Servlet过滤器工作过程

新建一个javaweb项目。如下的web.xml配置。

<?xml version="1.0" encoding="UTF-8"?>  <web-app 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"           version="3.0">      <filter>          <filter-name>PerformanceFilter</filter-name>          <filter-class>com.lyx.filter.PerformanceFilter</filter-class>      </filter>      <filter>          <filter-name>PerformanceFilter2</filter-name>          <filter-class>com.lyx.filter.PerformanceFilter2</filter-class>      </filter>      <filter-mapping>          <filter-name>PerformanceFilter</filter-name>          <url-pattern>/PerformanceServlet</url-pattern>      </filter-mapping>      <filter-mapping>          <filter-name>PerformanceFilter2</filter-name>          <url-pattern>/PerformanceServlet</url-pattern>      </filter-mapping>      <servlet>          <servlet-name>PerformanceServlet</servlet-name>          <servlet-class>com.lyx.servlet.PerformanceServlet</servlet-class>      </servlet>      <servlet-mapping>          <servlet-name>PerformanceServlet</servlet-name>          <url-pattern>/PerformanceServlet</url-pattern>      </servlet-mapping>  </web-app>

有两个filter，其中PerformanceFilter如下：

package com.lyx.filter;    import javax.servlet.*;  import javax.servlet.http.HttpServletRequest;  import java.io.IOException;    public class PerformanceFilter implements Filter {        private FilterConfig config;        public PerformanceFilter() {      }        @Override      public void destroy() {      }        @Override      public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {          HttpServletRequest httpServletRequest = (HttpServletRequest) request;          System.out.println("=======filter1 start=====" + httpServletRequest.getRequestURI());          chain.doFilter(request, response);  //把处理权交给下一个filter          System.out.println("=======filter1 end=======" + httpServletRequest.getRequestURI());      }        @Override      public void init(FilterConfig fConfig) throws ServletException {          this.config = fConfig;      }  }

PerformanceFilter2如下：

package com.lyx.filter;    import javax.servlet.*;  import javax.servlet.http.HttpServletRequest;  import java.io.IOException;    /**   * 在这个filter中统计请求到响应的时间   */  public class PerformanceFilter2 implements Filter {      public void destroy() {      }        public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {          HttpServletRequest httpServletRequest = (HttpServletRequest) req;          System.out.println("=======filter2 start=====" + httpServletRequest.getRequestURI());            long start = System.currentTimeMillis();          chain.doFilter(req, resp);          long end = System.currentTimeMillis();          System.out.println("total=" + (end - start));            System.out.println("=======filter2 end=======" + httpServletRequest.getRequestURI());      }        public void init(FilterConfig config) throws ServletException {        }    }

其中servlet如下：

package com.lyx.servlet;    import java.io.IOException;  import java.io.PrintWriter;  import javax.servlet.ServletException;  import javax.servlet.annotation.WebServlet;  import javax.servlet.http.HttpServlet;  import javax.servlet.http.HttpServletRequest;  import javax.servlet.http.HttpServletResponse;      @WebServlet(name = "PerformanceServlet",          urlPatterns = {"/PerformanceServlet"},          loadOnStartup = 1)  public class PerformanceServlet extends HttpServlet {      private static final long serialVersionUID = 1L;        public PerformanceServlet() {          super();      }          @Override      protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {          response.setContentType("text/html;charset=UTF-8");          PrintWriter out = response.getWriter();          String name = request.getParameter("name");            out.println("<html>");          out.println("<head>");          out.println("<title>Hello Servlet</title>");          out.println("</head>");          out.println("<body>");          out.println("<h1> Hello! " + name + " !</h1>");          out.println("</body>");          out.println("</html>");          out.close();      }  }

ok，启动web程序，查看后台打印情况：

=======filter1 start=====/PerformanceServlet

=======filter2 start=====/PerformanceServlet

total=2

=======filter2 end=======/PerformanceServlet

=======filter1 end=======/PerformanceServlet

很直观的反映了filter的执行过程，尤其当多个filter匹配时。。。

总结多个filter的执行过程：

过滤器的执行流程了：执行第一个过滤器的chain.doFilter()之前的代码——>第二个过滤器的chain.doFilter()之前的代码——>……——>第n个过滤器的chain.doFilter()之前的代码——>所请求servlet的service()方法中的代码——>所请求servlet的doGet()或doPost()方法中的代码——>第n个过滤器的chain.doFilter()之后的代码——>……——>第二个过滤器的chain.doFilter()之后的代码——>第一个过滤器的chain.doFilter()之后的代码。

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来自：http://my.oschina.net/xinxingegeya/blog/323763