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GH-2890: Document a bridge for subflow starts

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Fixes https://github.com/spring-projects/spring-integration/issues/2890

Explain in a docs why and how a `bridge` appears in the flow when we
declare a subflow for mapping

* Upgrade to Kotlin `1.3.50`

* Upgrade to Kotlin `1.3.50`
* Polishing Doc according PR comments

Doc polishing
This commit is contained in:
Artem Bilan
2019-08-21 10:03:10 -04:00
committed by Gary Russell
parent 075086775d
commit 5b43b6c7dd
2 changed files with 19 additions and 1 deletions
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2
build.gradle
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@@ -1,5 +1,5 @@
buildscript {
ext.kotlinVersion = '1.3.40'
ext.kotlinVersion = '1.3.50'
repositories {
maven { url 'https://repo.spring.io/plugins-release' }
}

18
src/reference/asciidoc/dsl.adoc
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@@ -816,6 +816,24 @@ When you configure a sub-flow as a lambda, the framework handles the request-rep
Sub-flows can be nested to any depth, but we do not recommend doing so.
In fact, even in the router case, adding complex sub-flows within a flow would quickly begin to look like a plate of spaghetti and be difficult for a human to parse.
[NOTE]
====
In cases where the DSL supports a subflow configuration, when a channel is normally needed for the component being configured, and that subflow starts with a `channel()` element, the framework implicitly places a `bridge()` between the component output channel and the flow's input channel.
For example, in this `filter` definition:
[source,java]
----
.filter(p -> p instanceof String, e -> e
.discardFlow(df -> df
.channel(MessageChannels.queue())
...)
----
the Framework internally creates a `DirectChannel` bean for injecting into the `MessageFilter.discardChannel`.
Then it wraps the subflow into an `IntegrationFlow` starting with this implicit channel for the subscription and places a `bridge` before the `channel()` specified in the flow.
When an existing `IntegrationFlow` bean is used as a subflow reference (instead of an inline subflow, e.g. a lambda), there is no such bridge required because the framework can resolve the first channel from the flow bean.
With an inline subflow, the input channel is not yet available.
====
[[java-dsl-protocol-adapters]]
=== Using Protocol Adapters