-name>LoginFilter</filter-name>
<filter-class>com.imooc.filter.LoginFilter</filter-class>
<init-param>
<param-name>noLoginPaths</param-name>
<param-value>login.jsp;fail.jsp;LoginServlet</param-value>
</init-param>
<init-param>
<param-name>charaset</param-name>
<param-value>GBK</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>LoginFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
</web-app>
web.xml
