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作者:Jacky    发表时间:2018-10-08 23:03:56     消息  查看  搜索  好友  邮件  复制  引用

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之一,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、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,Filter接口源代码:public abstract interface Filter{    public abstract void init(FilterConfig paramFilterConfig) throws ServletException;    public abstract void doFilter(ServletRequest paramServletRequest, ServletResponse paramServletResponse, FilterChain         paramFilterChain) throws IOException, ServletException;    public abstract void destroy();}
二、Filter是如何实现拦截的?(Filter的工作原理)
  Filter接口中有一个doFilter方法,当我们编写好Filter,并配置对哪个web资源进行拦截后,WEB服务器每次在调用web资源的service方法之前,
都会先调用一下filter的doFilter方法,因此,在该方法内编写代码可达到如下目的:
    调用目标资源之前,让一段代码执行。
    是否调用目标资源(即是否让用户访问web资源)。
    调用目标资源之后,让一段代码执行。
  web服务器在调用doFilter方法时,会传递一个filterChain对象进来,filterChain对象是filter接口中最重要的一个对象,它也提供了一个
doFilter方法,开发人员可以根据需求决定是否调用此方法,调用该方法,则web服务器就会调用web资源的service方法,即web资源就会被访问,
否则web资源不会被访问。
三、Filter开发流程
3.1、Filter开发步骤
Filter开发分为2步:
 * 编写java类实现Filter接口,并实现其doFilter方法。
&#160;* 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。过滤器范例:
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;/** * @author yangcq * @description 过滤器Filter的工作原理 */public class FilterTest implements Filter{     public void destroy() {        System.out.println("----Filter销毁----");    }     public void doFilter(ServletRequest request, ServletResponse response,FilterChain filterChain) throws IOException, ServletException {        // 对request、response进行一些预处理        request.setCharacterEncoding("UTF-8");        response.setCharacterEncoding("UTF-8");        response.setContentType("text/html;charset=UTF-8");        System.out.println("----调用service之前执行一段代码----");        filterChain.doFilter(request, response); // 执行目标资源,放行        System.out.println("----调用service之后执行一段代码----");    }     public void init(FilterConfig arg0) throws ServletException {        System.out.println("----Filter初始化----");    }}
在web. xml中配置过滤器:<?xml version="1.0" encoding="UTF-8"?><web-app version="3.0"     xmlns="http://java.sun.com/xml/ns/javaee"     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee     http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">  <display-name></display-name>      <welcome-file-list>    <welcome-file>index.jsp</welcome-file>  </welcome-file-list>  <!--配置过滤器-->  <filter>      <filter-name>FilterTest</filter-name>      <filter-class>com.yangcq.filter.FilterTest</filter-class>  </filter>  <!--映射过滤器-->  <filter-mapping>      <filter-name>FilterTest</filter-name>      <!--“/*”表示拦截所有的请求 -->      <url-pattern>/*</url-pattern>  </filter-mapping></web-app>
3.2、Filter链
  在一个web应用中,可以开发编写多个Filter,这些Filter组合起来称之为一个Filter链。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链的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,Spring框架下,过滤器的配置
&#160;&#160;&#160; 如果项目中使用了Spring框架,那么,很多过滤器都不用自己来写了,Spring为我们写好了一些常用的过滤器。下面我们就以字符编码的
过滤器CharacterEncodingFilter为例,来看一下Spring框架下,如果配置过滤器。    <filter>        <filter-name>encodingFilter</filter-name>        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>        <init-param>            <param-name>encoding</param-name>            <param-value>UTF-8</param-value>        </init-param>        <init-param>            <param-name>forceEncoding</param-name>            <param-value>true</param-value>        </init-param>    </filter>        <filter-mapping>        <filter-name>encodingFilter</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping>
很简单吧,这样几行配置代码,就完成了从全局控制字符编码的功能。接下来,我们看一下CharacterEncodingFilter这个过滤器的关键代码,感受
一下,大师的风采,如果我们写过滤器的话,可以以此为范例。package org.springframework.web.filter;import java.io.IOException;import javax.servlet.FilterChain;import javax.servlet.ServletException;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.springframework.util.ClassUtils;public class CharacterEncodingFilter extends OncePerRequestFilter{    private static final boolean responseSetCharacterEncodingAvailable = ClassUtils.hasMethod(        class$javax$servlet$http$HttpServletResponse, "setCharacterEncoding", new Class[] { String.class });    // 需要设置的编码方式,为了支持可配置,Spring把编码方式设置成了一个变量    private String encoding;    // 是否强制使用统一编码,也是为了支持可配置    private boolean forceEncoding;    // 构造器,在这里,Spring把forceEncoding的值默认设置成了false    public CharacterEncodingFilter(){        this.forceEncoding = false;    }    // encoding/forceEncoding的setter方法    public void setEncoding(String encoding){        this.encoding = encoding;    }    public void setForceEncoding(boolean forceEncoding){        this.forceEncoding = forceEncoding;    }    // Spring通过GenericFilterBean抽象类,对Filter接口进行了整合,    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)        throws ServletException, IOException{        if ((this.encoding != null) && (((this.forceEncoding) || (request.getCharacterEncoding() == null)))) {            request.setCharacterEncoding(this.encoding);            if ((this.forceEncoding) && (responseSetCharacterEncodingAvailable)) {                response.setCharacterEncoding(this.encoding);            }        }        filterChain.doFilter(request, response);    }}
GenericFilterBean类:
public abstract class GenericFilterBean implements Filter, BeanNameAware, ServletContextAware, InitializingBean, DisposableBean
还没有过瘾,那就再看一个项目中使用过的一个过滤器:InvilidCharacterFilter(防止脚本攻击的过滤器) import java.io.IOException;import java.util.Enumeration;import javax.servlet.FilterChain;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.apache.commons.lang.StringUtils;import org.springframework.web.filter.CharacterEncodingFilter;/* * InvalidCharacterFilter:过滤request请求中的非法字符,防止脚本攻击 * InvalidCharacterFilter继承了Spring框架的CharacterEncodingFilter过滤器,当然,我们也可以自己实现这样一个过滤器 */public class InvalidCharacterFilter extends CharacterEncodingFilter{    // 需要过滤的非法字符    private static String[] invalidCharacter = new String[]{        "script","select","insert","document","window","function",        "delete","update","prompt","alert","create","alter",        "drop","iframe","link","where","replace","function","onabort",        "onactivate","onafterprint","onafterupdate","onbeforeactivate",        "onbeforecopy","onbeforecut","onbeforedeactivateonfocus",        "onkeydown","onkeypress","onkeyup","onload",        "expression","applet","layer","ilayeditfocus","onbeforepaste",        "onbeforeprint","onbeforeunload","onbeforeupdate",        "onblur","onbounce","oncellchange","oncontextmenu",        "oncontrolselect","oncopy","oncut","ondataavailable",        "ondatasetchanged","ondatasetcomplete","ondeactivate",        "ondrag","ondrop","onerror","onfilterchange","onfinish","onhelp",        "onlayoutcomplete","onlosecapture","onmouse","ote",        "onpropertychange","onreadystatechange","onreset","onresize",        "onresizeend","onresizestart","onrow","onscroll",        "onselect","onstaronsubmit","onunload","IMgsrc","infarction"    };     protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)            throws ServletException, IOException{           String parameterName = null;        String parameterValue = null;        // 获取请求的参数        @SuppressWarnings("unchecked")        Enumeration<String> allParameter = request.getParameterNames();        while(allParameter.hasMoreElements()){            parameterName = allParameter.nextElement();            parameterValue = request.getParameter(parameterName);            if(null != parameterValue){                for(String str : invalidCharacter){                    if (StringUtils.containsIgnoreCase(parameterValue, str)){                        request.setAttribute("errorMessage", "非法字符:" + str);                        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/error.jsp");                        requestDispatcher.forward(request, response);                        return;                    }                }            }        }        super.doFilterInternal(request, response, filterChain);    }}
接下来需要在web.xml中进行配置:    <filter>        <filter-name>InvalidCharacterFilter</filter-name>        <filter-class>com.yangcq.filter.InvalidCharacterFilter</filter-class>    </filter>    <filter-mapping>        <filter-name>InvalidCharacterFilter</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping>
如果我们不使用Spring的CharacterEncodingFilter类,可以自己来写。import java.io.IOException;import java.util.Enumeration;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import org.apache.commons.lang.StringUtils;/** * SelfDefineInvalidCharacterFilter:过滤request请求中的非法字符,防止脚本攻击 */public class SelfDefineInvalidCharacterFilter implements Filter{     public void destroy() {            }     public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException {        String parameterName = null;        String parameterValue = null;        // 获取请求的参数        @SuppressWarnings("unchecked")        Enumeration<String> allParameter = request.getParameterNames();        while(allParameter.hasMoreElements()){            parameterName = allParameter.nextElement();            parameterValue = request.getParameter(parameterName);            if(null != parameterValue){                for(String str : invalidCharacter){                    if (StringUtils.containsIgnoreCase(parameterValue, str)){                        request.setAttribute("errorMessage", "非法字符:" + str);                        RequestDispatcher requestDispatcher = request.getRequestDispatcher("/error.jsp");                        requestDispatcher.forward(request, response);                        return;                    }                }            }        }        filterChain.doFilter(request, response); // 执行目标资源,放行    }     public void init(FilterConfig filterConfig) throws ServletException {            }    // 需要过滤的非法字符    private static String[] invalidCharacter = new String[]{        "script","select","insert","document","window","function",        "delete","update","prompt","alert","create","alter",        "drop","iframe","link","where","replace","function","onabort",        "onactivate","onafterprint","onafterupdate","onbeforeactivate",        "onbeforecopy","onbeforecut","onbeforedeactivateonfocus",        "onkeydown","onkeypress","onkeyup","onload",        "expression","applet","layer","ilayeditfocus","onbeforepaste",        "onbeforeprint","onbeforeunload","onbeforeupdate",        "onblur","onbounce","oncellchange","oncontextmenu",        "oncontrolselect","oncopy","oncut","ondataavailable",        "ondatasetchanged","ondatasetcomplete","ondeactivate",        "ondrag","ondrop","onerror","onfilterchange","onfinish","onhelp",        "onlayoutcomplete","onlosecapture","onmouse","ote",        "onpropertychange","onreadystatechange","onreset","onresize",        "onresizeend","onresizestart","onrow","onscroll",        "onselect","onstaronsubmit","onunload","IMgsrc","infarction"    }; }
接下来需要在web.xml中进行配置:    <filter>        <filter-name>SelfDefineInvalidCharacterFilter</filter-name>        <filter-class>com.yangcq.filter.SelfDefineInvalidCharacterFilter</filter-class>    </filter>    <filter-mapping>        <filter-name>SelfDefineInvalidCharacterFilter</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping>
五、Filter的生命周期
5.1、Filter的创建
  Filter的创建和销毁由web服务器负责。 web应用程序启动时,web服务器将创建Filter的实例对象,并调用其init方法,完成对象的初始化
功能,从而为后续的用户请求作好拦截的准备工作,filter对象只会创建一次,init方法也只会执行一次。通过init方法的参数,可获得代表当前
filter配置信息的FilterConfig对象。
5.2、Filter的销毁
  web容器调用destroy方法销毁Filter。destroy方法在Filter的生命周期中仅执行一次。在destroy方法中,可以释放过滤器使用的资源。
5.3、FilterConfig接口
  用户在配置filter时,可以使用<init-param>为filter配置一些初始化参数,当web容器实例化Filter对象,调用其init方法时,会把封装了
filter初始化参数的filterConfig对象传递进来。因此开发人员在编写filter时,通过filterConfig对象的方法,就可获得:
  String getFilterName():得到filter的名称。
  String getInitParameter(String name): 返回在部署描述中指定名称的初始化参数的值。如果不存在返回null.
  Enumeration getInitParameterNames():返回过滤器的所有初始化参数的名字的枚举集合。
  public ServletContext getServletContext():返回Servlet上下文对象的引用。
示例:利用FilterConfig得到filter配置信息package com.yangcq.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;public class FilterTest implements Filter {     /* 过滤器初始化     * @see javax.servlet.Filter#init(javax.servlet.FilterConfig)     */    @Override    public void init(FilterConfig filterConfig) throws ServletException {        System.out.println("----过滤器初始化----");        /**         *  <filter>                  <filter-name>FilterTest</filter-name>                  <filter-class>com.yangcq.filter.FilterTest</filter-class>                  <!--配置FilterTest过滤器的初始化参数-->                  <init-param>                      <description>FilterTest</description>                      <param-name>name</param-name>                      <param-value>gacl</param-value>                  </init-param>                  <init-param>                      <description>配置FilterTest过滤器的初始化参数</description>                      <param-name>like</param-name>                      <param-value>java</param-value>                  </init-param>            </filter>                         <filter-mapping>                  <filter-name>FilterDemo02</filter-name>                  <!--“/*”表示拦截所有的请求 -->                  <url-pattern>/*</url-pattern>             </filter-mapping>         */        //得到过滤器的名字        String filterName = filterConfig.getFilterName();        //得到在web.xml文件中配置的初始化参数        String initParam1 = filterConfig.getInitParameter("name");        String initParam2 = filterConfig.getInitParameter("like");        //返回过滤器的所有初始化参数的名字的枚举集合。        Enumeration<String> initParameterNames = filterConfig.getInitParameterNames();                System.out.println(filterName);        System.out.println(initParam1);        System.out.println(initParam2);        while (initParameterNames.hasMoreElements()) {            String paramName = (String) initParameterNames.nextElement();            System.out.println(paramName);        }    }     @Override    public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) throws IOException, ServletException {        System.out.println("FilterDemo02执行前!!!");        chain.doFilter(request, response);  //让目标资源执行,放行        System.out.println("FilterDemo02执行后!!!");    }     @Override    public void destroy() {        System.out.println("----过滤器销毁----");    }}
六、Filter的部署时的一些参数的含义
Filter的部署分为两个步骤:
  1、注册Filter
  2、映射Filter
6.1、注册Filter
  开发好Filter之后,需要在web.xml文件中进行注册,这样才能够被web服务器调用。在web.xml文件中注册Filter范例:<filter>    <description>过滤器名称</description>    <filter-name>自定义的名字</filter-name>    <filter-class>com.yangcq.filter.FilterTest</filter-class>    <!--配置FilterTest过滤器的初始化参数-->    <init-param>        <description>配置过滤器的初始化参数</description>        <param-name>name</param-name>        <param-value>gacl</param-value>    </init-param>    <init-param>        <description>配置FilterTest过滤器的初始化参数</description>        <param-name>like</param-name>        <param-value>java</param-value>    </init-param></filter>
&#160;&#160;&#160; <description>用于添加描述信息,该元素的内容可为空,<description>可以不配置。
  <filter-name>用于为过滤器指定一个名字,该元素的内容不能为空。
  <filter-class>元素用于指定过滤器的完整的限定类名。
  <init-param>元素用于为过滤器指定初始化参数,它的子元素<param-name>指定参数的名字,<param-value>指定参数的值。在过滤器中,
可以使用FilterConfig接口对象来访问初始化参数。如果过滤器不需要指定初始化参数,那么<init-param>元素可以不配置。
6.2、映射Filter
  在web.xml文件中注册了Filter之后,还要在web.xml文件中映射Filter
<!--映射过滤器-->
&#160; <filter-mapping>
&#160;&#160;&#160;&#160;&#160; <filter-name>FilterTest</filter-name>
&#160;&#160;&#160;&#160;&#160; <!--“/*”表示拦截所有的请求 -->
&#160;&#160;&#160;&#160;&#160; <url-pattern>/*</url-pattern>
&#160; </filter-mapping>
<filter-mapping>元素用于设置一个 Filter 所负责拦截的资源。一个Filter拦截的资源可通过两种方式来指定:Servlet 名称和资源访问的请求路径
  <filter-name>子元素用于设置filter的注册名称。该值必须是在<filter>元素中声明过的过滤器的名字
  <url-pattern>设置 filter 所拦截的请求路径(过滤器关联的URL样式)
  <servlet-name>指定过滤器所拦截的Servlet名称。
  <dispatcher>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。用户可以设置多个<dispatcher> 子元素用来指定 Filter 对资源的多种调用方式进行拦截。如下:
<filter-mapping>
&#160;&#160;&#160; <filter-name>testFilter</filter-name>
&#160;&#160;&#160; <url-pattern>/index.jsp</url-pattern>
&#160;&#160;&#160; <dispatcher>REQUEST</dispatcher>
&#160;&#160;&#160; <dispatcher>FORWARD</dispatcher>
</filter-mapping>
 <dispatcher> 子元素可以设置的值及其意义:
&#160;&#160;&#160; 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。
&#160;&#160;&#160; 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。
&#160;&#160;&#160; 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。
&#160;&#160;&#160; 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。




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