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Java Web总结十九Filter过滤器
一、Filter简介
Filter也称之为过滤器,它是Servlet技术中最激动人心的技术,Web开发人员通过Filter技术,对Web服务器管理的所有Web资源:例如Jsp,Servlet,静态图片文件或静态HTML文件等进行拦截,从而实现一些特殊的功能。例如实现URL级别的权限访问控制、过滤敏感词汇、自动登录、压缩响应信息等一些高级功能。
Servlet API中提供了一个Filter接口,开发Web应用时,如果编写的Java类实现了这个接口,则把这个Java类称之为过滤器Filter。通过Filter技术,开发人员可以实现用户在访问某个目标资源之前,对访问的请求和响应进行拦截,如下所示:
二、Filter开发步骤
1、编写Java类实现Filter接口,并实现doFilter方法。
2、在Web.xml文件中使用<filter>和<filter-mapping>元素对编写的Filter类进行注册,并设置它所能拦截的资源。
三、Filter链
1、在一个Web应用中,可以开发编写多个Filter,这些Filter组合起来称之为Filter链。
2、Web服务器根据Filter在web.xml文件中的注册顺序,决定先调用哪个Filter,当第一个Filter的doFilter方法被调用时,web服务器会创建一个代表Filter的FilterChain对象传递给该方法。在doFilter方法中,开发人员如果调用了FilterChain对象的doFilter方法,则web服务器会检查FilterChain对象中是否还有Filter,如果有,则调用第2个Filter,如果没有,则调用目标资源。
四、过滤器生命周期
空参构造() 1次
init() 1次
doFilter(请求,响应,过滤器链) N次,与请求次数有关
destory() 1次
Filter是一个单例
五、使用Filter读取配置文件信息
1、web.xml
<filter> <filter-name>FilterDemo4</filter-name> <filter-class>com.gnnuit.web.filter.FilterDemo4</filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param> </filter> <filter-mapping> <filter-name>FilterDemo4</filter-name> <url-pattern>/*</url-pattern> </filter-mapping>
2、FilterDemo4.java
package com.gnnuit.web.filter; import java.io.IOException; import java.util.Enumeration; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; //使用FilterConfig读取配置文件的信息 public class FilterDemo4 implements Filter { private FilterConfig filterConfig; @Override public void destroy() { // TODO Auto-generated method stub } @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // 方式一:读取一个 String encoding = filterConfig.getInitParameter("encoding"); response.setContentType("text/html;charset=" + encoding); // 方式二:读取多个 Enumeration<String> enums = filterConfig.getInitParameterNames(); while (enums.hasMoreElements()) { String key = enums.nextElement(); String value = filterConfig.getInitParameter(key); System.out.println(key + ":" + value); } chain.doFilter(request, response); } @Override public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; } }
五、Filter应用
1、统一Post请求中文字符编码的过滤器。(通过配置参数encoding指明使用何种字符编码,以处理表单form请求参数的中文问题)
(1)web.xml
<filter> <filter-name>FilterDemo5</filter-name> <filter-class>com.gnnuit.web.filter.FilterDemo5</filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param> </filter> <filter-mapping> <filter-name>FilterDemo5</filter-name> <url-pattern>/*</url-pattern> </filter-mapping>
(2)FilterDemo5.java
package com.gnnuit.web.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; //post请求中文字符编码的过滤器 public class FilterDemo5 implements Filter { private FilterConfig filterConfig; @Override public void destroy() { // TODO Auto-generated method stub } @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // 设置post请求编码 request.setCharacterEncoding("utf-8"); // 设置输出编码 String encoding = filterConfig.getInitParameter("encoding"); response.setContentType("text/html;charset=" + encoding); chain.doFilter(request, response); } @Override public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; } }
2、禁止浏览器缓存所有动态页面的过滤器
(1)web.xml
<filter> <filter-name>FilterDemo6</filter-name> <filter-class>com.gnnuit.web.filter.FilterDemo6</filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param> <init-param> <param-name>html</param-name> <param-value>86400</param-value> </init-param> </filter> <filter-mapping> <filter-name>FilterDemo6</filter-name> <url-pattern>/*</url-pattern> </filter-mapping>
(2)FilterDemo6.java
package com.gnnuit.web.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; //禁止浏览器缓存所有动态资源[例如JSP资源] //浏览器缓存所有静态资源,并指定缓存时间[例如html资源] public class FilterDemo6 implements Filter { private FilterConfig filterConfig; @Override public void destroy() { } @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest httpRequest = (HttpServletRequest) request; HttpServletResponse httpResponse = (HttpServletResponse) response; String uri = httpRequest.getRequestURI(); if (uri != null && uri.endsWith("jsp")) { httpResponse.setHeader("expires", "-1"); httpResponse.setHeader("cache-control", "no-cache"); httpResponse.setHeader("pragma", "no-cache"); } else if (uri != null && uri.endsWith(".html")) { String strHtml = filterConfig.getInitParameter("html"); long time = System.currentTimeMillis() + Integer.parseInt(strHtml) * 1000; // time为毫秒值 httpResponse.setDateHeader("expires", time); httpResponse.setHeader("cache-control", time / 1000 + ""); httpResponse.setHeader("pragma", time / 1000 + ""); } chain.doFilter(request, response); } @Override public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; } }