public void closeNewChannels() throws IOException {
        for (SocketChannel channel : newChannels) {
            channel.close();
        }
        newChannels.clear();
    }
Sets the output channel to which messages received on this server are echoed.
This is useful in tests where the clients sending the messages don't receive
the responses (eg. testing graceful close).
clients/src/test/java/org/apache/kafka/common/network/NioEchoServer.java