一般文件的的上传和下载可以使用基本的I/O流实现。但是从开发的效率和程序运行的效率方面考虑，一般会采用第三方的组件完成文件的上传，而文件的下载则不需要第三方组件。下面介绍Servlet使用第三方组件上传文件和使用Servlet下载文件。
1.下载上传组件
在实际的Java Web的实际开发中，一般使用commons-fileupload和commons-io组件来完成文件的上传功能，这两个组件都是Apache开发维护的
下载地址：commons.apache.org/proper/comm…
2.在项目中web-WebINF添加lib文件夹,然后将解压的jar包拷贝进来，还要右键项目添加依赖才会生效！
3.创建上传文件的页面(upload.jsp)
<%--
Created by IntelliJ IDEA.
User: qzf
Date: 2020/1/15
Time: 23:14
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>文件上传</title>
</head>
<body>
<form action="FileUpload" enctype="multipart/form-data" method="post">
<table cellpadding="0" cellspacing="0">
<tr>
<td>文件：</td>
<td><input type="file" name="file"> </td>
</tr>
<tr>
<td colspan="2" ><input type="submit" value="上传">
</td>
</tr>
</table>
</form>
</body>
</html>
4.创建上传文件的servlet.(FileUpload.jsp)
package servlet;
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileItemFactory;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import 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.File;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Iterator;
@WebServlet(name = "FileUpload")
public class FileUpload extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//判断文件是否上传
boolean isMultipart= ServletFileUpload.isMultipartContent(request);
if (isMultipart){
//创建文件上传对象
FileItemFactory factory=new DiskFileItemFactory();
ServletFileUpload upload=new ServletFileUpload(factory);
Iterator items;//存放解析表单的元素
try{
//解析表单提交的内容
items=upload.parseRequest(request).iterator();
while (items.hasNext()){
FileItem item=(FileItem)items.next();
if (!item.isFormField()){
//取出上传文件名称
String name=item.getName();
String fileName=name.substring(name.lastIndexOf("\\")+1,name.length());
//上传文件保存到服务器的路径
String path=request.getSession().getServletContext().getRealPath("upload")+ File.separatorChar+fileName;
//上传文件
File uploadedFile=new File(path);
item.write(uploadedFile);
//打印上传成功的信息
response.setContentType("text/html");
response.setCharacterEncoding("utf-8");
PrintWriter out=response.getWriter();
out.print("上传的文件为："+name+"<br>");
out.print("保存的地址："+path);
}
}
}catch(Exception e) {
e.printStackTrace();
}
}
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
5.配置web.xml文件
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
version="4.0">
<servlet>
<servlet-name>FileUpload</servlet-name>
<servlet-class>servlet.FileUpload</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>FileUpload</servlet-name>
<url-pattern>/FileUpload</url-pattern>
</servlet-mapping>
</web-app>
6.启动Tomcat服务器。在浏览器地址栏：
http://localhost:8081/upload.jsp
