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-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/servlet/WebDriverScope.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.
@@ -18,6 +18,8 @@ package org.springframework.boot.test.autoconfigure.web.servlet;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.openqa.selenium.WebDriver;
 
@@ -52,11 +54,17 @@ public class WebDriverScope implements Scope {
 	private static final String[] BEAN_CLASSES = { WEB_DRIVER_CLASS,
 			"org.springframework.test.web.servlet.htmlunit.webdriver.MockMvcHtmlUnitDriverBuilder" };
 
+	/**
+	 * Guards access to {@link #instances}.
+	 */
+	private final Lock instancesLock = new ReentrantLock();
+
 	private final Map<String, Object> instances = new HashMap<>();
 
 	@Override
 	public Object get(String name, ObjectFactory<?> objectFactory) {
-		synchronized (this.instances) {
+		this.instancesLock.lock();
+		try {
 			Object instance = this.instances.get(name);
 			if (instance == null) {
 				instance = objectFactory.getObject();
@@ -64,13 +72,20 @@ public class WebDriverScope implements Scope {
 			}
 			return instance;
 		}
+		finally {
+			this.instancesLock.unlock();
+		}
 	}
 
 	@Override
 	public Object remove(String name) {
-		synchronized (this.instances) {
+		this.instancesLock.lock();
+		try {
 			return this.instances.remove(name);
 		}
+		finally {
+			this.instancesLock.unlock();
+		}
 	}
 
 	@Override
@@ -93,7 +108,8 @@ public class WebDriverScope implements Scope {
 	 */
 	boolean reset() {
 		boolean reset = false;
-		synchronized (this.instances) {
+		this.instancesLock.lock();
+		try {
 			for (Object instance : this.instances.values()) {
 				reset = true;
 				if (instance instanceof WebDriver webDriver) {
@@ -102,6 +118,9 @@ public class WebDriverScope implements Scope {
 			}
 			this.instances.clear();
 		}
+		finally {
+			this.instancesLock.unlock();
+		}
 		return reset;
 	}