Prevent keep alive thread from blocking the AOT processing

Instead of creating the thread directly in the constructor, the thread
is now created when the context is refreshed and stopped when the
context is closed.

As AOT processing never refreshes the context, the thread is never
started and can't block the AOT processing task.

Closes spring-projectsgh-38531

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -32,6 +32,7 @@
 import java.util.Properties;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.Stream;
 
 import org.apache.commons.logging.Log;
@@ -67,7 +68,9 @@
 import org.springframework.context.annotation.ClassPathBeanDefinitionScanner;
 import org.springframework.context.annotation.ConfigurationClassPostProcessor;
 import org.springframework.context.aot.AotApplicationContextInitializer;
+import org.springframework.context.event.ApplicationContextEvent;
 import org.springframework.context.event.ContextClosedEvent;
+import org.springframework.context.event.ContextRefreshedEvent;
 import org.springframework.context.support.AbstractApplicationContext;
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.core.GenericTypeResolver;
@@ -417,9 +420,7 @@ private void prepareContext(DefaultBootstrapContext bootstrapContext, Configurab
 			context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
 		}
 		if (this.keepAlive) {
-			KeepAlive keepAlive = new KeepAlive();
-			keepAlive.start();
-			context.addApplicationListener(keepAlive);
+			context.addApplicationListener(new KeepAlive());
 		}
 		context.addBeanFactoryPostProcessor(new PropertySourceOrderingBeanFactoryPostProcessor(context));
 		if (!AotDetector.useGeneratedArtifacts()) {
@@ -1635,31 +1636,47 @@ public SpringApplicationRunListener getRunListener(SpringApplication springAppli
 	}
 
 	/**
-	 * A non-daemon thread to keep the JVM alive. Reacts to {@link ContextClosedEvent} to
-	 * stop itself when the application context is closed.
+	 * Starts a non-daemon thread to keep the JVM alive on {@link ContextRefreshedEvent}.
+	 * Stops the thread on {@link ContextClosedEvent}.
 	 */
-	private static final class KeepAlive extends Thread implements ApplicationListener<ContextClosedEvent> {
+	private static final class KeepAlive implements ApplicationListener<ApplicationContextEvent> {
 
-		KeepAlive() {
-			setName("keep-alive");
-			setDaemon(false);
-		}
+		private final AtomicReference<Thread> thread = new AtomicReference<>();
 
 		@Override
-		public void onApplicationEvent(ContextClosedEvent event) {
-			interrupt();
+		public void onApplicationEvent(ApplicationContextEvent event) {
+			if (event instanceof ContextRefreshedEvent) {
+				startKeepAliveThread();
+			}
+			else if (event instanceof ContextClosedEvent) {
+				stopKeepAliveThread();
+			}
 		}
 
-		@Override
-		public void run() {
-			while (true) {
-				try {
-					Thread.sleep(Long.MAX_VALUE);
-				}
-				catch (InterruptedException ex) {
-					break;
+		private void startKeepAliveThread() {
+			Thread thread = new Thread(() -> {
+				while (true) {
+					try {
+						Thread.sleep(Long.MAX_VALUE);
+					}
+					catch (InterruptedException ex) {
+						break;
+					}
 				}
+			});
+			if (this.thread.compareAndSet(null, thread)) {
+				thread.setDaemon(false);
+				thread.setName("keep-alive");
+				thread.start();
+			}
+		}
+
+		private void stopKeepAliveThread() {
+			Thread thread = this.thread.getAndSet(null);
+			if (thread == null) {
+				return;
 			}
+			thread.interrupt();
 		}
 
 	}