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重温java web过滤器filter

發布時間：2025/3/20 编程问答 22 豆豆

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1、Filter是什么？

Filter 技術是servlet 2.3 新增加的功能。Filter翻譯過來的意思就是過濾器，能對客戶端的消息進行預處理，然后將請求轉發給其它的web組件，可以對ServletHttpRequest和ServletHttpResponse進行修改和檢查。例如：在Filter中可以檢查用戶是否登錄，對未登錄的用戶跳轉到登陸界面。

2、過濾器快速入門

要定義一個過濾器，則需要實現javax.servlet.Filter接口，一個過濾器中包含了三個與生命周期相關的方法：

void init(FilterConfig? config)? 過濾器初始化時執行，FilterConfig 可以用來獲取過濾器的初始化參數。
void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

過濾器過濾請求時執行，包含了request和response，chain用來是否執行下一步請求。

destroy()?? web容器（tomcat）停止時執行

??? 第一步：創建DemoFilter.java

package cn.zq.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 DemoServlet implements Filter{ public void init(FilterConfig config) throws ServletException {System.out.println("DemoServlet.init...");} public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException { System.out.println("DemoServlet.doFilter...");System.out.println("this = " + this);} public void destroy() {System.out.println("DemoServlet.destroy...");}}

第二步：在web.xml文件中添加如下配置：

?<filter><filter-name>demo</filter-name><filter-class>cn.zq.filter.DemoServlet</filter-class></filter><filter-mapping><filter-name>demo</filter-name><url-pattern>/*</url-pattern></filter-mapping>

url-pattern配置為/*，表示過濾所有請求。

啟動tomcat，可以看到如下輸出：

說明Filter的init方法在web容器啟動的時候執行，讀者可以自行驗證destroy()方法會在web容器停止時執行，訪問主頁：

在訪問主頁的時候瀏覽器是一片空白的，控制臺輸出上面的消息，Filter只會實例化一次，為什么我們訪問不到我們要訪問的內容呢？只需要在doFilter方法中加入如下的語句就可以了：

chain.doFilter(req, resp);

FilterChain（過濾器鏈）是用來干什么的呢？這個對象只包含一個void doFilter(ServletRequest request, ServletResponse response)方法，Filter調用此方法去調用下一個web組件（Filter,Servlet等），如果不調用此方法，那么下一個web組件不會被執行。

再放行之前，我們可以在Filter中設置響應頭信息，如下：

?? ???? resp.setContentType("text/html;charset=UTF-8");chain.doFilter(req, resp);

過濾器的過濾過程如下：

過濾器中的各項配置：

配置初始化參數：

<filter><filter-name>demo</filter-name><filter-class>cn.zq.filter.DemoServlet</filter-class><init-param><param-name>encoding</param-name><param-value>UTF-8</param-value></init-param><init-param><param-name>name</param-name><param-value>RiccioZhang</param-value></init-param></filter><filter-mapping><filter-name>demo</filter-name><url-pattern>/*</url-pattern></filter-mapping>

然后在init方法中獲取：

public void init(FilterConfig config) throws ServletException {System.out.println("DemoServlet.init...");String encoding = config.getInitParameter("encoding");String name = config.getInitParameter("name");System.out.println("encoding="+encoding);System.out.println("name="+name);}<filter><filter-name>demo</filter-name><filter-class>cn.zq.filter.DemoServlet</filter-class></filter><filter-mapping><filter-name>demo</filter-name><url-pattern>/*</url-pattern><servlet-name>DemoServlet</servlet-name><dispatcher>ERROR</dispatcher></filter-mapping>

3、Filter的應用

為了便于編寫Filter，本節的所有應用中提供了一個通過的Filter的實現：

package cn.zq.filter; import java.io.IOException; import java.io.Serializable; import java.util.Enumeration; 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; /** * 通用的Filter的實現 *@author zq * */public abstract class GenericFilter implements Filter,FilterConfig, Serializable{ private static final long serialVersionUID = 5497978960987185665L; private FilterConfig filterConfig; /*** 需要初始化，應該覆蓋整個方法*/public void init(){} public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig;init();} abstract public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException; public void destroy() {}public String getFilterName() { return getFilterConfig().getFilterName();} public String getInitParameter(String name) { return getFilterConfig().getInitParameter(name);} public Enumeration<String> getInitParameterNames() { return getFilterConfig().getInitParameterNames();} public ServletContext getServletContext() { return getFilterConfig().getServletContext();} public FilterConfig getFilterConfig() { return filterConfig;} }package cn.zq.filter; import java.io.IOException; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public abstract class HttpFilter extends GenericFilter{ private static final long serialVersionUID = 1029993995265394412L; public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {HttpServletRequest req = (HttpServletRequest) request;HttpServletResponse resp = (HttpServletResponse) response;doFilter(req, resp, chain);} protected abstract void doFilter(HttpServletRequest request, HttpServletResponse response,FilterChain chain) throws IOException, ServletException; }

3.1、解決GET和POST獲取參數的亂碼問題

第一步：創建Filter

package cn.zq.filter; import java.io.IOException; import java.io.UnsupportedEncodingException; import java.util.Iterator; import java.util.Map; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletRequestWrapper; import javax.servlet.http.HttpServletResponse; /** * 處理字符編碼的Filter * @author zq * */public class CharacterEncodingFilter extends HttpFilter{ private static final long serialVersionUID = -4329981031091311164L; private String characterEncoding = "UTF-8"; public void init() {String ce = getInitParameter("characterEncoding"); if(ce != null && !ce.equals("")){characterEncoding = ce.toUpperCase();}} public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException {request.setCharacterEncoding(characterEncoding);response.setContentType("text/html;charset="+characterEncoding); //優化：只對GET請求的request進行包裝if(request.getMethod().equals("GET")){request = new ParameterHandlerRequest(request);}chain.doFilter(request, response);} private class ParameterHandlerRequest extends HttpServletRequestWrapper{ public ParameterHandlerRequest(HttpServletRequest request) { super(request);} public String getParameter(String name) {String value = super.getParameter(name); return getString(value);} private String getString(String value){ if(value != null){ try {value = ?new String( value.getBytes("ISO-8859-1"), getRequest().getCharacterEncoding() );} catch (UnsupportedEncodingException e) {e.printStackTrace();}} return value;} public String[] getParameterValues(String name) {String[] values = super.getParameterValues(name); if(null != values){ for(int i = 0; i < values.length; i++){values[i] = getString(values[i]);}} return values;} public Map<String, String[]> getParameterMap() {Map<String, String[]> paramMap = super.getParameterMap();Iterator<String[]> it = paramMap.values().iterator(); while(it.hasNext()){String[] values = it.next(); if(null != values){ for(int i = 0; i < values.length; i++){values[i] = getString(values[i]);}}} return paramMap;}} }

第二步：編寫配置文件

?<filter><filter-name>CharacterEncodingFilter</filter-name><filter-class>cn.zq.filter.CharacterEncodingFilter</filter-class></filter><filter-mapping><filter-name>CharacterEncodingFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping>

這個過濾器應該配置在所有過濾器的前面

第三步：測試

package cn.zq.servlet; import java.io.IOException; import java.util.Iterator; import java.util.Map; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class DemoServlet extends HttpServlet {private static final long serialVersionUID = -4363281555738840730L;public void doGet(HttpServletRequest request, HttpServletResponse response)?? ??? ??? ?throws ServletException, IOException {doPost(request, response);}public void doPost(HttpServletRequest request, HttpServletResponse response)?? ??? ??? ?throws ServletException, IOException {System.out.println("--------"+request.getMethod()+"--------");System.out.println("request = " + request);System.out.println("name = " + request.getParameter("name"));String[] names = request.getParameterValues("name");if(names != null && names.length > 0){System.out.println("names[0] = " + names[0]);}Map<String, String[]> parameterMap = request.getParameterMap();for(Iterator<String> it = parameterMap.keySet().iterator();it.hasNext();){String key = it.next();String[] values = parameterMap.get(key);System.out.println(key+"="+values[0] );}}} <servlet><servlet-name>DemoServlet</servlet-name><servlet-class>cn.zq.servlet.DemoServlet</servlet-class></servlet><servlet-mapping><servlet-name>DemoServlet</servlet-name><url-pattern>/servlet/DemoServlet</url-pattern></servlet-mapping><%@ 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%>"><title>My JSP 'index.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"></head><body><a href="<%=basePath%>servlet/DemoServlet?name=中國">Click</a><form action="<%=basePath%>servlet/DemoServlet" method="post"><input type="text" name="name"><br/><input type="submit"/></form></body></html>

訪問并測試：

這個過濾器應該被配置在所有過濾器的前面，就能解決全站的亂碼了，這樣就不用重復的編寫解決亂碼問題的代碼了。

3.2、設置所有的jsp頁面不緩存

因為jsp頁面的有些內容是動態生成的，所有混成jsp頁面的意義不大，我們通常會設置這些jsp頁面不緩存。

第一步：開發Filter

package cn.zq.filter; import java.io.IOException; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class DynamicPageCacheFilter extends HttpFilter { /** * */private static final long serialVersionUID = -5449451659530735173L; public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException { //設置3個響應頭response.setHeader("pragma", "no-cache");response.setHeader("cache-control", "no-cache");response.setDateHeader("expires", 0);chain.doFilter(request, response);}}

第二步：配置web.xml

?<filter><filter-name>DynamicPageCacheFilter</filter-name><filter-class> cn.zq.filter.DynamicPageCacheFilter</filter-class></filter><filter-mapping><filter-name>DynamicPageCacheFilter</filter-name><url-pattern>*.jsp</url-pattern><dispatcher>REQUEST</dispatcher><dispatcher>FORWARD</dispatcher><dispatcher>INCLUDE</dispatcher></filter-mapping

第三步：打開ie，清空所有的緩存，cookie，訪問本項目的jsp文件看是否有緩存文件，將Filter拿到，再訪問看是否有緩存文件。

控制是否緩存，也可以在jsp頁面中加入這幾個頭

3.3、控制靜態頁面緩存（如html，圖片）

第一步：編寫Filter

package cn.zq.filter; import java.io.IOException; import java.util.Calendar; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class StaticContentCacheFilter extends HttpFilter{ /** * */private static final long serialVersionUID = 7660878144738222823L; @Overridepublic void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException { /**讓圖片緩存一個月，html文件緩存一個星期 ,*具體的相關信息可以通過配置文件來配置。*/String requestURI = request.getRequestURI(); long time = 0; int day = 0; if(requestURI.endsWith(".jpg")){day = 30;}else if(requestURI.endsWith(".html")){day = 7;}Calendar calendar = Calendar.getInstance();calendar.add(Calendar.DATE, day);time = calendar.getTimeInMillis();response.setDateHeader("expires", time);chain.doFilter(request, response);}}

第二步：配置

?<filter><filter-name>StaticContentCacheFilter</filter-name><filter-class>cn.zq.filter.StaticContentCacheFilter</filter-class></filter><filter-mapping><filter-name>StaticContentCacheFilter</filter-name><url-pattern>*.html</url-pattern><url-pattern>*.jpg</url-pattern> ? </filter-mapping>

第三步：測試

請求資源，再次請求。查看狀態碼為304，及緩存文件的日期為N天以后。這是返回的狀態碼：HTTP/1.1 304 Not Modified

3.4 驗證用戶是否登錄

第一步：開發filter

package cn.zq.filter; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; /** * 驗證用戶是否登錄的過濾器 * @author Riccio Zhang * */public class LoginFilter extends HttpFilter{ private static final long serialVersionUID = -6363929637537263967L; protected void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException {HttpSession session = request.getSession();Object user = session.getAttribute("user"); //沒有找到user，則說明用戶沒有登錄，轉到登錄頁面讓用戶登錄if(user == null){PrintWriter out = response.getWriter();out.print("<script>" + ?"alert('您還未登錄!');" + ?"window.location.href='"+request.getContextPath()+"/login.jsp'" + ?"</script>"); return;}chain.doFilter(request, response);}}

第二步：開發登錄功能，配置web.xml

package cn.zq.servlet; import java.io.IOException; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class LoginServlet extends HttpServlet { private static final long serialVersionUID = 3059445154848670189L; public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException { //讓用戶退出登錄request.getSession().invalidate();response.sendRedirect(request.getContextPath() + "/login.jsp");} public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException { /** 讓用戶登錄成功*/String username = request.getParameter("username"); if(username == null || username.length() == 0){request.setAttribute("msg", "用戶名不能為空！");request.getRequestDispatcher("/login.jsp").forward(request, response); return;}request.getSession().setAttribute("user", username); //重定向到主頁response.sendRedirect(request.getContextPath() + "/page/index.jsp");}}

登錄頁面：/login.jsp

<%@ 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><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"></head><body><p style="color:red;font: 12px;">${requestScope.msg }</p><form action="<%=path%>/login" method="post">username : <input type="text" name="username" ><br/><input type="submit" value="Sign in"></form></body></html>

登錄成功跳轉頁面：/page/index.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@ taglib uri="http://java.sun.com/jsp/jstl/core" ?prefix="c"%><%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%>"><title>My JSP 'index.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"></head><body>歡迎你,<c:out value="${user }"></c:out><br/><a href="<%=path%>/login">退出</a></body></html> <filter><filter-name>LoginFilter</filter-name><filter-class>cn.zq.filter.LoginFilter</filter-class></filter><filter-mapping><filter-name>LoginFilter</filter-name><url-pattern>/page/*</url-pattern> ? </filter-mapping><servlet-mapping><servlet-name>DemoServlet</servlet-name><url-pattern>/servlet/DemoServlet</url-pattern></servlet-mapping><servlet-mapping><servlet-name>LoginServlet</servlet-name><url-pattern>/login</url-pattern></servlet-mapping>

第三步：測試，

在訪問/page/index.jsp頁面時，未登錄是否會跳轉到登錄頁面。登錄時顯示用戶的名字。

3.5、自動登錄

自動登錄是為了讓用戶下次訪問時，不用輸入用戶名和密碼。將用戶的信息保存到cookie中，下次直接從cookie中取。

第一步：開發登錄頁面

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%><%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><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"></head><body><c:choose><c:when test="${empty user }"><p style="color:red;font: 12px;">${requestScope.msg }</p><form action="<%=path%>/login" method="post">username : <input type="text" name="username" ><br/><fieldset><legend>自動登錄</legend><input type="radio" name="day" value="0">不自動登錄<br/><input type="radio" name="day" value="1">1天<br/><input type="radio" name="day" value="7">一個星期<br/><input type="radio" name="day" value="30">一個月<br/></fieldset><input type="submit" value="Sign in"></form></c:when><c:otherwise>歡迎您,<c:out value="${user }"></c:out><br/><a href="<c:url value='/login' />">退出</a></c:otherwise></c:choose></body></html>

第二步：開發登錄servlet

package cn.zq.servlet; import java.io.IOException; import java.net.URLEncoder; import javax.servlet.ServletException; import javax.servlet.http.Cookie; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class LoginServlet extends HttpServlet { /** * */private static final long serialVersionUID = 3059445154848670189L; public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException { //讓用戶退出登錄request.getSession().invalidate(); //刪除cookieCookie cookie = new Cookie("autoLogin", ""); /*** 0表示刪除文件和緩存* -1表示刪除文件，但是還有緩存*/cookie.setMaxAge(0);cookie.setPath("/");response.addCookie(cookie);response.sendRedirect(request.getContextPath() + "/login.jsp");} public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException { /** 讓用戶登錄成功*/String username = request.getParameter("username"); if(username == null || username.length() == 0){request.setAttribute("msg", "用戶名不能為空！");request.getRequestDispatcher("/login.jsp").forward(request, response); return;} int day = 0;String auto = request.getParameter("day"); try {day = Integer.parseInt(auto);} catch (Exception e) {} //對中文要進行編碼Cookie cookie = new Cookie("autoLogin", URLEncoder.encode(username, request.getCharacterEncoding()));cookie.setMaxAge(day*24*3600);cookie.setPath("/");response.addCookie(cookie);request.getSession().setAttribute("user", username);response.sendRedirect(request.getContextPath() + "/page/index.jsp");}} ?<servlet-mapping><servlet-name>DemoServlet</servlet-name><url-pattern>/servlet/DemoServlet</url-pattern></servlet-mapping><servlet-mapping><servlet-name>LoginServlet</servlet-name><url-pattern>/login</url-pattern></servlet-mapping>

第三步：開發自動登錄過濾器

package cn.zq.filter; import java.io.IOException; import java.net.URLDecoder; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.Cookie; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; public class AutoLoginFilter extends HttpFilter{ private static final long serialVersionUID = 5891858915933022714L; @Overridepublic void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException { /** 優化：當用戶手動登錄或退出時，就不需要自動登錄，* 并且用戶已經登錄，也不需要自動登錄，自動登錄的代碼是* 多此一舉*/HttpSession session = request.getSession();String requestURI = request.getRequestURI();Object user = session.getAttribute("user"); if(!requestURI.contains("/login") && user == null){ //獲取cookieCookie[] cookies = request.getCookies(); if(cookies != null){ for(Cookie c : cookies){ if("autoLogin".equals(c.getName())){String username = c.getValue();username = URLDecoder.decode(username, request.getCharacterEncoding());session.setAttribute("user", username); break;}}}}chain.doFilter(request, response);}} ?<filter><filter-name>AutoLoginFilter</filter-name><filter-class>cn.zq.filter.AutoLoginFilter</filter-class></filter><filter-mapping><filter-name>AutoLoginFilter</filter-name><url-pattern>/*</url-pattern> ?</filter-mapping>

3.6、過濾非法語句(臟話)

在過濾器中，包裝HttpServletRequest,修改getParameter方法

package cn.zq.filter; import java.io.IOException; import java.util.Arrays; import java.util.List; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletRequestWrapper; import javax.servlet.http.HttpServletResponse;public class DirtyWordsFilter extends HttpFilter{ private static final long serialVersionUID = -5025789414017693051L; public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)throws IOException, ServletException {request = new MyHttpServletRequest(request);chain.doFilter(request, response);}}class MyHttpServletRequest extends HttpServletRequestWrapper{List<String> dirtyWords = Arrays.asList(new String[]{"SB", "sb", "傻B", "2B"}); public MyHttpServletRequest(HttpServletRequest request) { super(request);} public String getParameter(String name) {String value = super.getParameter(name); if(value != null && value.length() > 0){ for(String dw : dirtyWords){value = value.replaceAll(dw, "***");}} return value;} }

3.7、全站壓縮

實現對輸出流的壓縮：

在tomcat將數據輸出到瀏覽器前，進行壓縮，可以減少傳送過去的數據，節約成本。如果在流量很少的情況下查看相同的內容和樂而不為呢？

思路：

在調用request.getOutputStream()或request.getWriter()時獲取自己的輸出流，將數據寫到事先準備的緩沖中。
在輸出完成后獲取我們自己的緩沖數據
然后在對緩沖的數據進行壓縮，在過濾器中將數據傳輸給瀏覽器

第一步：編寫壓縮數據的過濾器

package cn.zq.filter; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.OutputStreamWriter; import java.io.PrintWriter; import java.util.zip.GZIPOutputStream; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpServletResponseWrapper;public class GzipFilter extends HttpFilter{ private static final long serialVersionUID = 3410826595861585118L; public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException {String ac = request.getHeader("Accept-Encoding"); //支持gzip壓縮if(ac != null && ac.toLowerCase().indexOf("gzip") != -1){BufferedHttpServletResponse bRes = new BufferedHttpServletResponse(response);chain.doFilter(request, bRes); byte[] data = bRes.getData();System.out.println("->壓縮前數據大小：" + data.length);ByteArrayOutputStream bos = new ByteArrayOutputStream();GZIPOutputStream gout = new GZIPOutputStream(bos);gout.write(data);gout.close(); byte[] compressedData = bos.toByteArray();System.out.println("->壓縮后的數據大小："+compressedData.length); //設置頭信息response.setContentLength(compressedData.length);response.setHeader("Content-Encoding", "gzip");ServletOutputStream out = response.getOutputStream();out.write(compressedData);}else{chain.doFilter(request, response);}}} class BufferedHttpServletResponse extends HttpServletResponseWrapper{ private ByteArrayOutputStream buf = new ByteArrayOutputStream(); private PrintWriter pw; public BufferedHttpServletResponse(HttpServletResponse response) { super(response);} public PrintWriter getWriter() throws IOException {pw = new PrintWriter( new OutputStreamWriter(buf, getResponse().getCharacterEncoding())); return pw;}public ServletOutputStream getOutputStream() throws IOException {ServletOutputStream sos = new ServletOutputStream() { public void write(int b) throws IOException {buf.write(b);}}; return sos;} public byte[] getData(){ if(pw != null){pw.close();} return buf.toByteArray();} }

第二步：配置對所有的jsp進行壓縮

? <filter><filter-name>GzipFilter</filter-name><filter-class>cn.zq.filter.GzipFilter</filter-class></filter><filter-mapping><filter-name>GzipFilter</filter-name><servlet-name>DemoServlet</servlet-name><url-pattern>*.jsp</url-pattern></filter-mapping>

第三步：測試壓縮過濾器

使用壓縮過濾器應該注意：應該只用這個壓縮過濾器對文本進行壓縮，例如jsp,html,css,js等進行壓縮，對視頻和圖片的壓縮率很低，不要用來壓縮視頻和圖片，如果是下載，那也不應該用來壓縮，這樣不但壓縮率很低，而且還有可能讓服務器奔潰。

關于壓縮過濾器的優化：

在doFilter方法中先將數據拿出來，然后放到GzipOutputStream中進行壓縮，然后得到壓縮后的字節再輸出給客戶端，這樣2次都得到了字節，假如數據量較大，這2次都會占用較多的內存，能不能從包裝的response拿出來時直接就是壓縮過后的數據呢？改造后的代碼如下：

package cn.zq.filter; import java.io.ByteArrayOutputStream; import java.io.IOException; import java.io.OutputStreamWriter; import java.io.PrintWriter; import java.util.zip.GZIPOutputStream; import javax.servlet.FilterChain; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpServletResponseWrapper; public class GzipFilter extends HttpFilter{ private static final long serialVersionUID = 3410826595861585118L; public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)throws IOException, ServletException {String ac = request.getHeader("Accept-Encoding"); //支持gzip壓縮if(ac != null && ac.toLowerCase().indexOf("gzip") != -1){BufferedHttpServletResponse bRes = new BufferedHttpServletResponse(response);chain.doFilter(request, bRes); byte[] compressedData = bRes.getData(); //設置頭信息response.setContentLength(compressedData.length);response.setHeader("Content-Encoding", "gzip");ServletOutputStream out = response.getOutputStream();out.write(compressedData);}else{chain.doFilter(request, response);}}}class BufferedHttpServletResponse extends HttpServletResponseWrapper{ private ByteArrayOutputStream buf = new ByteArrayOutputStream(); private GZIPOutputStream gout; private PrintWriter pw; public BufferedHttpServletResponse(HttpServletResponse response) throws IOException { super(response);gout = new GZIPOutputStream(buf);} public PrintWriter getWriter() throws IOException {pw = new PrintWriter( new OutputStreamWriter(gout, getResponse().getCharacterEncoding())); return pw;} public ServletOutputStream getOutputStream() throws IOException {ServletOutputStream sos = new ServletOutputStream() { public void write(int b) throws IOException {gout.write(b);}}; return sos;} public byte[] getData() throws IOException{ if(pw != null){pw.close();}gout.close(); return buf.toByteArray();} }

4.總結

利用Filter能對請求和響應進行預處理，在到達目標組件之前，對強求進行處理，諸如：對請求頭和響應頭進行處理。充分的利用了包裝器設計模式，對request或response進行包裝，對其方法進行增強。假如我們拒絕某個請求，就可以寫一個過濾器對不希望的請求不放行，即不執行chain.doFilter(request, response)方法，過濾器能幫助我們干很多的事情。

作者：RiccioZhang 出處：https://blog.csdn.net/ricciozhang/article/details/43833401

????覺得有用就轉發分享一下吧

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