SO-27405495.java

@Configuration
@EnableIntegration
public class IntegrationConfig { 

    ...

    @Bean
    public IntegrationFlow reqFlow() {

        return IntegrationFlows
            .from("request.ch")
            .enrichHeaders(e -> { 
                e.headerChannelsToString(); 
            })
            .enrichHeaders(e -> { 
                e.messageProcessor(m -> { 
                    Map<String, Object> map = new HashMap<String, Object>(1);
                    map.put(JmsHeaders.CORRELATION_ID, m.getHeaders().get(MessageHeaders.REPLY_CHANNEL));
                    return map;
                } );
            })              
            .routeToRecipients(r -> {
                r.ignoreSendFailures(false);
                r.recipient("jms.req.ch", "true");
                r.recipient("jms.agg.ch", "true");
            })
            .get();
    }

    @Bean
    public IntegrationFlow jmsReqFlow() {

        return IntegrationFlows
            .from("jms.req.ch")
            .enrichHeaders( e -> { e.headerChannelsToString(); })
            .handle(Jms.outboundAdapter(cachingConnectionFactory)
                    .destination("TEST_REQUEST_CH")).get();
    }

    @Bean
    public IntegrationFlow jmsPostReqFlow() {

        return IntegrationFlows
            .from("jms.req.ch")
            .handle("postSendService", "postSendProcess")
            .get();
    }

    @Bean
    public IntegrationFlow jmsResFlow() {

        return IntegrationFlows
            .from(Jms.inboundAdapter(cachingConnectionFactory).destination(
                    "TEST_RESPONSE_CH"),
                    c -> c.poller(Pollers.fixedRate(1000).maxMessagesPerPoll(10)))
            .channel("jms.agg.ch").get();
    }

    @Bean
    public IntegrationFlow jmsAggFlow() {

        return IntegrationFlows
            .from("jms.agg.ch")
            .aggregate(a -> { 
                a.outputProcessor(g -> {
                    List<Message<?>> l = new ArrayList<Message<?>>(g.getMessages());

                    Message<?> firstMessage = l.get(0);
                    Message<?> lastMessage = (l.size() > 1) ? l.get(l.size() - 1) : firstMessage;

                    Message<?> messageOut = MessageBuilder.fromMessage(lastMessage)
                            .setHeader(MessageHeaders.REPLY_CHANNEL, (String) firstMessage.getHeaders().getReplyChannel())
                            .build();

                     return messageOut;
                }); 
                a.correlationStrategy( m -> {
                    return m.getHeaders().get(JmsHeaders.CORRELATION_ID);
                });
                a.releaseStrategy(g -> {
                    return g.size() == 2;
                });
                a.groupTimeout(45000);
                a.sendPartialResultOnExpiry(false);
                a.discardChannel("jms.agg.timeout.ch");
            }, null)
            .channel("response.ch")
            .get();
        }
    }

    @Bean
    public IntegrationFlow jmsAggTimeoutFlow() {
        return IntegrationFlows
            .from("jms.agg.timeout.ch")
            .handle(Message.class, (m, n) -> { 
                MessageTimeoutException mExc = new MessageTimeoutException(m);
                Message<?> mOut = MessageBuilder.withPayload(mExc)
                        .copyHeaders(m.getHeaders())
                        .build();
                return mOut; 
            })
            .channel("error.ch")
            .get();
    }
}

https://gist.github.com/artembilan/9ffba1bb0b18842e27cf#file-so-27405495-java-L18: no reason to use here messageProcessor. The CORRELATION_ID can be populated as normal .header(). At the same time you can just use IntegrationMessageHeaderAccessor.CORRELATION_ID and don't pollute JMS header. Using standard IntegrationMessageHeaderAccessor.CORRELATION_ID there was no reason to have .correlationStrategy() on the aggregator.
https://gist.github.com/artembilan/9ffba1bb0b18842e27cf#file-so-27405495-java-L35: this is redundant.
From other side, if you don't use Gateway, there is no reason to .headerChannelsToString(), because there is no TemporaryReplyChannel.
https://gist.github.com/artembilan/9ffba1bb0b18842e27cf#file-so-27405495-java-L80: it can be just simple Lambda expression:a.releaseStrategy(g -> g.size() == 2);
https://gist.github.com/artembilan/9ffba1bb0b18842e27cf#file-so-27405495-java-L98: would be better if you use here ErrorMessage:

 .handle(Message.class, (m, h) -> new ErrorMessage(new MessageTimeoutException(m), h)) 

That's all.

Have a good time!