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Kotlin DSL: handle(handler: (P, MessageHeaders) -> Any) endpoint should not require output channel if lambda returns void #3344

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ikarsokolov opened this issue Jul 16, 2020 · 5 comments · Fixed by #3346
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Kotlin DSL: handle(handler: (P, MessageHeaders) -> Any) endpoint should not require output channel if lambda returns void #3344

ikarsokolov opened this issue Jul 16, 2020 · 5 comments · Fixed by #3346
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@artembilan
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in: dsl type: enhancement
Milestone
5.4 M2

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@ikarsokolov
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ikarsokolov commented Jul 16, 2020 •
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Expected Behavior

handle<T> { payload, headers -> .. } should work without output channel configured if lambda returns void.
Like handle { message -> .. } currently does.

Current Behavior

integrationFlow(inputChannel) {
    wireTap {
        transform(Transformers.toJson(NODE))
        handle<ObjectNode> { payload, headers ->
            logger.debug(...)
        }
    }
    ...
}

throws DestinationResolutionException: no output-channel or replyChannel header available. While

integrationFlow(inputChannel) {
    wireTap {
        transform(Transformers.toJson(NODE))
        handle { message ->
            logger.debug(...)
        }
    }
    ...
}

works as intended.

Context
Now I am using this workaround, not sure if it is the best way to go:

handle<ObjectNode> { payload, headers ->
    logger.debug(...)
}
channel(nullChannel)
@ikarsokolov ikarsokolov added status: waiting-for-triage The issue need to be evaluated and its future decided type: enhancement labels Jul 16, 2020
@artembilan
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Well, since your lambda doesn't return anything (because of void logger.debug()), but that function still expect from us Any as a return, Kotlin just plays smart and build for us kotlin.Unit into the return statement which essentially becomes as a reply payload.

For now it looks like we need to treat that kotlin.Unit in our reflectional method invocation code as a void and do nothing ans we do with the void.

If there are better ideas how to make this Kotlin stuff working, I'm fully open for suggestion.

Thanks.

@artembilan
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For more Kotlin background: https://medium.com/thoughts-overflow/kotlin-has-nothing-but-there-is-nothing-like-nothing-in-java-cab98e4f4d26

@artembilan artembilan added this to the 5.4 M2 milestone Jul 17, 2020
@artembilan artembilan added backport 5.3.x and removed status: waiting-for-triage The issue need to be evaluated and its future decided labels Jul 17, 2020
@artembilan artembilan self-assigned this Jul 17, 2020
artembilan added a commit to artembilan/spring-integration that referenced this issue Jul 17, 2020
@artembilan
spring-projectsGH-3344: Treat kotlin.Unit return as null in MMIH
e69f4d3 
Fixes spring-projects#3344

When function lambda doesn't return anything (e.g. a `void` method call is the last one),
Kotlin produces a `kotlin.Unit` instance as a return value which is not null and produced
as a reply message payload.

* Fix `MessagingMethodInvokerHelper` to treat a `kotlin.Unit` as `null` for reply
making Kotlin lambdas working the same way as Java lambdas when we don't return anything
from from there

**Cherry-pick to `5.3.x`**
@artembilan artembilan mentioned this issue Jul 17, 2020
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@artembilan
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See PR on the matter: #3346

@ikarsokolov
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@artembilan I think you nailed it.

Another possible idea I have is to add to DSL handle(handler: (P, MessageHeaders) -> Unit) but I think this is not possible because of the type erasure and the two handle functions will be shadowed.

@artembilan
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True.
I think we can live with my fix for a while.
We still need to have Java interoperability.

garyrussell pushed a commit that referenced this issue Jul 20, 2020
@artembilan
GH-3344: Treat kotlin.Unit return as null in MMIH (#3346)
6a865b5 
* GH-3344: Treat kotlin.Unit return as null in MMIH

Fixes #3344

When function lambda doesn't return anything (e.g. a `void` method call is the last one),
Kotlin produces a `kotlin.Unit` instance as a return value which is not null and produced
as a reply message payload.

* Fix `MessagingMethodInvokerHelper` to treat a `kotlin.Unit` as `null` for reply
making Kotlin lambdas working the same way as Java lambdas when we don't return anything
from from there

**Cherry-pick to `5.3.x`**

* * Introduce `ClassUtils.isKotlinUnit(Class)` API;
use it in the `MessagingMethodInvokerHelper` instead of
`.getName().equals()`

* * Fix since on new `isKotlinUnit()` API
garyrussell pushed a commit that referenced this issue Jul 20, 2020
@artembilan @garyrussell
GH-3344: Treat kotlin.Unit return as null in MMIH (#3346)
dfa6c84 
* GH-3344: Treat kotlin.Unit return as null in MMIH

Fixes #3344

When function lambda doesn't return anything (e.g. a `void` method call is the last one),
Kotlin produces a `kotlin.Unit` instance as a return value which is not null and produced
as a reply message payload.

* Fix `MessagingMethodInvokerHelper` to treat a `kotlin.Unit` as `null` for reply
making Kotlin lambdas working the same way as Java lambdas when we don't return anything
from from there

**Cherry-pick to `5.3.x`**

* * Introduce `ClassUtils.isKotlinUnit(Class)` API;
use it in the `MessagingMethodInvokerHelper` instead of
`.getName().equals()`

* * Fix since on new `isKotlinUnit()` API
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GH-3344: Treat kotlin.Unit return as null in MMIH artembilan/spring-integration
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@ikarsokolov @artembilan