Improve null-safety of core/spring-boot-test

See gh-46926

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

@@ -40,7 +40,7 @@ import org.springframework.util.ClassUtils;
  */
 public final class AnnotatedClassFinder {
 
-	private static final Map<String, Class<?>> cache = Collections.synchronizedMap(new Cache(40));
+	private static final Map<String, @Nullable Class<?>> cache = Collections.synchronizedMap(new Cache(40));
 
 	private final Class<? extends Annotation> annotationType;
 
@@ -110,7 +110,7 @@ public final class AnnotatedClassFinder {
 	/**
 	 * Cache implementation based on {@link LinkedHashMap}.
 	 */
-	private static class Cache extends LinkedHashMap<String, Class<?>> {
+	private static class Cache extends LinkedHashMap<String, @Nullable Class<?>> {
 
 		private final int maxSize;
 
@@ -120,7 +120,7 @@ public final class AnnotatedClassFinder {
 		}
 
 		@Override
-		protected boolean removeEldestEntry(Map.Entry<String, Class<?>> eldest) {
+		protected boolean removeEldestEntry(Map.Entry<String, @Nullable Class<?>> eldest) {
 			return size() > this.maxSize;
 		}
 

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

@@ -553,8 +553,10 @@ public class SpringBootContextLoader extends AbstractContextLoader implements Ao
 
 				@Override
 				public void failed(@Nullable ConfigurableApplicationContext context, Throwable exception) {
+					if (context != null) {
 						ContextLoaderHook.this.failedContexts.add(context);
 					}
+				}
 
 			};
 		}

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

@@ -432,7 +432,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		this.runnerConfiguration.beanRegistrations.forEach((registration) -> registration.apply(context));
 		this.runnerConfiguration.initializers.forEach((initializer) -> initializer.initialize(context));
 		if (!CollectionUtils.isEmpty(this.runnerConfiguration.configurations)) {
-			BiConsumer<Class<?>, String> registrar = getRegistrar(context);
+			BiConsumer<Class<?>, @Nullable String> registrar = getRegistrar(context);
 			for (Configurations configurations : Configurations.collate(this.runnerConfiguration.configurations)) {
 				for (Class<?> beanClass : Configurations.getClasses(configurations)) {
 					String beanName = configurations.getBeanName(beanClass);
@@ -445,7 +445,7 @@ public abstract class AbstractApplicationContextRunner<SELF extends AbstractAppl
 		}
 	}
 
-	private BiConsumer<Class<?>, String> getRegistrar(C context) {
+	private BiConsumer<Class<?>, @Nullable String> getRegistrar(C context) {
 		if (context instanceof BeanDefinitionRegistry registry) {
 			return new AnnotatedBeanDefinitionReader(registry, context.getEnvironment())::registerBean;
 		}

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

@@ -139,7 +139,10 @@ class OutputCapture implements CapturedOutput {
 	 */
 	void reset() {
 		clearExisting();
-		this.systemCaptures.peek().reset();
+		SystemCapture peeked = this.systemCaptures.peek();
+		if (peeked != null) {
+			peeked.reset();
+		}
 	}
 
 	void clearExisting() {

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

@@ -56,9 +56,9 @@ public final class TestPropertyValues {
 
 	private static final TestPropertyValues EMPTY = new TestPropertyValues(Collections.emptyMap());
 
-	private final Map<String, Object> properties;
+	private final Map<String, @Nullable Object> properties;
 
-	private TestPropertyValues(Map<String, Object> properties) {
+	private TestPropertyValues(Map<String, @Nullable Object> properties) {
 		this.properties = Collections.unmodifiableMap(properties);
 	}
 
@@ -117,7 +117,7 @@ public final class TestPropertyValues {
 		if (stream == null) {
 			return this;
 		}
-		Map<String, Object> properties = new LinkedHashMap<>(this.properties);
+		Map<String, @Nullable Object> properties = new LinkedHashMap<>(this.properties);
 		stream.map(mapper).filter(Objects::nonNull).forEach((pair) -> pair.addTo(properties));
 		return new TestPropertyValues(properties);
 	}
@@ -175,7 +175,7 @@ public final class TestPropertyValues {
 	public void applyToSystemProperties(Runnable action) {
 		applyToSystemProperties(() -> {
 			action.run();
-			return null;
+			return new Object();
 		});
 	}
 
@@ -332,7 +332,7 @@ public final class TestPropertyValues {
 			this.value = value;
 		}
 
-		public void addTo(Map<String, Object> properties) {
+		public void addTo(Map<String, @Nullable Object> properties) {
 			properties.put(this.name, this.value);
 		}