Java Web 学习笔记

过滤器

Filter

功能：

1、用来拦截传入的请求和传出的相应。

2、修改或以某种方式处理正在客户端和服务端之间交换的数据流。

如何使用？

与使用Servlet类似，Filter是JavaWeb提供的一个借口，开发者只需要自定义一个类并且实现该接口即可

1package com.janeroad.filter;
2
3import javax.servlet.*; 
4import java.io.IOException;
5
6public class CharacterFilter implements Filter { [@Override](https://my.oschina.net/u/1162528) public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { servletRequest.setCharacterEncoding("UTF-8"); filterChain.doFilter(servletRequest,servletResponse); } }

web.xml中配置Filter

1<filter> 
2	<filter-name>charcater</filter-name> <filter-class>com.southwind.filter.CharacterFilter</filter-class> </filter> <filter-mapping> <filter-name>charcater</filter-name> <url-pattern>/login</url-pattern> <url-pattern>/test</url-pattern> </filter-mapping>

注意：doFilter方法中处理完业务逻辑之后，必须添加 filterChain.doFilter(servletRequest,servletResponse);

否则请求/相应无法向后传递，一直停留在过滤器中。

Filter的生命周期

当Tomcat启动时，通过反射机制调用Filter的无参构造函数创建实例化对象，同时调用init方法实现初始化，doFilter 方法调用多次，当Tomcat服务关闭的时候，调用destory来销毁Filter对象

无参构造函数：只调用一次，当Tomcat启动时调用（Filter一定要进行配置）

init方法：只调用一次，当Filter的实例化对象创建完成之后调用

doFilter：调用多次，访问Filter的业务逻辑都写在Filter中

destory：只调用一次，Tomcat关闭时调用

同时配置多个Filter，Filter的调用顺序是由web.xml中的配置顺序来决定的，写在上面的配置先调用，因为web.xml是从上到下顺序读取的

1<filter> 
2	<filter-name>my</filter-name> <filter-class>com.southwind.filter.MyFilter</filter-class> </filter> <filter-mapping> <filter-name>my</filter-name> <url-pattern>/login</url-pattern> </filter-mapping> <filter> <filter-name>charcater</filter-name> <filter-class>com.southwind.filter.CharacterFilter</filter-class> </filter> <filter-mapping> <filter-name>charcater</filter-name> <url-pattern>/login</url-pattern> <url-pattern>/test</url-pattern> </filter-mapping>

1、MyFilter

2、CharacterFilter

也可以通过注解的方式来简化web.xml中的配置

1<filter>  
2	<filter-name>my</filter-name> <filter-class>com.southwind.filter.MyFilter</filter-class> </filter> <filter-mapping> <filter-name>my</filter-name> <url-pattern>/login</url-pattern> </filter-mapping>

等于

1@WebFilter("/login")
2public class MyFilter implements Filter { }

实际开发中Filter的使用场景：

1、统一处理中文乱码

2、屏蔽敏感词

1package com.southwind.filter;
2
3import javax.servlet.*;
4import javax.servlet.annotation.WebFilter; 
5import java.io.IOException;
6
7@WebFilter("/test") public class WordFilter implements Filter { [@Override](https://my.oschina.net/u/1162528) public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { servletRequest.setCharacterEncoding("UTF-8"); //将"敏感词"替换成*** String name = servletRequest.getParameter("name"); name = name.replaceAll("敏感词","***"); servletRequest.setAttribute("name",name); filterChain.doFilter(servletRequest,servletResponse); } } 
8
9import javax.servlet.ServletException;
10import javax.servlet.ServletException; 
11import javax.servlet.annotation.WebServlet; 
12import javax.servlet.http.HttpServlet; 
13import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; @WebServlet("/test") public class TestServlet extends HttpServlet { @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String name = (String) req.getAttribute("name"); System.out.println("servlet:"+name); } }

3、控制资源的访问权限

1package com.southwind.filter;
2import javax.servlet.*; 
3import javax.servlet.annotation.WebFilter; 
4import javax.servlet.http.HttpServletRequest; 
5import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; import java.io.IOException; @WebFilter("/download.jsp") public class DownloadFilter implements Filter { @Override public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { HttpServletRequest request = (HttpServletRequest) servletRequest; HttpServletResponse response = (HttpServletResponse) servletResponse; HttpSession session = request.getSession(); String name = (String) session.getAttribute("name"); if(name == null){ //不是登录状态 response.sendRedirect("/login.jsp"); }else{ filterChain.doFilter(servletRequest,servletResponse); } } }

文件上传下载

1、input的type设置file

2、form表单的method设置post，get请求会将文件名传给服务端，而不是文件本身

3、form表单的enctype设置multipart/form-data，以二进制的形式传输数据

1<%@ page contentType="text/html;charset=UTF-8" language="java" %>
2<html> 
3<head>    
4	<title>Title</title>
5</head> 
6<body>    
7	<form enctype="multipart/form-data" action="/upload" method="post"> <input name="desc" type="text"/><br/> <input name="text" type="file"/><br/> <input type="submit" value="上传"/> </form> </body> </html>

Servlet

fileupload组件可以将所有的请求信息都解析成FileItem对象，可以通过FileItem对象的操作完成上传，面向对象的思想

1package com.janeroad.servlet;
2
3import org.apache.commons.fileupload.FileItem;
4import org.apache.commons.fileupload.FileUploadException; 
5import org.apache.commons.fileupload.disk.DiskFileItemFactory;
6import org.apache.commons.fileupload.servlet.ServletFileUpload; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.*; import java.util.List; @WebServlet("/upload") public class UploadServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { try { DiskFileItemFactory fileItemFactory = new DiskFileItemFactory(); ServletFileUpload servletFileUpload = new ServletFileUpload(fileItemFactory); List<FileItem> list = servletFileUpload.parseRequest(req); for(FileItem fileItem : list){ if(fileItem.isFormField()){ String name = fileItem.getFieldName(); String value = fileItem.getString("UTF-8"); System.out.println(name+":"+value); }else{ String fileName = fileItem.getName(); long size = fileItem.getSize(); System.out.println(fileName+":"+size+"Byte"); InputStream inputStream = fileItem.getInputStream(); String path = req.getServletContext().getRealPath("file/"+fileName); OutputStream outputStream = new FileOutputStream(path); int temp = 0; while((temp = inputStream.read())!=-1){ outputStream.write(temp); } outputStream.close(); inputStream.close(); System.out.println("上传成功"); } } } catch (FileUploadException e) { e.printStackTrace(); } } }

来源：真相只有一个

Java Web 学习笔记

过滤器

Filter的生命周期

文件上传下载

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