Servlet中Filter详解及demo

骑猪看日落 2022-09-21 01:44 12阅读 0赞

# 目录 #

*  目录
 *  Filter过滤器的功能
 *  Filter过滤器必须要实现的方法
 *  多个Filter过滤器的执行顺序
 *  demo

# Filter(过滤器)的功能 #

Filter顾名思义就是针对浏览器的请求进行过滤，甚至是拦截。它可以在一个请求到达servlet之前进行请求的预处理，在一个请求离开servlet后，到达浏览器之前进行后处理。多个Filter可以组成Filter Chain（过滤器链）。  
Filter可以完全控制一个请求，在一个Filter调用chain.doFilter()之前，此次请求是不会离开本Filter的。

Filter(过滤器)的工作流程是：  
1、在Servlet Request到达时拦截，并做处理；  
2、调用chain.doFilter()将Servlet Request传递下去；  
3、在Servlet Response离开之前拦截，并做后处理。

# Filter（过滤器）必须要实现的方法 #

*  public void init(FilterConfig config)。在Filter的生命周期中只执行一次，在服务器（如tomcat）加载war包时就执行。参数FilterConfig是根据web.xml文件中的配置生成的配置信息，可以用来初始化本Filter的私有属性。
 *  public void destroy()。也是只执行一次，做资源释放。
 *  public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)。这个函数中调用chain.doFilter(req, resp)之前的代码属于对request的预处理，调用chain.doFilter(req, resp)之后的代码属于对response的后处理。不调用此代码就表示拦截，不向后传递。

# 多个Filter（过滤器）的执行顺序 #

当一个工程中有多个Filter（过滤器）时，Filter的执行顺序一定得弄清楚。是根据在web.xml中配置的先后顺序，类似于进出栈的顺序来执行的。  
例如，下面demo中在web.xml中配置了两个filter和一个servlet，两个Filter的配置顺序是filter1在前，filter2在后。执行顺序如下图：

Created with Raphaël 2.1.0浏览器浏览器Filter1Filter1Filter2Filter2ServletServletHttpServletRequestHttpServletRequestHttpServletRequestHttpServletResponseHttpServletResponseHttpServletResponse

# demo #

web.xml里面配置了两个filter和一个servlet：

<?xml version="1.0" encoding="UTF-8"?>
    <web-app version="2.5" 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_2_5.xsd">
    
    
      <filter>
        <filter-name>filter1</filter-name>
        <filter-class>com.zcx.filterTest.filter1</filter-class>
        <init-param>
            
            <param-name>paraName</param-name>
            <param-value>paraValue</param-value>
        </init-param>
      </filter>
    
      <filter-mapping>
        <filter-name>filter1</filter-name>
        <url-pattern>/*</url-pattern>   
      </filter-mapping>
    
      <filter>
        <filter-name>filter2</filter-name>
        <filter-class>com.zcx.filterTest.filter2</filter-class>
      </filter>
    
      <filter-mapping>
        <filter-name>filter2</filter-name>
        <url-pattern>/*</url-pattern>
      </filter-mapping>
    
      <servlet>
            <servlet-name>Servlet1</servlet-name>
            <servlet-class>com.zcx.filterTest.Servlet1</servlet-class>
      </servlet>
    
      <servlet-mapping>
            <servlet-name>Servlet1</servlet-name> 
            <url-pattern>/*</url-pattern> 
      </servlet-mapping>
    
    </web-app>

新建filter1：

package com.zcx.filterTest;
    
    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{ 
        private String paraName;
    
        public void init(FilterConfig config) throws ServletException{
            paraName = config.getInitParameter("paraName");
            System.out.println("init filter1, paraName=" + paraName);
        }
    
        public void doFilter(ServletRequest req, ServletResponse resp,
                FilterChain chain) throws IOException, ServletException{
            System.out.println("Before filter1, paraName=" + paraName);
            chain.doFilter(req, resp);
            System.out.println("After filter1, paraName=" + paraName);
        }
    
        public void destroy(){
            System.out.println("Destory filter1");
            paraName = null;
        }
    }

新建filter2：

package com.zcx.filterTest;
    
    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 init(FilterConfig config) throws ServletException{
            System.out.println("init filter2");
        }
    
        public void doFilter(ServletRequest req, ServletResponse resp,
                FilterChain chain) throws IOException, ServletException{
            System.out.println("Before filter2");
            chain.doFilter(req, resp);
            System.out.println("After filter2");
        }
    
        public void destroy(){
            System.out.println("Destory filter2");
        }
    }

新建一个servlet：

package com.zcx.filterTest;
    
    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 Servlet1 extends HttpServlet{ 
        /** * */
        private static final long serialVersionUID = 4657145885789315799L;
    
        @Override  
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)  
                throws ServletException, IOException {  
            System.out.println("doget...");  
        }  
    
        @Override  
        protected void doPost(HttpServletRequest req, HttpServletResponse resp)  
                throws ServletException, IOException {  
            System.out.println("dopost...");  
        }  
    }

在tomcat部署此工程时进行初始化工作，控制台输出：  
init filter1, paraName=paraValue  
init filter2  
在浏览器中访问此工程时，控制台输出：  
Before filter1, paraName=paraValue  
Before filter2  
doget…  
After filter2  
After filter1, paraName=paraValue