Revise synchronized blocks

- Replace synchronized with Lock when guarding long-running operations
- Remove unnecessary synchronization in FileSystemWatcher
- Replace HashMap with ConcurrentHashMap in Restarter
- Remove unnecessary locking on AtomicBoolean in
  SpringApplicationBuilder
- Remove unnecessary locking in SimpleFormatter

Closes gh-36670

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/template/TemplateAvailabilityProviders.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2022 the original author or authors.
+ * Copyright 2012-2023 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.
@@ -22,6 +22,8 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.env.Environment;
@@ -52,7 +54,13 @@ public class TemplateAvailabilityProviders {
 	private final Map<String, TemplateAvailabilityProvider> resolved = new ConcurrentHashMap<>(CACHE_LIMIT);
 
 	/**
-	 * Map from view name resolve template view, synchronized when accessed.
+	 * Guards access to {@link #cache}.
+	 */
+	private final Lock cacheLock = new ReentrantLock();
+
+	/**
+	 * Map from view name resolve template view, protected by {@link #cacheLock} when
+	 * accessed.
 	 */
 	private final Map<String, TemplateAvailabilityProvider> cache = new LinkedHashMap<>(CACHE_LIMIT, 0.75f, true) {
 
@@ -133,12 +141,16 @@ public class TemplateAvailabilityProviders {
 		}
 		TemplateAvailabilityProvider provider = this.resolved.get(view);
 		if (provider == null) {
-			synchronized (this.cache) {
+			this.cacheLock.lock();
+			try {
 				provider = findProvider(view, environment, classLoader, resourceLoader);
 				provider = (provider != null) ? provider : NONE;
 				this.resolved.put(view, provider);
 				this.cache.put(view, provider);
 			}
+			finally {
+				this.cacheLock.unlock();
+			}
 		}
 		return (provider != NONE) ? provider : null;
 	}