Ambari 2.1 does not support JDK 1.6. If you are currently running JDK 1.6, you must upgrade to JDK 1.7 prior to upgrading Ambari and prior to performing this HDP 2.3 Stack upgrade. If you plan to upgrade your existing JDK to JDK 1.8, do so only after upgrading Ambari and after performing this HDP 2.3 Stack upgrade. Do not move to JDK 1.8 until your cluster is safely on HDP 2.3.