04_过滤器Filter_03_多个Filter的执行顺序

【Filter链】

*在一个web应用中，可以开发编写多个Filter，这些Filter组合起来称为一个Filter链。

*web服务器根据Filter在web.xml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，则调用第二个filter，如果没有，则调用目标资源。

【工程截图】

设计了两个Filter和一个Servlet，访问Servlet时，查看Filter的执行顺序。

【web.xml】

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>FilterDemo01</display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>

  <!-- 过滤顺序：谁的写在上面，谁先被过滤 -->
  <filter>
    <filter-name>FilterTest01</filter-name>
    <filter-class>com.Higgin.web.filter.FilterTest01</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>FilterTest01</filter-name>
    <url-pattern>/*</url-pattern> <!-- 过滤所有 -->
  </filter-mapping>

  <filter>
    <filter-name>FilterTest02</filter-name>
    <filter-class>com.Higgin.web.filter.FilterTest02</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>FilterTest02</filter-name>
    <url-pattern>/*</url-pattern> <!-- 过滤所有 -->
  </filter-mapping>

</web-app>

【FilterTest01.java】

package com.Higgin.web.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 FilterTest01 implements Filter{

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        System.out.println("FilterTest01 执行前---");

        chain.doFilter(request, response);//让目标资源执行，即：放行

        System.out.println("FilterTest01 执行后---");
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {

    }

    @Override
    public void destroy() {

    }
}

【FilterTest02.java】

package com.Higgin.web.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 FilterTest02 implements Filter{

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        System.out.println("FilterTest02 执行前---");

        chain.doFilter(request, response);  //放行

        System.out.println("FilterTest02 执行后---");
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {

    }

    @Override
    public void destroy() {

    }

}

【ServletTest01.java】

package com.Higgin.web.servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletTest01
 */
@WebServlet("/ServletTest01")
public class ServletTest01 extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public ServletTest01() {
        super();
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //response.getWriter().write("中国 加油！China Come on!");
System.out.println("执行ServletTest01---");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

}

【运行结果】

在浏览器中输入：http://localhost:8080/FilterDemo01/ServletTest01

查看控制台输出结果

自己可以尝试分别注释FilterTest01和FilterTest02中的chain.doFilter方法，用Junit查看具体的执行过程。