code: void registerSyncConsumer(MessageConsumer consumer) {
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              SibTr.entry(this, tc, "registerSyncConsumer", consumer);
  
          // ensure that movement between lists is atomic
          synchronized (consumerListsLock) {
              // remove the consumer from the async list
              asyncConsumers.remove(consumer);
              // add the consumer to the sync list if it's not
              // already present
              if (!syncConsumers.contains(consumer)) {
                  syncConsumers.add(consumer);
              }
          }
  
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              SibTr.exit(this, tc, "registerSyncConsumer");
      }
docs: Place the specified consumer in the synchronous list
  This will be called from the MessageConsumer.setMessageListener()
  method when the application specifies a null message listener.
  This method will tolerate multiple calls specifying the same consumer.
  
  @param consumer The MessageConsumer instance that asserts it is
  synchronous.