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：　　

     
    
      <%--提交普通键值对数据--%>
      
      
    
      <%--上传文件--%>
      
      
    
        
     

　　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
     
       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_data1value = helloworld======================================上传文件数据fieldName = uploadFilefileName = JVM指令.txtsizeInBytes = 28733