Filter过滤器讲解

Filter过滤器它是JavaWeb的三大组件之一，三大组件分别是：Servlet程序、Listener监听器、Filter过滤器
Filter过滤器它是JavaEE的规划，也是接口
Filter过滤器它的作用是：请求拦截

拦截请求常见的有：权限检查、日记操作、事物管理等等 二.Filter过滤器拦截

在WEB-INF的目录下，有很多资源，如html、jsp、txt、jpg等，都必须是用户登录之后才允许访问
简单的实现不用Filter过滤器就是可以看Session中是否包含用户登录信息即可
先从jsp文件里面进行Session的判断，是否有用户的cookie信息，但这种只能在jsp文件里面而html是不能写Java代码的
这就需要到Filter过滤器

<%--
  Created by IntelliJ IDEA.
  User: 程序员小徐同学
  Date: 2022-01-12
  Time: 16:41
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>

  
    
    
    
  
  
  
开始了新的测试
  <%
    Object user = session.getAttribute("user");
    if (user == null) {
      request.getRequestDispatcher("/loGin.jsp").forward(request,response);
      return;
    }
  %>
这是index.jsp页面
  

<%--
  Created by IntelliJ IDEA.
  User: 86176
  Date: 2022-01-12
  Time: 16:50
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>


    


这是登录页面

    用户名:  

    密 码: 

    

Filter过滤器执行的方式

获取用户cookie

<%--
  Created by IntelliJ IDEA.
  User: 86176
  Date: 2022-01-12
  Time: 16:50
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>


    


这是登录页面

    用户名:  

    密 码: 

    

配置下的Servlet响应

public class LoginServlet  extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        resp.setContentType("text/html;character=UTF-8");
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        if ("root".equals(username) && "root".equals(password)){
            req.getSession().setAttribute("user",username);
            resp.getWriter().write("登录成功");
        } else {
            req.getRequestDispatcher("/loGin.jsp").forward(req,resp);
        }
    }
}

给Servlet程序配置和Filter过滤器也进行配置，配置Filter就是要拦截的目标路径

 
   
       
       imFilter
       
       com.Demo.Filter.imFilter
   
    
    
        
        imFilter
        
        /admin/*
    
    
        LoginServlet
        com.Demo.Filter.LoginServlet
    
    
        LoginServlet
        /loginServlet
    

Filter过滤器代码段

public class imFilter  implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    public void destroy() {
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
        HttpSession session = httpServletRequest.getSession();
        Object user = session.getAttribute("user");
        //如果等于null ,说明还没有登录
        if (user == null) {
            servletRequest.getRequestDispatcher("/loGin.jsp").forward(servletRequest,servletResponse);
            return;
        } else {
            //不等于空，让用户继续向下访问资源
            filterChain.doFilter(servletRequest,servletResponse);
        }
    }
}

Filter过滤器包含几个方法

构造器方法init初始化方法doFilter过滤器方法destory销毁

public class imFilter  implements Filter {
    public imFilter() {
        System.out.println("Filter构造器方法");
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("Filter初始化方法");
    }
    @Override
    public void destroy() {
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("doFilter拦截方法");
    }
     @Override
    public void destory() {
        System.out.println("Filter的destory方法");
    }
}

FilterConfig类就是Filter过滤器的配置文件
Tomcat每次创建Filter的时候，也会同时创建一个FilterConfig类，这里包含了Filter配置文件的配置信息

FilterConfig类的作用是获取Filter过滤器的配置内容

获取Filter的名称Filter-name内容获取在Filter中配置的init-param初始化参数获取ServletContext对象

public class imFilter  implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("Filter-name的值是" + filterConfig.getFilterName());
        //获取在web.xml中的init-param初始化参数
        System.out.println("初始化参数username" + filterConfig.getInitParameter("username"));
        //获取ServletContext对象
        System.out.println(filterConfig.getServletContext());
    }
    }

FilterChain过滤链就是多个过滤器进行

FilterChain.doFilter（）方法作用

执行下一个Filter过滤器（如果有Filter）执行目录资源（没有Filter）所有Filter和目标资源默认都执行在同一个线程中 、多个Filter在执行时他们的顺序是由web.xml的配置顺序决定多个Filter共同执行的时候，他们都使用同一个Request对象

Filter的java代码

public class Filter1 implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("Filter2前置代码");
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("Filter2后置代码");
    }
}

public class imFilter  implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }
    @Override
    public void destroy() {
    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("Filter1前置代码");
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("Filter1后置代码");
    }
    public void destory() {
        System.out.println("Filter的destory方法");
    }
}

两个Filter‘在web.xml的配置

   
       
       imFilter
       
       com.Demo.Filter.imFilter
   
    
    
        
        imFilter
        
        /loGin.jsp
    
    
        Filter1
        com.Demo.Filter.Filter1
    
   
       Filter1
       /loGin.jsp