Method to check if Prefix Exists ?

[helio...@gmail.com]

Hello everybody,

I am using the jets3t library v0.9.4 for some time now, with a fair amount of success.

I know how to deal with existence or not of objects, but I cannot find an efficient way to deal with how to check for the existence of *prefixes*.

This is what I have:

public static boolean prefixExists(S3Service service, String bucket, String prefix) {

    S3Object[] objects = new S3Object[0];
    try {
        objects = service.listObjects(bucket, prefix, null, 2);    // 2 is the maxListingLength. Could possibly be '1' instead.      
    } catch (S3ServiceException e) {
        // some error handling
    }
    return objects.length > 0;
}

I was hoping that calling this for prefix="results/2016-10-24" would yield true if and only if there are actual objects under it.
Which certainly works, except that my arbitrary limit ( 2 ) does not really get acknowledged.
In fact, i get returned all 3996 objects that lie under the given prefix. This is wasteful and slow, because some other "folders" would list tens of thousands of contained objects, while a single one would have sufficed.

Is there a reason why the limit doesn't work?
Or, better yet, is there a more efficient way to ascertain the existence of a prefix?

Thank you all for reading!

[helio...@gmail.com]

Replying to my own post, as I figured it out.
Turns out I was just being silly and using the wrong method. The correct way to do the more efficient checking is via the chunk listing, as follows:

public static boolean prefixExists(S3Service service, String bucket, String prefix) {

    StorageObject[] objects = new StorageObject[0];
    try {
        objects = service.listObjectsChunked(bucket, prefix, null, 1, null, false).getObjects();
    } catch (ServiceException e) {

        // some error handling
    }
    return objects.length > 0;
}

Hopes this ends up being of some use to future list members!

[James Murty]

Hi,

Thanks for following up with a solution to your question. The `listObjectsChunked` method is indeed the right one to use to have more control over the listing operation, and avoid JetS3t automatically performing multiple requests to list *all* the items in the bucket.

You could use the slightly simpler version of `listObjectsChunked` without the last `false`, but it's better to be explicit about what you are doing so what you have is good.

Cheers,

James

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