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/src/main/java/org/springframework/boot/test/mock/web/SpringBootMockServletContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 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.
@@ -21,6 +21,8 @@ import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.nio.file.Files;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.core.io.FileSystemResourceLoader;
 import org.springframework.core.io.Resource;
@@ -44,6 +46,11 @@ public class SpringBootMockServletContext extends MockServletContext {
 
 	private File emptyRootDirectory;
 
+	/**
+	 * Guards access to {@link #emptyRootDirectory}.
+	 */
+	private final Lock emptyRootDirectoryLock = new ReentrantLock();
+
 	public SpringBootMockServletContext(String resourceBasePath) {
 		this(resourceBasePath, new FileSystemResourceLoader());
 	}
@@ -91,19 +98,21 @@ public class SpringBootMockServletContext extends MockServletContext {
 		if (resource == null && "/".equals(path)) {
 			// Liquibase assumes that "/" always exists, if we don't have a directory
 			// use a temporary location.
+			this.emptyRootDirectoryLock.lock();
 			try {
 				if (this.emptyRootDirectory == null) {
-					synchronized (this) {
-						File tempDirectory = Files.createTempDirectory("spr-servlet").toFile();
-						tempDirectory.deleteOnExit();
-						this.emptyRootDirectory = tempDirectory;
-					}
+					File tempDirectory = Files.createTempDirectory("spr-servlet").toFile();
+					tempDirectory.deleteOnExit();
+					this.emptyRootDirectory = tempDirectory;
 				}
 				return this.emptyRootDirectory.toURI().toURL();
 			}
 			catch (IOException ex) {
 				// Ignore
 			}
+			finally {
+				this.emptyRootDirectoryLock.unlock();
+			}
 		}
 		return resource;
 	}