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如圖所示，對(duì)于一個(gè)指定了過濾器的Web資源（Servlet、JSP、HTML）來說，過濾器攔在客戶端與Web資源之間，進(jìn)出都要經(jīng)過過濾器。
當(dāng)請(qǐng)求到來時(shí)，先經(jīng)過過濾器，過濾器可以檢查、修改請(qǐng)求的內(nèi)容再發(fā)給對(duì)應(yīng)的Web資源，甚至可以根據(jù)請(qǐng)求的內(nèi)容禁止客戶端訪問對(duì)應(yīng)的Web資源。
Web資源產(chǎn)生的應(yīng)答也要先經(jīng)過過濾器，過濾器可以檢查、修改應(yīng)答的內(nèi)容再發(fā)給客戶端。
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Filter接口
過濾器必須實(shí)現(xiàn)javax.servlet.Filter接口，這個(gè)接口包含3個(gè)方法：init、doFilter、destroy。
init(FilterConfig filterConfig) ：Java EE容器創(chuàng)建過濾器實(shí)例后調(diào)用這個(gè)方法，用于為過濾器做準(zhǔn)備工作，可以從filterConfig對(duì)象讀取配置文件web.xml中為該過濾器設(shè)置的初始化參數(shù)。
doFilter(ServletRequest request, ServletResponse response, FilterChain chain) ：過濾操作在這個(gè)方法里實(shí)現(xiàn)，包括檢查、修改請(qǐng)求對(duì)象，檢查修改應(yīng)答對(duì)象。參數(shù)request是請(qǐng)求對(duì)象，response是應(yīng)答對(duì)象，chain用于訪問后續(xù)過濾器。
destroy() ：Java EE容器在銷毀過濾器實(shí)例前調(diào)用這個(gè)方法，用于釋放資源。
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>myweb</display-name>
<filter>
<filter-name>filter1</filter-name>
<filter-class>hb.filter.Filter1</filter-class>
</filter>
<filter-mapping>
<filter-name>filter1</filter-name>
<url-pattern>/filter/*</url-pattern>
</filter-mapping>
<filter>
<filter-name>filter2</filter-name>
<filter-class>hb.filter.Filter2</filter-class>
</filter>
<filter-mapping>
<filter-name>filter2</filter-name>
<url-pattern>/filter/*</url-pattern>
</filter-mapping>
</web-app>
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package hb.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 Filter1 implements Filter {
public void destroy() {
System.out.println("Filter1 destory");
}
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain chain) throws IOException, ServletException {
System.out.println("Filter1 doFilter");
chain.doFilter(arg0, arg1);
}
public void init(FilterConfig arg0) throws ServletException {
System.out.println("Filter1 init");
}
}
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package hb.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 Filter2 implements Filter{
public void destroy() {
System.out.println("Filter2 destory");
}
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException {
System.out.println("Filter2 doFilter");
}
public void init(FilterConfig arg0) throws ServletException {
System.out.println("Filter2 init");
}
}
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<filter>
<filter-name>myFilter</filter-name>
<filter-class>xx.MyFilter</filte-class>
</filter>
<filter-mapping>
<filter-name>myFilter</filter-name>
<servlet-name>目標(biāo)資源一</servlet-name>
<dispatcher>REQUEST</dispatcher>
</filter-mapping>
<filter-mapping>
<filter-name>myFilter</filter-name>
<servlet-name>目標(biāo)資源二</servlet-name>
<dispatcher>REQUEST</dispatcher>
</filter-mapping>
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<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>myweb</display-name>
<servlet>
<display-name>MyServlet</display-name>
<servlet-name>MyServlet</servlet-name>
<servlet-class>hb.filter.MyServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>MyServlet</servlet-name>
<url-pattern>/MyServlet.do</url-pattern>
</servlet-mapping>
<filter>
<filter-name>filter3</filter-name>
<filter-class>hb.filter.Filter3</filter-class>
</filter>
<filter-mapping>
<filter-name>filter3</filter-name>
<servlet-name>MyServlet</servlet-name>
</filter-mapping>
</web-app>
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package hb.filter;
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 MyServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
public MyServlet() {
super();
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doPost(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
System.out.println("servlet");
response.sendRedirect("index.jsp");
}
}
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package hb.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 Filter3 implements Filter {
public void destroy() {
System.out.println("Filter3 destory");
}
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain chain) throws IOException, ServletException {
System.out.println("Filter3 doFilter");
chain.doFilter(arg0, arg1);//不能少，否則無法到servlet處理
}
public void init(FilterConfig arg0) throws ServletException {
System.out.println("Filter3 init");
}
}
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