Java三大器之過濾器(Filter)的工作原理和程式碼演示
一、Filter簡介
Filter也稱之為過濾器,它是Servlet技術中最激動人心的技術之一,WEB開發人員通過Filter技術,對web伺服器管理的所有web資源:例如Jsp,
Servlet, 靜態圖片檔案或靜態html檔案等進行攔截,從而實現一些特殊的功能。例如實現URL級別的許可權訪問控制、過濾敏感詞彙、壓縮響應資訊等
一些高階功能。
Servlet API中提供了一個Filter介面,開發web應用時,如果編寫的Java類實現了這個介面,則把這個java類稱之為過濾器Filter。通過Filter
技術,開發人員可以實現使用者在訪問某個目標資源之前,對訪問的請求和響應進行攔截,Filter介面原始碼:
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是如何實現攔截的?(Filter的工作原理)
Filter介面中有一個doFilter方法,當我們編寫好Filter,並配置對哪個web資源進行攔截後,WEB伺服器每次在呼叫web資源的service方法之前,
都會先呼叫一下filter的doFilter方法,因此,在該方法內編寫程式碼可達到如下目的:
呼叫目標資源之前,讓一段程式碼執行。
是否呼叫目標資源(即是否讓使用者訪問web資源)。
呼叫目標資源之後,讓一段程式碼執行。
web伺服器在呼叫doFilter方法時,會傳遞一個filterChain物件進來,filterChain物件是filter介面中最重要的一個物件,它也提供了一個
否則web資源不會被訪問。
三、Filter開發流程
3.1、Filter開發步驟
Filter開發分為2步:
* 編寫java類實現Filter介面,並實現其doFilter方法。
* 在web.xml 檔案中使用<filter>和<filter-mapping>元素對編寫的filter類進行註冊,並設定它所能攔截的資源。
過濾器範例:
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 過濾器Filter的工作原理 */ public class FilterTest implements Filter{ public void destroy() { System.out.println("----Filter銷燬----"); } public void doFilter(ServletRequest request, ServletResponse response,FilterChain filterChain) throws IOException, ServletException { // 對request、response進行一些預處理 request.setCharacterEncoding("UTF-8"); response.setCharacterEncoding("UTF-8"); response.setContentType("text/html;charset=UTF-8"); System.out.println("----呼叫service之前執行一段程式碼----"); filterChain.doFilter(request, response); // 執行目標資源,放行 System.out.println("----呼叫service之後執行一段程式碼----"); } public void init(FilterConfig arg0) throws ServletException { System.out.println("----Filter初始化----"); } }
在web. xml中配置過濾器:
<?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> <!--配置過濾器--> <filter> <filter-name>FilterTest</filter-name> <filter-class>com.yangcq.filter.FilterTest</filter-class> </filter> <!--對映過濾器--> <filter-mapping> <filter-name>FilterTest</filter-name> <!--“/*”表示攔截所有的請求 --> <url-pattern>/*</url-pattern> </filter-mapping> </web-app>
3.2、Filter鏈
在一個web應用中,可以開發編寫多個Filter,這些Filter組合起來稱之為一個Filter鏈。web伺服器根據Filter在web.xml檔案中的註冊順序,
決定先呼叫哪個Filter,當第一個Filter的doFilter方法被呼叫時,web伺服器會建立一個代表Filter鏈的FilterChain物件傳遞給該方法。在doFilter
方法中,開發人員如果呼叫了FilterChain物件的doFilter方法,則web伺服器會檢查FilterChain物件中是否還有filter,如果有,則呼叫第2個filter,
如果沒有,則呼叫目標資源。
四,Spring框架下,過濾器的配置
如果專案中使用了Spring框架,那麼,很多過濾器都不用自己來寫了,Spring為我們寫好了一些常用的過濾器。下面我們就以字元編碼的
過濾器CharacterEncodingFilter為例,來看一下Spring框架下,如果配置過濾器。
<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>
很簡單吧,這樣幾行配置程式碼,就完成了從全域性控制字元編碼的功能。接下來,我們看一下CharacterEncodingFilter這個過濾器的關鍵程式碼,感受
一下,大師的風采,如果我們寫過濾器的話,可以以此為範例。
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 }); // 需要設定的編碼方式,為了支援可配置,Spring把編碼方式設定成了一個變數 private String encoding; // 是否強制使用統一編碼,也是為了支援可配置 private boolean forceEncoding; // 構造器,在這裡,Spring把forceEncoding的值預設設定成了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通過GenericFilterBean抽象類,對Filter介面進行了整合, 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類:
public abstract class GenericFilterBean implements Filter, BeanNameAware, ServletContextAware, InitializingBean, DisposableBean
還沒有過癮,那就再看一個專案中使用過的一個過濾器:InvilidCharacterFilter(防止指令碼攻擊的過濾器)
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:過濾request請求中的非法字元,防止指令碼攻擊 * InvalidCharacterFilter繼承了Spring框架的CharacterEncodingFilter過濾器,當然,我們也可以自己實現這樣一個過濾器 */ public class InvalidCharacterFilter extends CharacterEncodingFilter{ // 需要過濾的非法字元 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; // 獲取請求的引數 @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", "非法字元:" + str); RequestDispatcher requestDispatcher = request.getRequestDispatcher("/error.jsp"); requestDispatcher.forward(request, response); return; } } } } super.doFilterInternal(request, response, filterChain); } }
接下來需要在web.xml中進行配置:
<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>
如果我們不使用Spring的CharacterEncodingFilter類,可以自己來寫。
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:過濾request請求中的非法字元,防止指令碼攻擊 */ 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; // 獲取請求的引數 @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", "非法字元:" + str); RequestDispatcher requestDispatcher = request.getRequestDispatcher("/error.jsp"); requestDispatcher.forward(request, response); return; } } } } filterChain.doFilter(request, response); // 執行目標資源,放行 } public void init(FilterConfig filterConfig) throws ServletException { } // 需要過濾的非法字元 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" }; }
接下來需要在web.xml中進行配置:
<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的生命週期
5.1、Filter的建立
Filter的建立和銷燬由web伺服器負責。 web應用程式啟動時,web伺服器將建立Filter的例項物件,並呼叫其init方法,完成物件的初始化
功能,從而為後續的使用者請求作好攔截的準備工作,filter物件只會建立一次,init方法也只會執行一次。通過init方法的引數,可獲得代表當前
filter配置資訊的FilterConfig物件。
5.2、Filter的銷燬
web容器呼叫destroy方法銷燬Filter。destroy方法在Filter的生命週期中僅執行一次。在destroy方法中,可以釋放過濾器使用的資源。
5.3、FilterConfig介面
使用者在配置filter時,可以使用<init-param>為filter配置一些初始化引數,當web容器例項化Filter物件,呼叫其init方法時,會把封裝了
filter初始化引數的filterConfig物件傳遞進來。因此開發人員在編寫filter時,通過filterConfig物件的方法,就可獲得:
String getFilterName():得到filter的名稱。
String getInitParameter(String name): 返回在部署描述中指定名稱的初始化引數的值。如果不存在返回null.
Enumeration getInitParameterNames():返回過濾器的所有初始化引數的名字的列舉集合。
public ServletContext getServletContext():返回Servlet上下文物件的引用。
示例:利用FilterConfig得到filter配置資訊
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 { /* 過濾器初始化 * @see javax.servlet.Filter#init(javax.servlet.FilterConfig) */ @Override public void init(FilterConfig filterConfig) throws ServletException { System.out.println("----過濾器初始化----"); /** * <filter> <filter-name>FilterTest</filter-name> <filter-class>com.yangcq.filter.FilterTest</filter-class> <!--配置FilterTest過濾器的初始化引數--> <init-param> <description>FilterTest</description> <param-name>name</param-name> <param-value>gacl</param-value> </init-param> <init-param> <description>配置FilterTest過濾器的初始化引數</description> <param-name>like</param-name> <param-value>java</param-value> </init-param> </filter> <filter-mapping> <filter-name>FilterDemo02</filter-name> <!--“/*”表示攔截所有的請求 --> <url-pattern>/*</url-pattern> </filter-mapping> */ //得到過濾器的名字 String filterName = filterConfig.getFilterName(); //得到在web.xml檔案中配置的初始化引數 String initParam1 = filterConfig.getInitParameter("name"); String initParam2 = filterConfig.getInitParameter("like"); //返回過濾器的所有初始化引數的名字的列舉集合。 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執行前!!!"); chain.doFilter(request, response); //讓目標資源執行,放行 System.out.println("FilterDemo02執行後!!!"); } @Override public void destroy() { System.out.println("----過濾器銷燬----"); } }
六、Filter的部署時的一些引數的含義
Filter的部署分為兩個步驟:
1、註冊Filter
2、對映Filter
6.1、註冊Filter
開發好Filter之後,需要在web.xml檔案中進行註冊,這樣才能夠被web伺服器呼叫。在web.xml檔案中註冊Filter範例:
<filter> <description>過濾器名稱</description> <filter-name>自定義的名字</filter-name> <filter-class>com.yangcq.filter.FilterTest</filter-class> <!--配置FilterTest過濾器的初始化引數--> <init-param> <description>配置過濾器的初始化引數</description> <param-name>name</param-name> <param-value>gacl</param-value> </init-param> <init-param> <description>配置FilterTest過濾器的初始化引數</description> <param-name>like</param-name> <param-value>java</param-value> </init-param> </filter>
<description>用於新增描述資訊,該元素的內容可為空,<description>可以不配置。
<filter-name>用於為過濾器指定一個名字,該元素的內容不能為空。
<filter-class>元素用於指定過濾器的完整的限定類名。
<init-param>元素用於為過濾器指定初始化引數,它的子元素<param-name>指定引數的名字,<param-value>指定引數的值。在過濾器中,
可以使用FilterConfig介面物件來訪問初始化引數。如果過濾器不需要指定初始化引數,那麼<init-param>元素可以不配置。
6.2、對映Filter
在web.xml檔案中註冊了Filter之後,還要在web.xml檔案中對映Filter
<!--對映過濾器-->
<filter-mapping>
<filter-name>FilterTest</filter-name>
<!--“/*”表示攔截所有的請求 -->
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>元素用於設定一個 Filter 所負責攔截的資源。一個Filter攔截的資源可通過兩種方式來指定:Servlet 名稱和資源訪問的請求路徑
<filter-name>子元素用於設定filter的註冊名稱。該值必須是在<filter>元素中宣告過的過濾器的名字
<url-pattern>設定 filter 所攔截的請求路徑(過濾器關聯的URL樣式)
<servlet-name>指定過濾器所攔截的Servlet名稱。
<dispatcher>指定過濾器所攔截的資源被 Servlet 容器呼叫的方式,可以是REQUEST,INCLUDE,FORWARD和ERROR之一,預設REQUEST。使用者可以設定多個<dispatcher> 子元素用來指定 Filter 對資源的多種呼叫方式進行攔截。如下:
<filter-mapping>
<filter-name>testFilter</filter-name>
<url-pattern>/index.jsp</url-pattern>
<dispatcher>REQUEST</dispatcher>
<dispatcher>FORWARD</dispatcher>
</filter-mapping>
<dispatcher> 子元素可以設定的值及其意義:
REQUEST:當用戶直接訪問頁面時,Web容器將會呼叫過濾器。如果目標資源是通過RequestDispatcher的include()或forward()方法訪問
時,那麼該過濾器就不會被呼叫。
INCLUDE:如果目標資源是通過RequestDispatcher的include()方法訪問時,那麼該過濾器將被呼叫。除此之外,該過濾器不會被呼叫。
FORWARD:如果目標資源是通過RequestDispatcher的forward()方法訪問時,那麼該過濾器將被呼叫,除此之外,該過濾器不會被呼叫。
ERROR:如果目標資源是通過宣告式異常處理機制呼叫時,那麼該過濾器將被呼叫。除此之外,過濾器不會被呼叫。
轉:https://www.cnblogs.com/austinspark-jessylu/p/7454573.html