Amazon Simple Storage Service (S3)

http://docs.aws.amazon.com/AmazonS3/latest/dev/ShareObjectPreSignedURL.html

All objects by default are private. Only the object owner has permission to access these objects. However, the object owner can optionally share objects with others by creating a pre-signed URL, using their own security credentials, to grant time-limited permission to download the objects.

When you create a pre-signed URL for your object, you must provide your security credentials, specify a bucket name, an object key, specify the HTTP method (GET to download the object) and expiration date and time. The pre-signed URLs are valid only for the specified duration.

Anyone who receives the pre-signed URL can then access the object. 

http://docs.aws.amazon.com/AmazonS3/latest/dev/PresignedUrlUploadObject.html

A pre-signed URL gives you access to the object identified in the URL, provided that the creator of the pre-signed URL has permissions to access that object. That is, if you receive a pre-signed URL to upload an object, you can upload the object only if the creator of the pre-signed URL has the necessary permissions to upload that object.

All objects and buckets by default are private. The pre-signed URLs are useful if you want your user/customer to be able upload a specific object to your bucket, but you don't require them to have AWS security credentials or permissions

https://github.com/kbastani/spring-boot-starter-amazon-s3

http://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/credentials.html

AmazonS3 s3Client = AmazonS3ClientBuilder.standard()
                       .withCredentials(new EnvironmentVariableCredentialsProvider())
                       .build();

http://stackoverflow.com/questions/41951978/amazons3clientcredentials-is-deprecated

You can either use AmazonS3ClientBuilder or AwsClientBuilder as alternatives.

For S3, simplest would be with AmazonS3ClientBuilder,

BasicAWSCredentials creds = new BasicAWSCredentials("access_key", "secret_key"); 
AmazonS3 s3Client = AmazonS3ClientBuilder.standard().withCredentials(new AWSStaticCredentialsProvider(creds)).build();

http://stackoverflow.com/questions/8303011/how-to-check-if-a-specified-key-exists-in-a-given-s3-bucket-using-java

There's now a doesObjectExist method in the official Java API.