Answer by ObjectiveTruth for RequestFactory is null inside Confluence Custom Seraph Authenticator

I managed to find an alternate solution to communicate with an external service

So, a few things for those looking at this:

1. Seraph requires any java jar, not specifically a atlassian-plugin
2. You can use any method you wish to communicate with the outside world

Saying that, you can use the build in HttpUrlConnection in Java. Its super gross to use but works perfectly. I'd abandon the idea of using SAL inside of seraph if someone finds this thread

Here's an example to get started:

try
    {
      // create the HttpURLConnection
      url = new URL(desiredUrl);
      HttpURLConnection connection = (HttpURLConnection) url.openConnection();

      // just want to do an HTTP GET here
      connection.setRequestMethod("GET");

      // uncomment this if you want to write output to this url
      //connection.setDoOutput(true);

      // give it 15 seconds to respond
      connection.setReadTimeout(15*1000);
      connection.connect();

      // read the output from the server
      reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
      stringBuilder = new StringBuilder();

      String line = null;
      while ((line = reader.readLine()) != null)
      {
        stringBuilder.append(line + "\n");
      }
      return stringBuilder.toString();
    }
    catch (Exception e)
    {
      e.printStackTrace();
      throw e;
    }

Copy/pasta from

https://alvinalexander.com/blog/post/java/how-open-url-read-contents-httpurl-connection-java

But you can find many examples online on how to use this