'Write-Only' over a TCP connection with Sp

Question

I am using int-ip:tcp-connection-factory and int-ip:tcp-outbound-gateway to communicate with an external server. The protocol for most of the services offered by the server follows the standard request-response style... which is working great. However, there are a few situations where I only need to send a request and no response is expected.

My question is, how do I configure my channels and connections so that I can specify whether or not to wait for a response? Currently I can't find a way and Spring always blocks after sending the request even if I am not expecting a response.

EDIT: As suggested, I have used a tcp-outbound-channel-adapter. My config file has only the followings:

<int:channel id="requestChannel" />

<int-ip:tcp-outbound-channel-adapter
    channel="requestChannel" connection-factory="client" />

<int-ip:tcp-connection-factory id="client"
    type="client" host="smtp.gmail.com" port="587" single-use="false"
    so-timeout="10000" />

And here's my Main class:

public final class Main {

    private static final Logger LOGGER = Logger.getLogger(Main.class);

    public static void main(final String... args) throws IOException {
        LOGGER.debug("entered main()...");

        final AbstractApplicationContext context = new ClassPathXmlApplicationContext(
                "classpath:*-context.xml");

        MessageChannel requestChannel = context.getBean("requestChannel", MessageChannel.class);
        requestChannel.send(MessageBuilder.withPayload("QUIT").build());

        LOGGER.debug("exiting main()...");
    }

}

Finally this is what I get in my log:

11:57:15.877 INFO  [main][com.together.email.Main] entered main()...
11:57:16.295 INFO  [main][org.springframework.integration.config.xml.DefaultConfiguringBeanFactoryPostProcessor] No bean named 'errorChannel' has been explicitly defined. Therefore, a default PublishSubscribeChannel will be created.
11:57:16.295 INFO  [main][org.springframework.integration.config.xml.DefaultConfiguringBeanFactoryPostProcessor] No bean named 'taskScheduler' has been explicitly defined. Therefore, a default ThreadPoolTaskScheduler will be created.
11:57:16.480 INFO  [main][org.springframework.integration.ip.tcp.connection.TcpNetClientConnectionFactory] started client
11:57:16.480 INFO  [main][org.springframework.integration.endpoint.EventDrivenConsumer] Adding {ip:tcp-outbound-channel-adapter} as a subscriber to the 'requestChannel' channel
11:57:16.480 INFO  [main][org.springframework.integration.channel.DirectChannel] Channel 'requestChannel' has 1 subscriber(s).
11:57:16.480 INFO  [main][org.springframework.integration.endpoint.EventDrivenConsumer] started org.springframework.integration.config.ConsumerEndpointFactoryBean#0
11:57:16.480 INFO  [main][org.springframework.integration.endpoint.EventDrivenConsumer] Adding {logging-channel-adapter:_org.springframework.integration.errorLogger} as a subscriber to the 'errorChannel' channel
11:57:16.481 INFO  [main][org.springframework.integration.channel.PublishSubscribeChannel] Channel 'errorChannel' has 1 subscriber(s).
11:57:16.481 INFO  [main][org.springframework.integration.endpoint.EventDrivenConsumer] started _org.springframework.integration.errorLogger
11:57:16.509 DEBUG [main][org.springframework.integration.channel.DirectChannel] preSend on channel 'requestChannel', message: [Payload=QUIT][Headers={timestamp=1381021036509, id=860ebe82-06c6-4393-95c7-0ece1a0a0e5d}]
11:57:16.509 DEBUG [main][org.springframework.integration.ip.tcp.TcpSendingMessageHandler] org.springframework.integration.ip.tcp.TcpSendingMessageHandler#0 received message: [Payload=QUIT][Headers={timestamp=1381021036509, id=860ebe82-06c6-4393-95c7-0ece1a0a0e5d}]
11:57:16.509 DEBUG [main][org.springframework.integration.ip.tcp.connection.TcpNetClientConnectionFactory] Opening new socket connection to smtp.gmail.com:587
11:57:16.745 DEBUG [main][org.springframework.integration.ip.tcp.connection.TcpNetConnection] New connection smtp.gmail.com:587:550c9b68-10a0-442d-b65d-d25d28df306b
11:57:16.748 DEBUG [main][org.springframework.integration.ip.tcp.TcpSendingMessageHandler] Got Connection smtp.gmail.com:587:550c9b68-10a0-442d-b65d-d25d28df306b
11:57:16.749 DEBUG [pool-1-thread-1][org.springframework.integration.ip.tcp.connection.TcpNetConnection] TcpListener exiting - no listener and not single use
11:57:16.750 DEBUG [main][org.springframework.integration.ip.tcp.connection.TcpNetConnection] Message sent [Payload=QUIT][Headers={timestamp=1381021036509, id=860ebe82-06c6-4393-95c7-0ece1a0a0e5d}]
11:57:16.750 DEBUG [main][org.springframework.integration.channel.DirectChannel] postSend (sent=true) on channel 'requestChannel', message: [Payload=QUIT][Headers={timestamp=1381021036509, id=860ebe82-06c6-4393-95c7-0ece1a0a0e5d}]
11:57:16.751 INFO  [main][com.together.email.Main] exiting main()...

I have set Spring's logging to debug level so that I can give you more details. As you can see from the last line of the log, my main exits. However, unfortunately the application doesn't terminate as [pool-1-thread-1] continues running. This thread comes to life as soon as send is invoked on requestChannel. Any idea what's going on here?

[In this example, I am sending an SMTP QUIT message as soon as the application connects to Google. In practice, I would actually not start with a QUIT. For example, at the beginning I may start with a HELO message. I have tried hooking in a tcp-inbound-channel-adapter to get the message response and that works great. The problem is with messages where I don't expect a reply.]

Answer 1

So, I suggest you to inject into the <int-ip:tcp-connection-factory> some 'fake' task-executor:

public class NullExecutor implements Executor {

    public void execute(Runnable command) {}
}

In this case your connection from AbstractClientConnectionFactory#obtainConnection() won't be configured (runned) to read from socket.

However System.exit(0); as last line of your main is enough. In this case all thread will be terminated, if they aren't daemons.