Filter入门学习

（一）Filter介绍

1.概念：filter是对客户端访问资源的过滤，符合条件放行，不符合条件不放行，并且可以对目标资源访问前后进行逻辑处理

2.快速入门

步骤：（重要），另外初始化方法，销毁方法和doFilter()方法的执行时间也必须要知道，在代码中有注释笔记

编写一个过滤器的类实现Filter接口

实现接口中尚未实现的方法(着重实现doFilter方法)

在web.xml中进行配置(主要是配置要对哪些资源进行过滤)

（二）示例及笔记

（a）QuickFilterOne.java

package cn.shu.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 QuickFilterOne implements Filter{
	/*FIlter对象销毁时执行的，关闭服务器执行该方法*/
	@Override
	public void destroy() {
		System.out.println("destroy执行...");
	}

	@Override
	/*只要访问某资源配置了filter过滤器，每次访问该资源时，都会执行doFilter方法
	 * 
	 * doFilter是Filter的核心过滤方法
	 * request :内部封装的是客户端http请求的内容
	 * response:代表的是响应
	 * FilterChain:过滤器链对象，知道所有filter的引用（所有filter都在FilterChain中有注册）和顺序*/
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		System.out.println("QuickFilterOne running");
		/*放行请求，此处的doFilter方法只有两个参数，区别于三个参数*/
		chain.doFilter(request, response);
		/*此处会放行请求，给下一个Filter，决定filter执行先后顺序的
		 * 因素是web.xml配置文件中filter-mapping的位置，若存在多个
		 * filter-mapping配置，则filter-mapping配置写在前面的filter先执行*/
	}
	
	//Filter创建时执行init方法，
	/*服务器一启动就创建QuickFilterOne过滤器对象，并执行初始化init方法*/
	@Override
	public void init(FilterConfig arg0) throws ServletException {
		/*1.获得web.xml中filter的名称<filter-name>QuickFilterOne</filter-name>*/
		System.out.println(arg0.getFilterName());
		/*2.可以获得当前filter初始化参数
		 * 需在web.xml中进行配置
		 * <filter>
    <filter-name>QuickFilterOne</filter-name>
    <filter-class>cn.shu.web.filter.QuickFilterOne</filter-class>
  <init-param>
  	<param-name>hehe</param-name>
  	<param-value>HEHE</param-value>
  </init-param>
  </filter>*/
		System.out.println(arg0.getInitParameter("hehe"));//HEHE
		
		/*3.获得servletContext*/
		System.out.println(arg0.getServletContext());
		System.out.println("init执行...");
	}

}

(b)QuickFilterTwo.java

package cn.shu.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 QuickFilterTwo implements Filter{

	@Override
	public void destroy() {
		
	}

	@Override
	public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
			throws IOException, ServletException {
		System.out.println("QuickFilterTwo running....");
		/*若此处不写放行语句，chain.doFilter(request, response);
		 * 启动项目，在浏览器中访问ServletOne资源，是访问不到的
		 * 控制台会打印：QuickFilterOne running
                      QuickFilterTwo running....
                      因为访问资源过了FIlterOne，却被FilterTwo过滤了
        
        filter可以做系统的权限访问模块
		 * */
	}

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

}

(c)ServletOne.java

package cn.shu.web.servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletOne
 */
public class ServletOne extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletOne() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletResponse request, HttpServletResponse response) throws ServletException, IOException {
		System.out.println("ServletOne running..");
		 //response.getWriter().write("ServletOne running..");
		
		//response.getWriter().append("Served at: ").append(request.getContextPath());
	 //((ServletRequest) request).getRequestDispatcher("/servlet2").forward((ServletRequest) request, request);
	
	request.sendRedirect(((HttpServletRequest) request).getContextPath()+"/servlet2");;
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

(d)ServletTwo.java

package cn.shu.web.servlet;

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

/**
 * Servlet implementation class ServletTwo
 */
public class ServletTwo extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletTwo() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		System.out.println("Servlet2 running...");
	
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

(e)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_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>Filter</display-name>
  <filter>
    <filter-name>QuickFilterOne</filter-name>
    <filter-class>cn.shu.web.filter.QuickFilterOne</filter-class>
    <init-param>
      <param-name>hehe</param-name>
      <param-value>HEHE</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>QuickFilterOne</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
  </filter-mapping>
 <!--  <filter>
    <filter-name>QuickFilterTwo</filter-name>
    <filter-class>cn.shu.web.filter.QuickFilterTwo</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>QuickFilterTwo</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping> -->
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>ServletOne</display-name>
    <servlet-name>ServletOne</servlet-name>
    <servlet-class>cn.shu.web.servlet.ServletOne</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletOne</servlet-name>
    <url-pattern>/servletOne</url-pattern>
  </servlet-mapping>
  <servlet>
    <description></description>
    <display-name>ServletTwo</display-name>
    <servlet-name>ServletTwo</servlet-name>
    <servlet-class>cn.shu.web.servlet.ServletTwo</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletTwo</servlet-name>
    <url-pattern>/servletTwo</url-pattern>
  </servlet-mapping>
</web-app>

说明：

在web.xml配置url-pattern配置时

完全匹配 /servlet1----直接访问/servlet1资源

目录匹配 /shu/* ----访问shu包下的

扩展名匹配 *.abc *.jsp-----访问后缀为abc和jsp的文件

在访问某个资源前，可以对其配置过滤器，若不符合条件不让其访问，可以配置一个过滤器，也可以配置多个过滤器，若其中任何一个过滤器不放行，该请求访问资源就会失败，过滤器访问的顺序由web.xml配置文件中过滤器的filter-mapping的前后顺序决定，filter-mapping配置在前，则首先执行，chain链对象知道所有的filter执行顺序，因为所有的filter都会在其中注册。

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