Filter过滤器的使用！！！

什么是过滤器

当浏览器向服务器发送请求的时候，过滤器可以将请求拦截下来，完成一些特殊的功能，比如：编码设置、权限校验、日志记录等。

过滤器执行流程

过滤器的使用：

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by.servlet;

import javax.servlet.*;
import java.io.IOException;

/**
 * <p>Project: JavaWeb-Servlet - FilterDome</p>
 * <p>Powered by scl On 2023-12-27 14:45:18</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class FilterDome implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");//设置请求的编码格式
        servletResponse.setContentType("text/html;charset=utf-8");//设置相应的编码格式

        System.out.println("---------come in----------");
        filterChain.doFilter(servletRequest,servletResponse);//过滤器放行
        System.out.println("------------good by------------");
    }

    @Override
    public void destroy() {

    }
}

测试代码：

/*
 * Copyright (c) 2020, 2023,  All rights reserved.
 *
 */
package com.by.servlet;


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

/**
 * <p>Project: JavaWeb-Servlet - FileterTest</p>
 * <p>Powered by scl On 2023-12-27 14:48:04</p>
 * <p>描述：<p>
 *
 * @author 孙臣龙 [1846080280@qq.com]
 * @version 1.0
 * @since 17
 */
public class FilterTest extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        resp.getWriter().write(username+password);
    }
}

表单代码：

<%--
  Created by IntelliJ IDEA.
  User: admin
  Date: 2023/12/25
  Time: 15:28
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  <form action="filter" method="get">
    <input type="text" name="username">
    <input type="text" name="password">
    <input type="submit" value="登录">
  </form>
  </body>
</html>

配置文件web.xml：

<!--过滤器文件-->
    <filter>
        <filter-name>FilterDome</filter-name>
        <filter-class>com.by.servlet.FilterDome</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDome</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <!--过滤器测试文件-->
    <servlet>
        <servlet-name>filterTest</servlet-name>
        <servlet-class>com.by.servlet.FilterTest</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>filterTest</servlet-name>
        <url-pattern>/filter</url-pattern>
    </servlet-mapping>

使用过滤器需要注意的事项：

1.过滤器必须实现Filter接口。

2.过滤器拦截的请求执行完毕之后，必须要放行，否则我们的请求就不会被执行。

 filterChain.doFilter(request,response); //过滤器放行

3.我们可以使用@WebFilter来配置过滤器要拦截的资源，当然我们也可以通过xml的方式配置过滤器。

  <filter>
        <filter-name>filter</filter-name>
        <filter-class>com.by.servlet.FilterDemo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>