如图所示，对于一个指定了过滤器的Web资源（Servlet、JSP、HTML）来说，过滤器拦在客户端与Web资源之间，进出都要经过过滤器。
当请求到来时，先经过过滤器，过滤器可以检查、修改请求的内容再发给对应的Web资源，甚至可以根据请求的内容禁止客户端访问对应的Web资源。
Web资源产生的应答也要先经过过滤器，过滤器可以检查、修改应答的内容再发给客户端。
Filter接口
过滤器必须实现javax.servlet.Filter接口，这个接口包含3个方法：init、doFilter、destroy。
init(FilterConfig filterConfig) ：Java EE容器创建过滤器实例后调用这个方法，用于为过滤器做准备工作，可以从filterConfig对象读取配置文件web.xml中为该过滤器设置的初始化参数。
doFilter(ServletRequest request, ServletResponse response, FilterChain chain) ：过滤操作在这个方法里实现，包括检查、修改请求对象，检查修改应答对象。参数request是请求对象，response是应答对象，chain用于访问后续过滤器。
destroy() ：Java EE容器在销毁过滤器实例前调用这个方法，用于释放资源。
<?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>
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");
}
}
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");
}
}
启动服务器，运行结果如下：
1、启动服务器，打印
Filter1 init
Filter2 init
2、在浏览器地址栏中输入 http://localhost:8080/myweb/filter/ 打印
Filter1 doFilter
Filter2 doFilter
1、以url-pattern方式配置的filter运行时肯定是先于servlet-name方式配置的filter
2、以url-pattern方式配置的filter中，如果有多个与当前请求匹配，则按web.xml中filter-mapping出现的顺序来运行
3、对于以servlet-name方式配置的filter，如果有多个与当前请求匹配，也是按web.xml中filter-mapping出现的顺序来运行
<filter>
<filter-name>myFilter</filter-name>
<filter-class>xx.MyFilter</filte-class>
</filter>
<filter-mapping>
<filter-name>myFilter</filter-name>
<servlet-name>目标资源一</servlet-name>
<dispatcher>REQUEST</dispatcher>
</filter-mapping>
<filter-mapping>
<filter-name>myFilter</filter-name>
<servlet-name>目标资源二</servlet-name>
<dispatcher>REQUEST</dispatcher>
</filter-mapping>
这种方式的配置，说明只有直接访问该目标资源时该过滤器才会起作用，对转发到该目标资源的请求将忽略不处理。
那如果我想对转发到目标资源二的请求进行过滤，那怎么办呢？答案见，下一种过滤器，forward过滤器。
<?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>
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");
}
}
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，否则无法到servlet处理
}
public void init(FilterConfig arg0) throws ServletException {
System.out.println("Filter3 init");
}
}
浏览器地址栏中输入：
http://localhost:8080/myweb/MyServlet.do
打印结果：
Filter3 doFilter
servlet
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