Polish Liveness and Readiness support

This commit moves the core Liveness and Readiness support to its own
`availability` package. We've made this a core concept independent of
Kubernetes.

Spring Boot now produces `LivenessStateChanged` and
`ReadinessStateChanged` events as part of the typical application
lifecycle.

Liveness and Readiness Probes (`HealthIndicator` components and health
groups) are still configured only when deployed on Kubernetes.

This commit also improves the documentation around Probes best practices
and container lifecycle considerations.

See gh-19593

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/availability/ApplicationAvailabilityAutoConfiguration.java

@@ -14,34 +14,25 @@
  * limitations under the License.
  */
 
-package org.springframework.boot.autoconfigure.kubernetes;
+package org.springframework.boot.autoconfigure.availability;
 
-import org.springframework.boot.autoconfigure.condition.ConditionalOnCloudPlatform;
-import org.springframework.boot.cloud.CloudPlatform;
-import org.springframework.boot.kubernetes.ApplicationStateProvider;
-import org.springframework.boot.kubernetes.SpringApplicationEventListener;
+import org.springframework.boot.availability.ApplicationAvailabilityProvider;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
 /**
  * {@link org.springframework.boot.autoconfigure.EnableAutoConfiguration} for
- * {@link ApplicationStateProvider}.
+ * {@link ApplicationAvailabilityProvider}.
  *
  * @author Brian Clozel
  * @since 2.3.0
  */
 @Configuration(proxyBeanMethods = false)
-@ConditionalOnCloudPlatform(CloudPlatform.KUBERNETES)
-public class ApplicationStateAutoConfiguration {
+public class ApplicationAvailabilityAutoConfiguration {
 
 	@Bean
-	public ApplicationStateProvider applicationStateProvider() {
-		return new ApplicationStateProvider();
-	}
-
-	@Bean
-	public SpringApplicationEventListener springApplicationEventListener() {
-		return new SpringApplicationEventListener();
+	public ApplicationAvailabilityProvider applicationAvailabilityProvider() {
+		return new ApplicationAvailabilityProvider();
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/availability/package-info.java

@@ -15,6 +15,6 @@
  */
 
 /**
- * Auto-configuration for Kubernetes application features.
+ * Auto-configuration for application availability features.
  */
-package org.springframework.boot.autoconfigure.kubernetes;
+package org.springframework.boot.autoconfigure.availability;

spring-boot-project/spring-boot-autoconfigure/src/main/resources/META-INF/spring.factories

@@ -89,7 +89,7 @@ org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
 org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
 org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
 org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
-org.springframework.boot.autoconfigure.kubernetes.ApplicationStateAutoConfiguration,\
+org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration,\
 org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
 org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
 org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/kubernetes/ApplicationAvailabilityAutoConfigurationTests.java

@@ -19,33 +19,25 @@ package org.springframework.boot.autoconfigure.kubernetes;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.boot.autoconfigure.AutoConfigurations;
-import org.springframework.boot.kubernetes.ApplicationStateProvider;
-import org.springframework.boot.kubernetes.SpringApplicationEventListener;
+import org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration;
+import org.springframework.boot.availability.ApplicationAvailabilityProvider;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
- * Tests for {@link ApplicationStateAutoConfiguration}
+ * Tests for {@link ApplicationAvailabilityAutoConfiguration}
  *
  * @author Brian Clozel
  */
-class ApplicationStateAutoConfigurationTests {
+class ApplicationAvailabilityAutoConfigurationTests {
 
 	private ApplicationContextRunner contextRunner = new ApplicationContextRunner()
-			.withConfiguration(AutoConfigurations.of(ApplicationStateAutoConfiguration.class));
+			.withConfiguration(AutoConfigurations.of(ApplicationAvailabilityAutoConfiguration.class));
 
 	@Test
-	void disabledWhenNotDeployedOnKubernetes() {
-		this.contextRunner.run(((context) -> assertThat(context).doesNotHaveBean(ApplicationStateProvider.class)
-				.doesNotHaveBean(SpringApplicationEventListener.class)));
-	}
-
-	@Test
-	void enabledWhenDeployedOnKubernetes() {
-		this.contextRunner.withPropertyValues("spring.main.cloud-platform:kubernetes")
-				.run(((context) -> assertThat(context).hasSingleBean(ApplicationStateProvider.class)
-						.hasSingleBean(SpringApplicationEventListener.class)));
+	void providerIsPresent() {
+		this.contextRunner.run(((context) -> assertThat(context).hasSingleBean(ApplicationAvailabilityProvider.class)));
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/websocket/servlet/WebSocketMessagingAutoConfigurationTests.java

@@ -92,7 +92,9 @@ class WebSocketMessagingAutoConfigurationTests {
 
 	@AfterEach
 	void tearDown() {
-		this.context.close();
+		if (this.context.isActive()) {
+			this.context.close();
+		}
 		this.sockJsClient.stop();
 	}