Add nullability annotations to module/spring-boot-devtools

See gh-46587

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/RemoteUrlPropertyExtractor.java

@@ -21,6 +21,8 @@ import java.net.URISyntaxException;
 import java.util.Collections;
 import java.util.Map;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.core.Ordered;
@@ -59,7 +61,7 @@ class RemoteUrlPropertyExtractor implements ApplicationListener<ApplicationEnvir
 		environment.getPropertySources().addLast(propertySource);
 	}
 
-	private String cleanRemoteUrl(String url) {
+	private @Nullable String cleanRemoteUrl(@Nullable String url) {
 		if (StringUtils.hasText(url) && url.endsWith("/")) {
 			return url.substring(0, url.length() - 1);
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/ConditionEvaluationDeltaLoggingListener.java

@@ -41,6 +41,7 @@ class ConditionEvaluationDeltaLoggingListener
 
 	private static final Log logger = LogFactory.getLog(ConditionEvaluationDeltaLoggingListener.class);
 
+	@SuppressWarnings("NullAway.Init")
 	private volatile ApplicationContext context;
 
 	@Override

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/DevToolsDataSourceAutoConfiguration.java

@@ -25,10 +25,12 @@ import java.util.Set;
 import javax.sql.DataSource;
 
 import org.apache.derby.jdbc.EmbeddedDriver;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.annotation.AnnotatedBeanDefinition;
 import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionMessage;
@@ -50,6 +52,7 @@ import org.springframework.context.annotation.Import;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.orm.jpa.AbstractEntityManagerFactoryBean;
 import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;
+import org.springframework.util.Assert;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for DevTools-specific
@@ -136,13 +139,13 @@ public final class DevToolsDataSourceAutoConfiguration {
 			HSQLDB("jdbc:hsqldb:mem:", Set.of("org.hsqldb.jdbcDriver", "org.hsqldb.jdbc.JDBCDriver",
 					"org.hsqldb.jdbc.pool.JDBCXADataSource"));
 
-			private final String urlPrefix;
+			private final @Nullable String urlPrefix;
 
 			private final ShutdownHandler shutdownHandler;
 
 			private final Set<String> driverClassNames;
 
-			InMemoryDatabase(String urlPrefix, Set<String> driverClassNames) {
+			InMemoryDatabase(@Nullable String urlPrefix, Set<String> driverClassNames) {
 				this(urlPrefix, driverClassNames, (dataSource) -> {
 					try (Connection connection = dataSource.getConnection()) {
 						try (Statement statement = connection.createStatement()) {
@@ -152,7 +155,8 @@ public final class DevToolsDataSourceAutoConfiguration {
 				});
 			}
 
-			InMemoryDatabase(String urlPrefix, Set<String> driverClassNames, ShutdownHandler shutdownHandler) {
+			InMemoryDatabase(@Nullable String urlPrefix, Set<String> driverClassNames,
+					ShutdownHandler shutdownHandler) {
 				this.urlPrefix = urlPrefix;
 				this.driverClassNames = driverClassNames;
 				this.shutdownHandler = shutdownHandler;
@@ -189,11 +193,13 @@ public final class DevToolsDataSourceAutoConfiguration {
 		@Override
 		public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
 			ConditionMessage.Builder message = ConditionMessage.forCondition("DevTools DataSource Condition");
-			String[] dataSourceBeanNames = context.getBeanFactory().getBeanNamesForType(DataSource.class, true, false);
+			ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
+			Assert.state(beanFactory != null, "'beanFactory' must not be null");
+			String[] dataSourceBeanNames = beanFactory.getBeanNamesForType(DataSource.class, true, false);
 			if (dataSourceBeanNames.length != 1) {
 				return ConditionOutcome.noMatch(message.didNotFind("a single DataSource bean").atAll());
 			}
-			if (context.getBeanFactory().getBeanNamesForType(DataSourceProperties.class, true, false).length != 1) {
+			if (beanFactory.getBeanNamesForType(DataSourceProperties.class, true, false).length != 1) {
 				return ConditionOutcome.noMatch(message.didNotFind("a single DataSourceProperties bean").atAll());
 			}
 			BeanDefinition dataSourceDefinition = context.getRegistry().getBeanDefinition(dataSourceBeanNames[0]);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/DevToolsProperties.java

@@ -21,6 +21,8 @@ import java.time.Duration;
 import java.util.ArrayList;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.NestedConfigurationProperty;
 import org.springframework.util.StringUtils;
@@ -76,7 +78,7 @@ public class DevToolsProperties {
 		/**
 		 * Additional patterns that should be excluded from triggering a full restart.
 		 */
-		private String additionalExclude;
+		private @Nullable String additionalExclude;
 
 		/**
 		 * Amount of time to wait between polling for classpath changes.
@@ -94,7 +96,7 @@ public class DevToolsProperties {
 		 * a simple name (without any path) of a file that appears on your classpath. If
 		 * not specified, any classpath file change triggers the restart.
 		 */
-		private String triggerFile;
+		private @Nullable String triggerFile;
 
 		/**
 		 * Additional paths to watch for changes.
@@ -133,11 +135,11 @@ public class DevToolsProperties {
 			this.exclude = exclude;
 		}
 
-		public String getAdditionalExclude() {
+		public @Nullable String getAdditionalExclude() {
 			return this.additionalExclude;
 		}
 
-		public void setAdditionalExclude(String additionalExclude) {
+		public void setAdditionalExclude(@Nullable String additionalExclude) {
 			this.additionalExclude = additionalExclude;
 		}
 
@@ -157,11 +159,11 @@ public class DevToolsProperties {
 			this.quietPeriod = quietPeriod;
 		}
 
-		public String getTriggerFile() {
+		public @Nullable String getTriggerFile() {
 			return this.triggerFile;
 		}
 
-		public void setTriggerFile(String triggerFile) {
+		public void setTriggerFile(@Nullable String triggerFile) {
 			this.triggerFile = triggerFile;
 		}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/DevToolsR2dbcAutoConfiguration.java

@@ -18,12 +18,14 @@ package org.springframework.boot.devtools.autoconfigure;
 
 import io.r2dbc.spi.Connection;
 import io.r2dbc.spi.ConnectionFactory;
+import org.jspecify.annotations.Nullable;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Mono;
 
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.annotation.AnnotatedBeanDefinition;
 import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionMessage;
@@ -40,6 +42,7 @@ import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.ConfigurationCondition;
 import org.springframework.core.type.AnnotatedTypeMetadata;
 import org.springframework.core.type.MethodMetadata;
+import org.springframework.util.Assert;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for DevTools-specific R2DBC
@@ -99,7 +102,7 @@ public final class DevToolsR2dbcAutoConfiguration {
 			return closeConnection(connection, null);
 		}
 
-		private Publisher<Void> closeConnection(Connection connection, Throwable ex) {
+		private Publisher<Void> closeConnection(Connection connection, @Nullable Throwable ex) {
 			return connection.close();
 		}
 
@@ -115,7 +118,9 @@ public final class DevToolsR2dbcAutoConfiguration {
 		@Override
 		public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
 			ConditionMessage.Builder message = ConditionMessage.forCondition("DevTools ConnectionFactory Condition");
-			String[] beanNames = context.getBeanFactory().getBeanNamesForType(ConnectionFactory.class, true, false);
+			ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
+			Assert.state(beanFactory != null, "'beanFactory' must not be null");
+			String[] beanNames = beanFactory.getBeanNamesForType(ConnectionFactory.class, true, false);
 			if (beanNames.length != 1) {
 				return ConditionOutcome.noMatch(message.didNotFind("a single ConnectionFactory bean").atAll());
 			}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/LocalDevToolsAutoConfiguration.java

@@ -22,6 +22,7 @@ import java.util.List;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
@@ -49,6 +50,7 @@ import org.springframework.context.event.ContextRefreshedEvent;
 import org.springframework.context.event.GenericApplicationListener;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.log.LogMessage;
+import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
 /**
@@ -116,6 +118,7 @@ public final class LocalDevToolsAutoConfiguration {
 		ClassPathFileSystemWatcher classPathFileSystemWatcher(FileSystemWatcherFactory fileSystemWatcherFactory,
 				ClassPathRestartStrategy classPathRestartStrategy) {
 			URL[] urls = Restarter.getInstance().getInitialUrls();
+			Assert.state(urls != null, "'urls' must not be null");
 			ClassPathFileSystemWatcher watcher = new ClassPathFileSystemWatcher(fileSystemWatcherFactory,
 					classPathRestartStrategy, urls);
 			watcher.setStopWatcherOnRestart(true);
@@ -176,7 +179,7 @@ public final class LocalDevToolsAutoConfiguration {
 		}
 
 		@Override
-		public boolean supportsSourceType(Class<?> sourceType) {
+		public boolean supportsSourceType(@Nullable Class<?> sourceType) {
 			return true;
 		}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/OptionalLiveReloadServer.java

@@ -18,6 +18,7 @@ package org.springframework.boot.devtools.autoconfigure;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.boot.devtools.livereload.LiveReloadServer;
@@ -34,13 +35,13 @@ public class OptionalLiveReloadServer implements InitializingBean {
 
 	private static final Log logger = LogFactory.getLog(OptionalLiveReloadServer.class);
 
-	private LiveReloadServer server;
+	private @Nullable LiveReloadServer server;
 
 	/**
 	 * Create a new {@link OptionalLiveReloadServer} instance.
 	 * @param server the server to manage or {@code null}
 	 */
-	public OptionalLiveReloadServer(LiveReloadServer server) {
+	public OptionalLiveReloadServer(@Nullable LiveReloadServer server) {
 		this.server = server;
 	}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/RemoteDevToolsAutoConfiguration.java

@@ -48,6 +48,7 @@ import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
 import org.springframework.core.log.LogMessage;
 import org.springframework.http.server.ServerHttpRequest;
+import org.springframework.util.Assert;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for remote development support.
@@ -78,7 +79,9 @@ public final class RemoteDevToolsAutoConfiguration {
 	@ConditionalOnMissingBean
 	AccessManager remoteDevToolsAccessManager() {
 		RemoteDevToolsProperties remoteProperties = this.properties.getRemote();
-		return new HttpHeaderAccessManager(remoteProperties.getSecretHeaderName(), remoteProperties.getSecret());
+		String secret = remoteProperties.getSecret();
+		Assert.state(secret != null, "'secret' must not be null");
+		return new HttpHeaderAccessManager(remoteProperties.getSecretHeaderName(), secret);
 	}
 
 	@Bean

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/RemoteDevToolsProperties.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.autoconfigure;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Configuration properties for remote Spring Boot applications.
  *
@@ -39,7 +41,7 @@ public class RemoteDevToolsProperties {
 	 * A shared secret required to establish a connection (required to enable remote
 	 * support).
 	 */
-	private String secret;
+	private @Nullable String secret;
 
 	/**
 	 * HTTP header used to transfer the shared secret.
@@ -58,11 +60,11 @@ public class RemoteDevToolsProperties {
 		this.contextPath = contextPath;
 	}
 
-	public String getSecret() {
+	public @Nullable String getSecret() {
 		return this.secret;
 	}
 
-	public void setSecret(String secret) {
+	public void setSecret(@Nullable String secret) {
 		this.secret = secret;
 	}
 
@@ -104,26 +106,26 @@ public class RemoteDevToolsProperties {
 		/**
 		 * The host of the proxy to use to connect to the remote application.
 		 */
-		private String host;
+		private @Nullable String host;
 
 		/**
 		 * The port of the proxy to use to connect to the remote application.
 		 */
-		private Integer port;
+		private @Nullable Integer port;
 
-		public String getHost() {
+		public @Nullable String getHost() {
 			return this.host;
 		}
 
-		public void setHost(String host) {
+		public void setHost(@Nullable String host) {
 			this.host = host;
 		}
 
-		public Integer getPort() {
+		public @Nullable Integer getPort() {
 			return this.port;
 		}
 
-		public void setPort(Integer port) {
+		public void setPort(@Nullable Integer port) {
 			this.port = port;
 		}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/autoconfigure/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Auto-configuration for {@code spring-boot-devtools}.
  */
+@NullMarked
 package org.springframework.boot.devtools.autoconfigure;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathDirectories.java

@@ -25,6 +25,7 @@ import java.util.List;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.log.LogMessage;
 import org.springframework.util.ResourceUtils;
@@ -41,7 +42,7 @@ public class ClassPathDirectories implements Iterable<File> {
 
 	private final List<File> directories = new ArrayList<>();
 
-	public ClassPathDirectories(URL[] urls) {
+	public ClassPathDirectories(URL @Nullable [] urls) {
 		if (urls != null) {
 			addUrls(urls);
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathFileChangeListener.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.classpath;
 
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.devtools.filewatch.ChangedFile;
 import org.springframework.boot.devtools.filewatch.ChangedFiles;
 import org.springframework.boot.devtools.filewatch.FileChangeListener;
@@ -39,7 +41,7 @@ class ClassPathFileChangeListener implements FileChangeListener {
 
 	private final ClassPathRestartStrategy restartStrategy;
 
-	private final FileSystemWatcher fileSystemWatcherToStop;
+	private final @Nullable FileSystemWatcher fileSystemWatcherToStop;
 
 	/**
 	 * Create a new {@link ClassPathFileChangeListener} instance.
@@ -49,7 +51,7 @@ class ClassPathFileChangeListener implements FileChangeListener {
 	 * {@code null})
 	 */
 	ClassPathFileChangeListener(ApplicationEventPublisher eventPublisher, ClassPathRestartStrategy restartStrategy,
-			FileSystemWatcher fileSystemWatcherToStop) {
+			@Nullable FileSystemWatcher fileSystemWatcherToStop) {
 		Assert.notNull(eventPublisher, "'eventPublisher' must not be null");
 		Assert.notNull(restartStrategy, "'restartStrategy' must not be null");
 		this.eventPublisher = eventPublisher;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/ClassPathFileSystemWatcher.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.classpath;
 
 import java.net.URL;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.InitializingBean;
@@ -39,8 +41,9 @@ public class ClassPathFileSystemWatcher implements InitializingBean, DisposableB
 
 	private final FileSystemWatcher fileSystemWatcher;
 
-	private final ClassPathRestartStrategy restartStrategy;
+	private final @Nullable ClassPathRestartStrategy restartStrategy;
 
+	@SuppressWarnings("NullAway.Init")
 	private ApplicationContext applicationContext;
 
 	private boolean stopWatcherOnRestart;
@@ -53,7 +56,7 @@ public class ClassPathFileSystemWatcher implements InitializingBean, DisposableB
 	 * @param urls the URLs to watch
 	 */
 	public ClassPathFileSystemWatcher(FileSystemWatcherFactory fileSystemWatcherFactory,
-			ClassPathRestartStrategy restartStrategy, URL[] urls) {
+			@Nullable ClassPathRestartStrategy restartStrategy, URL[] urls) {
 		Assert.notNull(fileSystemWatcherFactory, "'fileSystemWatcherFactory' must not be null");
 		Assert.notNull(urls, "'urls' must not be null");
 		this.fileSystemWatcher = fileSystemWatcherFactory.getFileSystemWatcher();

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/classpath/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Support for classpath monitoring.
  */
+@NullMarked
 package org.springframework.boot.devtools.classpath;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/env/DevToolsHomePropertiesPostProcessor.java

@@ -29,6 +29,8 @@ import java.util.Set;
 import java.util.function.Function;
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.devtools.system.DevToolsEnablementDeducer;
 import org.springframework.boot.env.EnvironmentPostProcessor;
@@ -137,14 +139,14 @@ public class DevToolsHomePropertiesPostProcessor implements EnvironmentPostProce
 			.anyMatch((fileExtension) -> StringUtils.endsWithIgnoreCase(name, fileExtension));
 	}
 
-	protected File getHomeDirectory() {
+	protected @Nullable File getHomeDirectory() {
 		return getHomeDirectory(() -> this.environmentVariables.get("SPRING_DEVTOOLS_HOME"),
 				() -> this.systemProperties.getProperty("spring.devtools.home"),
 				() -> this.systemProperties.getProperty("user.home"));
 	}
 
 	@SafeVarargs
-	private File getHomeDirectory(Supplier<String>... pathSuppliers) {
+	private @Nullable File getHomeDirectory(Supplier<String>... pathSuppliers) {
 		for (Supplier<String> pathSupplier : pathSuppliers) {
 			String path = pathSupplier.get();
 			if (StringUtils.hasText(path)) {

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/env/DevToolsPropertyDefaultsPostProcessor.java

@@ -24,6 +24,7 @@ import java.util.Map;
 import java.util.Properties;
 
 import org.apache.commons.logging.Log;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.devtools.logger.DevToolsLogFactory;
@@ -123,7 +124,7 @@ public class DevToolsPropertyDefaultsPostProcessor implements EnvironmentPostPro
 		return false;
 	}
 
-	private Class<?> resolveClassName(String candidate, ClassLoader classLoader) {
+	private @Nullable Class<?> resolveClassName(String candidate, ClassLoader classLoader) {
 		try {
 			return ClassUtils.resolveClassName(candidate, classLoader);
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/env/package-info.java

@@ -18,4 +18,7 @@
  * DevTools classes relating to Spring Framework's
  * {@link org.springframework.core.env.Environment}.
  */
+@NullMarked
 package org.springframework.boot.devtools.env;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/ChangedFile.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.filewatch;
 
 import java.io.File;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
@@ -82,7 +84,7 @@ public final class ChangedFile {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (obj == this) {
 			return true;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/ChangedFiles.java

@@ -21,6 +21,8 @@ import java.util.Collections;
 import java.util.Iterator;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * A collections of files from a specific source directory that have changed.
  *
@@ -62,7 +64,7 @@ public final class ChangedFiles implements Iterable<ChangedFile> {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (obj == null) {
 			return false;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/DirectorySnapshot.java

@@ -27,6 +27,8 @@ import java.util.LinkedHashSet;
 import java.util.Map;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.devtools.filewatch.ChangedFile.Type;
 import org.springframework.util.Assert;
 
@@ -73,7 +75,7 @@ class DirectorySnapshot {
 		}
 	}
 
-	ChangedFiles getChangedFiles(DirectorySnapshot snapshot, FileFilter triggerFilter) {
+	ChangedFiles getChangedFiles(DirectorySnapshot snapshot, @Nullable FileFilter triggerFilter) {
 		Assert.notNull(snapshot, "'snapshot' must not be null");
 		File directory = this.directory;
 		Assert.isTrue(snapshot.directory.equals(directory),
@@ -99,7 +101,7 @@ class DirectorySnapshot {
 		return new ChangedFiles(directory, changes);
 	}
 
-	private boolean acceptChangedFile(FileFilter triggerFilter, FileSnapshot file) {
+	private boolean acceptChangedFile(@Nullable FileFilter triggerFilter, FileSnapshot file) {
 		return (triggerFilter == null || !triggerFilter.accept(file.getFile()));
 	}
 
@@ -112,7 +114,7 @@ class DirectorySnapshot {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (this == obj) {
 			return true;
 		}
@@ -125,7 +127,7 @@ class DirectorySnapshot {
 		return super.equals(obj);
 	}
 
-	boolean equals(DirectorySnapshot other, FileFilter filter) {
+	boolean equals(DirectorySnapshot other, @Nullable FileFilter filter) {
 		if (this.directory.equals(other.directory)) {
 			Set<FileSnapshot> ourFiles = filter(this.files, filter);
 			Set<FileSnapshot> otherFiles = filter(other.files, filter);
@@ -134,7 +136,7 @@ class DirectorySnapshot {
 		return false;
 	}
 
-	private Set<FileSnapshot> filter(Set<FileSnapshot> source, FileFilter filter) {
+	private Set<FileSnapshot> filter(Set<FileSnapshot> source, @Nullable FileFilter filter) {
 		if (filter == null) {
 			return source;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSnapshot.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.filewatch;
 
 import java.io.File;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -49,7 +51,7 @@ class FileSnapshot {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		if (this == obj) {
 			return true;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/FileSystemWatcher.java

@@ -30,6 +30,8 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -58,11 +60,11 @@ public class FileSystemWatcher {
 
 	private final AtomicInteger remainingScans = new AtomicInteger(-1);
 
-	private final Map<File, DirectorySnapshot> directories = new HashMap<>();
+	private final Map<File, @Nullable DirectorySnapshot> directories = new HashMap<>();
 
-	private Thread watchThread;
+	private @Nullable Thread watchThread;
 
-	private FileFilter triggerFilter;
+	private @Nullable FileFilter triggerFilter;
 
 	private final Object monitor = new Object();
 
@@ -94,7 +96,7 @@ public class FileSystemWatcher {
 	 * @since 2.4.0
 	 */
 	public FileSystemWatcher(boolean daemon, Duration pollInterval, Duration quietPeriod,
-			SnapshotStateRepository snapshotStateRepository) {
+			@Nullable SnapshotStateRepository snapshotStateRepository) {
 		Assert.notNull(pollInterval, "'pollInterval' must not be null");
 		Assert.notNull(quietPeriod, "'quietPeriod' must not be null");
 		Assert.isTrue(pollInterval.toMillis() > 0, "'pollInterval' must be positive");
@@ -151,7 +153,7 @@ public class FileSystemWatcher {
 	 * Set an optional {@link FileFilter} used to limit the files that trigger a change.
 	 * @param triggerFilter a trigger filter or null
 	 */
-	public void setTriggerFilter(FileFilter triggerFilter) {
+	public void setTriggerFilter(@Nullable FileFilter triggerFilter) {
 		synchronized (this.monitor) {
 			this.triggerFilter = triggerFilter;
 		}
@@ -227,7 +229,7 @@ public class FileSystemWatcher {
 
 		private final List<FileChangeListener> listeners;
 
-		private final FileFilter triggerFilter;
+		private final @Nullable FileFilter triggerFilter;
 
 		private final long pollInterval;
 
@@ -237,9 +239,9 @@ public class FileSystemWatcher {
 
 		private final SnapshotStateRepository snapshotStateRepository;
 
-		private Watcher(AtomicInteger remainingScans, List<FileChangeListener> listeners, FileFilter triggerFilter,
-				long pollInterval, long quietPeriod, Map<File, DirectorySnapshot> directories,
-				SnapshotStateRepository snapshotStateRepository) {
+		private Watcher(AtomicInteger remainingScans, List<FileChangeListener> listeners,
+				@Nullable FileFilter triggerFilter, long pollInterval, long quietPeriod,
+				Map<File, DirectorySnapshot> directories, SnapshotStateRepository snapshotStateRepository) {
 			this.remainingScans = remainingScans;
 			this.listeners = listeners;
 			this.triggerFilter = triggerFilter;
@@ -289,6 +291,7 @@ public class FileSystemWatcher {
 			for (Map.Entry<File, DirectorySnapshot> entry : previous.entrySet()) {
 				DirectorySnapshot previousDirectory = entry.getValue();
 				DirectorySnapshot currentDirectory = current.get(entry.getKey());
+				Assert.state(currentDirectory != null, "'currentDirectory' must not be null");
 				if (!previousDirectory.equals(currentDirectory, this.triggerFilter)) {
 					return true;
 				}
@@ -310,6 +313,7 @@ public class FileSystemWatcher {
 			for (DirectorySnapshot snapshot : snapshots) {
 				DirectorySnapshot previous = this.directories.get(snapshot.getDirectory());
 				updated.put(snapshot.getDirectory(), snapshot);
+				Assert.state(previous != null, "'previous' must not be null");
 				ChangedFiles changedFiles = previous.getChangedFiles(snapshot, this.triggerFilter);
 				if (!changedFiles.getFiles().isEmpty()) {
 					changeSet.add(changedFiles);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/SnapshotStateRepository.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.filewatch;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Repository used by {@link FileSystemWatcher} to save file/directory snapshots across
  * restarts.
@@ -35,7 +37,7 @@ public interface SnapshotStateRepository {
 		}
 
 		@Override
-		public Object restore() {
+		public @Nullable Object restore() {
 			return null;
 		}
 
@@ -57,6 +59,6 @@ public interface SnapshotStateRepository {
 	 * Restore any previously saved state.
 	 * @return the previously saved state or {@code null}
 	 */
-	Object restore();
+	@Nullable Object restore();
 
 }

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/StaticSnapshotStateRepository.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.filewatch;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * {@link SnapshotStateRepository} that uses a single static instance.
  *
@@ -25,7 +27,7 @@ class StaticSnapshotStateRepository implements SnapshotStateRepository {
 
 	static final StaticSnapshotStateRepository INSTANCE = new StaticSnapshotStateRepository();
 
-	private volatile Object state;
+	private volatile @Nullable Object state;
 
 	@Override
 	public void save(Object state) {
@@ -33,7 +35,7 @@ class StaticSnapshotStateRepository implements SnapshotStateRepository {
 	}
 
 	@Override
-	public Object restore() {
+	public @Nullable Object restore() {
 		return this.state;
 	}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/filewatch/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Class to watch the local filesystem for changes.
  */
+@NullMarked
 package org.springframework.boot.devtools.filewatch;
+
+import org.jspecify.annotations.NullMarked;

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

@@ -32,6 +32,7 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.log.LogMessage;
 import org.springframework.util.Assert;
@@ -63,9 +64,9 @@ public class LiveReloadServer {
 
 	private final ThreadFactory threadFactory;
 
-	private ServerSocket serverSocket;
+	private @Nullable ServerSocket serverSocket;
 
-	private Thread listenThread;
+	private @Nullable Thread listenThread;
 
 	/**
 	 * Create a new {@link LiveReloadServer} listening on the default port.
@@ -140,6 +141,7 @@ public class LiveReloadServer {
 	}
 
 	private void acceptConnections() {
+		Assert.state(this.serverSocket != null, "'serverSocket' must not be null");
 		do {
 			try {
 				Socket socket = this.serverSocket.accept();
@@ -173,6 +175,7 @@ public class LiveReloadServer {
 				catch (InterruptedException ex) {
 					Thread.currentThread().interrupt();
 				}
+				Assert.state(this.serverSocket != null, "'serverSocket' must not be null");
 				this.serverSocket.close();
 				try {
 					this.listenThread.join();

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/livereload/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Support for the livereload protocol.
  */
+@NullMarked
 package org.springframework.boot.devtools.livereload;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/logger/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Devtools specific logging concerns.
  */
+@NullMarked
 package org.springframework.boot.devtools.logger;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Spring Boot developer tools.
  */
+@NullMarked
 package org.springframework.boot.devtools;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/client/ClassPathChangeUploader.java

@@ -155,6 +155,7 @@ public class ClassPathChangeUploader implements ApplicationListener<ClassPathCha
 
 	private ClassLoaderFile asClassLoaderFile(ChangedFile changedFile) throws IOException {
 		ClassLoaderFile.Kind kind = TYPE_MAPPINGS.get(changedFile.getType());
+		Assert.state(kind != null, "'kind' must not be null");
 		byte[] bytes = (kind != Kind.DELETED) ? FileCopyUtils.copyToByteArray(changedFile.getFile()) : null;
 		long lastModified = (kind != Kind.DELETED) ? changedFile.getFile().lastModified() : System.currentTimeMillis();
 		return new ClassLoaderFile(kind, lastModified, bytes);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/client/RemoteClientConfiguration.java

@@ -26,6 +26,7 @@ import java.util.concurrent.Executors;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.beans.factory.ObjectProvider;
@@ -77,7 +78,7 @@ public class RemoteClientConfiguration implements InitializingBean {
 	private final DevToolsProperties properties;
 
 	@Value("${remoteUrl}")
-	private String remoteUrl;
+	private @Nullable String remoteUrl;
 
 	public RemoteClientConfiguration(DevToolsProperties properties) {
 		this.properties = properties;
@@ -119,7 +120,7 @@ public class RemoteClientConfiguration implements InitializingBean {
 		if (!remoteProperties.getRestart().isEnabled()) {
 			logger.warn("Remote restart is disabled.");
 		}
-		if (!this.remoteUrl.startsWith("https://")) {
+		if (this.remoteUrl == null || !this.remoteUrl.startsWith("https://")) {
 			logger.warn(LogMessage.format(
 					"The connection to %s is insecure. You should use a URL starting with 'https://'.",
 					this.remoteUrl));

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/client/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Client support for a remotely running Spring Boot application.
  */
+@NullMarked
 package org.springframework.boot.devtools.remote.client;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/HandlerMapper.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.remote.server;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.http.server.ServerHttpRequest;
 
 /**
@@ -33,6 +35,6 @@ public interface HandlerMapper {
 	 * @param request the request
 	 * @return a {@link Handler} or {@code null}
 	 */
-	Handler getHandler(ServerHttpRequest request);
+	@Nullable Handler getHandler(ServerHttpRequest request);
 
 }

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/UrlHandlerMapper.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.remote.server;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.util.Assert;
 
@@ -45,7 +47,7 @@ public class UrlHandlerMapper implements HandlerMapper {
 	}
 
 	@Override
-	public Handler getHandler(ServerHttpRequest request) {
+	public @Nullable Handler getHandler(ServerHttpRequest request) {
 		if (this.requestUri.equals(request.getURI().getPath())) {
 			return this.handler;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/remote/server/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Server support for a remotely running Spring Boot application.
  */
+@NullMarked
 package org.springframework.boot.devtools.remote.server;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/AgentReloader.java

@@ -20,6 +20,8 @@ import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.ClassUtils;
 
 /**
@@ -52,7 +54,7 @@ public abstract class AgentReloader {
 				|| isActive(ClassLoader.getSystemClassLoader());
 	}
 
-	private static boolean isActive(ClassLoader classLoader) {
+	private static boolean isActive(@Nullable ClassLoader classLoader) {
 		for (String agentClass : AGENT_CLASSES) {
 			if (ClassUtils.isPresent(agentClass, classLoader)) {
 				return true;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/ClassLoaderFilesResourcePatternResolver.java

@@ -27,6 +27,8 @@ import java.util.List;
 import java.util.Map.Entry;
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.devtools.restart.classloader.ClassLoaderFile;
 import org.springframework.boot.devtools.restart.classloader.ClassLoaderFile.Kind;
 import org.springframework.boot.devtools.restart.classloader.ClassLoaderFileURLStreamHandler;
@@ -77,7 +79,7 @@ final class ClassLoaderFilesResourcePatternResolver implements ResourcePatternRe
 			.getResourcePatternResolver(applicationContext, retrieveResourceLoader(applicationContext));
 	}
 
-	private ResourceLoader retrieveResourceLoader(ApplicationContext applicationContext) {
+	private @Nullable ResourceLoader retrieveResourceLoader(ApplicationContext applicationContext) {
 		Field field = ReflectionUtils.findField(applicationContext.getClass(), "resourceLoader", ResourceLoader.class);
 		if (field == null) {
 			return null;
@@ -94,7 +96,7 @@ final class ClassLoaderFilesResourcePatternResolver implements ResourcePatternRe
 	}
 
 	@Override
-	public ClassLoader getClassLoader() {
+	public @Nullable ClassLoader getClassLoader() {
 		return this.patternResolverDelegate.getClassLoader();
 	}
 
@@ -200,7 +202,7 @@ final class ClassLoaderFilesResourcePatternResolver implements ResourcePatternRe
 	private static class ResourcePatternResolverFactory {
 
 		ResourcePatternResolver getResourcePatternResolver(AbstractApplicationContext applicationContext,
-				ResourceLoader resourceLoader) {
+				@Nullable ResourceLoader resourceLoader) {
 			ResourceLoader targetResourceLoader = (resourceLoader != null) ? resourceLoader
 					: new ApplicationContextResourceLoader(applicationContext::getProtocolResolvers);
 			return new PathMatchingResourcePatternResolver(targetResourceLoader);
@@ -216,7 +218,7 @@ final class ClassLoaderFilesResourcePatternResolver implements ResourcePatternRe
 
 		@Override
 		public ResourcePatternResolver getResourcePatternResolver(AbstractApplicationContext applicationContext,
-				ResourceLoader resourceLoader) {
+				@Nullable ResourceLoader resourceLoader) {
 			if (applicationContext instanceof WebApplicationContext) {
 				return getServletContextResourcePatternResolver(applicationContext, resourceLoader);
 			}
@@ -224,7 +226,7 @@ final class ClassLoaderFilesResourcePatternResolver implements ResourcePatternRe
 		}
 
 		private ResourcePatternResolver getServletContextResourcePatternResolver(
-				AbstractApplicationContext applicationContext, ResourceLoader resourceLoader) {
+				AbstractApplicationContext applicationContext, @Nullable ResourceLoader resourceLoader) {
 			ResourceLoader targetResourceLoader = (resourceLoader != null) ? resourceLoader
 					: new WebApplicationContextResourceLoader(applicationContext::getProtocolResolvers,
 							(WebApplicationContext) applicationContext);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/DefaultRestartInitializer.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.restart;
 
 import java.net.URL;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.devtools.system.DevToolsEnablementDeducer;
 
 /**
@@ -32,7 +34,7 @@ import org.springframework.boot.devtools.system.DevToolsEnablementDeducer;
 public class DefaultRestartInitializer implements RestartInitializer {
 
 	@Override
-	public URL[] getInitialUrls(Thread thread) {
+	public URL @Nullable [] getInitialUrls(Thread thread) {
 		if (!isMain(thread)) {
 			return null;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/MainMethod.java

@@ -19,6 +19,8 @@ package org.springframework.boot.devtools.restart;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -57,7 +59,7 @@ class MainMethod {
 		return className.startsWith("org.springframework.boot.loader.");
 	}
 
-	private Method getMainMethod(StackTraceElement element) {
+	private @Nullable Method getMainMethod(StackTraceElement element) {
 		try {
 			Class<?> elementClass = Class.forName(element.getClassName());
 			Method method = elementClass.getDeclaredMethod("main", String[].class);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/OnInitializedRestarterCondition.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.restart;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.autoconfigure.condition.ConditionMessage;
 import org.springframework.boot.autoconfigure.condition.ConditionOutcome;
 import org.springframework.boot.autoconfigure.condition.SpringBootCondition;
@@ -44,7 +46,7 @@ class OnInitializedRestarterCondition extends SpringBootCondition {
 		return ConditionOutcome.match(message.because("available and initialized"));
 	}
 
-	private Restarter getRestarter() {
+	private @Nullable Restarter getRestarter() {
 		try {
 			return Restarter.getInstance();
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartInitializer.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.restart;
 
 import java.net.URL;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Strategy interface used to initialize a {@link Restarter}.
  *
@@ -39,6 +41,6 @@ public interface RestartInitializer {
 	 * @param thread the source thread
 	 * @return initial URLs or {@code null}
 	 */
-	URL[] getInitialUrls(Thread thread);
+	URL @Nullable [] getInitialUrls(Thread thread);
 
 }

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartLauncher.java

@@ -18,6 +18,8 @@ package org.springframework.boot.devtools.restart;
 
 import java.lang.reflect.Method;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Thread used to launch a restarted application.
  *
@@ -29,7 +31,7 @@ class RestartLauncher extends Thread {
 
 	private final String[] args;
 
-	private Throwable error;
+	private @Nullable Throwable error;
 
 	RestartLauncher(ClassLoader classLoader, String mainClassName, String[] args,
 			UncaughtExceptionHandler exceptionHandler) {
@@ -55,7 +57,7 @@ class RestartLauncher extends Thread {
 		}
 	}
 
-	Throwable getError() {
+	@Nullable Throwable getError() {
 		return this.error;
 	}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/RestartScopeInitializer.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.restart;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.factory.ObjectFactory;
 import org.springframework.beans.factory.config.Scope;
 import org.springframework.context.ApplicationContextInitializer;
@@ -54,12 +56,12 @@ public class RestartScopeInitializer implements ApplicationContextInitializer<Co
 		}
 
 		@Override
-		public Object resolveContextualObject(String key) {
+		public @Nullable Object resolveContextualObject(String key) {
 			return null;
 		}
 
 		@Override
-		public String getConversationId() {
+		public @Nullable String getConversationId() {
 			return null;
 		}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/Restarter.java

@@ -38,6 +38,7 @@ import java.util.concurrent.locks.ReentrantLock;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.CachedIntrospectionResults;
 import org.springframework.beans.factory.ObjectFactory;
@@ -86,7 +87,7 @@ public class Restarter {
 
 	private static final String[] NO_ARGS = {};
 
-	private static Restarter instance;
+	private static @Nullable Restarter instance;
 
 	private final Set<URL> urls = new LinkedHashSet<>();
 
@@ -106,9 +107,9 @@ public class Restarter {
 
 	private boolean enabled = true;
 
-	private final URL[] initialUrls;
+	private final URL @Nullable [] initialUrls;
 
-	private final String mainClassName;
+	private final @Nullable String mainClassName;
 
 	private final ClassLoader applicationClassLoader;
 
@@ -145,7 +146,7 @@ public class Restarter {
 		this.leakSafeThreads.add(new LeakSafeThread());
 	}
 
-	private String getMainClassName(Thread thread) {
+	private @Nullable String getMainClassName(Thread thread) {
 		try {
 			return new MainMethod(thread).getDeclaringClassName();
 		}
@@ -170,7 +171,7 @@ public class Restarter {
 			getLeakSafeThread().callAndWait(() -> {
 				start(FailureHandler.NONE);
 				cleanupCaches();
-				return null;
+				return new Object();
 			});
 		}
 		catch (Exception ex) {
@@ -249,7 +250,7 @@ public class Restarter {
 		getLeakSafeThread().call(() -> {
 			Restarter.this.stop();
 			Restarter.this.start(failureHandler);
-			return null;
+			return new Object();
 		});
 	}
 
@@ -271,7 +272,7 @@ public class Restarter {
 		while (true);
 	}
 
-	private Throwable doStart() throws Exception {
+	private @Nullable Throwable doStart() throws Exception {
 		Assert.state(this.mainClassName != null, "Unable to find the main class to restart");
 		URL[] urls = this.urls.toArray(new URL[0]);
 		ClassLoaderFiles updatedFiles = new ClassLoaderFiles(this.classLoaderFiles);
@@ -288,7 +289,8 @@ public class Restarter {
 	 * @return any exception that caused the launch to fail or {@code null}
 	 * @throws Exception in case of errors
 	 */
-	protected Throwable relaunch(ClassLoader classLoader) throws Exception {
+	protected @Nullable Throwable relaunch(ClassLoader classLoader) throws Exception {
+		Assert.state(this.mainClassName != null, "'mainClassName' must not be null");
 		RestartLauncher launcher = new RestartLauncher(classLoader, this.mainClassName, this.args,
 				this.exceptionHandler);
 		launcher.start();
@@ -417,7 +419,7 @@ public class Restarter {
 		this.rootContexts.add(applicationContext);
 	}
 
-	void remove(ConfigurableApplicationContext applicationContext) {
+	void remove(@Nullable ConfigurableApplicationContext applicationContext) {
 		if (applicationContext != null) {
 			this.rootContexts.remove(applicationContext);
 		}
@@ -456,7 +458,7 @@ public class Restarter {
 	 * Return the initial set of URLs as configured by the {@link RestartInitializer}.
 	 * @return the initial URLs or {@code null}
 	 */
-	public URL[] getInitialUrls() {
+	public URL @Nullable [] getInitialUrls() {
 		return this.initialUrls;
 	}
 
@@ -575,9 +577,9 @@ public class Restarter {
 	 */
 	private class LeakSafeThread extends Thread {
 
-		private Callable<?> callable;
+		private @Nullable Callable<?> callable;
 
-		private Object result;
+		private @Nullable Object result;
 
 		LeakSafeThread() {
 			setDaemon(false);
@@ -594,6 +596,7 @@ public class Restarter {
 			start();
 			try {
 				join();
+				Assert.state(this.result != null, "'this.result' must not be null");
 				return (V) this.result;
 			}
 			catch (InterruptedException ex) {
@@ -609,6 +612,7 @@ public class Restarter {
 			// RestartClassLoader
 			try {
 				Restarter.this.leakSafeThreads.put(new LeakSafeThread());
+				Assert.state(this.callable != null, "'callable' must not be null");
 				this.result = this.callable.call();
 			}
 			catch (Exception ex) {

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/SilentExitExceptionHandler.java

@@ -20,6 +20,8 @@ import java.lang.Thread.UncaughtExceptionHandler;
 import java.lang.reflect.InvocationTargetException;
 import java.util.Arrays;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * {@link UncaughtExceptionHandler} decorator that allows a thread to exit silently.
  *
@@ -28,9 +30,9 @@ import java.util.Arrays;
  */
 class SilentExitExceptionHandler implements UncaughtExceptionHandler {
 
-	private final UncaughtExceptionHandler delegate;
+	private final @Nullable UncaughtExceptionHandler delegate;
 
-	SilentExitExceptionHandler(UncaughtExceptionHandler delegate) {
+	SilentExitExceptionHandler(@Nullable UncaughtExceptionHandler delegate) {
 		this.delegate = delegate;
 	}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFile.java

@@ -16,8 +16,11 @@
 
 package org.springframework.boot.devtools.restart.classloader;
 
+import java.io.Serial;
 import java.io.Serializable;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -30,11 +33,12 @@ import org.springframework.util.Assert;
  */
 public class ClassLoaderFile implements Serializable {
 
+	@Serial
 	private static final long serialVersionUID = 1;
 
 	private final Kind kind;
 
-	private final byte[] contents;
+	private final byte @Nullable [] contents;
 
 	private final long lastModified;
 
@@ -53,7 +57,7 @@ public class ClassLoaderFile implements Serializable {
 	 * @param lastModified the last modified time
 	 * @param contents the file contents
 	 */
-	public ClassLoaderFile(Kind kind, long lastModified, byte[] contents) {
+	public ClassLoaderFile(Kind kind, long lastModified, byte @Nullable [] contents) {
 		Assert.notNull(kind, "'kind' must not be null");
 		if (kind == Kind.DELETED) {
 			Assert.isTrue(contents == null, "'contents' must be null");
@@ -87,7 +91,7 @@ public class ClassLoaderFile implements Serializable {
 	 * {@link #getKind()} is {@link Kind#DELETED}.
 	 * @return the contents or {@code null}
 	 */
-	public byte[] getContents() {
+	public byte @Nullable [] getContents() {
 		return this.contents;
 	}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFileRepository.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.devtools.restart.classloader;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * A container for files that may be served from a {@link ClassLoader}. Can be used to
  * represent files that have been added, modified or deleted since the original JAR was
@@ -39,6 +41,6 @@ public interface ClassLoaderFileRepository {
 	 * @param name the name of the file
 	 * @return a {@link ClassLoaderFile} or {@code null}
 	 */
-	ClassLoaderFile getFile(String name);
+	@Nullable ClassLoaderFile getFile(String name);
 
 }

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/ClassLoaderFiles.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.devtools.restart.classloader;
 
+import java.io.Serial;
 import java.io.Serializable;
 import java.util.Collection;
 import java.util.Collections;
@@ -26,6 +27,8 @@ import java.util.Set;
 
 import javax.management.loading.ClassLoaderRepository;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.Assert;
 
 /**
@@ -133,7 +136,7 @@ public class ClassLoaderFiles implements ClassLoaderFileRepository, Serializable
 	}
 
 	@Override
-	public ClassLoaderFile getFile(String name) {
+	public @Nullable ClassLoaderFile getFile(String name) {
 		for (SourceDirectory sourceDirectory : this.sourceDirectories.values()) {
 			ClassLoaderFile file = sourceDirectory.get(name);
 			if (file != null) {
@@ -148,6 +151,7 @@ public class ClassLoaderFiles implements ClassLoaderFileRepository, Serializable
 	 */
 	public static class SourceDirectory implements Serializable {
 
+		@Serial
 		private static final long serialVersionUID = 1;
 
 		private final String name;
@@ -170,7 +174,7 @@ public class ClassLoaderFiles implements ClassLoaderFileRepository, Serializable
 			this.files.remove(name);
 		}
 
-		protected final ClassLoaderFile get(String name) {
+		protected final @Nullable ClassLoaderFile get(String name) {
 			return this.files.get(name);
 		}
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/RestartClassLoader.java

@@ -23,6 +23,8 @@ import java.net.URLClassLoader;
 import java.security.ProtectionDomain;
 import java.util.Enumeration;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.devtools.restart.classloader.ClassLoaderFile.Kind;
 import org.springframework.core.SmartClassLoader;
 import org.springframework.util.Assert;
@@ -80,7 +82,7 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 	}
 
 	@Override
-	public URL getResource(String name) {
+	public @Nullable URL getResource(String name) {
 		ClassLoaderFile file = this.updatedFiles.getFile(name);
 		if (file != null && file.getKind() == Kind.DELETED) {
 			return null;
@@ -93,7 +95,7 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 	}
 
 	@Override
-	public URL findResource(String name) {
+	public @Nullable URL findResource(String name) {
 		final ClassLoaderFile file = this.updatedFiles.getFile(name);
 		if (file == null) {
 			return super.findResource(name);
@@ -139,11 +141,12 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 			throw new ClassNotFoundException(name);
 		}
 		byte[] bytes = file.getContents();
+		Assert.state(bytes != null, "'bytes' must not be null");
 		return defineClass(name, bytes, 0, bytes.length);
 	}
 
 	@Override
-	public Class<?> publicDefineClass(String name, byte[] b, ProtectionDomain protectionDomain) {
+	public Class<?> publicDefineClass(String name, byte[] b, @Nullable ProtectionDomain protectionDomain) {
 		return defineClass(name, b, 0, b.length, protectionDomain);
 	}
 
@@ -171,11 +174,11 @@ public class RestartClassLoader extends URLClassLoader implements SmartClassLoad
 	 */
 	private static class CompoundEnumeration<E> implements Enumeration<E> {
 
-		private E firstElement;
+		private @Nullable E firstElement;
 
 		private final Enumeration<E> enumeration;
 
-		CompoundEnumeration(E firstElement, Enumeration<E> enumeration) {
+		CompoundEnumeration(@Nullable E firstElement, Enumeration<E> enumeration) {
 			this.firstElement = firstElement;
 			this.enumeration = enumeration;
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/classloader/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Classloaders used for reload support.
  */
+@NullMarked
 package org.springframework.boot.devtools.restart.classloader;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Application restart support.
  */
+@NullMarked
 package org.springframework.boot.devtools.restart;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/DefaultSourceDirectoryUrlFilter.java

@@ -20,6 +20,8 @@ import java.net.URL;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.util.StringUtils;
 
 /**
@@ -46,7 +48,7 @@ public class DefaultSourceDirectoryUrlFilter implements SourceDirectoryUrlFilter
 		return isMatch(sourceDirectory, jarName);
 	}
 
-	private String getJarName(URL url) {
+	private @Nullable String getJarName(URL url) {
 		Matcher matcher = URL_MODULE_PATTERN.matcher(url.toString());
 		if (matcher.find()) {
 			return matcher.group(1);

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/RestartServer.java

@@ -107,7 +107,9 @@ public class RestartServer {
 				if (classLoaderFile.getKind() == Kind.DELETED) {
 					return file.delete();
 				}
-				FileCopyUtils.copy(classLoaderFile.getContents(), file);
+				byte[] contents = classLoaderFile.getContents();
+				Assert.state(contents != null, "'contents' must not be null");
+				FileCopyUtils.copy(contents, file);
 				return true;
 			}
 		}

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/server/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Remote restart server.
  */
+@NullMarked
 package org.springframework.boot.devtools.restart.server;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/settings/DevToolsSettings.java

@@ -25,6 +25,7 @@ import java.util.Map;
 import java.util.regex.Pattern;
 
 import org.apache.commons.logging.Log;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.devtools.logger.DevToolsLogFactory;
 import org.springframework.core.io.UrlResource;
@@ -45,7 +46,7 @@ public class DevToolsSettings {
 
 	private static final Log logger = DevToolsLogFactory.getLog(DevToolsSettings.class);
 
-	private static DevToolsSettings settings;
+	private static @Nullable DevToolsSettings settings;
 
 	private final List<Pattern> restartIncludePatterns = new ArrayList<>();
 

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/settings/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Classes for loading DevTools settings.
  */
+@NullMarked
 package org.springframework.boot.devtools.settings;
+
+import org.jspecify.annotations.NullMarked;

module/spring-boot-devtools/src/main/java/org/springframework/boot/devtools/system/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Devtools system support classes.
  */
+@NullMarked
 package org.springframework.boot.devtools.system;
+
+import org.jspecify.annotations.NullMarked;