Polish multiple root contexts in `Restarter`

See gh-7335
See gh-7336

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartApplicationListener.java

@@ -45,15 +45,14 @@ public class RestartApplicationListener
 			onApplicationStartingEvent((ApplicationStartingEvent) event);
 		}
 		if (event instanceof ApplicationPreparedEvent) {
-			Restarter.getInstance()
-					.prepare(((ApplicationPreparedEvent) event).getApplicationContext());
+			onApplicationPreparedEvent((ApplicationPreparedEvent) event);
 		}
 		if (event instanceof ApplicationReadyEvent
 				|| event instanceof ApplicationFailedEvent) {
 			Restarter.getInstance().finish();
-			if (event instanceof ApplicationFailedEvent) {
-				Restarter.getInstance().remove(((ApplicationFailedEvent) event).getApplicationContext());
 		}
+		if (event instanceof ApplicationFailedEvent) {
+			onApplicationFailedEvent((ApplicationFailedEvent) event);
 		}
 	}
 
@@ -72,6 +71,14 @@ public class RestartApplicationListener
 		}
 	}
 
+	private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
+		Restarter.getInstance().prepare(event.getApplicationContext());
+	}
+
+	private void onApplicationFailedEvent(ApplicationFailedEvent event) {
+		Restarter.getInstance().remove(event.getApplicationContext());
+	}
+
 	@Override
 	public int getOrder() {
 		return this.order;

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/Restarter.java

@@ -52,7 +52,6 @@ import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.util.Assert;
-import org.springframework.util.CollectionUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -316,11 +315,9 @@ public class Restarter {
 		this.logger.debug("Stopping application");
 		this.stopLock.lock();
 		try {
-			if (!CollectionUtils.isEmpty(this.rootContexts)) {
-				for (ConfigurableApplicationContext rootContext : this.rootContexts) {
-					rootContext.close();
-				}
-				this.rootContexts.clear();
+			for (ConfigurableApplicationContext context : this.rootContexts) {
+				context.close();
+				this.rootContexts.remove(context);
 			}
 			cleanupCaches();
 			if (this.forceReferenceCleanup) {