java如何实现文件的上传和下载功能 java文件传输的基础操作教程

multipart/form-data

HttpServletRequest

getPart()

request.getPart("fileInputName")

Part

Part

getInputStream()

FileOutputStream

// 示例：Servlet中的文件上传处理
@WebServlet("/upload")
@MultipartConfig // 标注这是一个multipart请求
public class FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String uploadPath = getServletContext().getRealPath("") + File.separator + "uploads";
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) {
            uploadDir.mkdir();
        }

        try {
            Part filePart = request.getPart("file"); // "file"是前端input标签的name属性值
            String fileName = filePart.getSubmittedFileName(); // 获取原始文件名
            if (fileName != null && !fileName.isEmpty()) {
                // 确保文件名安全，避免路径遍历攻击
                fileName = new File(fileName).getName(); 
                String filePath = uploadPath + File.separator + fileName;

                // 将文件写入服务器磁盘
                try (InputStream input = filePart.getInputStream();
                     FileOutputStream output = new FileOutputStream(filePath)) {
                    byte[] buffer = new byte[1024];
                    int bytesRead;
                    while ((bytesRead = input.read(buffer)) != -1) {
                        output.write(buffer, 0, bytesRead);
                    }
                }
                response.getWriter().println("文件上传成功: " + fileName);
            } else {
                response.getWriter().println("未选择文件或文件名为空。");
            }
        } catch (Exception e) {
            response.getWriter().println("文件上传失败: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

Content-Type

Content-Disposition

OutputStream

HttpServletResponse

// 示例：Servlet中的文件下载处理
@WebServlet("/download")
public class FileDownloadServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String fileName = request.getParameter("name"); // 假设通过参数传递文件名
        if (fileName == null || fileName.isEmpty()) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "文件名不能为空。");
            return;
        }

        String filePath = getServletContext().getRealPath("") + File.separator + "uploads" + File.separator + fileName;
        File downloadFile = new File(filePath);

        if (!downloadFile.exists() || !downloadFile.isFile()) {
            response.sendError(HttpServletResponse.SC_NOT_FOUND, "文件未找到或不是有效文件。");
            return;
        }

        // 设置响应头
        response.setContentType(getServletContext().getMimeType(fileName)); // 根据文件名获取MIME类型
        response.setHeader("Content-Disposition", "attachment; filename=\"" + new String(fileName.getBytes("UTF-8"), "ISO-8859-1") + "\"");
        response.setContentLength((int) downloadFile.length());

        // 将文件内容写入响应流
        try (FileInputStream fileInput = new FileInputStream(downloadFile);
             BufferedInputStream bufferedInput = new BufferedInputStream(fileInput);
             OutputStream output = response.getOutputStream()) {
            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = bufferedInput.read(buffer)) != -1) {
                output.write(buffer, 0, bytesRead);
            }
            output.flush();
        } catch (IOException e) {
            // 客户端可能中断下载，这里捕获并处理
            System.err.println("文件下载过程中发生IO错误: " + e.getMessage());
        }
    }
}

Part.getInputStream()

@RequestParam

MultipartFile

MultipartFile

Tomcat

Jetty

CommonsMultipartResolver

StandardServletMultipartResolver

multipart/form-data

// 示例：Spring Boot/Spring MVC 中的文件上传
@RestController
@RequestMapping("/api/files")
public class FileController {

    private final String UPLOAD_DIR = "uploads/"; // 假设文件上传到应用根目录下的uploads文件夹

    @PostMapping("/upload")
    public ResponseEntity<String> uploadFile(@RequestParam("file") MultipartFile file) {
        if (file.isEmpty()) {
            return ResponseEntity.badRequest().body("请选择一个文件上传。");
        }
        try {
            // 获取文件原始名称
            String originalFilename = file.getOriginalFilename();
            // 确保文件名安全，避免路径遍历
            String safeFilename = new File(originalFilename).getName(); 
            String filePath = UPLOAD_DIR + safeFilename;

            // 获取应用程序的实际路径（如果需要保存到相对路径）
            // 例如：ResourceUtils.getURL("classpath:").getPath() + UPLOAD_DIR + safeFilename;
            // 但更推荐直接配置一个绝对路径或者使用云存储

            // 将文件保存到服务器
            file.transferTo(new File(filePath)); 

            return ResponseEntity.ok("文件上传成功: " + originalFilename);
        } catch (IOException e) {
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("文件上传失败: " + e.getMessage());
        }
    }
}

ResponseEntity<Resource>

HttpServletResponse

ResponseEntity<Resource>

FileSystemResource

ByteArrayResource

// 示例：Spring Boot/Spring MVC 中的文件下载
@RestController
@RequestMapping("/api/files")
public class FileController {

    private final String UPLOAD_DIR = "uploads/";

    @GetMapping("/download")
    public ResponseEntity<Resource> downloadFile(@RequestParam("name") String fileName) {
        try {
            // 确保文件名安全
            String safeFilename = new File(fileName).getName();
            Path filePath = Paths.get(UPLOAD_DIR).resolve(safeFilename).normalize();
            Resource resource = new UrlResource(filePath.toUri());

            if (resource.exists() && resource.isReadable()) {
                String contentType = Files.probeContentType(filePath); // 尝试获取MIME类型
                if (contentType == null) {
                    contentType = "application/octet-stream"; // 默认MIME类型
                }

                return ResponseEntity.ok()
                        .contentType(MediaType.parseMediaType(contentType))
                        .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + resource.getFilename() + "\"")
                        .body(resource);
            } else {
                return ResponseEntity.notFound().build();
            }
        } catch (MalformedURLException e) {
            return ResponseEntity.badRequest().body(null);
        } catch (IOException e) {
            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(null);
        }
    }
}