@VisibleForTesting
  Subject getCurrentUser() {
    return SecurityUtils.getSubject();
  }
It first looks the shiro subject in AccessControlContext since JMX will use multiple threads to
process operations from the same client, then it looks into Shiro's thead context.

@return the shiro subject, null if security is not enabled
geode-core/src/main/java/org/apache/geode/internal/security/IntegratedSecurityService.java