文件上传与下载可谓上网中的常见现象。apache为我们准备了用于文件上传与下载的两个jar包（commons-fileupload-1.2.1.jar，commons-io-1.4.jar）。我们在web项目中，将这两个jar包添加依赖，就可以使用客户端上传文件、服务器端下载文件的功能。

　　我们假设一个前端页面（如jsp页面）向服务器发起一个上传文件的请求，这时，表单必须必须用POST方式提交，设置表单提交格式为enctype=”multipart/form-data”，表示表单将能够提交字节数据，而不是键值对；服务器获取数据的时候，就不能request.getParameter了，必须用流获取；我们在服务器端用工具类库，对接收到的数据进行解析分析，及文件转储。

　　前端页面的表单如下：　　

<form action="UploadServlet" method="post" enctype="multipart/form-data">
    <input type="text" name="in_data1"><%--提交普通键值对数据--%><br><br>
    <input type="file" name="uploadFile"><%--上传文件--%><br><br>
    <input type="submit" value="提交">
  </form>

　　接下来是处理文件上传与下载的核心工具类：

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileItemFactory;
import org.apache.commons.fileupload.FileUploadException;
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.util.Iterator;
import java.util.List;
/**
 * 处理文件上传与下载
 */
@WebServlet(name = "UploadServlet",urlPatterns = "/UploadServlet")
public class UploadServlet extends HttpServlet {
    private String dirPath=null;
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //处理中文文件名乱码
        request.setCharacterEncoding("UTF-8");
        genUploadDir(request);//创建文件上传的根目录
        // 检查前端发来的请求是否文件上传请求
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);
        if (isMultipart) {
            //符合文件上传的要求,则进行文件的解析和转储
            // Create a factory for disk-based file items
            FileItemFactory factory = new DiskFileItemFactory();
            // Create a new file upload handler
            ServletFileUpload upload = new ServletFileUpload(factory);
          // Parse the request
            try {
                List<FileItem> items = upload.parseRequest(request);
                Iterator iter = items.iterator();
                while (iter.hasNext()) {
                    FileItem item = (FileItem) iter.next();
                    if (item.isFormField()) {
                        //如果是一个普通表单域
                        processFormField(item);
                    } else {
                        //如果是文件域
                        processUploadedFile(item);
                    }
                }
                response.getWriter().write("successful");
            } catch (FileUploadException e) {
                e.printStackTrace();
            }
        }
    }
    //创建我们文件上传的根目录
    private void genUploadDir(HttpServletRequest request) {
        if (dirPath==null) {
            dirPath =request.getServletContext().getRealPath("/WEB-INF/upload/");
            File dir=new File(dirPath);
            //如果根目录不存在
            if (!dir.exists()) {
                //则新建该文件夹
                dir.mkdirs();
            }
        }
    }

    private void processUploadedFile(FileItem item) {
        String fieldName = item.getFieldName();
        System.out.println("fieldName = " + fieldName);
        String fileName = item.getName();
        System.out.println("fileName = " + fileName);
        long sizeInBytes = item.getSize();
        System.out.println("sizeInBytes = " + sizeInBytes);
        File uploadedFile = new File(dirPath,fileName);
        try {
            item.write(uploadedFile);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void processFormField(FileItem item) {
        String name = item.getFieldName();
        System.out.println("name = " + name);
        String value = item.getString();
        System.out.println("value = " + value);
        System.out.println("======================================");
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request,response);
    }
}

　　在idea中运行tomcat服务器后，在前端表单中填入数据与上传的文件，服务器控制台显示文件上传成功：

键值对数据
name = in_data1
value = helloworld
======================================
上传文件数据
fieldName = uploadFile
fileName = JVM指令.txt
sizeInBytes = 28733