How to write a string to Amazon S3 bucket?

What is the best way to upload data without creating file?

If you meant without creating a file on S3, well, you can't really do that. On Amazon S3, the only way to store data is as files, or using more accurate terminology, objects. An object can contain from 1 byte zero bytes to 5 terabytes of data, and is stored in a bucket. Amazon's S3 homepage lays out the basic facts quite clearly. (For other data storing options on AWS, you might want to read e.g. about SimpleDB.)

If you meant without creating a local temporary file, then the answer depends on what library/tool you are using. (As RickMeasham suggested, please add more details!) With the s3cmd tool, for example, you can't skip creating temp file, while with the JetS3t Java library uploading a String directly would be easy:

// (First init s3Service and testBucket)
S3Object stringObject = new S3Object("HelloWorld.txt", "Hello World!");
s3Service.putObject(testBucket, stringObject);

Doesn't look as nice, but here is how you can do it using Amazons Java client, probably what JetS3t does behind the scenes anyway.

private boolean putArtistPage(AmazonS3 s3,String bucketName, String key, String webpage)
    {
        try
        {
            byte[]                  contentAsBytes = webpage.getBytes("UTF-8");
            ByteArrayInputStream    contentsAsStream      = new ByteArrayInputStream(contentAsBytes);
            ObjectMetadata          md = new ObjectMetadata();
            md.setContentLength(contentAsBytes.length);
            s3.putObject(new PutObjectRequest(bucketname, key, contentsAsStream, md));
            return true;
        }
        catch(AmazonServiceException e)
        {
            log.log(Level.SEVERE, e.getMessage(), e);
            return false;
        }
        catch(Exception ex)
        {
            log.log(Level.SEVERE, ex.getMessage(), ex);
            return false;
        }
    }

There is an overload for the AmazonS3.putObject method that accepts the bucket string, a key string, and a string of text content. I hadn't seen mention of it on stack overflow so I'm putting this here. It's going to be similar @Jonik's answer, but without the additional dependency.

AmazonS3 s3client = AmazonS3ClientBuilder.standard().withRegion(Regions.US_EAST_1).build();
s3client.putObject(bucket, key, contents);