JavaWeb 文件上传和下载

JavaWeb 文件上传和下载
首先去apache官网下载commons-io.jar和commons-fileupload.jar,下载后导入到项目中去
apache官网都有相关文档，左侧的user guide 有关于上传下载实现的相关代码

upload.html
enctype="multipart/form-data"必须加上

<form action="uploadServlet?action=upload" method="post" enctype="multipart/form-data">
			<input type="file" name="filename" id="filename"/><br/>
			<input type="submit"/>
	</form>

uploadServlet.java

public class UploadServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(req, resp);
	}
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		req.setCharacterEncoding("utf-8");
		resp.setCharacterEncoding("utf-8");
		String action = req.getParameter("action");
		ServletContext servletContext = null;
		String realPath = null;  //图片文件夹路径
		PrintWriter pw = null;
		switch(action) {
		//文件上传
		case "upload":
			DiskFileItemFactory diskFileItemFactory = new DiskFileItemFactory();
			//配置临时存储库
			servletContext = this.getServletConfig().getServletContext();
			File repository = (File) servletContext.getAttribute("javax.servlet.context.tempdir");
			diskFileItemFactory.setRepository(repository);
			//创建文件上传程序对象
			ServletFileUpload upload = new ServletFileUpload(diskFileItemFactory);
			List<FileItem> items = null;
			try {
				//解析request发送的请求
				items = upload.parseRequest(req);
			} catch (FileUploadException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			Iterator<FileItem> iterator = items.iterator();
			while(iterator.hasNext()) {
				FileItem item = iterator.next();
				if(item.isFormField()) {
					//表单内容
				}else {
					//表单文件流内容
					String fieldName = item.getFieldName();
					String fileName = item.getName();
					String contextType = item.getContentType();
					//判断item是否在内存中
					boolean isInMemory = item.isInMemory();
					//获取item的大小
					long sizeInBytes = item.getSize();
					//系统毫秒数作每个文件的唯一标识符
					long seconds = System.currentTimeMillis();
					//获取文件后缀名
					String fileType = fileName.substring(fileName.lastIndexOf('.'), fileName.length());
					//设置文件上传的位置
					//servletContext.getRealPah() 获取服务器上真实绝对路径
					//通过系统时间生成唯一的文件名，存储在up文件夹下
					String FileWay = servletContext.getRealPath("/up/")+seconds+fileType;
					File uploadFile = new File(FileWay);
					try {
						//将文件上传至指定文件夹
						item.write(uploadFile); 
					} catch (Exception e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
 				}
			}
			break;
		//文件下载
		case "download":
			String filename = req.getParameter("filename");
			System.out.println(filename);
			//下载文件保存到本地的名称
			resp.setHeader("content-disposition", "attachment;filename="+URLEncoder.encode(filename,"utf-8"));
			OutputStream outputStream = resp.getOutputStream();
			servletContext = this.getServletContext();
			String realpath = servletContext.getRealPath("/");
			//filename要和tomcat中上传文件夹中的文件名保持一致，否则会出现FileNotFoundException
			FileInputStream fileInputStream = new FileInputStream(realpath+filename);
			byte[] b = new byte[1024];
			int i;
			while((i=fileInputStream.read(b,0,b.length))!= -1) {
				outputStream.write(b);
				b = new byte[1024];
			}
			fileInputStream.close();
			outputStream.flush();
			outputStream.close();
			break;
		default:
			break;
		}
	}
}

注意：上传文件是使用的是文件在本地的绝对路径，下载文件时使用的是文件在服务器中的相对路径，文件存储在tomcat服务器中的一个文件夹中而不是项目中的文件夹，访问该文件夹可以通过servletContext.getRealPah() 获取服务器上真实绝对路径