@Override
    public synchronized Transaction onClose(final Consumer<Transaction> consumer) {
        throw new UnsupportedOperationException("Threaded transactions are open when created and in manual mode");
    }
The nature of threaded transactions are such that they are always open when created and manual in nature,
therefore setting this value is not required.

@throws UnsupportedOperationException
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/AbstractThreadedTransaction.java