Filter过滤器

一.Filter什么是过滤器

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: 2022-01-12Time: 16:41To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html><head><title>Cookie</title><base href="http://localhost:8888/day2/"><style type="text/css">ul li {list-style: none;}</style></head><body><h3>开始了新的测试</h3><%Object user = session.getAttribute("user");if (user == null) {request.getRequestDispatcher("/loGin.jsp").forward(request,response);return;}%>
这是index.jsp页面</body>
</html>

<%--Created by IntelliJ IDEA.User: 86176Date: 2022-01-12Time: 16:50To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head><title>Title</title>
</head>
<body>
<h4>这是登录页面</h4>
<form>用户名: <input type="text" name="username"> <br>密 码: <input type="password" name="password" value=""><br><input type="submit" value="登录">
</form>
</body>
</html>

Filter过滤器执行的方式

获取用户Cookie

<%--Created by IntelliJ IDEA.User: 86176Date: 2022-01-12Time: 16:50To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head><title>Title</title>
</head>
<body>
<h4>这是登录页面</h4>
<form action="http://localhost:8888/day2/loginServlet" method="get">用户名: <input type="text" name="username"> <br>密 码: <input type="password" name="password" value=""><br><input type="submit" value="登录">
</form>
</body>
</html>

配置下的Servlet响应

public class LoginServlet  extends HttpServlet {@Overrideprotected 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

 <!--Filter标签用于配置一个Filter过滤器--><filter><!--给Filter起一个别名--><filter-name>imFilter</filter-name><!--配置Filter的全类名--><filter-class>com.Demo.Filter.imFilter</filter-class></filter><!--Filter-mapping 配置Filter过滤器的拦截路径--><filter-mapping><!--Filter-name表示当前拦截那个路径给Filter过滤使用--><filter-name>imFilter</filter-name><!--urlpattern配置拦截路径斜杠就是表示http://ip:port/工程路径/  映射到web目录下--><url-pattern>/admin/*</url-pattern></filter-mapping><servlet><servlet-name>LoginServlet</servlet-name><servlet-class>com.Demo.Filter.LoginServlet</servlet-class></servlet><servlet-mapping><servlet-name>LoginServlet</servlet-name><url-pattern>/loginServlet</url-pattern></servlet-mapping>

Filter过滤器代码段


public class imFilter  implements Filter {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {}@Overridepublic void destroy() {}@Overridepublic 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);}}
}

三…Filter的生命周期

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

四.FilterConfig类

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 {@Overridepublic 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());}}

五.FilterChain过滤链

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 {@Overridepublic 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 {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {}@Overridepublic void destroy() {}@Overridepublic 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

<!--Filter标签用于配置一个Filter过滤器--><filter><!--给Filter起一个别名--><filter-name>imFilter</filter-name><!--配置Filter的全类名--><filter-class>com.Demo.Filter.imFilter</filter-class></filter><!--Filter-mapping 配置Filter过滤器的拦截路径--><filter-mapping><!--Filter-name表示当前拦截那个路径给Filter过滤使用--><filter-name>imFilter</filter-name><!--urlpattern配置拦截路径斜杠就是表示http://ip:port/工程路径/  映射到web目录下--><url-pattern>/loGin.jsp</url-pattern></filter-mapping><filter><filter-name>Filter1</filter-name><filter-class>com.Demo.Filter.Filter1</filter-class></filter><filter-mapping><filter-name>Filter1</filter-name><url-pattern>/loGin.jsp</url-pattern></filter-mapping>

六.还有什么问题可以留言给我，会尽心解答的，觉得文章不错的话点个赞把