I'm pretty sure it is using slf4j for logging.

https://github.com/sailthru/sailthru-java-client/blob/a424d0a3bdd8cc37bfe36bbf57cedd99291be984/src/test/com/sailthru/client/SailthruUtilTest.java

https://github.com/sailthru/sailthru-java-client/blob/8e45c455e367435e03a4ec6c9babe6885a8efba3/src/main/com/sailthru/client/handler/response/JsonResponse.java

https://github.com/sailthru/sailthru-java-client/blob/2804402e32ae8405c0c47a79d0d422e2fd9d6b4c/src/main/com/sailthru/client/exceptions/ApiException.java