Add nullability annotations to core/spring-boot-test

See gh-46587

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/AnnotatedClassFinder.java

@@ -22,6 +22,8 @@ import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Set;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider;
 import org.springframework.core.type.filter.AnnotationTypeFilter;
@@ -63,7 +65,7 @@ public final class AnnotatedClassFinder {
 	 * @return the first {@link Class} annotated with the target annotation within the
 	 * hierarchy defined by the given {@code source} or {@code null} if none is found.
 	 */
-	public Class<?> findFromClass(Class<?> source) {
+	public @Nullable Class<?> findFromClass(Class<?> source) {
 		Assert.notNull(source, "'source' must not be null");
 		return findFromPackage(ClassUtils.getPackageName(source));
 	}
@@ -75,7 +77,7 @@ public final class AnnotatedClassFinder {
 	 * @return the first {@link Class} annotated with the target annotation within the
 	 * hierarchy defined by the given {@code source} or {@code null} if none is found.
 	 */
-	public Class<?> findFromPackage(String source) {
+	public @Nullable Class<?> findFromPackage(String source) {
 		Assert.notNull(source, "'source' must not be null");
 		Class<?> configuration = cache.get(source);
 		if (configuration == null) {
@@ -85,13 +87,15 @@ public final class AnnotatedClassFinder {
 		return configuration;
 	}
 
-	private Class<?> scanPackage(String source) {
+	private @Nullable Class<?> scanPackage(String source) {
 		while (!source.isEmpty()) {
 			Set<BeanDefinition> components = this.scanner.findCandidateComponents(source);
 			if (!components.isEmpty()) {
 				Assert.state(components.size() == 1, () -> "Found multiple @" + this.annotationType.getSimpleName()
 						+ " annotated classes " + components);
-				return ClassUtils.resolveClassName(components.iterator().next().getBeanClassName(), null);
+				String beanClassName = components.iterator().next().getBeanClassName();
+				Assert.state(beanClassName != null, "'beanClassName' must not be null");
+				return ClassUtils.resolveClassName(beanClassName, null);
 			}
 			source = getParentPackage(source);
 		}

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/FilteredClassLoader.java

@@ -27,6 +27,8 @@ import java.util.Collections;
 import java.util.Enumeration;
 import java.util.function.Predicate;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.SmartClassLoader;
 import org.springframework.core.io.ClassPathResource;
 
@@ -117,7 +119,7 @@ public class FilteredClassLoader extends URLClassLoader implements SmartClassLoa
 	}
 
 	@Override
-	public URL getResource(String name) {
+	public @Nullable URL getResource(String name) {
 		for (Predicate<String> filter : this.resourcesFilters) {
 			if (filter.test(name)) {
 				return null;
@@ -137,7 +139,7 @@ public class FilteredClassLoader extends URLClassLoader implements SmartClassLoa
 	}
 
 	@Override
-	public InputStream getResourceAsStream(String name) {
+	public @Nullable InputStream getResourceAsStream(String name) {
 		for (Predicate<String> filter : this.resourcesFilters) {
 			if (filter.test(name)) {
 				return null;
@@ -147,7 +149,7 @@ public class FilteredClassLoader extends URLClassLoader implements SmartClassLoa
 	}
 
 	@Override
-	public Class<?> publicDefineClass(String name, byte[] b, ProtectionDomain protectionDomain) {
+	public Class<?> publicDefineClass(String name, byte[] b, @Nullable ProtectionDomain protectionDomain) {
 		for (Predicate<String> filter : this.classesFilters) {
 			if (filter.test(name)) {
 				throw new IllegalArgumentException(String.format("Defining class with name %s is not supported", name));

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/ImportsContextCustomizer.java

@@ -24,6 +24,8 @@ import java.util.LinkedHashSet;
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
@@ -156,6 +158,7 @@ class ImportsContextCustomizer implements ContextCustomizer {
 
 		private static final String[] NO_IMPORTS = {};
 
+		@SuppressWarnings("NullAway.Init")
 		private ConfigurableListableBeanFactory beanFactory;
 
 		@Override
@@ -254,7 +257,7 @@ class ImportsContextCustomizer implements ContextCustomizer {
 			return ANNOTATION_FILTERS.stream().anyMatch((filter) -> filter.matches(typeName));
 		}
 
-		private Set<Object> determineImports(MergedAnnotations annotations, Class<?> testClass) {
+		private @Nullable Set<Object> determineImports(MergedAnnotations annotations, Class<?> testClass) {
 			Set<Object> determinedImports = new LinkedHashSet<>();
 			AnnotationMetadata metadata = AnnotationMetadata.introspect(testClass);
 			for (MergedAnnotation<Import> annotation : annotations.stream(Import.class).toList()) {
@@ -269,7 +272,7 @@ class ImportsContextCustomizer implements ContextCustomizer {
 			return determinedImports;
 		}
 
-		private Set<Object> determineImports(Class<?> source, AnnotationMetadata metadata) {
+		private @Nullable Set<Object> determineImports(Class<?> source, AnnotationMetadata metadata) {
 			if (DeterminableImports.class.isAssignableFrom(source)) {
 				// We can determine the imports
 				return ((DeterminableImports) instantiate(source)).determineImports(metadata);

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/ImportsContextCustomizerFactory.java

@@ -19,6 +19,8 @@ package org.springframework.boot.test.context;
 import java.lang.reflect.Method;
 import java.util.List;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.aot.AotDetector;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Import;
@@ -41,7 +43,7 @@ import org.springframework.util.ReflectionUtils;
 class ImportsContextCustomizerFactory implements ContextCustomizerFactory {
 
 	@Override
-	public ContextCustomizer createContextCustomizer(Class<?> testClass,
+	public @Nullable ContextCustomizer createContextCustomizer(Class<?> testClass,
 			List<ContextConfigurationAttributes> configAttributes) {
 		if (AotDetector.useGeneratedArtifacts()) {
 			return null;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootContextLoader.java

@@ -23,6 +23,8 @@ import java.util.Collections;
 import java.util.List;
 import java.util.function.Consumer;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.aot.hint.ExecutableMode;
 import org.springframework.aot.hint.RuntimeHints;
 import org.springframework.beans.BeanUtils;
@@ -105,6 +107,8 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 	private static final Consumer<SpringApplication> ALREADY_CONFIGURED = (springApplication) -> {
 	};
 
+	private static final Object NONE = new Object();
+
 	@Override
 	public ApplicationContext loadContext(MergedContextConfiguration mergedConfig) throws Exception {
 		return loadContext(mergedConfig, Mode.STANDARD, null, null);
@@ -123,8 +127,8 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 	}
 
 	private ApplicationContext loadContext(MergedContextConfiguration mergedConfig, Mode mode,
-			ApplicationContextInitializer<ConfigurableApplicationContext> initializer, RuntimeHints runtimeHints)
-			throws Exception {
+			@Nullable ApplicationContextInitializer<ConfigurableApplicationContext> initializer,
+			@Nullable RuntimeHints runtimeHints) throws Exception {
 		assertHasClassesOrLocations(mergedConfig);
 		SpringBootTestAnnotation annotation = SpringBootTestAnnotation.get(mergedConfig);
 		String[] args = annotation.getArgs();
@@ -153,7 +157,7 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 						+ SpringVersion.getVersion() + ").");
 	}
 
-	private Method getMainMethod(MergedContextConfiguration mergedConfig, UseMainMethod useMainMethod) {
+	private @Nullable Method getMainMethod(MergedContextConfiguration mergedConfig, UseMainMethod useMainMethod) {
 		if (useMainMethod == UseMainMethod.NEVER) {
 			return null;
 		}
@@ -167,14 +171,16 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 				"Cannot use main method as no @SpringBootConfiguration-annotated class is available");
 		Method mainMethod = findMainMethod(springBootConfiguration);
 		Assert.state(mainMethod != null || useMainMethod == UseMainMethod.WHEN_AVAILABLE,
-				() -> "Main method not found on '%s'".formatted(springBootConfiguration.getName()));
+				() -> "Main method not found on '%s'"
+					.formatted((springBootConfiguration != null) ? springBootConfiguration.getName() : null));
 		return mainMethod;
 	}
 
-	private static Method findMainMethod(Class<?> type) {
+	private static @Nullable Method findMainMethod(@Nullable Class<?> type) {
 		Method mainMethod = (type != null) ? ReflectionUtils.findMethod(type, "main", String[].class) : null;
 		if (mainMethod == null && KotlinDetector.isKotlinPresent()) {
 			try {
+				Assert.state(type != null, "'type' must not be null");
 				Class<?> kotlinClass = ClassUtils.forName(type.getName() + "Kt", type.getClassLoader());
 				mainMethod = ReflectionUtils.findMethod(kotlinClass, "main", String[].class);
 			}
@@ -285,7 +291,7 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 	 * method if you need a custom environment.
 	 * @return a {@link ConfigurableEnvironment} instance
 	 */
-	protected ConfigurableEnvironment getEnvironment() {
+	protected @Nullable ConfigurableEnvironment getEnvironment() {
 		return null;
 	}
 
@@ -462,9 +468,9 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 	private static class ParentContextApplicationContextInitializer
 			implements ApplicationContextInitializer<ConfigurableApplicationContext> {
 
-		private final ApplicationContext parent;
+		private final @Nullable ApplicationContext parent;
 
-		ParentContextApplicationContextInitializer(ApplicationContext parent) {
+		ParentContextApplicationContextInitializer(@Nullable ApplicationContext parent) {
 			this.parent = parent;
 		}
 
@@ -507,7 +513,7 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 
 		private final Mode mode;
 
-		private final ApplicationContextInitializer<ConfigurableApplicationContext> initializer;
+		private final @Nullable ApplicationContextInitializer<ConfigurableApplicationContext> initializer;
 
 		private final Consumer<SpringApplication> configurer;
 
@@ -515,7 +521,8 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 
 		private final List<ApplicationContext> failedContexts = Collections.synchronizedList(new ArrayList<>());
 
-		ContextLoaderHook(Mode mode, ApplicationContextInitializer<ConfigurableApplicationContext> initializer,
+		ContextLoaderHook(Mode mode,
+				@Nullable ApplicationContextInitializer<ConfigurableApplicationContext> initializer,
 				Consumer<SpringApplication> configurer) {
 			this.mode = mode;
 			this.initializer = initializer;
@@ -530,6 +537,7 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 				public void starting(ConfigurableBootstrapContext bootstrapContext) {
 					ContextLoaderHook.this.configurer.accept(application);
 					if (ContextLoaderHook.this.mode == Mode.AOT_RUNTIME) {
+						Assert.state(ContextLoaderHook.this.initializer != null, "'initializer' must not be null");
 						application.addInitializers(
 								(AotApplicationContextInitializer<?>) ContextLoaderHook.this.initializer::initialize);
 					}
@@ -544,24 +552,24 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 				}
 
 				@Override
-				public void failed(ConfigurableApplicationContext context, Throwable exception) {
+				public void failed(@Nullable ConfigurableApplicationContext context, Throwable exception) {
 					ContextLoaderHook.this.failedContexts.add(context);
 				}
 
 			};
 		}
 
-		private <T> ApplicationContext runMain(Runnable action) throws Exception {
+		private ApplicationContext runMain(Runnable action) throws Exception {
 			return run(() -> {
 				action.run();
-				return null;
+				return NONE;
 			});
 		}
 
-		private ApplicationContext run(ThrowingSupplier<ConfigurableApplicationContext> action) throws Exception {
+		private ApplicationContext run(ThrowingSupplier<?> action) throws Exception {
 			try {
-				ConfigurableApplicationContext context = SpringApplication.withHook(this, action);
-				if (context != null) {
+				Object result = SpringApplication.withHook(this, action);
+				if (result instanceof ApplicationContext context) {
 					return context;
 				}
 			}

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTestAnnotation.java

@@ -19,6 +19,8 @@ package org.springframework.boot.test.context;
 import java.util.Arrays;
 import java.util.Objects;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.test.context.SpringBootTest.UseMainMethod;
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -52,7 +54,7 @@ class SpringBootTestAnnotation implements ContextCustomizer {
 		this(TestContextAnnotationUtils.findMergedAnnotation(testClass, SpringBootTest.class));
 	}
 
-	private SpringBootTestAnnotation(SpringBootTest annotation) {
+	private SpringBootTestAnnotation(@Nullable SpringBootTest annotation) {
 		this.args = (annotation != null) ? annotation.args() : NO_ARGS;
 		this.webEnvironment = (annotation != null) ? annotation.webEnvironment() : WebEnvironment.NONE;
 		this.useMainMethod = (annotation != null) ? annotation.useMainMethod() : UseMainMethod.NEVER;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTestContextBootstrapper.java

@@ -25,6 +25,7 @@ import java.util.Set;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.boot.WebApplicationType;
@@ -287,7 +288,7 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 	 * the bootstrapper class as a property.
 	 * @return the differentiator or {@code null}
 	 */
-	protected String getDifferentiatorPropertySourceProperty() {
+	protected @Nullable String getDifferentiatorPropertySourceProperty() {
 		return getClass().getName() + "=true";
 	}
 
@@ -320,22 +321,22 @@ public class SpringBootTestContextBootstrapper extends DefaultTestContextBootstr
 	 * @param testClass the source test class
 	 * @return the {@link WebEnvironment} or {@code null}
 	 */
-	protected WebEnvironment getWebEnvironment(Class<?> testClass) {
+	protected @Nullable WebEnvironment getWebEnvironment(Class<?> testClass) {
 		SpringBootTest annotation = getAnnotation(testClass);
 		return (annotation != null) ? annotation.webEnvironment() : null;
 	}
 
-	protected Class<?>[] getClasses(Class<?> testClass) {
+	protected Class<?> @Nullable [] getClasses(Class<?> testClass) {
 		SpringBootTest annotation = getAnnotation(testClass);
 		return (annotation != null) ? annotation.classes() : null;
 	}
 
-	protected String[] getProperties(Class<?> testClass) {
+	protected String @Nullable [] getProperties(Class<?> testClass) {
 		SpringBootTest annotation = getAnnotation(testClass);
 		return (annotation != null) ? annotation.properties() : null;
 	}
 
-	protected SpringBootTest getAnnotation(Class<?> testClass) {
+	protected @Nullable SpringBootTest getAnnotation(Class<?> testClass) {
 		return TestContextAnnotationUtils.findMergedAnnotation(testClass, SpringBootTest.class);
 	}
 

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/ApplicationContextAssert.java

@@ -29,6 +29,7 @@ import org.assertj.core.api.AbstractThrowableAssert;
 import org.assertj.core.api.Assertions;
 import org.assertj.core.api.MapAssert;
 import org.assertj.core.error.BasicErrorMessageFactory;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
@@ -54,14 +55,14 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class ApplicationContextAssert<C extends ApplicationContext>
 		extends AbstractAssert<ApplicationContextAssert<C>, C> {
 
-	private final Throwable startupFailure;
+	private final @Nullable Throwable startupFailure;
 
 	/**
 	 * Create a new {@link ApplicationContextAssert} instance.
 	 * @param applicationContext the source application context
 	 * @param startupFailure the startup failure or {@code null}
 	 */
-	ApplicationContextAssert(C applicationContext, Throwable startupFailure) {
+	ApplicationContextAssert(C applicationContext, @Nullable Throwable startupFailure) {
 		super(applicationContext, ApplicationContextAssert.class);
 		Assert.notNull(applicationContext, "'applicationContext' must not be null");
 		this.startupFailure = startupFailure;
@@ -80,7 +81,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	 */
 	public ApplicationContextAssert<C> hasBean(String name) {
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to have bean named:%n <%s>", name));
+			throwAssertionError(
+					contextFailedToStartWhenExpecting(this.startupFailure, "to have bean named:%n <%s>", name));
 		}
 		if (findBean(name) == null) {
 			throwAssertionError(new BasicErrorMessageFactory(
@@ -123,7 +125,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	public ApplicationContextAssert<C> hasSingleBean(Class<?> type, Scope scope) {
 		Assert.notNull(scope, "'scope' must not be null");
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to have a single bean of type:%n <%s>", type));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
+					"to have a single bean of type:%n <%s>", type));
 		}
 		String[] names = scope.getBeanNamesForType(getApplicationContext(), type);
 		if (names.length == 0) {
@@ -170,7 +173,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	public ApplicationContextAssert<C> doesNotHaveBean(Class<?> type, Scope scope) {
 		Assert.notNull(scope, "'scope' must not be null");
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("not to have any beans of type:%n <%s>", type));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
+					"not to have any beans of type:%n <%s>", type));
 		}
 		String[] names = scope.getBeanNamesForType(getApplicationContext(), type);
 		if (names.length > 0) {
@@ -194,7 +198,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	 */
 	public ApplicationContextAssert<C> doesNotHaveBean(String name) {
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("not to have any beans of name:%n <%s>", name));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
+					"not to have any beans of name:%n <%s>", name));
 		}
 		try {
 			Object bean = getApplicationContext().getBean(name);
@@ -221,7 +226,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	 */
 	public <T> AbstractObjectArrayAssert<?, String> getBeanNames(Class<T> type) {
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to get beans names with type:%n <%s>", type));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
+					"to get beans names with type:%n <%s>", type));
 		}
 		return Assertions.assertThat(getApplicationContext().getBeanNamesForType(type))
 			.as("Bean names of type <%s> from <%s>", type, getApplicationContext());
@@ -267,7 +273,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	public <T> AbstractObjectAssert<?, T> getBean(Class<T> type, Scope scope) {
 		Assert.notNull(scope, "'scope' must not be null");
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to contain bean of type:%n <%s>", type));
+			throwAssertionError(
+					contextFailedToStartWhenExpecting(this.startupFailure, "to contain bean of type:%n <%s>", type));
 		}
 		String[] names = scope.getBeanNamesForType(getApplicationContext(), type);
 		String name = (names.length > 0) ? getPrimary(names, scope) : null;
@@ -280,7 +287,7 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 		return Assertions.assertThat(bean).as("Bean of type <%s> from <%s>", type, getApplicationContext());
 	}
 
-	private String getPrimary(String[] names, Scope scope) {
+	private @Nullable String getPrimary(String[] names, Scope scope) {
 		if (names.length == 1) {
 			return names[0];
 		}
@@ -325,7 +332,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	 */
 	public AbstractObjectAssert<?, Object> getBean(String name) {
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to contain a bean of name:%n <%s>", name));
+			throwAssertionError(
+					contextFailedToStartWhenExpecting(this.startupFailure, "to contain a bean of name:%n <%s>", name));
 		}
 		Object bean = findBean(name);
 		return Assertions.assertThat(bean).as("Bean of name <%s> from <%s>", name, getApplicationContext());
@@ -351,8 +359,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	@SuppressWarnings("unchecked")
 	public <T> AbstractObjectAssert<?, T> getBean(String name, Class<T> type) {
 		if (this.startupFailure != null) {
-			throwAssertionError(
-					contextFailedToStartWhenExpecting("to contain a bean of name:%n <%s> (%s)", name, type));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure,
+					"to contain a bean of name:%n <%s> (%s)", name, type));
 		}
 		Object bean = findBean(name);
 		if (bean != null && type != null && !type.isInstance(bean)) {
@@ -364,7 +372,7 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 			.as("Bean of name <%s> and type <%s> from <%s>", name, type, getApplicationContext());
 	}
 
-	private Object findBean(String name) {
+	private @Nullable Object findBean(String name) {
 		try {
 			return getApplicationContext().getBean(name);
 		}
@@ -409,7 +417,8 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	public <T> MapAssert<String, T> getBeans(Class<T> type, Scope scope) {
 		Assert.notNull(scope, "'scope' must not be null");
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to get beans of type:%n <%s>", type));
+			throwAssertionError(
+					contextFailedToStartWhenExpecting(this.startupFailure, "to get beans of type:%n <%s>", type));
 		}
 		return Assertions.assertThat(scope.getBeansOfType(getApplicationContext(), type))
 			.as("Beans of type <%s> from <%s>", type, getApplicationContext());
@@ -456,7 +465,7 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 	 */
 	public ApplicationContextAssert<C> hasNotFailed() {
 		if (this.startupFailure != null) {
-			throwAssertionError(contextFailedToStartWhenExpecting("to have not failed"));
+			throwAssertionError(contextFailedToStartWhenExpecting(this.startupFailure, "to have not failed"));
 		}
 		return this;
 	}
@@ -465,12 +474,13 @@ public class ApplicationContextAssert<C extends ApplicationContext>
 		return this.actual;
 	}
 
-	protected final Throwable getStartupFailure() {
+	protected final @Nullable Throwable getStartupFailure() {
 		return this.startupFailure;
 	}
 
-	private ContextFailedToStart<C> contextFailedToStartWhenExpecting(String expectationFormat, Object... arguments) {
-		return new ContextFailedToStart<>(getApplicationContext(), this.startupFailure, expectationFormat, arguments);
+	private ContextFailedToStart<C> contextFailedToStartWhenExpecting(Throwable startupFailure,
+			String expectationFormat, Object... arguments) {
+		return new ContextFailedToStart<>(getApplicationContext(), startupFailure, expectationFormat, arguments);
 	}
 
 	/**

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/ApplicationContextAssertProvider.java

@@ -22,6 +22,7 @@ import java.util.Arrays;
 import java.util.function.Supplier;
 
 import org.assertj.core.api.AssertProvider;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.context.ApplicationContext;
 import org.springframework.util.Assert;
@@ -86,7 +87,7 @@ public interface ApplicationContextAssertProvider<C extends ApplicationContext>
 	 * context started without issue.
 	 * @return the startup failure or {@code null}
 	 */
-	Throwable getStartupFailure();
+	@Nullable Throwable getStartupFailure();
 
 	@Override
 	void close();

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/AssertProviderApplicationContextInvocationHandler.java

@@ -24,6 +24,8 @@ import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.context.ApplicationContext;
 import org.springframework.core.style.ToStringCreator;
 import org.springframework.util.Assert;
@@ -39,9 +41,9 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 
 	private final Class<?> applicationContextType;
 
-	private final ApplicationContext applicationContext;
+	private final @Nullable ApplicationContext applicationContext;
 
-	private final RuntimeException startupFailure;
+	private final @Nullable RuntimeException startupFailure;
 
 	AssertProviderApplicationContextInvocationHandler(Class<?> applicationContextType, Supplier<?> contextSupplier) {
 		this.applicationContextType = applicationContextType;
@@ -66,7 +68,7 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 	}
 
 	@Override
-	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
+	public @Nullable Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
 		if (isToString(method)) {
 			return toString();
 		}
@@ -95,6 +97,7 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 			return "Unstarted application context " + this.applicationContextType.getName() + "[startupFailure="
 					+ this.startupFailure.getClass().getName() + "]";
 		}
+		Assert.state(this.applicationContext != null, "'applicationContext' must not be null");
 		ToStringCreator builder = new ToStringCreator(this.applicationContext)
 			.append("id", this.applicationContext.getId())
 			.append("applicationName", this.applicationContext.getApplicationName())
@@ -119,7 +122,7 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 		return ("getStartupFailure".equals(method.getName()) && method.getParameterCount() == 0);
 	}
 
-	private Object getStartupFailure() {
+	private @Nullable Object getStartupFailure() {
 		return this.startupFailure;
 	}
 
@@ -135,7 +138,7 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 		return ("close".equals(method.getName()) && method.getParameterCount() == 0);
 	}
 
-	private Object invokeClose() throws IOException {
+	private @Nullable Object invokeClose() throws IOException {
 		if (this.applicationContext instanceof Closeable closeable) {
 			closeable.close();
 		}
@@ -155,6 +158,7 @@ class AssertProviderApplicationContextInvocationHandler implements InvocationHan
 		if (this.startupFailure != null) {
 			throw new IllegalStateException(this + " failed to start", this.startupFailure);
 		}
+		Assert.state(this.applicationContext != null, "'applicationContext' must not be null");
 		return this.applicationContext;
 	}
 

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/assertj/package-info.java

@@ -17,4 +17,7 @@
 /**
  * AssertJ support for ApplicationContexts.
  */
+@NullMarked
 package org.springframework.boot.test.context.assertj;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/filter/TestTypeExcludeFilter.java

@@ -18,6 +18,8 @@ package org.springframework.boot.test.context.filter;
 
 import java.io.IOException;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.boot.context.TypeExcludeFilter;
 import org.springframework.boot.test.context.TestComponent;
@@ -68,7 +70,7 @@ class TestTypeExcludeFilter extends TypeExcludeFilter {
 	}
 
 	@Override
-	public boolean equals(Object obj) {
+	public boolean equals(@Nullable Object obj) {
 		return (obj != null) && (getClass() == obj.getClass());
 	}
 

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/filter/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Test support for {@link org.springframework.boot.context.TypeExcludeFilter}.
  */
+@NullMarked
 package org.springframework.boot.test.context.filter;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/package-info.java

@@ -18,4 +18,7 @@
  * Classes and annotations related to configuring Spring's {@code ApplicationContext} for
  * tests.
  */
+@NullMarked
 package org.springframework.boot.test.context;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/AbstractApplicationContextRunner.java

@@ -24,6 +24,8 @@ import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.BeanDefinitionCustomizer;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
@@ -213,7 +215,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @return a new instance with the updated class loader
 	 * @see FilteredClassLoader
 	 */
-	public SELF withClassLoader(ClassLoader classLoader) {
+	public SELF withClassLoader(@Nullable ClassLoader classLoader) {
 		return newInstance(this.runnerConfiguration.withClassLoader(classLoader));
 	}
 
@@ -258,7 +260,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @param <T> the type of the bean
 	 * @return a new instance with the updated bean
 	 */
-	public <T> SELF withBean(String name, Class<T> type, Object... constructorArgs) {
+	public <T> SELF withBean(@Nullable String name, Class<T> type, Object... constructorArgs) {
 		return newInstance(this.runnerConfiguration.withBean(name, type, constructorArgs));
 	}
 
@@ -295,7 +297,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 	 * @param <T> the type of the bean
 	 * @return a new instance with the updated bean
 	 */
-	public <T> SELF withBean(String name, Class<T> type, Supplier<T> supplier,
+	public <T> SELF withBean(@Nullable String name, Class<T> type, Supplier<T> supplier,
 			BeanDefinitionCustomizer... customizers) {
 		return newInstance(this.runnerConfiguration.withBean(name, type, supplier, customizers));
 	}
@@ -370,7 +372,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		}
 	}
 
-	private void withContextClassLoader(ClassLoader classLoader, Runnable action) {
+	private void withContextClassLoader(@Nullable ClassLoader classLoader, Runnable action) {
 		if (classLoader == null) {
 			action.run();
 		}
@@ -392,6 +394,8 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		ResolvableType resolvableType = ResolvableType.forClass(AbstractApplicationContextRunner.class, getClass());
 		Class<A> assertType = (Class<A>) resolvableType.resolveGeneric(1);
 		Class<C> contextType = (Class<C>) resolvableType.resolveGeneric(2);
+		Assert.state(assertType != null, "'assertType' must not be null");
+		Assert.state(contextType != null, "'contextType' must not be null");
 		return ApplicationContextAssertProvider.get(assertType, contextType, () -> createAndLoadContext(refresh),
 				this.runnerConfiguration.additionalContextInterfaces);
 	}
@@ -471,11 +475,11 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 
 		Consumer<GenericApplicationContext> registrar;
 
-		public BeanRegistration(String name, Class<T> type, Object... constructorArgs) {
+		public BeanRegistration(@Nullable String name, Class<T> type, Object... constructorArgs) {
 			this.registrar = (context) -> context.registerBean(name, type, constructorArgs);
 		}
 
-		public BeanRegistration(String name, Class<T> type, Supplier<T> supplier,
+		public BeanRegistration(@Nullable String name, Class<T> type, Supplier<T> supplier,
 				BeanDefinitionCustomizer... customizers) {
 			this.registrar = (context) -> context.registerBean(name, type, supplier, customizers);
 		}
@@ -503,9 +507,9 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 
 		private TestPropertyValues systemProperties = TestPropertyValues.empty();
 
-		private ClassLoader classLoader;
+		private @Nullable ClassLoader classLoader;
 
-		private ApplicationContext parent;
+		private @Nullable ApplicationContext parent;
 
 		private List<BeanRegistration<?>> beanRegistrations = Collections.emptyList();
 
@@ -561,7 +565,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 			return config;
 		}
 
-		private RunnerConfiguration<C> withClassLoader(ClassLoader classLoader) {
+		private RunnerConfiguration<C> withClassLoader(@Nullable ClassLoader classLoader) {
 			RunnerConfiguration<C> config = new RunnerConfiguration<>(this);
 			config.classLoader = classLoader;
 			return config;
@@ -573,14 +577,14 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 			return config;
 		}
 
-		private <T> RunnerConfiguration<C> withBean(String name, Class<T> type, Object... constructorArgs) {
+		private <T> RunnerConfiguration<C> withBean(@Nullable String name, Class<T> type, Object... constructorArgs) {
 			RunnerConfiguration<C> config = new RunnerConfiguration<>(this);
 			config.beanRegistrations = add(config.beanRegistrations,
 					new BeanRegistration<>(name, type, constructorArgs));
 			return config;
 		}
 
-		private <T> RunnerConfiguration<C> withBean(String name, Class<T> type, Supplier<T> supplier,
+		private <T> RunnerConfiguration<C> withBean(@Nullable String name, Class<T> type, Supplier<T> supplier,
 				BeanDefinitionCustomizer... customizers) {
 			RunnerConfiguration<C> config = new RunnerConfiguration<>(this);
 			config.beanRegistrations = add(config.beanRegistrations,

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/WebApplicationContextRunner.java

@@ -18,8 +18,11 @@ package org.springframework.boot.test.context.runner;
 
 import java.util.function.Supplier;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.test.context.assertj.AssertableWebApplicationContext;
 import org.springframework.boot.web.context.servlet.AnnotationConfigServletWebApplicationContext;
+import org.springframework.lang.Contract;
 import org.springframework.mock.web.MockServletContext;
 import org.springframework.web.context.ConfigurableWebApplicationContext;
 import org.springframework.web.context.WebApplicationContext;
@@ -81,8 +84,9 @@ public final class WebApplicationContextRunner extends
 	 * @param contextFactory the context factory to decorate
 	 * @return an updated supplier that will set the {@link MockServletContext}
 	 */
-	public static Supplier<ConfigurableWebApplicationContext> withMockServletContext(
-			Supplier<ConfigurableWebApplicationContext> contextFactory) {
+	@Contract("!null -> !null")
+	public static @Nullable Supplier<ConfigurableWebApplicationContext> withMockServletContext(
+			@Nullable Supplier<ConfigurableWebApplicationContext> contextFactory) {
 		return (contextFactory != null) ? () -> {
 			ConfigurableWebApplicationContext context = contextFactory.get();
 			context.setServletContext(new MockServletContext());

core/spring-boot-test/src/main/java/org/springframework/boot/test/context/runner/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Test utilities to run application contexts for testing.
  */
+@NullMarked
 package org.springframework.boot.test.context.runner;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/AbstractJsonMarshalTester.java

@@ -27,6 +27,7 @@ import java.io.StringReader;
 import java.lang.reflect.Field;
 
 import org.assertj.core.api.Assertions;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.ObjectFactory;
 import org.springframework.core.ResolvableType;
@@ -68,9 +69,9 @@ import org.springframework.util.ReflectionUtils;
  */
 public abstract class AbstractJsonMarshalTester<T> {
 
-	private Class<?> resourceLoadClass;
+	private @Nullable Class<?> resourceLoadClass;
 
-	private ResolvableType type;
+	private @Nullable ResolvableType type;
 
 	/**
 	 * Create a new uninitialized {@link AbstractJsonMarshalTester} instance.
@@ -107,18 +108,30 @@ public abstract class AbstractJsonMarshalTester<T> {
 	 * Return the type under test.
 	 * @return the type under test
 	 */
-	protected final ResolvableType getType() {
+	protected final @Nullable ResolvableType getType() {
 		return this.type;
 	}
 
+	private ResolvableType getTypeNotNull() {
+		ResolvableType type = getType();
+		Assert.state(type != null, "Instance has not been initialized");
+		return type;
+	}
+
 	/**
 	 * Return class used to load relative resources.
 	 * @return the resource load class
 	 */
-	protected final Class<?> getResourceLoadClass() {
+	protected final @Nullable Class<?> getResourceLoadClass() {
 		return this.resourceLoadClass;
 	}
 
+	private Class<?> getResourceLoadClassNotNull() {
+		Class<?> resourceLoadClass = getResourceLoadClass();
+		Assert.state(resourceLoadClass != null, "Instance has not been initialized");
+		return resourceLoadClass;
+	}
+
 	/**
 	 * Return {@link JsonContent} from writing the specific value.
 	 * @param value the value to write
@@ -128,7 +141,7 @@ public abstract class AbstractJsonMarshalTester<T> {
 	public JsonContent<T> write(T value) throws IOException {
 		verify();
 		Assert.notNull(value, "'value' must not be null");
-		String json = writeObject(value, this.type);
+		String json = writeObject(value, getTypeNotNull());
 		return getJsonContent(json);
 	}
 
@@ -140,7 +153,7 @@ public abstract class AbstractJsonMarshalTester<T> {
 	 * @since 2.1.5
 	 */
 	protected JsonContent<T> getJsonContent(String json) {
-		return new JsonContent<>(getResourceLoadClass(), getType(), json);
+		return new JsonContent<>(getResourceLoadClassNotNull(), getType(), json);
 	}
 
 	/**
@@ -281,7 +294,7 @@ public abstract class AbstractJsonMarshalTester<T> {
 		verify();
 		Assert.notNull(resource, "'resource' must not be null");
 		InputStream inputStream = resource.getInputStream();
-		T object = readObject(inputStream, this.type);
+		T object = readObject(inputStream, getTypeNotNull());
 		closeQuietly(inputStream);
 		return new ObjectContent<>(this.type, object);
 	}
@@ -306,7 +319,7 @@ public abstract class AbstractJsonMarshalTester<T> {
 	public ObjectContent<T> read(Reader reader) throws IOException {
 		verify();
 		Assert.notNull(reader, "'reader' must not be null");
-		T object = readObject(reader, this.type);
+		T object = readObject(reader, getTypeNotNull());
 		closeQuietly(reader);
 		return new ObjectContent<>(this.type, object);
 	}

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/BasicJsonTester.java

@@ -20,6 +20,8 @@ import java.io.File;
 import java.io.InputStream;
 import java.nio.charset.Charset;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.io.Resource;
 import org.springframework.util.Assert;
 
@@ -47,7 +49,7 @@ import org.springframework.util.Assert;
  */
 public class BasicJsonTester {
 
-	private JsonLoader loader;
+	private @Nullable JsonLoader loader;
 
 	/**
 	 * Create a new uninitialized {@link BasicJsonTester} instance.
@@ -69,7 +71,7 @@ public class BasicJsonTester {
 	 * @param charset the charset used to load resources
 	 * @since 1.4.1
 	 */
-	public BasicJsonTester(Class<?> resourceLoadClass, Charset charset) {
+	public BasicJsonTester(Class<?> resourceLoadClass, @Nullable Charset charset) {
 		Assert.notNull(resourceLoadClass, "'resourceLoadClass' must not be null");
 		this.loader = new JsonLoader(resourceLoadClass, charset);
 	}
@@ -91,7 +93,7 @@ public class BasicJsonTester {
 	 * @param charset the charset used when loading relative classpath resources
 	 * @since 1.4.1
 	 */
-	protected final void initialize(Class<?> resourceLoadClass, Charset charset) {
+	protected final void initialize(Class<?> resourceLoadClass, @Nullable Charset charset) {
 		if (this.loader == null) {
 			this.loader = new JsonLoader(resourceLoadClass, charset);
 		}
@@ -105,8 +107,8 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(CharSequence source) {
-		verify();
-		return getJsonContent(this.loader.getJson(source));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(source));
 	}
 
 	/**
@@ -116,8 +118,8 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(String path, Class<?> resourceLoadClass) {
-		verify();
-		return getJsonContent(this.loader.getJson(path, resourceLoadClass));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(path, resourceLoadClass));
 	}
 
 	/**
@@ -126,8 +128,8 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(byte[] source) {
-		verify();
-		return getJsonContent(this.loader.getJson(source));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(source));
 	}
 
 	/**
@@ -136,8 +138,8 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(File source) {
-		verify();
-		return getJsonContent(this.loader.getJson(source));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(source));
 	}
 
 	/**
@@ -146,8 +148,8 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(InputStream source) {
-		verify();
-		return getJsonContent(this.loader.getJson(source));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(source));
 	}
 
 	/**
@@ -156,16 +158,17 @@ public class BasicJsonTester {
 	 * @return the JSON content
 	 */
 	public JsonContent<Object> from(Resource source) {
-		verify();
-		return getJsonContent(this.loader.getJson(source));
+		JsonLoader loader = verify();
+		return getJsonContent(loader, loader.getJson(source));
 	}
 
-	private void verify() {
+	private JsonLoader verify() {
 		Assert.state(this.loader != null, "Uninitialized BasicJsonTester");
+		return this.loader;
 	}
 
-	private JsonContent<Object> getJsonContent(String json) {
-		return new JsonContent<>(this.loader.getResourceLoadClass(), null, json);
+	private JsonContent<Object> getJsonContent(JsonLoader loader, String json) {
+		return new JsonContent<>(loader.getResourceLoadClass(), null, json);
 	}
 
 }

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/JacksonTester.java

@@ -27,6 +27,7 @@ import com.fasterxml.jackson.databind.ObjectWriter;
 import com.jayway.jsonpath.Configuration;
 import com.jayway.jsonpath.spi.json.JacksonJsonProvider;
 import com.jayway.jsonpath.spi.mapper.JacksonMappingProvider;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.ObjectFactory;
 import org.springframework.core.ResolvableType;
@@ -66,7 +67,7 @@ public class JacksonTester<T> extends AbstractJsonMarshalTester<T> {
 
 	private final ObjectMapper objectMapper;
 
-	private Class<?> view;
+	private @Nullable Class<?> view;
 
 	/**
 	 * Create a new {@link JacksonTester} instance.
@@ -87,7 +88,8 @@ public class JacksonTester<T> extends AbstractJsonMarshalTester<T> {
 		this(resourceLoadClass, type, objectMapper, null);
 	}
 
-	public JacksonTester(Class<?> resourceLoadClass, ResolvableType type, ObjectMapper objectMapper, Class<?> view) {
+	public JacksonTester(Class<?> resourceLoadClass, ResolvableType type, ObjectMapper objectMapper,
+			@Nullable Class<?> view) {
 		super(resourceLoadClass, type);
 		Assert.notNull(objectMapper, "'objectMapper' must not be null");
 		this.objectMapper = objectMapper;
@@ -100,7 +102,9 @@ public class JacksonTester<T> extends AbstractJsonMarshalTester<T> {
 			.jsonProvider(new JacksonJsonProvider(this.objectMapper))
 			.mappingProvider(new JacksonMappingProvider(this.objectMapper))
 			.build();
-		return new JsonContent<>(getResourceLoadClass(), getType(), json, configuration);
+		Class<?> resourceLoadClass = getResourceLoadClass();
+		Assert.state(resourceLoadClass != null, "'resourceLoadClass' must not be null");
+		return new JsonContent<>(resourceLoadClass, getType(), json, configuration);
 	}
 
 	@Override
@@ -167,7 +171,11 @@ public class JacksonTester<T> extends AbstractJsonMarshalTester<T> {
 	 * @return the new instance
 	 */
 	public JacksonTester<T> forView(Class<?> view) {
-		return new JacksonTester<>(getResourceLoadClass(), getType(), this.objectMapper, view);
+		Class<?> resourceLoadClass = getResourceLoadClass();
+		ResolvableType type = getType();
+		Assert.state(resourceLoadClass != null, "'resourceLoadClass' must not be null");
+		Assert.state(type != null, "'type' must not be null");
+		return new JacksonTester<>(resourceLoadClass, type, this.objectMapper, view);
 	}
 
 	/**

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/JsonContent.java

@@ -18,6 +18,7 @@ package org.springframework.boot.test.json;
 
 import com.jayway.jsonpath.Configuration;
 import org.assertj.core.api.AssertProvider;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.util.Assert;
@@ -36,7 +37,7 @@ public final class JsonContent<T> implements AssertProvider<JsonContentAssert> {
 
 	private final Class<?> resourceLoadClass;
 
-	private final ResolvableType type;
+	private final @Nullable ResolvableType type;
 
 	private final String json;
 
@@ -48,7 +49,7 @@ public final class JsonContent<T> implements AssertProvider<JsonContentAssert> {
 	 * @param type the type under test (or {@code null} if not known)
 	 * @param json the actual JSON content
 	 */
-	public JsonContent(Class<?> resourceLoadClass, ResolvableType type, String json) {
+	public JsonContent(Class<?> resourceLoadClass, @Nullable ResolvableType type, String json) {
 		this(resourceLoadClass, type, json, Configuration.defaultConfiguration());
 	}
 
@@ -59,7 +60,7 @@ public final class JsonContent<T> implements AssertProvider<JsonContentAssert> {
 	 * @param json the actual JSON content
 	 * @param configuration the JsonPath configuration
 	 */
-	JsonContent(Class<?> resourceLoadClass, ResolvableType type, String json, Configuration configuration) {
+	JsonContent(Class<?> resourceLoadClass, @Nullable ResolvableType type, String json, Configuration configuration) {
 		Assert.notNull(resourceLoadClass, "'resourceLoadClass' must not be null");
 		Assert.notNull(json, "'json' must not be null");
 		Assert.notNull(configuration, "'configuration' must not be null");

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/JsonContentAssert.java

@@ -33,6 +33,7 @@ import org.assertj.core.api.Assert;
 import org.assertj.core.api.Assertions;
 import org.assertj.core.api.ListAssert;
 import org.assertj.core.api.MapAssert;
+import org.jspecify.annotations.Nullable;
 import org.skyscreamer.jsonassert.JSONCompare;
 import org.skyscreamer.jsonassert.JSONCompareMode;
 import org.skyscreamer.jsonassert.JSONCompareResult;
@@ -74,7 +75,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @param json the actual JSON content
 	 * @since 1.4.1
 	 */
-	public JsonContentAssert(Class<?> resourceLoadClass, Charset charset, CharSequence json) {
+	public JsonContentAssert(Class<?> resourceLoadClass, @Nullable Charset charset, CharSequence json) {
 		this(resourceLoadClass, charset, json, Configuration.defaultConfiguration());
 	}
 
@@ -86,7 +87,8 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @param json the actual JSON content
 	 * @param configuration the json-path configuration
 	 */
-	JsonContentAssert(Class<?> resourceLoadClass, Charset charset, CharSequence json, Configuration configuration) {
+	JsonContentAssert(Class<?> resourceLoadClass, @Nullable Charset charset, CharSequence json,
+			Configuration configuration) {
 		super(json, JsonContentAssert.class);
 		this.configuration = configuration;
 		this.loader = new JsonLoader(resourceLoadClass, charset);
@@ -98,7 +100,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @see org.assertj.core.api.AbstractAssert#isEqualTo(java.lang.Object)
 	 */
 	@Override
-	public JsonContentAssert isEqualTo(Object expected) {
+	public JsonContentAssert isEqualTo(@Nullable Object expected) {
 		if (expected == null || expected instanceof CharSequence) {
 			return isEqualToJson((CharSequence) expected);
 		}
@@ -115,7 +117,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 			return isEqualToJson(resource);
 		}
 		failWithMessage("Unsupported type for JSON assert %s", expected.getClass());
-		return null;
+		return this;
 	}
 
 	/**
@@ -128,7 +130,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @return {@code this} assertion object
 	 * @throws AssertionError if the actual JSON value is not equal to the given one
 	 */
-	public JsonContentAssert isEqualToJson(CharSequence expected) {
+	public JsonContentAssert isEqualToJson(@Nullable CharSequence expected) {
 		String expectedJson = this.loader.getJson(expected);
 		return assertNotFailed(compare(expectedJson, JSONCompareMode.LENIENT));
 	}
@@ -426,7 +428,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @see org.assertj.core.api.AbstractAssert#isEqualTo(java.lang.Object)
 	 */
 	@Override
-	public JsonContentAssert isNotEqualTo(Object expected) {
+	public JsonContentAssert isNotEqualTo(@Nullable Object expected) {
 		if (expected == null || expected instanceof CharSequence) {
 			return isNotEqualToJson((CharSequence) expected);
 		}
@@ -443,7 +445,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 			return isNotEqualToJson(resource);
 		}
 		failWithMessage("Unsupported type for JSON assert %s", expected.getClass());
-		return null;
+		return this;
 	}
 
 	/**
@@ -456,7 +458,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	 * @return {@code this} assertion object
 	 * @throws AssertionError if the actual JSON value is equal to the given one
 	 */
-	public JsonContentAssert isNotEqualToJson(CharSequence expected) {
+	public JsonContentAssert isNotEqualToJson(@Nullable CharSequence expected) {
 		String expectedJson = this.loader.getJson(expected);
 		return assertNotPassed(compare(expectedJson, JSONCompareMode.LENIENT));
 	}
@@ -985,7 +987,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 	}
 
 	@SuppressWarnings("unchecked")
-	private <T> T extractingJsonPathValue(CharSequence expression, Object[] args, Class<T> type,
+	private <T> @Nullable T extractingJsonPathValue(CharSequence expression, Object[] args, Class<T> type,
 			String expectedDescription) {
 		JsonPathValue value = new JsonPathValue(expression, args);
 		if (value.getValue(false) != null) {
@@ -994,7 +996,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 		return (T) value.getValue(false);
 	}
 
-	private JSONCompareResult compare(CharSequence expectedJson, JSONCompareMode compareMode) {
+	private JSONCompareResult compare(@Nullable CharSequence expectedJson, JSONCompareMode compareMode) {
 		if (this.actual == null) {
 			return compareForNull(expectedJson);
 		}
@@ -1010,7 +1012,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 		}
 	}
 
-	private JSONCompareResult compare(CharSequence expectedJson, JSONComparator comparator) {
+	private JSONCompareResult compare(@Nullable CharSequence expectedJson, JSONComparator comparator) {
 		if (this.actual == null) {
 			return compareForNull(expectedJson);
 		}
@@ -1026,7 +1028,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 		}
 	}
 
-	private JSONCompareResult compareForNull(CharSequence expectedJson) {
+	private JSONCompareResult compareForNull(@Nullable CharSequence expectedJson) {
 		JSONCompareResult result = new JSONCompareResult();
 		result.passed();
 		if (expectedJson != null) {
@@ -1099,7 +1101,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 			}
 		}
 
-		void assertHasValue(Class<?> type, String expectedDescription) {
+		void assertHasValue(@Nullable Class<?> type, String expectedDescription) {
 			Object value = getValue(true);
 			if (value == null || isIndefiniteAndEmpty()) {
 				failWithNoValueMessage();
@@ -1128,7 +1130,7 @@ public class JsonContentAssert extends AbstractAssert<JsonContentAssert, CharSeq
 			return ObjectUtils.isEmpty(getValue(false));
 		}
 
-		Object getValue(boolean required) {
+		@Nullable Object getValue(boolean required) {
 			try {
 				return read();
 			}

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/JsonLoader.java

@@ -25,8 +25,11 @@ import java.io.InputStreamReader;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
+import org.springframework.lang.Contract;
 import org.springframework.util.FileCopyUtils;
 
 /**
@@ -41,7 +44,7 @@ class JsonLoader {
 
 	private final Charset charset;
 
-	JsonLoader(Class<?> resourceLoadClass, Charset charset) {
+	JsonLoader(Class<?> resourceLoadClass, @Nullable Charset charset) {
 		this.resourceLoadClass = resourceLoadClass;
 		this.charset = (charset != null) ? charset : StandardCharsets.UTF_8;
 	}
@@ -50,7 +53,8 @@ class JsonLoader {
 		return this.resourceLoadClass;
 	}
 
-	String getJson(CharSequence source) {
+	@Contract("!null -> !null")
+	@Nullable String getJson(@Nullable CharSequence source) {
 		if (source == null) {
 			return null;
 		}

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/ObjectContent.java

@@ -17,6 +17,7 @@
 package org.springframework.boot.test.json;
 
 import org.assertj.core.api.AssertProvider;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.util.Assert;
@@ -32,7 +33,7 @@ import org.springframework.util.Assert;
  */
 public final class ObjectContent<T> implements AssertProvider<ObjectContentAssert<T>> {
 
-	private final ResolvableType type;
+	private final @Nullable ResolvableType type;
 
 	private final T object;
 
@@ -41,7 +42,7 @@ public final class ObjectContent<T> implements AssertProvider<ObjectContentAsser
 	 * @param type the type under test (or {@code null} if not known)
 	 * @param object the actual object content
 	 */
-	public ObjectContent(ResolvableType type, T object) {
+	public ObjectContent(@Nullable ResolvableType type, T object) {
 		Assert.notNull(object, "'object' must not be null");
 		this.type = type;
 		this.object = object;

core/spring-boot-test/src/main/java/org/springframework/boot/test/json/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Support for testing JSON.
  */
+@NullMarked
 package org.springframework.boot.test.json;
+
+import org.jspecify.annotations.NullMarked;

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

@@ -22,6 +22,8 @@ import java.net.MalformedURLException;
 import java.net.URL;
 import java.nio.file.Files;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.core.io.FileSystemResourceLoader;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.ResourceLoader;
@@ -42,7 +44,7 @@ public class SpringBootMockServletContext extends MockServletContext {
 
 	private final ResourceLoader resourceLoader;
 
-	private File emptyRootDirectory;
+	private @Nullable File emptyRootDirectory;
 
 	public SpringBootMockServletContext(String resourceBasePath) {
 		this(resourceBasePath, new FileSystemResourceLoader());
@@ -86,7 +88,7 @@ public class SpringBootMockServletContext extends MockServletContext {
 	}
 
 	@Override
-	public URL getResource(String path) throws MalformedURLException {
+	public @Nullable URL getResource(String path) throws MalformedURLException {
 		URL resource = super.getResource(path);
 		if (resource == null && "/".equals(path)) {
 			// Liquibase assumes that "/" always exists, if we don't have a directory

core/spring-boot-test/src/main/java/org/springframework/boot/test/mock/web/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Mock web classes specific to Spring Boot.
  */
+@NullMarked
 package org.springframework.boot.test.mock.web;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/rsocket/server/package-info.java

@@ -17,4 +17,7 @@
 /**
  * RSocket server test utilities and support classes.
  */
+@NullMarked
 package org.springframework.boot.test.rsocket.server;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/system/OutputCapture.java

@@ -27,6 +27,8 @@ import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.ansi.AnsiOutput;
 import org.springframework.boot.ansi.AnsiOutput.Enabled;
 import org.springframework.util.Assert;
@@ -47,7 +49,7 @@ class OutputCapture implements CapturedOutput {
 
 	private final Deque<SystemCapture> systemCaptures = new ArrayDeque<>();
 
-	private AnsiOutputState ansiOutputState;
+	private @Nullable AnsiOutputState ansiOutputState;
 
 	private final AtomicReference<String> out = new AtomicReference<>(null);
 
@@ -329,7 +331,7 @@ class OutputCapture implements CapturedOutput {
 			AnsiOutput.setEnabled(this.saved);
 		}
 
-		static AnsiOutputState saveAndDisable() {
+		static @Nullable AnsiOutputState saveAndDisable() {
 			if (!ClassUtils.isPresent("org.springframework.boot.ansi.AnsiOutput",
 					OutputCapture.class.getClassLoader())) {
 				return null;

core/spring-boot-test/src/main/java/org/springframework/boot/test/system/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Classes for {@link java.lang.System System}-related testing.
  */
+@NullMarked
 package org.springframework.boot.test.system;
+
+import org.jspecify.annotations.NullMarked;

core/spring-boot-test/src/main/java/org/springframework/boot/test/util/ApplicationContextTestUtils.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.test.util;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 
@@ -31,7 +33,7 @@ public abstract class ApplicationContextTestUtils {
 	 * Closes this {@link ApplicationContext} and its parent hierarchy if any.
 	 * @param context the context to close (can be {@code null})
 	 */
-	public static void closeAll(ApplicationContext context) {
+	public static void closeAll(@Nullable ApplicationContext context) {
 		if (context != null) {
 			if (context instanceof ConfigurableApplicationContext configurableContext) {
 				configurableContext.close();

core/spring-boot-test/src/main/java/org/springframework/boot/test/util/TestPropertyValues.java

@@ -27,6 +27,8 @@ import java.util.function.Function;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.context.properties.source.ConfigurationPropertySources;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
@@ -37,6 +39,7 @@ import org.springframework.core.env.MutablePropertySources;
 import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
 import org.springframework.core.env.SystemEnvironmentPropertySource;
+import org.springframework.lang.Contract;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
@@ -76,7 +79,7 @@ public final class TestPropertyValues {
 	 * @return a new {@link TestPropertyValues} instance
 	 * @since 2.4.0
 	 */
-	public TestPropertyValues and(Iterable<String> pairs) {
+	public TestPropertyValues and(@Nullable Iterable<String> pairs) {
 		return (pairs != null) ? and(StreamSupport.stream(pairs.spliterator(), false)) : this;
 	}
 
@@ -87,7 +90,7 @@ public final class TestPropertyValues {
 	 * @return a new {@link TestPropertyValues} instance
 	 * @since 2.4.0
 	 */
-	public TestPropertyValues and(Stream<String> pairs) {
+	public TestPropertyValues and(@Nullable Stream<String> pairs) {
 		return (pairs != null) ? and(pairs, Pair::parse) : this;
 	}
 
@@ -97,7 +100,7 @@ public final class TestPropertyValues {
 	 * @return a new {@link TestPropertyValues} instance
 	 * @since 2.4.0
 	 */
-	public TestPropertyValues and(Map<String, String> map) {
+	public TestPropertyValues and(@Nullable Map<String, String> map) {
 		return (map != null) ? and(map.entrySet().stream(), Pair::fromMapEntry) : this;
 	}
 
@@ -110,7 +113,7 @@ public final class TestPropertyValues {
 	 * @return a new {@link TestPropertyValues} instance
 	 * @since 2.4.0
 	 */
-	public <T> TestPropertyValues and(Stream<T> stream, Function<T, Pair> mapper) {
+	public <T> TestPropertyValues and(@Nullable Stream<T> stream, Function<T, @Nullable Pair> mapper) {
 		if (stream == null) {
 			return this;
 		}
@@ -200,8 +203,8 @@ public final class TestPropertyValues {
 
 	@SuppressWarnings("unchecked")
 	private void addToSources(MutablePropertySources sources, Type type, String name) {
-		if (sources.contains(name)) {
-			PropertySource<?> propertySource = sources.get(name);
+		PropertySource<?> propertySource = sources.get(name);
+		if (propertySource != null) {
 			if (propertySource.getClass() == type.getSourceClass()) {
 				((Map<String, Object>) propertySource.getSource()).putAll(this.properties);
 				return;
@@ -232,7 +235,7 @@ public final class TestPropertyValues {
 	 * environment
 	 * @return the new instance
 	 */
-	public static TestPropertyValues of(Iterable<String> pairs) {
+	public static TestPropertyValues of(@Nullable Iterable<String> pairs) {
 		return (pairs != null) ? of(StreamSupport.stream(pairs.spliterator(), false)) : empty();
 	}
 
@@ -244,7 +247,7 @@ public final class TestPropertyValues {
 	 * environment
 	 * @return the new instance
 	 */
-	public static TestPropertyValues of(Stream<String> pairs) {
+	public static TestPropertyValues of(@Nullable Stream<String> pairs) {
 		return (pairs != null) ? of(pairs, Pair::parse) : empty();
 	}
 
@@ -254,7 +257,7 @@ public final class TestPropertyValues {
 	 * @param map the map of properties that need to be added to the environment
 	 * @return the new instance
 	 */
-	public static TestPropertyValues of(Map<String, String> map) {
+	public static TestPropertyValues of(@Nullable Map<String, String> map) {
 		return (map != null) ? of(map.entrySet().stream(), Pair::fromMapEntry) : empty();
 	}
 
@@ -267,7 +270,7 @@ public final class TestPropertyValues {
 	 * {@link Pair}
 	 * @return the new instance
 	 */
-	public static <T> TestPropertyValues of(Stream<T> stream, Function<T, Pair> mapper) {
+	public static <T> TestPropertyValues of(@Nullable Stream<T> stream, Function<T, @Nullable Pair> mapper) {
 		return (stream != null) ? empty().and(stream, mapper) : empty();
 	}
 
@@ -297,9 +300,9 @@ public final class TestPropertyValues {
 
 		private final Class<? extends MapPropertySource> sourceClass;
 
-		private final String suffix;
+		private final @Nullable String suffix;
 
-		Type(Class<? extends MapPropertySource> sourceClass, String suffix) {
+		Type(Class<? extends MapPropertySource> sourceClass, @Nullable String suffix) {
 			this.sourceClass = sourceClass;
 			this.suffix = suffix;
 		}
@@ -321,9 +324,9 @@ public final class TestPropertyValues {
 
 		private final String name;
 
-		private final String value;
+		private final @Nullable String value;
 
-		private Pair(String name, String value) {
+		private Pair(String name, @Nullable String value) {
 			Assert.hasLength(name, "'name' must not be empty");
 			this.name = name;
 			this.value = value;
@@ -333,7 +336,7 @@ public final class TestPropertyValues {
 			properties.put(this.name, this.value);
 		}
 
-		public static Pair parse(String pair) {
+		public static @Nullable Pair parse(String pair) {
 			int index = getSeparatorIndex(pair);
 			String name = (index > 0) ? pair.substring(0, index) : pair;
 			String value = (index > 0) ? pair.substring(index + 1) : "";
@@ -358,7 +361,8 @@ public final class TestPropertyValues {
 		 * @return the {@link Pair} instance or {@code null}
 		 * @since 2.4.0
 		 */
-		public static Pair fromMapEntry(Map.Entry<String, String> entry) {
+		@Contract("!null -> !null")
+		public static @Nullable Pair fromMapEntry(Map.@Nullable Entry<String, String> entry) {
 			return (entry != null) ? of(entry.getKey(), entry.getValue()) : null;
 		}
 
@@ -369,8 +373,8 @@ public final class TestPropertyValues {
 		 * @return the {@link Pair} instance or {@code null}
 		 * @since 2.4.0
 		 */
-		public static Pair of(String name, String value) {
-			if (StringUtils.hasLength(name) || StringUtils.hasLength(value)) {
+		public static @Nullable Pair of(@Nullable String name, @Nullable String value) {
+			if (StringUtils.hasLength(name)) {
 				return new Pair(name, value);
 			}
 			return null;

core/spring-boot-test/src/main/java/org/springframework/boot/test/util/package-info.java

@@ -17,4 +17,7 @@
 /**
  * General purpose test utilities.
  */
+@NullMarked
 package org.springframework.boot.test.util;
+
+import org.jspecify.annotations.NullMarked;