如何知道对象是否已完全上传到 amazon s3？

Question

我们如何检查对象是否已完全上传到 Amazon s3？我正在从 android 应用程序将图像上传到 s3，然后调用 lambda 函数。我想在调用 lambda 之前确保该对象已上传到 s3。目前大文件上传会产生问题。

Answer 1

可以通过PutObjectResult查看，响应表示对象已成功存储。 Amazon S3 从不存储部分对象：如果您收到成功的响应，那么您可以确信整个对象都已存储。所以主要的关键播放器是 MD5 算法，使用它您可以确认您要上传的对象是否与上传的相同，为了让您知道任何文档的 MD5 值，假设它是您可以使用的图像在下面找到所以

`    package aws.example.s3;

    import java.io.File;
    import java.io.FileInputStream;
    import java.io.IOException;
    import java.security.MessageDigest;
    import java.security.NoSuchAlgorithmException;

    public class MD5demo {
      public static String MD5check(String path){
        // TODO Auto-generated method stub
        //Create checksum for this file
        File file = new File(path);
    
        //Get the checksum
        String checksum="null";
        try {
            //Use MD5 algorithm
            MessageDigest md5Digest;
            try {
                md5Digest = MessageDigest.getInstance("MD5");
                checksum = getFileChecksum(md5Digest, file);
                //see checksum
                //System.out.println("Checksum: "+checksum);
            } catch (NoSuchAlgorithmException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
         }catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return checksum;
         
    }
    
    private static String getFileChecksum(MessageDigest digest, File file) 
     throws IOException
    {
        //Get file input stream for reading the file content
        FileInputStream fis = new FileInputStream(file);
         
        //Create byte array to read data in chunks
        byte[] byteArray = new byte[1024];
        int bytesCount = 0;
          
        //Read file data and update in message digest
        while ((bytesCount = fis.read(byteArray)) != -1) {
            digest.update(byteArray, 0, bytesCount);
        };
         
        //close the stream; We don't need it now.
        fis.close();
         
        //Get the hash's bytes
        byte[] bytes = digest.digest();
         
        //This bytes[] has bytes in decimal format;
        //Convert it to hexadecimal format
        StringBuilder sb = new StringBuilder();
        for(int i=0; i< bytes.length ;i++)
        {
            sb.append(Integer.toString((bytes[i] & 0xff) + 0x100, 
      16).substring(1));
        }
         
        //return complete hash
       return sb.toString();
      }
}`

所以，你只需要在 MD5check(String path) 中传递文件路径，它就会返回 MD5 值

现在，在您正在编写代码以上传任何文档的班级中，只需使用以下代码

PutObjectResult putObjectResult = null;
    try  {
           putObjectResult = s3.putObject(new PutObjectRequest(<Enter your Bucket name here>, <KeyName> , <pass your file object here>));
    
           String MD5=MD5check(<pls enter your file path here>);  //you can pass your file also directly, for that please change the method implementation of MD5Demo class accepting a file instead of path
           
           System.out.println("Etag: "+putObjectResult.getETag()); //getting the Etag which is nothing but same as MD5 value of the file that got uploaded in S3
           System.out.println("MD5:  "+MD5);
           
           if(MD5.equals(putObjectResult.getETag())) {
               System.out.println("Congrats!! Document Uploaded Successfully");
           }else {
               System.out.println("Sorry! Could not upload the document");
           }

希望对你有帮助，如果你有任何建议，请告诉我:)

Answer 2

无需在将对象上传到 Amazon S3 后调用 AWS Lambda 函数，只需配置 S3 存储桶以触发 lambda 本身：

_{（来源：amazon.com）}

这样，只有当对象成功上传到 S3 时才会触发该功能，对您的应用程序的工作量较少。

见：Using AWS Lambda with Amazon S3

如果你真的想单独做，那么你可以使用ETag（这是一个MD5校验和）来确认文件已按预期上传。