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/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) {
+		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();
+				}
+			});
+
+			// Start the server to trigger initialization listeners
+			this.tomcat.start();
+
+			// We can re-throw failure exception directly in the main thread
+			rethrowDeferredStartupExceptions();
+
 			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();
-
-				// We can re-throw failure exception directly in the main thread
-				rethrowDeferredStartupExceptions();
-
-				try {
-					ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
-				}
-				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();
+				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