Springboot文件读写

@RestController
@RequestMapping(value = "/file")
public class FileController {
	/**
	 * 大文件读取（使用 StreamingResponseBody 实现流式下载）
	 */
	@GetMapping("/large-data")
	public StreamingResponseBody downloadLargeData(HttpServletResponse response) {
		// 设置响应头
		response.setContentType("text/csv");
		response.setHeader("Content-Disposition", "attachment; filename=\"large-data.csv\"");

		return new StreamingResponseBody() {
			@Override
			public void writeTo(OutputStream outputStream) throws IOException {
				// 模拟分页查询数据
				int pageSize = 10000; // 每页大小
				int page = 0;
				while (true) {
					// 从数据库或其他数据源中分页读取数据
					List<String> data = fetchDataFromDatabase(page, pageSize);
					if (data.isEmpty()) {
						break; // 数据读取完毕
					}

					// 将数据写入响应流
					for (String line : data) {
						outputStream.write((line + "\n").getBytes());
					}
					outputStream.flush();

					page++;
				}
			}
		};
	}

	// 模拟从数据库分页读取数据
	private List<String> fetchDataFromDatabase(int page, int pageSize) {
		// 这里可以替换为实际的数据库查询逻辑
		// 例如：SELECT * FROM large_table LIMIT pageSize OFFSET page * pageSize
		if (page >= 10) { // 假设总共有10页数据
			return List.of();
		}
		return List.of("Data line " + (page * pageSize + 1), "Data line " + (page * pageSize + 2));
	}

	/**
	 * 使用文件分块下载（文件已经存在）
	 *
	 * @param response
	 * @return
	 * @throws IOException
	 */
	@GetMapping("/large-file")
	public StreamingResponseBody downloadLargeFile(HttpServletResponse response) throws IOException {
		File file = new File("/path/to/large-file.zip");

		response.setContentType("application/octet-stream");
		response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");

		InputStream inputStream = new FileInputStream(file);
		return outputStream -> {
			int nRead;
			byte[] data = new byte[1024];
			while ((nRead = inputStream.read(data, 0, data.length)) != -1) {
				outputStream.write(data, 0, nRead);
			}
			inputStream.close();
		};
	}

	@GetMapping("/export1")
	public void export() {
		// 数据赋值
		List<ProductLabel> productLabelList = new ArrayList<>();
		ProductLabel productLabel1 = new ProductLabel();
		productLabel1.setLabelId("国补");
		productLabel1.setSequence(1);
		productLabelList.add(productLabel1);

		ProductLabel productLabel2 = new ProductLabel();
		productLabel2.setLabelId("高性价比");
		productLabel2.setSequence(2);
		productLabelList.add(productLabel2);

		ProductLabel productLabel3 = new ProductLabel();
		productLabel3.setLabelId("库存不多");
		productLabel3.setSequence(5);
		productLabelList.add(productLabel3);

		ProductInfo productInfo = new ProductInfo();
		productInfo.setProductId("1001");
		productInfo.setProductName("华为Mate 40");
		productInfo.setPrice(2999.00f);
		productInfo.setCreateTime(new Date());
		productInfo.setProductLabelList(productLabelList);

		// 创建ObjectMapper对象
		ObjectMapper objectMapper = new ObjectMapper();

		try {
			// 将实体对象转换为JSON字符串
			String jsonString = objectMapper.writeValueAsString(productInfo);
			System.out.println("JSON String: " + jsonString);

			// 将JSON字符串写入文件
			File file = new File("product.json"); // 不加路径，直接存储到项目更目录
			objectMapper.writeValue(file, productInfo); // 此处不能写入JSON字符串，否则文件会存储含转义符的字符数据

			System.out.println("JSON data written to file: " + file.getAbsolutePath());
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	/**
	 * 实体对象转换JSON格式导出
	 *
	 * @param request
	 * @param response
	 * @throws IOException
	 */
	@GetMapping("/export")
	public void export(HttpServletRequest request, HttpServletResponse response) throws IOException {
		// 数据赋值
		List<ProductLabel> productLabelList = new ArrayList<>();
		ProductLabel productLabel1 = new ProductLabel();
		productLabel1.setLabelId("国补");
		productLabel1.setSequence(1);
		productLabelList.add(productLabel1);

		ProductLabel productLabel2 = new ProductLabel();
		productLabel2.setLabelId("高性价比");
		productLabel2.setSequence(2);
		productLabelList.add(productLabel2);

		ProductLabel productLabel3 = new ProductLabel();
		productLabel3.setLabelId("库存不多");
		productLabel3.setSequence(5);
		productLabelList.add(productLabel3);

		ProductInfo productInfo = new ProductInfo();
		productInfo.setProductId("1001");
		productInfo.setProductName("华为Mate 40");
		productInfo.setPrice(2999.00f);
		productInfo.setCreateTime(new Date());
		productInfo.setProductLabelList(productLabelList);

		// 创建ObjectMapper对象
		ObjectMapper objectMapper = new ObjectMapper();
		// 下载文件
		// 设置响应头
		response.setContentType("application/octet-stream");
		response.setHeader("Content-Disposition", "attachment; filename=\"product.json\"");
		// 将实体对象写入响应流
		objectMapper.writeValue(response.getOutputStream(), productInfo);
	}

	/**
	 * 导入JSON文件再转换实体对象
	 *
	 * @param
	 * @return
	 * @date 2025/2/20 20:50
	 */
	@PostMapping("/upload")
	public String upload(@RequestParam("file") MultipartFile file) throws IOException {
		if (file.isEmpty()) {
			return "文件不能为空";
		}

		String jsonContent = new String(file.getBytes(), StandardCharsets.UTF_8);
		System.out.println("读取到的文件内容：" + jsonContent);
		ObjectMapper objectMapper = new ObjectMapper();
		ProductInfo productInfo = objectMapper.readValue(jsonContent, ProductInfo.class);
		String productId = productInfo.getProductId();
		List<ProductLabel> productLabelList = productInfo.getProductLabelList();
		System.out.println("取到了数据：" + productId);
		return "取到了数据";
	}

调用下载方法示例：