GH-9455: Introduce IntegrationKeepAlive (#9493)

Fixes: #9455
Issue link: https://github.com/spring-projects/spring-integration/issues/9455

* Add an `IntegrationKeepAlive` infrastructure bean to initiate a long-lived non-daemon thread
to keep application alive when it cannot be kept like that for various reason, but has to.
* Expose `spring.integration.keepAlive` global property to disable an `IntegrationKeepAlive` auto-startup
* Test and document the feature

src/reference/antora/modules/ROOT/nav.adoc

@@ -99,6 +99,7 @@
 ** xref:shutdown.adoc[]
 ** xref:graph.adoc[]
 ** xref:integration-graph-controller.adoc[]
+** xref:keep-alive.adoc[]
 * xref:reactive-streams.adoc[]
 * xref:native-aot.adoc[]
 * xref:endpoint-summary.adoc[]

src/reference/antora/modules/ROOT/pages/configuration/global-properties.adoc

@@ -19,6 +19,7 @@ spring.integration.endpoints.noAutoStartup= <7>
 spring.integration.channels.error.requireSubscribers=true <8>
 spring.integration.channels.error.ignoreFailures=true <9>
 spring.integration.endpoints.defaultTimeout=30000 <10>
+spring.integration.keepAlive=true <11>
 ----
 
 <1> When true, `input-channel` instances are automatically declared as `DirectChannel` instances when not explicitly found in the application context.
@@ -57,6 +58,11 @@ Since version 5.5.
 Default value is 30 seconds to avoid indefinite blocking.
 Can be configured to a negative value to restore infinite blocking behavior in endpoints.
 Since version 6.2.
+
+<11> Whether to start the `IntegrationKeepAlive`.
+Default is `true`, however depends on the beans in the application context.
+See xref:keep-alive.adoc[Keep Alive] for more information.
+Since version 6.4.
 ====
 
 These properties can be overridden by adding a `/META-INF/spring.integration.properties` file to the classpath or an `IntegrationContextUtils.INTEGRATION_GLOBAL_PROPERTIES_BEAN_NAME` bean for the `org.springframework.integration.context.IntegrationProperties` instance.
@@ -77,5 +83,6 @@ spring.integration.channels.maxBroadcastSubscribers=0x7fffffff
 spring.integration.readOnly.headers=
 spring.integration.messagingTemplate.throwExceptionOnLateReply=true
 spring.integration.endpoints.defaultTimeout=30000
+spring.integration.keepAlive=false
 ----
 

src/reference/antora/modules/ROOT/pages/keep-alive.adoc

@@ -0,0 +1,17 @@
+[[keep-alive]]
+= Integration Keep Alive
+
+Starting with version 6.4, Spring Integration provides an `IntegrationKeepAlive` infrastructure bean.
+It manages an `spring-integration-keep-alive` non-daemon forever thread which keeps an application running.
+In some use-cases, e.g. `WebSocketInboundChannelAdapter` based on the `StandardWebSocketClient` does not use non-daemon thread for session, therefore an application may exit prematurely.
+Or an application logic may have only service activators or outbound channel adapters which rely on some other interaction, but not loops from executors like the one from Web server.
+Or all the threads in the application are virtual.
+
+The `IntegrationKeepAlive` is started automatically only if `TaskScheduler` is configured for non-daemon threads and there is no `AbstractPollingEndpoint` beans in the application context.
+In case of the `TaskScheduler` bean configured for daemon or virtual threads, the `IntegrationKeepAlive` is started regardless of the presence for `AbstractPollingEndpoint` beans.
+
+This component can be disabled by the `spring.integration.keepAlive` global property.
+See xref:configuration/global-properties.adoc[Global Properties] for more information.
+The `IntegrationKeepAlive` can be injected in some service and stopped manually if there is no need to keep an application alive or such a status is managed somewhere else.
+
+See also Spring Boot https://docs.spring.io/spring-boot/reference/features/spring-application.html#features.spring-application.virtual-threads[Virtual Threads] documentation for a `spring.main.keep-alive` property.

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -27,6 +27,10 @@ The SpEL evaluation infrastructure now supports configuration for `IndexAccessor
 Also, an out-of-the-box `JsonIndexAccessor` is provided.
 See xref:spel.adoc[SpEL Support] for more information.
 
+The `IntegrationKeepAlive` component has been introduced.
+See xref:keep-alive.adoc[Integration Keep Alive] for more information.
+
+
 [[x6.4-general]]
 === General Changes