Message Handler Chain in Spring Boot - spring-integration

Question

Since Spring Boot recommends Java based configuration, I'm having trouble to convert the following xml based message handler chain config to Java based. Any help is appreciated.
<chain input-channel="incomingChannel" output-channel="completeChannel">
<splitter ref="itemSplitter" />
<transformer ref="transformer1" />
<transformer ref="transformer2" />
<aggregator ref="requestProcessor" />
<transformer ref="transformer3" />
<transformer ref="transformer4" />
I have tried to use IntegrationFlows to achieve the same as above.
#Bean
public IntegrationFlow incomingFlow(){
return IntegrationFlows.from(incomingChannel())
.split("itemSplitter","split")
.transform("transformer1")
.transform("transformer2")
.aggregate()//? need to figure out how to initialize this?
.transform("transformer3")
.transform("transformer4")
.channel(completeChannel())
.get();
}
But I got the following error
Failed to transform Message; nested exception is org.springframework.messaging.MessageHandlingException: Expression evaluation failed: locateItemEnrichmentTransformer; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1008E:(pos 0): Property or field 'transformer1' cannot be found on object of type 'org.springframework.messaging.support.GenericMessage' - maybe not public?
Hence I'm not sure if this is the equivalent way in Java code to translate the original chain xml config.
RequestProcessor (aggregator) implementation:
#Component
public class RequestProcessor {
/** The log. */
private static Log log = LogFactory.getLog(RequestProcessor.class);
#Aggregator
public Message<Requests> aggregate(#Headers Map<String, ?> headers, List<Request> requests) {
try {
return MessageBuilder.withPayload(new Requests(service.submit(requests, false, true)))
.copyHeaders(headers)
.build();
} catch (ClientException e) {
log.error(e.toString());
return null;
}
}
}

Answer 1

There is no obligation to convert the flow from XML to Java - you can use #ImportResource to pull in the XML.
It is certainly possible to wire up a MessageHandlerChain in java but as you have found, it's easier to use the Java DSL to replace a chain.
The
.transform("transformer1")
form of .transform() (1 String parameter) expects an expression, not a bean name.
You can use
.transform(transformer1())
Where transformer1() is your transformer #Bean.
EDIT
For the aggregator, if you are using Java 8...
.aggregate(a -> a.processor(requestProcessor()))
...for Java 7 or 6...
.aggregate(new Consumer<AggregatorSpec>() {
public void accept(AggregatorSpec a) {
a.processor(requestProcessor());
}
})