Log ApplicationAvailability state changes

Fixes gh-23098

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

@@ -19,6 +19,10 @@ package org.springframework.boot.availability;
 import java.util.HashMap;
 import java.util.Map;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
+import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.ApplicationListener;
 import org.springframework.util.Assert;
 
@@ -34,6 +38,8 @@ import org.springframework.util.Assert;
 public class ApplicationAvailabilityBean
 		implements ApplicationAvailability, ApplicationListener<AvailabilityChangeEvent<?>> {
 
+	private static final Log logger = LogFactory.getLog(ApplicationAvailability.class);
+
 	private final Map<Class<? extends AvailabilityState>, AvailabilityChangeEvent<?>> events = new HashMap<>();
 
 	@Override
@@ -58,10 +64,31 @@ public class ApplicationAvailabilityBean
 
 	@Override
 	public void onApplicationEvent(AvailabilityChangeEvent<?> event) {
+		logStateChange(event);
 		Class<? extends AvailabilityState> stateType = getStateType(event.getState());
 		this.events.put(stateType, event);
 	}
 
+	private void logStateChange(AvailabilityChangeEvent<?> event) {
+		Class<? extends AvailabilityState> stateType = getStateType(event.getState());
+		StringBuilder message = new StringBuilder(
+				"Application availability state " + stateType.getSimpleName() + " changed");
+		AvailabilityChangeEvent<? extends AvailabilityState> lastChangeEvent = getLastChangeEvent(stateType);
+		if (lastChangeEvent != null) {
+			message.append(" from " + lastChangeEvent.getState());
+		}
+		message.append(" to " + event.getState());
+		if (event.getSource() != null) {
+			if (event.getSource() instanceof Throwable) {
+				message.append(": " + event.getSource());
+			}
+			else if (!(event.getSource() instanceof ApplicationEventPublisher)) {
+				message.append(": " + event.getSource().getClass().getName());
+			}
+		}
+		logger.info(message);
+	}
+
 	@SuppressWarnings("unchecked")
 	private Class<? extends AvailabilityState> getStateType(AvailabilityState state) {
 		if (state instanceof Enum) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/availability/ApplicationAvailabilityBeanTests.java

@@ -16,9 +16,14 @@
 
 package org.springframework.boot.availability;
 
+import java.io.IOException;
+
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
+import org.springframework.boot.testsupport.system.CapturedOutput;
+import org.springframework.boot.testsupport.system.OutputCaptureExtension;
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -29,6 +34,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * @author Brian Clozel
  * @author Phillip Webb
  */
+@ExtendWith(OutputCaptureExtension.class)
 class ApplicationAvailabilityBeanTests {
 
 	private AnnotationConfigApplicationContext context;
@@ -87,6 +93,28 @@ class ApplicationAvailabilityBeanTests {
 		assertThat(this.availability.getLastChangeEvent(TestState.class)).isNotNull();
 	}
 
+	@Test
+	void stateChangesAreLogged(CapturedOutput output) {
+		AvailabilityChangeEvent.publish(this.context, LivenessState.CORRECT);
+		assertThat(output).contains("Application availability state LivenessState changed to CORRECT\n");
+		AvailabilityChangeEvent.publish(this.context, LivenessState.BROKEN);
+		assertThat(output).contains("Application availability state LivenessState changed from CORRECT to BROKEN\n");
+	}
+
+	@Test
+	void stateChangesAreLoggedWithExceptionSource(CapturedOutput output) {
+		AvailabilityChangeEvent.publish(this.context, new IOException("connection error"), LivenessState.BROKEN);
+		assertThat(output).contains("Application availability state LivenessState changed to BROKEN: "
+				+ "java.io.IOException: connection error\n");
+	}
+
+	@Test
+	void stateChangesAreLoggedWithOtherSource(CapturedOutput output) {
+		AvailabilityChangeEvent.publish(this.context, new CustomEventSource(), LivenessState.BROKEN);
+		assertThat(output).contains("Application availability state LivenessState changed to BROKEN: "
+				+ CustomEventSource.class.getName() + "\n");
+	}
+
 	enum TestState implements AvailabilityState {
 
 		ONE {
@@ -107,4 +135,8 @@ class ApplicationAvailabilityBeanTests {
 
 	}
 
+	static class CustomEventSource {
+
+	}
+
 }