<!-- https://mvnrepository.com/artifact/commons-fileupload/commons-fileupload -->
<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.4</version>
</dependency>

JSP

<form action="${pageContext.request.contextPath}/fileUpload" enctype="multipart/form-data" method="post">
    <input type="file" name="file"> <br/>
    <input type="submit" value="上传">
</form>

代码

// 上传文件存储目录
private static final String UPLOAD_DIRECTORY    = "/WEB-INF/upload";

// 上传配置
private static final int MAX_FILE_SIZE      = 1024 * 1024 * 40; // 40MB
private static final int MAX_REQUEST_SIZE   = 1024 * 1024 * 50; // 50MB

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // 检测是否为多媒体上传
    if (!ServletFileUpload.isMultipartContent(request)) {
        PrintWriter writer = response.getWriter();
        writer.println("Error: 表单必须包含 enctype=multipart/form-data");
        writer.flush();
        return;
    }

    // 创建上传文件的保存目录，建议在WEB-INF路径下
    String uploadPath = this.getServletContext().getRealPath("/WEB-INF/upload");
    File uploadDir = new File(uploadPath);
    if (!uploadDir.exists()) {
        uploadDir.mkdir();
    }

    DiskFileItemFactory factory = new DiskFileItemFactory();
    ServletFileUpload upload = new ServletFileUpload(factory);
    // 设置文件最大上传大小
    upload.setFileSizeMax(MAX_FILE_SIZE);
    // 中文处理
    upload.setHeaderEncoding("UTF-8");
    // 设置最大请求值 (包含文件和表单数据)
    upload.setSizeMax(MAX_REQUEST_SIZE);

    try {
        // 解析请求的内容提取文件数据
        List<FileItem> formItems = upload.parseRequest(request);

        if (formItems != null && formItems.size() > 0) {
            // 迭代表单数据
            for (FileItem item : formItems) {
                // 处理不在表单中的字段
                if (!item.isFormField()) {
                    String fileName = new File(item.getName()).getName();
                    String filePath = uploadPath + File.separator + fileName;
                    File storeFile = new File(filePath);
                    // 在控制台输出文件的上传路径
                    System.out.println(filePath);
                    // 保存文件到硬盘
                    item.write(storeFile);
                    request.setAttribute("message","文件上传成功!");
                }
            }
        }
    } catch (Exception ex) {
        request.setAttribute("message","错误信息: " + ex.getMessage());
    }
    // 跳转到 message.jsp
    request.getServletContext().getRequestDispatcher("/message.jsp").forward(request, response);
}