Question

We are using a java class to dowload a file from AWS s3 bucket with the following code

inputStream = AWSFileUtil.getInputStream(
            AWSConnectionUtil.getS3Object(null),
            "cdn.generalsentiment.com", filePath);

AWSFileUtil is a class which check the credentials and gets the inputstream from S3bucket using the getInputStream method.The filePath is the file inside cdn.generalsentiment.com bucket.

We want to write a method which can just check whether the particular file exists or not in the AWS S3 bucket and returns a boolean or some other value.

Please suggest me a solution for this.

public static boolean isValidFile(AmazonS3 s3,
        String bucketName,
        String path) throws AmazonClientException {
    try {
        ObjectMetadata objectMetadata =  
s3.getObjectMetadata("cdn.generalsentiment.com", path);
    } catch (NotFoundException nfe) {
        nfe.printStackTrace();
    }

    return true;
}

If the file exists it returns true, else it throws NotFoundException, which i want to catch and return the “isValidFile” method result as false.
Guys any other alternative for the method body or return type would be great.

The updated one

public static boolean doesFileExist(AmazonS3 s3,
        String bucketName,
        String path) throws AmazonClientException,
        AmazonServiceException {
    boolean isValidFile = true;
    try {
        ObjectMetadata objectMetadata = 
s3.getObjectMetadata("cdn.generalsentiment.com", path);

    } catch (NotFoundException nfe) {
        isValidFile = false;
    }
   catch (Exception exception) {
        exception.printStackTrace();
        isValidFile = false;
    }
    return isValidFile;
}

Answer 1

Daan’s answer using GET Bucket (List Objects) (via the respective wrapper from the AWS for Java, see below) is the most efficient approach to get the desired information for many objects at once (+1), you’ll need to post process the response accordingly of course.

This is done most easily via one of the respective methods of Class AmazonS3Client, e.g. listObjects(String bucketName):

AmazonS3 s3 = new AmazonS3Client(); // provide credentials, if need be
ObjectListing objectListing = s3.listObjects(new ListObjectsRequest()
        .withBucketName("cdn.generalsentiment.com");
for (S3ObjectSummary objectSummary : objectListing.getObjectSummaries()) {
    System.out.println(objectSummary.getKey());
}

Alternative

If you are only interested in a single object (file) at a time, using HEAD Object will be much more efficient, insofar you can deduce existence straight from the respective HTTP response code (see Error Responses for details), i.e. 404 Not Found for a response of NoSuchKey – The specified key does not exist.

Again, this is done most easily via Class AmazonS3Client, namely getObjectMetadata(String bucketName, String key), e.g.:

public static boolean isValidFile(AmazonS3 s3,
        String bucketName,
        String path) throws AmazonClientException, AmazonServiceException {
    boolean isValidFile = true;
    try {
        ObjectMetadata objectMetadata = s3.getObjectMetadata(bucketName, path);
    } catch (AmazonS3Exception s3e) {
        if (s3e.getStatusCode() == 404) {
        // i.e. 404: NoSuchKey - The specified key does not exist
            isValidFile = false;
        }
        else {
            throw s3e;    // rethrow all S3 exceptions other than 404   
        }
    }

    return isValidFile;
}