1. Filter:
1). Filter 是什么 ?
①. JavaWEB 的一个重要组件, 可以对发送到 Servlet 的请求进行拦截, 并对响应也进行拦截.
②. Filter 是实现了 Filter 接口的 Java 类.
③. Filter 需要在 web.xml 文件中进行配置和映射.
2). 如何创建一个 Filter, 并把他跑起来
①. 创建一个 Filter 类: 实现 Filter 接口: public class HelloFilter implements Filter
②. 在 web.xml 文件中配置并映射该 Filter.
其中 url-pattern 指定该 Filter 可以拦截哪些资源, 即可以通过哪些 url 访问到该 Filter
<!-- 注册Filter --> <filter> <filter-name>helloFilter</filter-name> <filter-class>com.aff.filter.HelloFilter</filter-class> <init-param> <param-name>name</param-name> <param-value>root</param-value> </init-param> </filter> <!-- 映射Filter --> <filter-mapping> <filter-name>helloFilter</filter-name> <url-pattern>/test.jsp</url-pattern> </filter-mapping>
3). Filter 相关的 API:
①. Filter 接口:
> public void init(FilterConfig filterConfig): 类似于 Servlet 的 init 方法.
在创建 Filter 对象(Filter 对象在 Servlet 容器加载当前 WEB 应用时即被创建)后,
立即被调用, 且只被调用一次. 该方法用于对当前的 Filter 进行初始化操作. Filter 实例是单例的.
FilterConfig 类似于 ServletConfig
可以在 web.xml 文件中配置当前 Filter 的初始化参数. 配置方式也和 Servlet 类似。
<filter> <filter-name>helloFilter</filter-name> <filter-class>com.aff.filter.HelloFilter</filter-class> <init-param> <param-name>name</param-name> <param-value>root</param-value> </init-param> </filter>
> public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain):
真正 Filter 的逻辑代码需要编写在该方法中. 每次拦截都会调用该方法.
FilterChain: Filter 链. 多个 Filter 可以构成一个 Filter 链.
- doFilter(ServletRequest request, ServletResponse response): 把请求传给 Filter 链的下一个 Filter,
若当前 Filter 是 Filter 链的最后一个 Filter, 将把请求给到目标 Serlvet(或 JSP)
- 多个 Filter 拦截的顺序和 <filter-mapping> 配置的顺序有关, 靠前的先被调用.
> public void destroy(): 释放当前 Filter 所占用的资源的方法. 在 Filter 被销毁之前被调用, 且只被调用一次.
4). <dispatcher> 元素: 指定过滤器所拦截的资源被 Servlet 容器调用的方式，
可以是REQUEST,INCLUDE,FORWARD和ERROR之一，默认REQUEST.
可以设置多个<dispatcher> 子元素用来指定 Filter 对资源的多种调用方式进行拦截
①. REQUEST：当用户直接访问页面时，Web容器将会调用过滤器。
如果目标资源是通过RequestDispatcher的include()或forward()方法访问时，
那么该过滤器就不会被调用。通过 GET 或 POST 请求直接访问。
②. FORWARD：如果目标资源是通过RequestDispatcher的forward()方法访问时，那么该过滤器将被调用，除此之外，该过滤器不会被调用。
或 <jsp:forward page="/..." /> 或 通过 page 指令的 errorPage 转发页面. <%@ page errorPage="test.jsp" %>
②. INCLUDE：如果目标资源是通过RequestDispatcher的include()方法访问时，那么该过滤器将被调用。除此之外，该过滤器不会被调用。
或 <jsp:include file="/..." />
④. ERROR：如果目标资源是通过声明式异常处理机制调用时，那么该过滤器将被调用。除此之外，过滤器不会被调用。
在 web.xml 文件中通过 error-page 节点进行声明:
<error-page> <exception-type>java.lang.ArithmeticException</exception-type> <location>/test.jsp</location> </error-page> <filter-mapping> <filter-name>secondFilter</filter-name> <url-pattern>/test.jsp</url-pattern> <dispatcher>REQUEST</dispatcher> <dispatcher>FORWARD</dispatcher> <dispatcher>INCLUDE</dispatcher> <dispatcher>ERROR</dispatcher> </filter-mapping>
练习
login.jsp
<%@ 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> <body> <font color=" red" >${message }</font> <br> <br> <form action="hello.jsp" method="post"> username: <input type="text" name="username" value="${param.username }"> password:<input type="password" name="password"/> <input type="submit" value="Submit"/> </form> </body> </html>
hello.jsp
<%@ 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> <body> Hello: ${param.username} </body> </html>
web.xml
<?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" version="3.1"> <!--初始化参数 --> <context-param> <param-name>password</param-name> <param-value>123456</param-value> </context-param> <filter> <display-name>UserNameFilter</display-name> <filter-name>UserNameFilter</filter-name> <filter-class>com.aff.filter.UserNameFilter</filter-class> <init-param> <param-name>username</param-name> <param-value>root</param-value> </init-param> </filter> <filter-mapping> <filter-name>UserNameFilter</filter-name> <url-pattern>/hello.jsp</url-pattern> </filter-mapping> <filter> <display-name>PasswordFilter</display-name> <filter-name>PasswordFilter</filter-name> <filter-class>com.aff.filter.PasswordFilter</filter-class> </filter> <filter-mapping> <filter-name>PasswordFilter</filter-name> <url-pattern>/hello.jsp</url-pattern> </filter-mapping> </web-app>
UserNameFilter
package com.aff.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; public class UserNameFilter implements Filter { public UserNameFilter() { } public void destroy() { } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { String initUser = filterConfig.getInitParameter("username"); String username = request.getParameter("username"); if (!initUser.equals(username)) { request.setAttribute("message", "用户名不正确"); request.getRequestDispatcher("/login.jsp").forward(request, response); return; } chain.doFilter(request, response); } private FilterConfig filterConfig; public void init(FilterConfig fConfig) throws ServletException { this.filterConfig = fConfig; } }
PasswordFilter
package com.aff.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; public class PasswordFilter implements Filter { public PasswordFilter() { } public void destroy() { } public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { String initPassword = filterConfig.getServletContext().getInitParameter("password"); String passsword = request.getParameter("password"); if (!initPassword.equals(passsword)) { request.setAttribute("message", "密码不正确"); request.getRequestDispatcher("/login.jsp").forward(request, response); return; } chain.doFilter(request, response); } private FilterConfig filterConfig; public void init(FilterConfig fConfig) throws ServletException { this.filterConfig = fConfig; } }