java断点续传前后端交互，后端用AmazonS3实现示例代码

以下是一个基于Amazon S3实现断点续传的示例代码，包括前端和后端的交互。

前端代码：

function uploadFile(file) {
  const CHUNK_SIZE = 1024 * 1024 * 5; // 5MB
  const bucketName = 'my-bucket';
  const s3 = new AWS.S3();
  const fileSize = file.size;
  const fileName = file.name;
  const uploadId = localStorage.getItem('uploadId') || null;
  const parts = [];
  let loaded = 0;

  if (!uploadId) {
    // 新建上传任务
    s3.createMultipartUpload({
      Bucket: bucketName,
      Key: fileName,
    }, (err, data) => {
      if (err) {
        console.error(err);
        return;
      }
      uploadId = data.UploadId;
      localStorage.setItem('uploadId', uploadId);
      uploadPart(1, 0);
    });
  } else {
    // 恢复上传任务
    s3.listParts({
      Bucket: bucketName,
      Key: fileName,
      UploadId: uploadId,
    }, (err, data) => {
      if (err) {
        console.error(err);
        return;
      }
      data.Parts.forEach((part) => {
        parts[part.PartNumber - 1] = {
          ETag: part.ETag,
          PartNumber: part.PartNumber,
        };
        loaded += part.Size;
      });
      uploadPart(parts.length + 1, loaded);
    });
  }

  function uploadPart(partNumber, start) {
    const end = Math.min(start + CHUNK_SIZE, fileSize);
    const chunk = file.slice(start, end);
    const params = {
      Body: chunk,
      Bucket: bucketName,
      Key: fileName,
      PartNumber: partNumber,
      UploadId: uploadId,
    };
    s3.uploadPart(params, (err, data) => {
      if (err) {
        console.error(err);
        return;
      }
      parts[partNumber - 1] = {
        ETag: data.ETag,
        PartNumber: partNumber,
      };
      loaded += chunk.size;
      if (end === fileSize) {
        completeUpload();
      } else {
        uploadPart(partNumber + 1, end);
      }
    });
  }

  function completeUpload() {
    s3.completeMultipartUpload({
      Bucket: bucketName,
      Key: fileName,
      UploadId: uploadId,
      MultipartUpload: {
        Parts: parts,
      },
    }, (err, data) => {
      if (err) {
        console.error(err);
        return;
      }
      localStorage.removeItem('uploadId');
      console.log(data);
    });
  }
}

后端代码：

@RestController
public class S3Controller {

    private final AmazonS3 s3Client;

    @Autowired
    public S3Controller(AmazonS3 s3Client) {
        this.s3Client = s3Client;
    }

    @PostMapping("/upload")
    public void upload(@RequestParam String fileName,
                       @RequestParam String uploadId,
                       @RequestParam(required = false) Integer partNumber,
                       @RequestParam(required = false) String eTag,
                       @RequestParam(required = false) MultipartFile file) throws IOException {
        if (file != null && !file.isEmpty()) {
            s3Client.uploadPart(new UploadPartRequest()
                    .withBucketName("my-bucket")
                    .withKey(fileName)
                    .withPartNumber(partNumber)
                    .withUploadId(uploadId)
                    .withInputStream(file.getInputStream())
                    .withPartSize(file.getSize()));
        } else {
            s3Client.completeMultipartUpload(new CompleteMultipartUploadRequest()
                    .withBucketName("my-bucket")
                    .withKey(fileName)
                    .withUploadId(uploadId)
                    .withPartETags(Collections.singletonList(new PartETag(partNumber, eTag))));
        }
    }

    @PostMapping("/create")
    public String create(@RequestParam String fileName) {
        InitiateMultipartUploadResult result = s3Client.initiateMultipartUpload(new InitiateMultipartUploadRequest("my-bucket", fileName));
        return result.getUploadId();
    }

    @GetMapping("/list")
    public List<MultipartUpload> list() {
        return s3Client.listMultipartUploads(new ListMultipartUploadsRequest("my-bucket")).getMultipartUploads();
    }
}

在这个示例代码中，我们使用AWS SDK for Java操作Amazon S3服务。前端代码负责将文件分成多个部分进行上传，后端代码负责接收并存储每个部分，并在最后将所有部分合并成一个完整的文件。通过使用AWS SDK提供的分块上传接口，我们可以轻松地实现断点续传功能。