java如何实现文件上传和下载功能？这个问题可能是我们日常学习或工作经常见到的。希望通过这个问题能让你收获颇深。下面是小编给大家带来的参考内容，让我们一起来看看吧！

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需要导入的jar包

运行截图

文件上传截图

文件下载截图

上传文件代码servlet

	@WebServlet(name = "UploadServlet",value = "/upload")
	@MultipartConfig(maxFileSize = 1024*1024*5,maxRequestSize = 1024*1024*20) //1 添加MultipartConfig注解
	public class UploadServlet extends HttpServlet {
	    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	        //存放文件的目录
	        String realPath = request.getServletContext().getRealPath("/WEB-INF/upload");
	        File dir=new File(realPath);
	        if(!dir.exists()){
	            dir.mkdirs();
	        }
	        List allowExts=new ArrayList();
	        allowExts.add("jpg");
	        allowExts.add("png");
	        allowExts.add("gif");
	
	        //1乱码
	        request.setCharacterEncoding("utf-8");
	        response.setContentType("text/html;charset=utf-8");
	        //2使用getParts()获取数据
	        Collection parts = request.getParts();
	        //3遍历
	        PrintWriter out = response.getWriter();
	        if(parts!=null&&parts.size()>0){
	            for (Part part : parts) {
	                //判断表单元素是普通字段，还是文件
	                String submittedFileName = part.getSubmittedFileName();
	                if(submittedFileName==null){//普通字段
	                    String name = part.getName();
	                    String value = request.getParameter(name);
	                    System.out.println(name+"..."+value);
	                }else{//文件
	
	                    //判断文件是否为""
	                    if(submittedFileName.equals("")){
	                        continue;
	                    }
	                    //System.out.println(submittedFileName);
	                    //从请求头中获取文件
	                    String dis = part.getHeader("content-disposition");
	                    String filename=dis.substring(dis.lastIndexOf("filename=")+10, dis.length()-1);
	                    filename=filename.substring(filename.lastIndexOf("\\")+1);
	                    System.out.println(filename);
	                    //获取文件名的后缀
	                    String ext=filename.substring(filename.lastIndexOf(".")+1);
	                    if(!allowExts.contains(ext)){
	                        out.println(filename+"不符合上传文件类型要求...");
	                        continue;
	                    }
	                    //把文件保存
	                    //1创建新的文件名
	                    String newFileName = UploadUtils.makeNewFileName(filename);
	                    //2创建新的路径
	                    String newPath = UploadUtils.makeNewPath(realPath, filename);
	                    part.write(newPath+File.separator+newFileName);
	                    //删除part
	                    part.delete();
	                    out.println("上传成功:"+filename);
	                }
	            }
	        }
	    }
	
	    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	        doPost(request,response);
	    }
	}

每个属性表示的内容

文件下载代码servlet

@WebServlet(name = "DownLoadServlet",value = "/download")
public class DownLoadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //乱码
        request.setCharacterEncoding("utf-8");
        //获取文件名
        String uuidFilename = request.getParameter("filename");//d578be74fd864ac2a879d77b07f13793_backg.jpg
        //去掉uuid
        String filename=uuidFilename.substring(uuidFilename.indexOf("_")+1);
        //存放文件的根路径
        String realPath = request.getServletContext().getRealPath("/WEB-INF/upload");
        //获取真正目录
        String path = UploadUtils.makeNewPath(realPath, filename);

        File file=new File(path+ File.separator+uuidFilename);
        if(file.exists()){
            response.setHeader("content-disposition", "attachment;filename="+ URLEncoder.encode(filename, "utf-8"));
            ServletOutputStream os = response.getOutputStream();
            FileInputStream fis=new FileInputStream(file);
            byte[] buf=new byte[1024*4];
            int len=0;
            while((len=fis.read(buf))!=-1){
                os.write(buf,0,len);
            }
        }else{
            response.setContentType("text/html;charset=utf-8");
            response.getWriter().write("文件不存在...");
        }

    }

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

每个属性表示的内容

读取下载文件servlet

@WebServlet(name = "ListFileServlet",value = "/listfile")
public class ListFileServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1读取可以被下载的文件
        String realPath = request.getServletContext().getRealPath("/WEB-INF/upload");
        HashMap map=new HashMap<>();
        UploadUtils.listFile(new File(realPath),map);
        //2放入域中
        request.setAttribute("map", map);
        //3转发
        request.getRequestDispatcher("/list.jsp").forward(request, response);

    }

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

工具类servlet

public class UploadUtils {
    public static void main(String[] args) {
        String s = makeNewFileName("aaa.jpg");
        System.out.println(s);
    }
    /**
     * 根据原始文件名产生一个新的文件名
     * @param filename
     * @return
     */
    public static String makeNewFileName(String filename){
        //UUID 统一唯一标识码
        String uuid = UUID.randomUUID().toString().replace("-", "");//默认32位的16进制
        return uuid+"_"+filename;
    }

    /**
     * 创建新的路径
     * @param path
     * @param filename
     * @return
     */
    public static String makeNewPath(String path,String filename){
        int num = filename.hashCode();//01101011001011011111111111 1111 0101 0101
        int path2=num&0xf;
        int path3=(num>>4)&0xf;
        String newPath=path+ File.separator+path2+File.separator+path3;
        File dir=new File(newPath);
        if(!dir.exists()){
            dir.mkdirs();
        }
        return newPath;
    }
    //遍历可以被下载的文件
    public static void listFile(File dir,HashMap map){
        File[] files = dir.listFiles();
        if(files!=null&&files.length>0){
            for (File file : files) {
                if(file.isDirectory()){
                    listFile(file, map);
                }else{
                    //文件
                    String uuidFilename=file.getName();
                    String filename=uuidFilename.substring(uuidFilename.indexOf("_")+1);
                    map.put(uuidFilename,filename);
                }
            }
        }
    }


}

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标题名称：java如何实现文件上传和下载功能
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