一、文件上传的原理分析
1、文件上传的必要前提
a、表单的method必须是post
b、表单的enctype属性必须是multipart/form-data类型的。
enctype默认值：application/x-www-form-urlencoded
作用：告知服务器，请求正文的MIME类型
application/x-www-form-urlencoded ： username=abc&password=123
ServletRequest.getParameter(String name);该方法是专门读取该类型的方法
multipart/form-data:
2、借助commons-fileupload组件实现文件的上传
a、拷贝jar包：commons-fileupload.jar commons-io.jar
b、实现原理
3、乱码问题
a、普通字段的乱码
FileItem.getString(String charset);编码要和客户端一致
b、上传的中文文件名乱码
request.setCharacterEncoding("UTF-8");编码要和客户端一致
4、具体实现
前台upload.jsp代码如下
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title>文件上传</title><meta http-equiv="pragma" content="no-cache"><meta http-equiv="cache-control" content="no-cache"><meta http-equiv="expires" content="0"><!--<link rel="stylesheet" type="text/css" href="styles.css">--></head><body><form action="${pageContext.request.contextPath}/servlet/UploadServlet3" method="post" enctype="multipart/form-data">name:<input name="name"/><br/>file1:<input type="file" name="f1"/><br/>file2:<input type="file" name="f2"/><br/><input type="submit" value="上传"></form></body></html>后台servlet代码
package com.itheima.servlet;import java.io.File;import java.io.IOException;import java.io.PrintWriter;import java.io.UnsupportedEncodingException;import java.util.List;import java.util.UUID;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.apache.commons.fileupload.FileItem;import org.apache.commons.fileupload.FileUploadBase;import org.apache.commons.fileupload.FileUploadException;import org.apache.commons.fileupload.disk.DiskFileItemFactory;import org.apache.commons.fileupload.servlet.ServletFileUpload;import org.apache.commons.io.FilenameUtils;//详解public class UploadServlet3 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8");response.setContentType("text/html;charset=UTF-8");PrintWriter out = response.getWriter();System.out.print(request.getRemoteAddr());boolean isMultipart = ServletFileUpload.isMultipartContent(request);if(!isMultipart){throw new RuntimeException("请检查您的表单的enctype属性，确定是multipart/form-data");}DiskFileItemFactory dfif = new DiskFileItemFactory();ServletFileUpload parser = new ServletFileUpload(dfif);// parser.setFileSizeMax(3*1024*1024);//设置单个文件上传的大小// parser.setSizeMax(6*1024*1024);//多文件上传时总大小限制List<FileItem> items = null;try {items = parser.parseRequest(request);}catch(FileUploadBase.FileSizeLimitExceededException e) {out.write("上传文件超出了3M");return;}catch(FileUploadBase.SizeLimitExceededException e){out.write("总文件超出了6M");return;}catch (FileUploadException e) {e.printStackTrace();throw new RuntimeException("解析上传内容失败，请重新试一下");}//处理请求内容if(items!=null){for(FileItem item:items){if(item.isFormField()){processFormField(item);}else{processUploadField(item);}}}out.write("上传成功！");}private void processUploadField(FileItem item) {try {String fileName = item.getName();//用户没有选择上传文件时if(fileName!=null&&!fileName.equals("")){fileName = UUID.randomUUID().toString()+"_"+FilenameUtils.getName(fileName);//扩展名String extension = FilenameUtils.getExtension(fileName);//MIME类型String contentType = item.getContentType();if(contentType.startsWith("image/")){//分目录存储：日期解决// Date now = new Date();// DateFormat df = new SimpleDateFormat("yyyy-MM-dd");// // String childDirectory = df.format(now);//按照文件名的hashCode计算存储目录String childDirectory = makeChildDirectory(getServletContext().getRealPath("/WEB-INF/files/"),fileName);String storeDirectoryPath = getServletContext().getRealPath("/WEB-INF/files/"+childDirectory);File storeDirectory = new File(storeDirectoryPath);if(!storeDirectory.exists()){storeDirectory.mkdirs();}System.out.println(fileName);item.write(new File(storeDirectoryPath+File.separator+fileName));//删除临时文件}}} catch (Exception e) {throw new RuntimeException("上传失败,请重试");}}//计算存放的子目录private String makeChildDirectory(String realPath, String fileName) {int hashCode = fileName.hashCode();int dir1 = hashCode&0xf;// 取1~4位int dir2 = (hashCode&0xf0)>>4;//取5~8位String directory = ""+dir1+File.separator+dir2;File file = new File(realPath,directory);if(!file.exists())file.mkdirs();return directory;}private void processFormField(FileItem item) {String fieldName = item.getFieldName();//字段名String fieldValue;try {fieldValue = item.getString("UTF-8");} catch (UnsupportedEncodingException e) {throw new RuntimeException("不支持UTF-8编码");}System.out.println(fieldName+"="+fieldValue);}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}5、关于临时文件问题
a、DiskFileItemFactory
public void setRepository(File repository)：设置临时文件的存放目录
public void setSizeThreshold(int sizeThreshold)：设置缓存的大小
b、
文件上传时，自己用IO流处理，一定要在流关闭后删除临时文件。FileItem.delete()
建议使用：FileItem.writer(File f).会自动删除临时文件。
6、限制文件的大小
a、
ServletFileUpload.setFileSizeMax(3*1024*1024);//设置单个文件上传的大小
b、
ServletFileUpload.setSizeMax(6*1024*1024);//多文件上传时总大小限制
以上所述是小编给大家介绍的commons fileupload实现文件上传的实例代码，希望对大家有所帮助，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对武林网网站的支持！