public static SaslServerAuthenticationProviders getInstance(Configuration conf) {
    SaslServerAuthenticationProviders providers = holder.get();
    if (null == providers) {
      synchronized (holder) {
        // Someone else beat us here
        providers = holder.get();
        if (null != providers) {
          return providers;
        }

        providers = createProviders(conf);
        holder.set(providers);
      }
    }
    return providers;
  }
Returns a singleton instance of {@link SaslServerAuthenticationProviders}.
hbase-server/src/main/java/org/apache/hadoop/hbase/security/provider/SaslServerAuthenticationProviders.java