过滤器

1. 过滤器位于客户端和Web应用程序之间，用于检查和修改两者之间流过的请求和响应。
2. 在请求到达Servlet/JSP之前，过滤器会截获请求。
3. 在响应送给客户端之前，过滤器截获响应
4. 多个过滤器形成一个过滤器链，过滤器链中不同过滤器的先后顺序由部署文件web.xml中过滤器映射<filter-mapping>的顺序决定。
5. 最先截获客户端请求的过滤器将最后截获Servlet/JSP的响应信息。
6. 我们也可以为一个Web应用组件部署多个过滤器链，每个过滤器只执行某个特定的操作或检查。这样请求在到达被访问的目标前，需要经过这个过滤器链。具体流程的话，我们可以参考递归。

具体实现

web.xml

<?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>WishingWall</display-name><!-- 敏感字过滤器 --><filter><filter-name>ContentFilter</filter-name><filter-class>com.etc.filter.ContentFilter</filter-class><init-param><param-name>filePath</param-name><param-value>/WEB-INF/words</param-value></init-param>
</filter><filter-mapping><filter-name>ContentFilter</filter-name><url-pattern>/*</url-pattern>
</filter-mapping></web-app>

ContentFilter

package com.yby.filter;import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;public class ContentFilter implements Filter{private Map<String, String> map=new HashMap<String, String>();@Overridepublic void init(FilterConfig config) throws ServletException {String filePath=config.getInitParameter("filePath");//从配置文件种获取到文件的相对路径/WEB-INF/wordsServletContext context=config.getServletContext();//读取上下文环境String realPath=context.getRealPath(filePath);//绝对路径FileReader fr=null;BufferedReader br=null;try {fr=new FileReader(realPath);br=new BufferedReader(fr);String line=null;while((line=br.readLine())!=null) {String[] str=line.split("==");//根据==进行拆分字符串map.put(str[0], str[1]);}} catch (Exception e) {e.printStackTrace();}finally {try {if(br!=null) {br.close();}if(fr!=null) {fr.close();}} catch (IOException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}@Overridepublic void doFilter(ServletRequest request, ServletResponse resonpse, FilterChain filterChain)throws IOException, ServletException {//处理请求中的数据HttpRequestWrapper hrw=new HttpRequestWrapper((HttpServletRequest)request);hrw.setMap(map);	filterChain.doFilter(hrw, resonpse);}@Overridepublic void destroy() {this.map=null;}}

HttpRequestWrapper

package com.yby.filter;import java.util.Iterator;
import java.util.Map;
import java.util.Set;import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
/*http请求包装类*/
public class HttpRequestWrapper extends HttpServletRequestWrapper {private Map<String, String> map=null;public HttpRequestWrapper(HttpServletRequest request) {super(request);}public String replace(String str) {StringBuffer sb=new StringBuffer(str);Set<String> keys=this.getMap().keySet();//key为敏感字集合Iterator<String> it=keys.iterator();String ss=null;//存储key变量while(it.hasNext()) {String key=it.next();int index=sb.indexOf(key);//查找字符串中是否存在需要替换的文字if(index!=-1) {if(key!=null) {ss=key;}sb.replace(index, index+key.length(),this.getMap().get(key) );}}if(ss!=null) {//确保已经替换完毕if(sb.toString().indexOf(ss)==-1) {return sb.toString();}else {return replace(sb.toString());}}return sb.toString();}//重写getParameter方法public String getParameter(String str) {if(str.equals("pager.offset")) {//分页的信息不做处理的话可以这么处理return super.getParameter(str);}else {String content=super.getParameter(str);return replace(content);//返回被替换的文字}}public Map<String, String> getMap() {return map;}public void setMap(Map<String, String> map) {this.map = map;}
}