Jonathan Lalou's blog

If you’re working with the AWS SDK for Java and you’ve seen the following log message:

WARN --- AmazonS3Client : No content length specified for stream data. Stream contents will be buffered in memory and could result in out of memory errors.

…you’re not alone. This warning might seem harmless at first, but it can lead to serious issues, especially in production environments.

What’s Really Happening?

This message appears when you upload a stream to Amazon S3 without explicitly setting the content length in the request metadata.

When that happens, the SDK doesn’t know how much data it’s about to upload, so it buffers the entire stream into memory before sending it to S3. If the stream is large, this could lead to:

• Excessive memory usage
• Slow performance
• OutOfMemoryError crashes

✅ How to Fix It

Whenever you upload a stream, make sure you calculate and set the content length using ObjectMetadata.

Example with Byte Array:

byte[] bytes = ...; // your content
ByteArrayInputStream inputStream = new ByteArrayInputStream(bytes);

ObjectMetadata metadata = new ObjectMetadata();
metadata.setContentLength(bytes.length);

PutObjectRequest request = new PutObjectRequest(bucketName, key, inputStream, metadata);
s3Client.putObject(request);

Example with File:

File file = new File("somefile.txt");
FileInputStream fileStream = new FileInputStream(file);

ObjectMetadata metadata = new ObjectMetadata();
metadata.setContentLength(file.length());

PutObjectRequest request = new PutObjectRequest(bucketName, key, fileStream, metadata);
s3Client.putObject(request);

What If You Don’t Know the Length?

Sometimes, you can’t know the content length ahead of time (e.g., you’re piping data from another service). In that case:

• Write the stream to a ByteArrayOutputStream first (good for small data)
• Use the S3 Multipart Upload API to stream large files without specifying the total size

Conclusion

Always set the content length when uploading to S3 via streams. It’s a small change that prevents large-scale problems down the road.

By taking care of this up front, you make your service safer, more memory-efficient, and more scalable.

Got questions or dealing with tricky S3 upload scenarios? Drop them in the comments!