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;
 	}

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.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.
@@ -158,9 +158,7 @@ public class FileSystemWatcher {
 	}
 
 	private void checkNotStarted() {
-		synchronized (this.monitor) {
-			Assert.state(this.watchThread == null, "FileSystemWatcher already started");
-		}
+		Assert.state(this.watchThread == null, "FileSystemWatcher already started");
 	}
 
 	/**

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/livereload/LiveReloadServer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 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.
@@ -29,6 +29,8 @@ import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -57,7 +59,12 @@ public class LiveReloadServer {
 
 	private final List<Connection> connections = new ArrayList<>();
 
-	private final Object monitor = new Object();
+	/**
+	 * Guards access to {@link #connections}.
+	 */
+	private final Lock connectionsLock = new ReentrantLock();
+
+	private final Lock lock = new ReentrantLock();
 
 	private final int port;
 
@@ -108,7 +115,8 @@ public class LiveReloadServer {
 	 * @throws IOException in case of I/O errors
 	 */
 	public int start() throws IOException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			Assert.state(!isStarted(), "Server already started");
 			logger.debug(LogMessage.format("Starting live reload server on port %s", this.port));
 			this.serverSocket = new ServerSocket(this.port);
@@ -119,6 +127,9 @@ public class LiveReloadServer {
 			this.listenThread.start();
 			return localPort;
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	/**
@@ -126,9 +137,13 @@ public class LiveReloadServer {
 	 * @return {@code true} if the server is running
 	 */
 	public boolean isStarted() {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			return this.listenThread != null;
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	/**
@@ -163,7 +178,8 @@ public class LiveReloadServer {
 	 * @throws IOException in case of I/O errors
 	 */
 	public void stop() throws IOException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.listenThread != null) {
 				closeAllConnections();
 				try {
@@ -184,22 +200,31 @@ public class LiveReloadServer {
 				this.serverSocket = null;
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	private void closeAllConnections() throws IOException {
-		synchronized (this.connections) {
+		this.connectionsLock.lock();
+		try {
 			for (Connection connection : this.connections) {
 				connection.close();
 			}
 		}
+		finally {
+			this.connectionsLock.unlock();
+		}
 	}
 
 	/**
 	 * Trigger livereload of all connected clients.
 	 */
 	public void triggerReload() {
-		synchronized (this.monitor) {
-			synchronized (this.connections) {
+		this.lock.lock();
+		try {
+			this.connectionsLock.lock();
+			try {
 				for (Connection connection : this.connections) {
 					try {
 						connection.triggerReload();
@@ -209,19 +234,33 @@ public class LiveReloadServer {
 					}
 				}
 			}
+			finally {
+				this.connectionsLock.unlock();
+			}
+		}
+		finally {
+			this.lock.unlock();
 		}
 	}
 
 	private void addConnection(Connection connection) {
-		synchronized (this.connections) {
+		this.connectionsLock.lock();
+		try {
 			this.connections.add(connection);
 		}
+		finally {
+			this.connectionsLock.unlock();
+		}
 	}
 
 	private void removeConnection(Connection connection) {
-		synchronized (this.connections) {
+		this.connectionsLock.lock();
+		try {
 			this.connections.remove(connection);
 		}
+		finally {
+			this.connectionsLock.unlock();
+		}
 	}
 
 	/**

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/Restarter.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.
@@ -22,7 +22,6 @@ import java.lang.reflect.Field;
 import java.net.URL;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.HashMap;
 import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
@@ -30,6 +29,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.BlockingDeque;
 import java.util.concurrent.Callable;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.LinkedBlockingDeque;
 import java.util.concurrent.ThreadFactory;
@@ -92,7 +92,7 @@ public class Restarter {
 
 	private final ClassLoaderFiles classLoaderFiles = new ClassLoaderFiles();
 
-	private final Map<String, Object> attributes = new HashMap<>();
+	private final Map<String, Object> attributes = new ConcurrentHashMap<>();
 
 	private final BlockingDeque<LeakSafeThread> leakSafeThreads = new LinkedBlockingDeque<>();
 
@@ -440,18 +440,11 @@ public class Restarter {
 	}
 
 	public Object getOrAddAttribute(String name, final ObjectFactory<?> objectFactory) {
-		synchronized (this.attributes) {
-			if (!this.attributes.containsKey(name)) {
-				this.attributes.put(name, objectFactory.getObject());
-			}
-			return this.attributes.get(name);
-		}
+		return this.attributes.computeIfAbsent(name, (ignore) -> objectFactory.getObject());
 	}
 
 	public Object removeAttribute(String name) {
-		synchronized (this.attributes) {
-			return this.attributes.remove(name);
-		}
+		return this.attributes.remove(name);
 	}
 
 	/**

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/client/TunnelClient.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 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.
@@ -25,6 +25,8 @@ import java.nio.channels.AsynchronousCloseException;
 import java.nio.channels.ServerSocketChannel;
 import java.nio.channels.SocketChannel;
 import java.nio.channels.WritableByteChannel;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -49,7 +51,7 @@ public class TunnelClient implements SmartInitializingSingleton {
 
 	private final TunnelClientListeners listeners = new TunnelClientListeners();
 
-	private final Object monitor = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private final int listenPort;
 
@@ -66,7 +68,8 @@ public class TunnelClient implements SmartInitializingSingleton {
 
 	@Override
 	public void afterSingletonsInstantiated() {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.serverThread == null) {
 				try {
 					start();
@@ -76,6 +79,9 @@ public class TunnelClient implements SmartInitializingSingleton {
 				}
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	/**
@@ -84,7 +90,8 @@ public class TunnelClient implements SmartInitializingSingleton {
 	 * @throws IOException in case of I/O errors
 	 */
 	public int start() throws IOException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			Assert.state(this.serverThread == null, "Server already started");
 			ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
 			serverSocketChannel.socket().bind(new InetSocketAddress(this.listenPort));
@@ -94,6 +101,9 @@ public class TunnelClient implements SmartInitializingSingleton {
 			this.serverThread.start();
 			return port;
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	/**
@@ -101,7 +111,8 @@ public class TunnelClient implements SmartInitializingSingleton {
 	 * @throws IOException in case of I/O errors
 	 */
 	public void stop() throws IOException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.serverThread != null) {
 				this.serverThread.close();
 				try {
@@ -113,12 +124,19 @@ public class TunnelClient implements SmartInitializingSingleton {
 				this.serverThread = null;
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	protected final ServerThread getServerThread() {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			return this.serverThread;
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	public void addListener(TunnelClientListener listener) {

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/payload/HttpTunnelPayloadForwarder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 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.
@@ -20,6 +20,8 @@ import java.io.IOException;
 import java.nio.channels.WritableByteChannel;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.util.Assert;
 
@@ -36,7 +38,7 @@ public class HttpTunnelPayloadForwarder {
 
 	private final Map<Long, HttpTunnelPayload> queue = new HashMap<>();
 
-	private final Object monitor = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private final WritableByteChannel targetChannel;
 
@@ -52,7 +54,8 @@ public class HttpTunnelPayloadForwarder {
 	}
 
 	public void forward(HttpTunnelPayload payload) throws IOException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			long seq = payload.getSequence();
 			if (this.lastRequestSeq != seq - 1) {
 				Assert.state(this.queue.size() < MAXIMUM_QUEUE_SIZE, "Too many messages queued");
@@ -67,6 +70,9 @@ public class HttpTunnelPayloadForwarder {
 				forward(queuedItem);
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 }

spring-boot-project/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/HttpTunnelServer.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.
@@ -25,6 +25,9 @@ import java.util.Deque;
 import java.util.Iterator;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -122,7 +125,12 @@ public class HttpTunnelServer {
 
 	private long disconnectTimeout = DEFAULT_DISCONNECT_TIMEOUT;
 
-	private volatile ServerThread serverThread;
+	/**
+	 * Guards access to {@link #serverThread}.
+	 */
+	private final Lock serverThreadLock = new ReentrantLock();
+
+	private ServerThread serverThread;
 
 	/**
 	 * Creates a new {@link HttpTunnelServer} instance.
@@ -164,7 +172,8 @@ public class HttpTunnelServer {
 	 * @throws IOException in case of I/O errors
 	 */
 	protected ServerThread getServerThread() throws IOException {
-		synchronized (this) {
+		this.serverThreadLock.lock();
+		try {
 			if (this.serverThread == null) {
 				ByteChannel channel = this.serverConnection.open(this.longPollTimeout);
 				this.serverThread = new ServerThread(channel);
@@ -172,15 +181,22 @@ public class HttpTunnelServer {
 			}
 			return this.serverThread;
 		}
+		finally {
+			this.serverThreadLock.unlock();
+		}
 	}
 
 	/**
 	 * Called when the server thread exits.
 	 */
 	void clearServerThread() {
-		synchronized (this) {
+		this.serverThreadLock.lock();
+		try {
 			this.serverThread = null;
 		}
+		finally {
+			this.serverThreadLock.unlock();
+		}
 	}
 
 	/**
@@ -210,6 +226,13 @@ public class HttpTunnelServer {
 
 		private final Deque<HttpConnection> httpConnections;
 
+		/**
+		 * Guards access to {@link #httpConnections}.
+		 */
+		private final Lock httpConnectionsLock = new ReentrantLock();
+
+		private final Condition httpConnectionsCondition = this.httpConnectionsLock.newCondition();
+
 		private final HttpTunnelPayloadForwarder payloadForwarder;
 
 		private boolean closed;
@@ -247,7 +270,8 @@ public class HttpTunnelServer {
 			while (this.targetServer.isOpen()) {
 				closeStaleHttpConnections();
 				ByteBuffer data = HttpTunnelPayload.getPayloadData(this.targetServer);
-				synchronized (this.httpConnections) {
+				this.httpConnectionsLock.lock();
+				try {
 					if (data != null) {
 						HttpTunnelPayload payload = new HttpTunnelPayload(this.responseSeq.incrementAndGet(), data);
 						payload.logIncoming();
@@ -255,15 +279,20 @@ public class HttpTunnelServer {
 						connection.respond(payload);
 					}
 				}
+				finally {
+					this.httpConnectionsLock.unlock();
+				}
 			}
 		}
 
 		private HttpConnection getOrWaitForHttpConnection() {
-			synchronized (this.httpConnections) {
+			this.httpConnectionsLock.lock();
+			try {
 				HttpConnection httpConnection = this.httpConnections.pollFirst();
 				while (httpConnection == null) {
 					try {
-						this.httpConnections.wait(HttpTunnelServer.this.longPollTimeout);
+						this.httpConnectionsCondition.await(HttpTunnelServer.this.longPollTimeout,
+								TimeUnit.MILLISECONDS);
 					}
 					catch (InterruptedException ex) {
 						Thread.currentThread().interrupt();
@@ -273,10 +302,14 @@ public class HttpTunnelServer {
 				}
 				return httpConnection;
 			}
+			finally {
+				this.httpConnectionsLock.unlock();
+			}
 		}
 
 		private void closeStaleHttpConnections() throws IOException {
-			synchronized (this.httpConnections) {
+			this.httpConnectionsLock.lock();
+			try {
 				checkNotDisconnected();
 				Iterator<HttpConnection> iterator = this.httpConnections.iterator();
 				while (iterator.hasNext()) {
@@ -287,6 +320,9 @@ public class HttpTunnelServer {
 					}
 				}
 			}
+			finally {
+				this.httpConnectionsLock.unlock();
+			}
 		}
 
 		private void checkNotDisconnected() {
@@ -298,7 +334,8 @@ public class HttpTunnelServer {
 		}
 
 		private void closeHttpConnections() {
-			synchronized (this.httpConnections) {
+			this.httpConnectionsLock.lock();
+			try {
 				while (!this.httpConnections.isEmpty()) {
 					try {
 						this.httpConnections.removeFirst().respond(HttpStatus.GONE);
@@ -308,6 +345,9 @@ public class HttpTunnelServer {
 					}
 				}
 			}
+			finally {
+				this.httpConnectionsLock.unlock();
+			}
 		}
 
 		private void closeTargetServer() {
@@ -328,13 +368,17 @@ public class HttpTunnelServer {
 			if (this.closed) {
 				httpConnection.respond(HttpStatus.GONE);
 			}
-			synchronized (this.httpConnections) {
+			this.httpConnectionsLock.lock();
+			try {
 				while (this.httpConnections.size() > 1) {
 					this.httpConnections.removeFirst().respond(HttpStatus.TOO_MANY_REQUESTS);
 				}
 				this.lastHttpRequestTime = System.currentTimeMillis();
 				this.httpConnections.addLast(httpConnection);
-				this.httpConnections.notify();
+				this.httpConnectionsCondition.signal();
+			}
+			finally {
+				this.httpConnectionsLock.unlock();
 			}
 			forwardToTargetServer(httpConnection);
 		}
@@ -368,6 +412,10 @@ public class HttpTunnelServer {
 
 		private volatile boolean complete = false;
 
+		private final Lock lock = new ReentrantLock();
+
+		private final Condition lockCondition = this.lock.newCondition();
+
 		public HttpConnection(ServerHttpRequest request, ServerHttpResponse response) {
 			this.createTime = System.currentTimeMillis();
 			this.request = request;
@@ -426,8 +474,12 @@ public class HttpTunnelServer {
 			if (this.async == null) {
 				while (!this.complete) {
 					try {
-						synchronized (this) {
-							wait(1000);
+						this.lock.lock();
+						try {
+							this.lockCondition.await(1, TimeUnit.SECONDS);
+						}
+						finally {
+							this.lock.unlock();
 						}
 					}
 					catch (InterruptedException ex) {
@@ -476,9 +528,13 @@ public class HttpTunnelServer {
 				this.async.complete();
 			}
 			else {
-				synchronized (this) {
+				this.lock.lock();
+				try {
 					this.complete = true;
-					notifyAll();
+					this.lockCondition.signalAll();
+				}
+				finally {
+					this.lock.unlock();
 				}
 			}
 		}

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;
 	}
 

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;
 	}

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/data/RandomAccessDataFile.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.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 /**
  * {@link RandomAccessData} implementation backed by a {@link RandomAccessFile}.
@@ -209,7 +211,7 @@ public class RandomAccessDataFile implements RandomAccessData {
 
 	private static final class FileAccess {
 
-		private final Object monitor = new Object();
+		private final Lock lock = new ReentrantLock();
 
 		private final File file;
 
@@ -221,11 +223,15 @@ public class RandomAccessDataFile implements RandomAccessData {
 		}
 
 		private int read(byte[] bytes, long position, int offset, int length) throws IOException {
-			synchronized (this.monitor) {
+			this.lock.lock();
+			try {
 				openIfNecessary();
 				this.randomAccessFile.seek(position);
 				return this.randomAccessFile.read(bytes, offset, length);
 			}
+			finally {
+				this.lock.unlock();
+			}
 		}
 
 		private void openIfNecessary() {
@@ -241,20 +247,28 @@ public class RandomAccessDataFile implements RandomAccessData {
 		}
 
 		private void close() throws IOException {
-			synchronized (this.monitor) {
+			this.lock.lock();
+			try {
 				if (this.randomAccessFile != null) {
 					this.randomAccessFile.close();
 					this.randomAccessFile = null;
 				}
 			}
+			finally {
+				this.lock.unlock();
+			}
 		}
 
 		private int readByte(long position) throws IOException {
-			synchronized (this.monitor) {
+			this.lock.lock();
+			try {
 				openIfNecessary();
 				this.randomAccessFile.seek(position);
 				return this.randomAccessFile.read();
 			}
+			finally {
+				this.lock.unlock();
+			}
 		}
 
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/builder/SpringApplicationBuilder.java

@@ -76,7 +76,7 @@ public class SpringApplicationBuilder {
 
 	private final SpringApplication application;
 
-	private ConfigurableApplicationContext context;
+	private volatile ConfigurableApplicationContext context;
 
 	private SpringApplicationBuilder parent;
 
@@ -145,10 +145,8 @@ public class SpringApplicationBuilder {
 		}
 		configureAsChildIfNecessary(args);
 		if (this.running.compareAndSet(false, true)) {
-			synchronized (this.running) {
-				// If not already running copy the sources over and then run.
-				this.context = build().run(args);
-			}
+			// If not already running copy the sources over and then run.
+			this.context = build().run(args);
 		}
 		return this.context;
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/ConfigTreePropertySource.java

@@ -29,6 +29,8 @@ import java.util.EnumSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Stream;
 
 import org.springframework.boot.convert.ApplicationConversionService;
@@ -258,6 +260,11 @@ public class ConfigTreePropertySource extends EnumerablePropertySource<Path> imp
 
 		private final Path path;
 
+		/**
+		 * Guards access to {@link #resource}.
+		 */
+		private final Lock resourceLock = new ReentrantLock();
+
 		private final Resource resource;
 
 		private final Origin origin;
@@ -341,11 +348,15 @@ public class ConfigTreePropertySource extends EnumerablePropertySource<Path> imp
 				}
 				if (this.content == null) {
 					assertStillExists();
-					synchronized (this.resource) {
+					this.resourceLock.lock();
+					try {
 						if (this.content == null) {
 							this.content = FileCopyUtils.copyToByteArray(this.resource.getInputStream());
 						}
 					}
+					finally {
+						this.resourceLock.unlock();
+					}
 				}
 				return this.content;
 			}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/java/SimpleFormatter.java

@@ -38,17 +38,15 @@ public class SimpleFormatter extends Formatter {
 
 	private final String pid = getOrUseDefault(LoggingSystemProperty.PID.getEnvironmentVariableName(), "????");
 
-	private final Date date = new Date();
-
 	@Override
-	public synchronized String format(LogRecord record) {
-		this.date.setTime(record.getMillis());
+	public String format(LogRecord record) {
+		Date date = new Date(record.getMillis());
 		String source = record.getLoggerName();
 		String message = formatMessage(record);
 		String throwable = getThrowable(record);
 		String thread = getThreadName();
-		return String.format(this.format, this.date, source, record.getLoggerName(),
-				record.getLevel().getLocalizedName(), message, throwable, thread, this.pid);
+		return String.format(this.format, date, source, record.getLoggerName(), record.getLevel().getLocalizedName(),
+				message, throwable, thread, this.pid);
 	}
 
 	private String getThrowable(LogRecord record) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/security/reactive/ApplicationContextServerWebExchangeMatcher.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.
@@ -16,6 +16,8 @@
 
 package org.springframework.boot.security.reactive;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Supplier;
 
 import reactor.core.publisher.Mono;
@@ -44,7 +46,7 @@ public abstract class ApplicationContextServerWebExchangeMatcher<C> implements S
 
 	private volatile Supplier<C> context;
 
-	private final Object contextLock = new Object();
+	private final Lock contextLock = new ReentrantLock();
 
 	public ApplicationContextServerWebExchangeMatcher(Class<? extends C> contextClass) {
 		Assert.notNull(contextClass, "Context class must not be null");
@@ -81,13 +83,17 @@ public abstract class ApplicationContextServerWebExchangeMatcher<C> implements S
 
 	protected Supplier<C> getContext(ServerWebExchange exchange) {
 		if (this.context == null) {
-			synchronized (this.contextLock) {
+			this.contextLock.lock();
+			try {
 				if (this.context == null) {
 					Supplier<C> createdContext = createContext(exchange);
 					initialized(createdContext);
 					this.context = createdContext;
 				}
 			}
+			finally {
+				this.contextLock.unlock();
+			}
 		}
 		return this.context;
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/security/servlet/ApplicationContextRequestMatcher.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.security.servlet;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Supplier;
 
 import jakarta.servlet.http.HttpServletRequest;
@@ -45,7 +47,7 @@ public abstract class ApplicationContextRequestMatcher<C> implements RequestMatc
 
 	private volatile boolean initialized;
 
-	private final Object initializeLock = new Object();
+	private final Lock initializeLock = new ReentrantLock();
 
 	public ApplicationContextRequestMatcher(Class<? extends C> contextClass) {
 		Assert.notNull(contextClass, "Context class must not be null");
@@ -61,12 +63,16 @@ public abstract class ApplicationContextRequestMatcher<C> implements RequestMatc
 		}
 		Supplier<C> context = () -> getContext(webApplicationContext);
 		if (!this.initialized) {
-			synchronized (this.initializeLock) {
+			this.initializeLock.lock();
+			try {
 				if (!this.initialized) {
 					initialized(context);
 					this.initialized = true;
 				}
 			}
+			finally {
+				this.initializeLock.unlock();
+			}
 		}
 		return matches(request, context);
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/system/ApplicationTemp.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.
@@ -28,6 +28,8 @@ import java.nio.file.attribute.PosixFilePermissions;
 import java.security.MessageDigest;
 import java.util.EnumSet;
 import java.util.HexFormat;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -51,6 +53,11 @@ public class ApplicationTemp {
 
 	private volatile Path path;
 
+	/**
+	 * Guards access to {@link #path}.
+	 */
+	private final Lock pathLock = new ReentrantLock();
+
 	/**
 	 * Create a new {@link ApplicationTemp} instance.
 	 */
@@ -90,10 +97,14 @@ public class ApplicationTemp {
 
 	private Path getPath() {
 		if (this.path == null) {
-			synchronized (this) {
+			this.pathLock.lock();
+			try {
 				String hash = HexFormat.of().withUpperCase().formatHex(generateHash(this.sourceClass));
 				this.path = createDirectory(getTempDirectory().resolve(hash));
 			}
+			finally {
+				this.pathLock.unlock();
+			}
 		}
 		return this.path;
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyWebServer.java

@@ -20,6 +20,8 @@ import java.io.IOException;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 
 import org.apache.commons.logging.Log;
@@ -57,7 +59,7 @@ public class JettyWebServer implements WebServer {
 
 	private static final Log logger = LogFactory.getLog(JettyWebServer.class);
 
-	private final Object monitor = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private final Server server;
 
@@ -113,21 +115,23 @@ public class JettyWebServer implements WebServer {
 	}
 
 	private void initialize() {
-		synchronized (this.monitor) {
-			try {
-				// Cache the connectors and then remove them to prevent requests being
-				// handled before the application context is ready.
-				this.connectors = this.server.getConnectors();
-				JettyWebServer.this.server.setConnectors(null);
-				// Start the server so that the ServletContext is available
-				this.server.start();
-				this.server.setStopAtShutdown(false);
-			}
-			catch (Throwable ex) {
-				// Ensure process isn't left running
-				stopSilently();
-				throw new WebServerException("Unable to start embedded Jetty web server", ex);
-			}
+		this.lock.lock();
+		try {
+			// Cache the connectors and then remove them to prevent requests being
+			// handled before the application context is ready.
+			this.connectors = this.server.getConnectors();
+			JettyWebServer.this.server.setConnectors(null);
+			// Start the server so that the ServletContext is available
+			this.server.start();
+			this.server.setStopAtShutdown(false);
+		}
+		catch (Throwable ex) {
+			// Ensure process isn't left running
+			stopSilently();
+			throw new WebServerException("Unable to start embedded Jetty web server", ex);
+		}
+		finally {
+			this.lock.unlock();
 		}
 	}
 
@@ -142,7 +146,8 @@ public class JettyWebServer implements WebServer {
 
 	@Override
 	public void start() throws WebServerException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.started) {
 				return;
 			}
@@ -179,6 +184,9 @@ public class JettyWebServer implements WebServer {
 				throw new WebServerException("Unable to start embedded Jetty server", ex);
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	String getStartedLogMessage() {
@@ -241,7 +249,8 @@ public class JettyWebServer implements WebServer {
 
 	@Override
 	public void stop() {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			this.started = false;
 			if (this.gracefulShutdown != null) {
 				this.gracefulShutdown.abort();
@@ -258,17 +267,22 @@ public class JettyWebServer implements WebServer {
 				throw new WebServerException("Unable to stop embedded Jetty server", ex);
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	@Override
 	public void destroy() {
-		synchronized (this.monitor) {
-			try {
-				this.server.stop();
-			}
-			catch (Exception ex) {
-				throw new WebServerException("Unable to destroy embedded Jetty server", ex);
-			}
+		this.lock.lock();
+		try {
+			this.server.stop();
+		}
+		catch (Exception ex) {
+			throw new WebServerException("Unable to destroy embedded Jetty server", ex);
+		}
+		finally {
+			this.lock.unlock();
 		}
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatWebServer.java

@@ -20,6 +20,8 @@ import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 
 import javax.naming.NamingException;
@@ -60,7 +62,7 @@ public class TomcatWebServer implements WebServer {
 
 	private static final AtomicInteger containerCounter = new AtomicInteger(-1);
 
-	private final Object monitor = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private final Map<Service, Connector[]> serviceConnectors = new HashMap<>();
 
@@ -106,41 +108,43 @@ public class TomcatWebServer implements WebServer {
 
 	private void initialize() throws WebServerException {
 		logger.info("Tomcat initialized with " + getPortsDescription(false));
-		synchronized (this.monitor) {
-			try {
-				addInstanceIdToEngineName();
-
-				Context context = findContext();
-				context.addLifecycleListener((event) -> {
-					if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {
-						// Remove service connectors so that protocol binding doesn't
-						// happen when the service is started.
-						removeServiceConnectors();
-					}
-				});
-
-				// Start the server to trigger initialization listeners
-				this.tomcat.start();
+		this.lock.lock();
+		try {
+			addInstanceIdToEngineName();
+
+			Context context = findContext();
+			context.addLifecycleListener((event) -> {
+				if (context.equals(event.getSource()) && Lifecycle.START_EVENT.equals(event.getType())) {
+					// Remove service connectors so that protocol binding doesn't
+					// happen when the service is started.
+					removeServiceConnectors();
+				}
+			});
 
-				// We can re-throw failure exception directly in the main thread
-				rethrowDeferredStartupExceptions();
+			// Start the server to trigger initialization listeners
+			this.tomcat.start();
 
-				try {
-					ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
-				}
-				catch (NamingException ex) {
-					// Naming is not enabled. Continue
-				}
+			// We can re-throw failure exception directly in the main thread
+			rethrowDeferredStartupExceptions();
 
-				// Unlike Jetty, all Tomcat threads are daemon threads. We create a
-				// blocking non-daemon to stop immediate shutdown
-				startDaemonAwaitThread();
+			try {
+				ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
 			}
-			catch (Exception ex) {
-				stopSilently();
-				destroySilently();
-				throw new WebServerException("Unable to start embedded Tomcat", ex);
+			catch (NamingException ex) {
+				// Naming is not enabled. Continue
 			}
+
+			// Unlike Jetty, all Tomcat threads are daemon threads. We create a
+			// blocking non-daemon to stop immediate shutdown
+			startDaemonAwaitThread();
+		}
+		catch (Exception ex) {
+			stopSilently();
+			destroySilently();
+			throw new WebServerException("Unable to start embedded Tomcat", ex);
+		}
+		finally {
+			this.lock.unlock();
 		}
 	}
 
@@ -205,7 +209,8 @@ public class TomcatWebServer implements WebServer {
 
 	@Override
 	public void start() throws WebServerException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.started) {
 				return;
 			}
@@ -233,6 +238,9 @@ public class TomcatWebServer implements WebServer {
 				ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	String getStartedLogMessage() {
@@ -324,7 +332,8 @@ public class TomcatWebServer implements WebServer {
 
 	@Override
 	public void stop() throws WebServerException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			boolean wasStarted = this.started;
 			try {
 				this.started = false;
@@ -342,6 +351,9 @@ public class TomcatWebServer implements WebServer {
 				}
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	@Override

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/undertow/UndertowWebServer.java

@@ -25,6 +25,8 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import io.undertow.Undertow;
 import io.undertow.server.HttpHandler;
@@ -63,7 +65,7 @@ public class UndertowWebServer implements WebServer {
 
 	private final AtomicReference<GracefulShutdownCallback> gracefulShutdownCallback = new AtomicReference<>();
 
-	private final Object monitor = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private final Undertow.Builder builder;
 
@@ -104,7 +106,8 @@ public class UndertowWebServer implements WebServer {
 
 	@Override
 	public void start() throws WebServerException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (this.started) {
 				return;
 			}
@@ -136,6 +139,9 @@ public class UndertowWebServer implements WebServer {
 				}
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	private void destroySilently() {
@@ -268,7 +274,8 @@ public class UndertowWebServer implements WebServer {
 
 	@Override
 	public void stop() throws WebServerException {
-		synchronized (this.monitor) {
+		this.lock.lock();
+		try {
 			if (!this.started) {
 				return;
 			}
@@ -286,6 +293,9 @@ public class UndertowWebServer implements WebServer {
 				throw new WebServerException("Unable to stop Undertow", ex);
 			}
 		}
+		finally {
+			this.lock.unlock();
+		}
 	}
 
 	@Override