java - How to write an S3 object to a file?

Question

What's the fastest way to write an S3 object (of which I have the key) to a file? I'm using Java.

Answer 1

Since Java 7 (published back in July 2011), there’s a better way: Files.copy() utility from java.util.nio.file.

Copies all bytes from an input stream to a file.

So you need neither an external library nor rolling your own byte array loops. Two examples below, both of which use the input stream from S3Object.getObjectContent().

InputStream in = s3Client.getObject("bucketName", "key").getObjectContent();

1) Write to a new file at specified path:

Files.copy(in, Paths.get("/my/path/file.jpg"));

2) Write to a temp file in system's default tmp location:

File tmp = File.createTempFile("s3test", "");
Files.copy(in, tmp.toPath(), StandardCopyOption.REPLACE_EXISTING);

(Without specifying the option to replace existing file, you'll get a FileAlreadyExistsException.)

Also note that getObjectContent() Javadocs urge you to close the input stream:

If you retrieve an S3Object, you should close this input stream as soon as possible, because the object contents aren't buffered in memory and stream directly from Amazon S3. Further, failure to close this stream can cause the request pool to become blocked.

So it should be safest to wrap everything in try-catch-finally, and do in.close(); in the finally block.

The above assumes that you use the official SDK from Amazon (aws-java-sdk-s3).