On a simple java desktop app, uploading an image to an S3-compliant API service is so simple. Each bucket are represented as folders, therefore no special approach is needed. Here is how it was done,

public AWSTestCase() {
	AwsClientBuilder.EndpointConfiguration endpoint = new AwsClientBuilder.EndpointConfiguration("http://localhost:8082/", "us-west-2");
	client = AmazonS3ClientBuilder.standard()
			.withPathStyleAccessEnabled(true)
			.withCredentials(new AWSStaticCredentialsProvider(new AnonymousAWSCredentials()))
			.withEndpointConfiguration(endpoint)
			.withClientConfiguration(new ClientConfiguration().withProtocol(Protocol.HTTP))
			.enablePathStyleAccess()
			.build();
}

@Test
public void testUploadToBucket() {
	client.putObject("bucket01", "jim.png", new File("d:\\jim.png"));
}

It will try to upload to below url,

http://localhost:8082/bucket01

But the same approach is not working on Android,

File file = new File(Environment.getExternalStorageDirectory(), attachmentModel.getLocalPath());

AWSCredentials credentials = new BasicAWSCredentials(ConstantCommon.ACCESS_KEY, ConstantCommon.SECRET_KEY);
ClientConfiguration clientConfig = new ClientConfiguration();
clientConfig.setProtocol(Protocol.HTTP);

AmazonS3Client client = new AmazonS3Client(credentials, clientConfig);
client.setEndpoint("http://localhost:8082/");
client.putObject("bucket01", attachmentModel.getRemotePath(), file);

It will gives error,

com.amazonaws.AmazonClientException: Unable to execute HTTP request: bucket01.localhost

Somehow app will try to upload to below url, because bucket name are treated as subdomain.

http://bucket01.localhost:8082/

Workaround is quite easy, setting bucket name as empty string and set bucket name as folder on endpoint should solve this issue.

public AWSTestCase() {
	client = new AmazonS3Client(new AnonymousAWSCredentials());
	client.setEndpoint("http://localhost:8082/bucket01");
}

@Test
public void testUploadToBucket() {
	client.putObject("", "jim.png", new File("d:\\jim.png"));
}

The content of my Gradle file,

    implementation 'com.amazonaws:aws-android-sdk-s3:2.9.2'
    implementation 'com.amazonaws:aws-android-sdk-core:2.16.5'