1.首页jsp 应该获取servlet请求转发过来的文件目录，显示文件名称，
注意：首先要进行逻辑判断，把list为空的情况筛选出来；
2.把转发来的list集合 展现在index页面中，在下载的超链接路径的参数传递文件名称，传到download的Servlet路径，然后在download的servlet的方法里面根据传来的名字，在资源目录里面找，如果存在该文件，则创建一个该文件的输入流进入内存，再通过response获得输出流，将输入流传到index页面
注意：
2.1应该设置传递的文件mime类型，resp.setContentType(contentType)，通过逻辑判断file的文件类型2.2如果浏览器获取到该流后，应该设置浏览器的打开方式，比如直接下载，就要设置http头信息，如果文件名称包含中文，就会报错，解决方法是：将文件名进行URL编码
String encodedFileName = URLEncoder.encode(submittedFileName, "UTF-8"); resp.setHeader("Content-Disposition","attachment;filename="+encodedFileName); 2.3 输出流最后应该flush ，输入流close关闭
3.文件上传 需要在表单里action里写上upload的servlet路径，然后在servlet里面接收传来的文件需要使用post请求，注意需要在servlet设置@MultipartConfig，
注意：先设置编码方式 再获取打印流
resp.setContentType("text/html;charset=UTF-8");
PrintWriter writer = resp.getWriter();
4.servlet继承HttpServlet（）
import javax.servlet.http.HttpServlet;
前端页面的代码
<%@ page import="cn.hutool.core.util.ArrayUtil" %>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<html>
<head>
<title>文件上传下载</title>
</head>
<body>
<h1>文件列表</h1>
<%
String[] list = (String[]) request.getAttribute("list");
%>
<% if (ArrayUtil.isNotEmpty(list)){
for (int i = 0; i<list.length; i++) {
%>
<a ><%=list[i]%></a>
<a href="/download?name=<%=list[i]%>">下载</a>
</br>
<%
}
}
%>
<hr>
<h2>文件上传</h2>
<form action="upload" method="post" enctype="multipart/form-data">
<input type="file" name="file">
<button>上传</button>
</form>
</body>
</html>
index的Servlet
package fileupdown;
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.File;
import java.io.IOException;
import java.net.URLEncoder;
@WebServlet("/index")
public class Index extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// super.service(req, resp);
//创建File对象，指向文件上传的目录
File file = new File("D:\\CodeAll\\code\\FirstWeb\\src\\main\\webapp\\resourse");
//获取该目录下的文件列表
String[] list = file.list();
req.setAttribute("list", list);
req.getRequestDispatcher("index.jsp").forward(req, resp);
}
}
upload的servlet
package fileupdown;
import javax.servlet.ServletException;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.Part;
import java.io.*;
@WebServlet("/upload")
@MultipartConfig
public class FileUpload extends HttpServlet {
@Override
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// 获取文件部分
Part part = req.getPart("file");
//获取上传的文件名
String submittedFileName = part.getSubmittedFileName();
// 获取文件内容流
InputStream fileContent = part.getInputStream();
OutputStream outputStream = new
FileOutputStream("D:\\CodeAll\\code\\FirstWeb\\src\\main\\webapp\\resourse\\"+submittedFileName);
byte[] bytes = new byte[1024];
int len = 0;
while (-1!= (len =fileContent.read(bytes) )){
outputStream.write(bytes,0,len);
}
resp.setContentType("text/html;charset=UTF-8");
PrintWriter writer = resp.getWriter();
writer.print("<html>");
writer.print("<body>");
writer.print("<h2>文件上传成功!<h2>");
writer.print("<h2>3s后返回下载列表<h2>");
writer.print("</body>");
writer.print("</html>");
System.out.println("打印上传成功");
// resp.setHeader("Refresh", "3; url=index.jsp");
resp.setHeader("Refresh", "3; url=./index");
// resp.sendRedirect("/index");
}
}
download的servlet
package fileupdown;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.Part;
import java.io.*;
import java.net.URLEncoder;
@WebServlet("/download")
@MultipartConfig
public class FileDownload extends HttpServlet {
@Override
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// super.service(req, resp);
//获取上传到服务器的文件名
String submittedFileName = req.getParameter("name");
File file = new File("D:\\CodeAll\\code\\FirstWeb\\src\\main\\webapp\\resourse\\" + submittedFileName);
if (file.exists()){
FileInputStream fileInputStream = new FileInputStream(file);
String contentType = "";
if (file.getName().endsWith(".jpg") || file.getName().endsWith(".jpeg")) {
contentType = "image/jpeg";
} else if (file.getName().endsWith(".png")) {
contentType = "image/png";
} else if (file.getName().endsWith(".gif")) {
contentType = "image/gif";
} else if (file.getName().endsWith(".pdf")) {
contentType = "application/pdf";
} else if (file.getName().endsWith(".doc") || file.getName().endsWith(".docx")) {
contentType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document";
} else if (file.getName().endsWith(".xls") || file.getName().endsWith(".xlsx")) {
contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
} else if (file.getName().endsWith(".zip")) {
contentType = "application/zip";
} else if (file.getName().endsWith(".txt")) {
contentType = "text/html;charset=UTF-8";
} else if (file.getName().endsWith(".doc") || file.getName().endsWith(".docx")) {
contentType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document";
}
resp.setContentType(contentType);
// 对文件名进行URL编码
String encodedFileName = URLEncoder.encode(submittedFileName, "UTF-8");
resp.setHeader("Content-Disposition", "attachment; filename="+encodedFileName);
ServletOutputStream outputStream = resp.getOutputStream();
byte[] bytes = new byte[1024];
int len = 0;
while (-1!=(len=fileInputStream.read(bytes))){
outputStream.write(bytes,0,len);
}
outputStream.flush();
fileInputStream.close();
}else {
//进行文件不存在的处理
}
}
}