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

spring-integration-core/src/test/java/org/springframework/integration/context/IntegrationContextTests.java

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
 import org.springframework.integration.endpoint.AbstractEndpoint;
+import org.springframework.integration.endpoint.management.IntegrationKeepAlive;
 import org.springframework.integration.test.util.TestUtils;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
 import org.springframework.test.annotation.DirtiesContext;
@@ -52,6 +53,9 @@ public class IntegrationContextTests {
 	@Autowired
 	private ThreadPoolTaskScheduler taskScheduler;
 
+	@Autowired
+	private IntegrationKeepAlive integrationKeepAlive;
+
 	@Test
 	public void testIntegrationContextComponents() {
 		assertThat(this.integrationProperties.isMessagingTemplateThrowExceptionOnLateReply()).isTrue();
@@ -62,6 +66,7 @@ public class IntegrationContextTests {
 		assertThat(this.serviceActivator.isRunning()).isFalse();
 		assertThat(this.serviceActivatorExplicit.isAutoStartup()).isTrue();
 		assertThat(this.serviceActivatorExplicit.isRunning()).isTrue();
+		assertThat(this.integrationKeepAlive.isRunning()).isTrue();
 	}
 
 }

spring-integration-core/src/test/java/org/springframework/integration/endpoint/IntegrationKeepAliveTests.java

@@ -0,0 +1,135 @@
+/*
+ * Copyright 2024 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.endpoint;
+
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.context.IntegrationContextUtils;
+import org.springframework.integration.context.IntegrationProperties;
+import org.springframework.integration.endpoint.management.IntegrationKeepAlive;
+import org.springframework.integration.test.util.TestUtils;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.awaitility.Awaitility.await;
+import static org.mockito.Mockito.mock;
+
+/**
+ * @author Artem Bilan
+ *
+ * @since 6.4
+ */
+@SpringJUnitConfig
+@DirtiesContext
+public class IntegrationKeepAliveTests {
+
+	@Test
+	void keepAliveIsActive(@Autowired IntegrationKeepAlive integrationKeepAlive) {
+		assertThat(integrationKeepAlive.isRunning()).isTrue();
+		Thread keepAliveThread = TestUtils.getPropertyValue(integrationKeepAlive, "keepAliveThread", Thread.class);
+		assertThat(keepAliveThread.isAlive()).isTrue();
+		integrationKeepAlive.stop();
+		await().untilAsserted(() -> assertThat(keepAliveThread.isAlive()).isFalse());
+		integrationKeepAlive.start();
+	}
+
+	@Configuration
+	@EnableIntegration
+	public static class TestConfiguration {
+
+	}
+
+	@Nested
+	@ContextConfiguration(classes = WithPollingEndpoint.WithPollingEndpointConfig.class)
+	class WithPollingEndpoint {
+
+		@Test
+		void keepAliveNotActive(@Autowired IntegrationKeepAlive integrationKeepAlive) {
+			assertThat(integrationKeepAlive.isRunning()).isFalse();
+		}
+
+		@Configuration
+		static class WithPollingEndpointConfig {
+
+			@Bean
+			AbstractPollingEndpoint mockPollingEndpoint() {
+				return mock();
+			}
+
+		}
+
+	}
+
+	@Nested
+	@ContextConfiguration(classes = WithDaemonTaskScheduler.WithDaemonTaskSchedulerConfig.class)
+	class WithDaemonTaskScheduler {
+
+		@Test
+		void keepAliveActive(@Autowired IntegrationKeepAlive integrationKeepAlive) {
+			assertThat(integrationKeepAlive.isRunning()).isTrue();
+		}
+
+		@Configuration
+		static class WithDaemonTaskSchedulerConfig {
+
+			@Bean
+			AbstractPollingEndpoint mockPollingEndpoint() {
+				return mock();
+			}
+
+			@Bean
+			String daemonSetter(ThreadPoolTaskScheduler taskScheduler) {
+				taskScheduler.setDaemon(true);
+				return null;
+			}
+
+		}
+
+	}
+
+	@Nested
+	@ContextConfiguration(classes = WithGlobalProperty.WithGlobalPropertyConfig.class)
+	class WithGlobalProperty {
+
+		@Test
+		void keepAliveNotActive(@Autowired IntegrationKeepAlive integrationKeepAlive) {
+			assertThat(integrationKeepAlive.isRunning()).isFalse();
+		}
+
+		@Configuration
+		static class WithGlobalPropertyConfig {
+
+			@Bean(IntegrationContextUtils.INTEGRATION_GLOBAL_PROPERTIES_BEAN_NAME)
+			static IntegrationProperties integrationProperties() {
+				IntegrationProperties integrationProperties = new IntegrationProperties();
+				integrationProperties.setKeepAlive(false);
+				return integrationProperties;
+			}
+
+		}
+
+	}
+
+}