.aggregate
https://docs.spring.io/spring-integration/reference/html/dsl.html#java-dsl-aggregators
    It aggregates a sequence of individual messages into a single message and is necessarily more complex.
    By default, an aggregator returns a message that contains a collection of payloads from incoming messages.
        @Bean
        public IntegrationFlow splitAggregateFlow() {
            return IntegrationFlow.from("splitAggregateInput")
                    .split()
                    .channel(MessageChannels.executor(this.taskExecutor()))
                    .resequence()
                    .aggregate()
                    .get();
        }
    The split() method splits the list into individual messages and sends them to the ExecutorChannel.
    The resequence() method reorders messages by sequence details found in the message headers.
    The aggregate() method collects those messages.

    However, you can change the default behavior by specifying a release strategy and correlation strategy, among other things.
    Consider the following example:

        .aggregate(aggregatorSpec ->
                aggregatorSpec
                    .correlationStrategy(m -> m.getHeaders().get("myCorrelationKey"))
                    .releaseStrategy(g -> g.size() > 10)
                    .messageStore(messageStore()))

    The preceding example correlates messages that have myCorrelationKey headers and releases the messages
    once at least ten have been accumulated.

https://docs.spring.io/spring-integration/reference/html/message-routing.html#aggregator

https://docs.spring.io/spring-integration/reference/html/aggregator.html
https://docs.spring.io/spring-integration/reference/html/aggregator.html#aggregator-annotations

2014
https://spring.io/blog/2014/11/25/spring-integration-java-dsl-line-by-line-tutorial