SnsTemplate provides functionality to check for topic existence

[hardikSinghBehl]

Type: Feature

Is your feature request related to a problem? Please describe.
When using SNS, I usually configure the SNS ARN using @ConfigurationProperties and pass it when using the SnsTemplate class. If an incorrect ARN is configured, the exception is thrown at runtime when the library tries to publish a message. To allow validation of SNS topic existence to be performed at application startup using ConstraintValidator, It would be great if a method boolean topicExists(String topicArn) is available in SnsTemplate class. This would be similar to the method boolean bucketExists(String bucketName) present in S3Template class.

Describe the solution you'd like
addition of method boolean topicExists(String topicArn) in SnsTemplate. This can be achieved using the read only action sns:ListTopics. Something like below

	public boolean topicExists(String topicArn) {
		Assert.notNull(topicArn, "topicArn is required");
		if (!isValidArnFormat(topicArn)) {
			throw SomeException();
		}
		final var snsTopics = getTopics();
		for (var snsTopic : snsTopics) {
			if (snsTopic.topicArn().equals(topicArn)) {
				return Boolean.TRUE;
			}
		}
		return Boolean.FALSE;
	}

	private List<Topic> getTopics() {
		final var snsTopics = new ArrayList<Topic>();
		ListTopicsResponse listTopicsResponse = snsClient.listTopics();
		snsTopics.addAll(listTopicsResponse.topics());

		while (listTopicsResponse.nextToken() != null) {
			var nextToken = listTopicsResponse.nextToken();
			listTopicsResponse = snsClient.listTopics(request -> request.nextToken(nextToken));
			snsTopics.addAll(listTopicsResponse.topics());
		}
		return snsTopics;
	}

Describe alternatives you've considered
Used the above solution in my application, but it'll be great if there was a concise way of doing this similar to S3Template

Additional context
Can submit PR if the feature is approved

[MatejNedic]

Hey @hardikSinghBehl ,

Interesting idea. I am up for it but I would suggest using getTopicAttribute call.

Looking forward to your PR! :)

[hardikSinghBehl]

sure @MatejNedic,

I've raised a PR and used the GetTopicAttributes call.