Spring Integration中最小流的意外行为

Question

I'm creating a new flow and started from almost empty definition

@Bean
public IntegrationFlow routerFlow() {
    return IntegrationFlows
            .from(routerInputChannel())
            .channel(routerOutputChannel())
            .get();
}

So far so good. But if I add simple wireTap line

@Bean
public IntegrationFlow routerFlow() {
    return IntegrationFlows
            .from(routerInputChannel())
            .wireTap(m -> System.out.println(m))
            .channel(routerOutputChannel())
            .get();
}

I received exception

Caused by: org.springframework.beans.factory.BeanCreationException: The 'IntegrationFlow' can't consist of only one 'MessageChannel'. Add at lest '.bridge()' EIP-method before the end of flow.
    at org.springframework.integration.dsl.IntegrationFlowDefinition.get(IntegrationFlowDefinition.java:2670)
    at org.springframework.integration.dsl.IntegrationFlowBuilder.get(IntegrationFlowBuilder.java:26)
    at org.springframework.integration.dsl.IntegrationFlowDefinition.wireTap(IntegrationFlowDefinition.java:329)
    at org.springframework.integration.dsl.IntegrationFlowDefinition.wireTap(IntegrationFlowDefinition.java:263)
    at com.xxx.xxx.xxx.xxx.config.RootConfiguration.routerFlow(RootConfiguration.java:29)

What is the reason of such exception?

I use Spring Integration 4.3.7.RELEASE and Spring Integration Java DSL 1.1.4.RELEASE

Answer 1

Well, that is indeed unexpected.

The wireTap() has signature:

public B wireTap(IntegrationFlow flow) {

Where IntegrationFlow is functional interface and therefore can be specified by the Lambda. At the same time the argument of that Lambda can really be sent to the System.out.println() . If we convert your Lambda to the interface implementation we'll see:

.wireTap(new IntegrationFlow() {

        @Override
        public void configure(IntegrationFlowDefinition<?> x) {
            System.out.println(x);
        }

})

As you see there is really no anything about the message you would expect with your Lambda for .handle() .

And you should see something like this in your logs:

org.springframework.integration.dsl.IntegrationFlowBuilder@6995bf68

That is exactly SOUT of the System.out.println(IntegrationFlowDefinition) ...

So, to fix your problem we should have something like this:

.wireTap(flow -> flow.handle(System.out::println))

I don't know what we can do from the Framework perspective. I'm pretty sure any @FunctionalInterface -based construction suffers the same problem if we provide a Lambda with the Object.toString() signature. That is just some nasty side-effect of the syntax sugar IMO :-).