重温java web过滤器filter
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发布日期:   2021-04-04
作者:   欧阳思海
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1、Filter是什么？

Filter 技术是servlet 2.3 新增加的功能。Filter翻译过来的意思就是过滤器，能对客户端的消息进行预处理，然后将请求转发给其它的web组件，可以对ServletHttpRequest和ServletHttpResponse进行修改和检查。例如：在Filter中可以检查用户是否登录，对未登录的用户跳转到登陆界面。

2、过滤器快速入门

要定义一个过滤器，则需要实现javax.servlet.Filter接口，一个过滤器中包含了三个与生命周期相关的方法：

  • void init(FilterConfig  config)  过滤器初始化时执行，FilterConfig 可以用来获取过滤器的初始化参数。- void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

  • 过滤器过滤请求时执行，包含了request和response，chain用来是否执行下一步请求。
  • destroy()   web容器（tomcat）停止时执行
        第一步：创建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配置为/*，表示过滤所有请求。

启动tomcat，可以看到如下输出：

说明Filter的init方法在web容器启动的时候执行，读者可以自行验证destroy()方法会在web容器停止时执行，访问主页：

在访问主页的时候浏览器是一片空白的，控制台输出上面的消息，Filter只会实例化一次，为什么我们访问不到我们要访问的内容呢？只需要在doFilter方法中加入如下的语句就可以了：


chain.doFilter(req, resp);

FilterChain（过滤器链）是用来干什么的呢？这个对象只包含一个void doFilter(ServletRequest request, ServletResponse response)方法，Filter调用此方法去调用下一个web组件（Filter,Servlet等），如果不调用此方法，那么下一个web组件不会被执行。

再放行之前，我们可以在Filter中设置响应头信息，如下：


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

过滤器的过滤过程如下：

过滤器中的各项配置：

  • 配置初始化参数：
    
 <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方法中获取：


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的名字 -->
      <filter-name>demo</filter-name>
      <!-- 类名 -->
      <filter-class>cn.zq.filter.DemoServlet</filter-class>
  </filter>
  <!-- 可以有多个 -->
  <filter-mapping>
      <!-- 对哪个filter进行配置 -->
      <filter-name>demo</filter-name>
      <!--           配置过滤的url，不能是/          其他与servlet配置类似       -->
      <url-pattern>/*</url-pattern>
      <!-- 
          根据名字配置对哪个servlet进行过滤
       -->
      <servlet-name>DemoServlet</servlet-name>
      <!-- 
          ERROR: <error-page>过来的请求
          FORWARD: 对转发过来的请求进行过滤，也就是对request.getRequestDispatcher(path).forward(request, response)
          INCLUDE:对request.getRequestDispatcher(path).include(request, response)过来的请求进行过滤
          REQUEST(默认): 对客户端的请求进行拦截
          可以配置多个
       -->
      <dispatcher>ERROR</dispatcher>
  </filter-mapping>

3、Filter的应用

为了便于编写Filter，本节的所有应用中提供了一个通过的Filter的实现：


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的实现 *
 @author zq 
 * 
 */
 
public abstract class GenericFilter implements Filter,FilterConfig, Serializable{    
    private static final long serialVersionUID = 5497978960987185665L;    
        private FilterConfig filterConfig;    
    /**
     * 需要初始化，应该覆盖整个方法
     */
    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、解决GET和POST获取参数的乱码问题

第一步：创建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;
/** 
* 处理字符编码的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);        
        //优化：只对GET请求的request进行包装
        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：编写配置文件


  <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>

这个过滤器应该配置在所有过滤器的前面

第三步：测试


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">
    <!--
    <link rel="stylesheet" type="text/css" href="styles.css">
    -->
  </head>
  
  <body>
   <a href="<%=basePath%>servlet/DemoServlet?name=中国">Click</a>
   <form action="<%=basePath%>servlet/DemoServlet" method="post">
           <input type="text" name="name"><br/>
           <input type="submit"/>
   </form>
  </body></html>

访问并测试：

这个过滤器应该被配置在所有过滤器的前面，就能解决全站的乱码了，这样就不用重复的编写解决乱码问题的代码了。

3.2、设置所有的jsp页面不缓存

因为jsp页面的有些内容是动态生成的，所有混成jsp页面的意义不大，我们通常会设置这些jsp页面不缓存。

第一步：开发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 {        
        //设置3个响应头
        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：打开ie，清空所有的缓存，cookie，访问本项目的jsp文件看是否有缓存文件，将Filter拿到，再访问看是否有缓存文件。

控制是否缓存，也可以在jsp页面中加入这几个头


<meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">

3.3、控制静态页面缓存（如html，图片）

第一步：编写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;    @Override
    public void doFilter(HttpServletRequest request,
            HttpServletResponse response, FilterChain chain)
            throws IOException, ServletException {        /*
         *让图片缓存一个月，html文件缓存一个星期 ,
         *具体的相关信息可以通过配置文件来配置。
         */
        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：测试

请求资源，再次请求。查看状态码为304，及缓存文件的日期为N天以后。这是返回的状态码：HTTP/1.1 304 Not Modified

3.4 验证用户是否登录

第一步：开发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;
/** 
* 验证用户是否登录的过滤器 
* @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");        //没有找到user，则说明用户没有登录，转到登录页面让用户登录
        if(user == null){
            PrintWriter out = response.getWriter();
            out.print("<script>" +                      "alert('您还未登录!');" +                      "window.location.href='"+request.getContextPath()+"/login.jsp'" +                      "</script>");            return;
        }
        chain.doFilter(request, response);
    }

}

第二步：开发登录功能，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 {        //让用户退出登录
        request.getSession().invalidate();
        response.sendRedirect(request.getContextPath() + "/login.jsp");
    }    

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {        /*
         * 让用户登录成功
         */
        String username = request.getParameter("username");        if(username == null || username.length() == 0){
            request.setAttribute("msg", "用户名不能为空！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);            return;
        }
        request.getSession().setAttribute("user", username);        
        //重定向到主页
        response.sendRedirect(request.getContextPath() + "/page/index.jsp");
    }

}

登录页面：/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>

登录成功跳转页面：/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">
    <!--
    <link rel="stylesheet" type="text/css" href="styles.css">
    -->
  </head>
  
  <body>
   
     欢迎你,<c:out value="${user }"></c:out><br/>
  <a href="<%=path%>/login">退出</a>
  </body></html>


 <!-- 对/page/*进行过滤 -->
  <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：测试，

在访问/page/index.jsp页面时，未登录是否会跳转到登录页面。登录时显示用户的名字。

3.5、自动登录

自动登录是为了让用户下次访问时，不用输入用户名和密码。将用户的信息保存到cookie中，下次直接从cookie中取。

第一步：开发登录页面


<%@ 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>自动登录</legend>
                        <input type="radio" name="day" value="0">不自动登录<br/>
                        <input type="radio" name="day" value="1">1天<br/>
                        <input type="radio" name="day" value="7">一个星期<br/>
                        <input type="radio" name="day" value="30">一个月<br/>
                    </fieldset>
                    <input type="submit" value="Sign in">
                </form>
            </c:when>
            <c:otherwise>
                欢迎您,<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：开发登录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 {        //让用户退出登录
        request.getSession().invalidate();        //删除cookie
        Cookie cookie = new Cookie("autoLogin", "");        
        /**
         * 0表示删除文件和缓存
         * -1表示删除文件，但是还有缓存
         */
        cookie.setMaxAge(0);
        cookie.setPath("/");
        response.addCookie(cookie);
        response.sendRedirect(request.getContextPath() + "/login.jsp");
    }    
        
        public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {        /*
         * 让用户登录成功
         */
        String username = request.getParameter("username");        if(username == null || username.length() == 0){
            request.setAttribute("msg", "用户名不能为空！");
            request.getRequestDispatcher("/login.jsp").forward(request, response);            return;
        }        int day = 0;
        String auto = request.getParameter("day");        try {
            day = Integer.parseInt(auto);
        } catch (Exception e) {
            
        }        
        //对中文要进行编码
        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：开发自动登录过滤器


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;    

    @Override
    public void doFilter(HttpServletRequest request,
            HttpServletResponse response, FilterChain chain)
            throws IOException, ServletException {        
        /*
         * 优化：当用户手动登录或退出时，就不需要自动登录，
         * 并且用户已经登录，也不需要自动登录，自动登录的代码是
         * 多此一举
         */
        HttpSession session = request.getSession();
        String requestURI = request.getRequestURI();
        Object user = session.getAttribute("user");        
        if(!requestURI.contains("/login") && user == null){            
            //获取cookie
            Cookie[] 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、过滤非法语句(脏话)

在过滤器中，包装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、全站压缩

实现对输出流的压缩：

在tomcat将数据输出到浏览器前，进行压缩，可以减少传送过去的数据，节约成本。如果在流量很少的情况下查看相同的内容和乐而不为呢？

思路：

  • 在调用request.getOutputStream()或request.getWriter()时获取自己的输出流，将数据写到事先准备的缓冲中。- 在输出完成后获取我们自己的缓冲数据- 然后在对缓冲的数据进行压缩，在过滤器中将数据传输给浏览器

  • 第一步：编写压缩数据的过滤器

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压缩
        if(ac != null && ac.toLowerCase().indexOf("gzip") != -1){
            BufferedHttpServletResponse bRes = new BufferedHttpServletResponse(response);
            chain.doFilter(request, bRes);            
            byte[] data = bRes.getData();
            System.out.println("->压缩前数据大小：" + data.length);
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            GZIPOutputStream gout = new GZIPOutputStream(bos);
            gout.write(data);
            gout.close();            
            byte[] compressedData = bos.toByteArray();
            System.out.println("->压缩后的数据大小："+compressedData.length);            
            //设置头信息
            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：配置对所有的jsp进行压缩


   <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：测试压缩过滤器

使用压缩过滤器应该注意：应该只用这个压缩过滤器对文本进行压缩，例如jsp,html,css,js等进行压缩，对视频和图片的压缩率很低，不要用来压缩视频和图片，如果是下载，那也不应该用来压缩，这样不但压缩率很低，而且还有可能让服务器奔溃。

关于压缩过滤器的优化：

在doFilter方法中先将数据拿出来，然后放到GzipOutputStream中进行压缩，然后得到压缩后的字节再输出给客户端，这样2次都得到了字节，假如数据量较大，这2次都会占用较多的内存，能不能从包装的response拿出来时直接就是压缩过后的数据呢？改造后的代码如下：


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压缩
        if(ac != null && ac.toLowerCase().indexOf("gzip") != -1){
            BufferedHttpServletResponse bRes = new BufferedHttpServletResponse(response);
            chain.doFilter(request, bRes);            
            byte[] compressedData = bRes.getData();            
            //设置头信息
            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.总结

利用Filter能对请求和响应进行预处理，在到达目标组件之前，对强求进行处理，诸如：对请求头和响应头进行处理。充分的利用了包装器设计模式，对request或response进行包装，对其方法进行增强。假如我们拒绝某个请求，就可以写一个过滤器对不希望的请求不放行，即不执行chain.doFilter(request, response)方法，过滤器能帮助我们干很多的事情。


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

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原文地址：https://sihai.blog.csdn.net/article/details/109464851

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