Document Kotlin DSL (#3210)

* Document Kotlin DSL

* Remove unused imports in `FunctionsTests.kt`

* * Fix typos in Docs

spring-integration-core/src/test/kotlin/org/springframework/integration/function/FunctionsTests.kt

@@ -35,11 +35,9 @@ import org.springframework.integration.annotation.Transformer
 import org.springframework.integration.channel.DirectChannel
 import org.springframework.integration.channel.QueueChannel
 import org.springframework.integration.config.EnableIntegration
-import org.springframework.integration.dsl.IntegrationFlows
 import org.springframework.integration.dsl.integrationFlow
 import org.springframework.integration.endpoint.SourcePollingChannelAdapter
 import org.springframework.integration.gateway.GatewayProxyFactoryBean
-import org.springframework.integration.test.util.OnlyOnceTrigger
 import org.springframework.messaging.Message
 import org.springframework.messaging.MessageChannel
 import org.springframework.messaging.PollableChannel

src/reference/asciidoc/dsl.adoc

@@ -3,6 +3,8 @@
 
 The Spring Integration Java configuration and DSL provides a set of convenient builders and a fluent API that lets you configure Spring Integration message flows from Spring `@Configuration` classes.
 
+(See also <<./kotlin-dsl.adoc#kotlin-dsl,Kotlin DSL>>.)
+
 The Java DSL for Spring Integration is essentially a facade for Spring Integration.
 The DSL provides a simple way to embed Spring Integration Message Flows into your application by using the fluent `Builder` pattern together with existing Java configuration from Spring Framework and Spring Integration.
 We also use and support lambdas (available with Java 8) to further simplify Java configuration.

src/reference/asciidoc/kotlin-dsl.adoc

@@ -0,0 +1,96 @@
+[[kotlin-dsl]]
+== Kotlin DSL
+
+The Kotlin DSL is a wrapper and extension to <<./dsl.adoc#java-dsl,Java DSL>> and aimed to make Spring Integration development on Kotlin as smooth and straightforward as possible with interoperability with the existing Java API and Kotlin language-specific structures.
+
+All you need to get started is just an import for `org.springframework.integration.dsl.integrationFlow` - an overloaded global function for Kotlin DSL.
+
+For `IntegrationFlow` definitions as lambdas we typically don't need anything else from Kotlin and just declare a bean like this:
+
+====
+[source, kotlin]
+----
+@Bean
+fun oddFlow() =
+IntegrationFlow { flow ->
+    flow.handle<Any> { _, _ -> "odd" }
+}
+----
+====
+
+In this case Kotlin understands that the lambda should be translated into `IntegrationFlow` anonymous instance and target Java DSL processor parses this construction properly into Java objects.
+
+As an alternative to the construction above and for consistency with use-cases explained below, a Kotlin-specif DSL should be used for declaring integration flows in the *builder* pattern style:
+
+====
+[source, kotlin]
+----
+@Bean
+fun flowLambda() =
+    integrationFlow {
+        filter<String> { it === "test" }
+        wireTap {
+                    handle { println(it.payload) }
+                }
+        transform<String, String> { it.toUpperCase() }
+    }
+----
+====
+
+Such a global `integrationFlow()` function expects a lambda in builder style for a `KotlinIntegrationFlowDefinition` (a Kotlin wrapper for the `IntegrationFlowDefinition`) and produces a regular `IntegrationFlow` lambda implementation.
+See more overloaded `integrationFlow()` variants below.
+
+Many other scenarios require an `IntegrationFlow` to be started from source of data (e.g. `JdbcPollingChannelAdapter`, `JmsInboundGateway` or just an existing `MessageChannel`).
+For this purpose, the Spring Integration Java DSL provides an `IntegrationFlows` factory with its large number of overloaded `from()` methods.
+This factory can be used in Kotlin as well:
+
+====
+[source, kotlin]
+----
+@Bean
+fun flowFromSupplier() =
+         IntegrationFlows.from<String>({ "bar" }) { e -> e.poller { p -> p.fixedDelay(10).maxMessagesPerPoll(1) } }
+                 .channel { c -> c.queue("fromSupplierQueue") }
+                 .get()
+----
+====
+
+But unfortunately not all `from()` methods are compatible with Kotlin structures.
+To fix the gap, this project provides a Kotlin DSL around an `IntegrationFlows` factory.
+It is implemented as a set of overloaded `integrationFlow()` functions.
+With a consumer for a `KotlinIntegrationFlowDefinition` to declare the rest of the flow as an `IntegrationFlow` lambda to reuse the mentioned above experience and also avoid `get()` call in the end.
+For example:
+
+====
+[source, kotlin]
+----
+@Bean
+fun functionFlow() =
+        integrationFlow<Function<String, String>>({ beanName("functionGateway") }) {
+            transform<String, String> { it.toUpperCase() }
+        }
+
+@Bean
+fun messageSourceFlow() =
+        integrationFlow(MessageProcessorMessageSource { "testSource" },
+                { poller { it.fixedDelay(10).maxMessagesPerPoll(1) } }) {
+            channel { queue("fromSupplierQueue") }
+        }
+----
+====
+
+In addition, Kotlin extensions are provided for the Java DSL API which needs some refinement for Kotlin structures.
+For example `IntegrationFlowDefinition<*>` requires a reifying for many methods with `Class<P>` argument:
+
+====
+[source, kotlin]
+----
+@Bean
+fun convertFlow() =
+    integrationFlow("convertFlowInput") {
+        convert<TestPojo>()
+    }
+----
+====
+
+

src/reference/asciidoc/whats-new.adoc

@@ -34,6 +34,12 @@ A new `IntegrationFlowExtension` API has been introduced to allow extension of t
 This also can be used to introduce customizers for any out-of-the-box `IntegrationComponentSpec` extensions.
 See <<./dsl.adoc#java-dsl-extensions,DSL Extensions>> for more information.
 
+[[x5.3-kotlin-dsl]]
+==== Kotlin DSL
+
+The Kotlin DSL for integration flow configurations has been introduced.
+See <<./kotlin-dsl.adoc#kotlin-dsl,Kotlin DSL Chapter>> for more information.
+
 [[x5.3-reactive-request-handler-advice]]
 ==== ReactiveRequestHandlerAdvice