Add nullability annotations to module/spring-boot-observation

See gh-46587

module/spring-boot-observation/src/main/java/org/springframework/boot/observation/autoconfigure/ObservationHandlerGroups.java

@@ -23,6 +23,7 @@ import java.util.List;
 
 import io.micrometer.observation.ObservationHandler;
 import io.micrometer.observation.ObservationRegistry.ObservationConfig;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.LinkedMultiValueMap;
@@ -52,8 +53,15 @@ class ObservationHandlerGroups {
 
 	void register(ObservationConfig config, List<ObservationHandler<?>> handlers) {
 		MultiValueMap<ObservationHandlerGroup, ObservationHandler<?>> grouped = new LinkedMultiValueMap<>();
+		List<ObservationHandler<?>> unclaimed = new ArrayList<>();
 		for (ObservationHandler<?> handler : handlers) {
-			grouped.add(findGroup(handler), handler);
+			ObservationHandlerGroup group = findGroup(handler);
+			if (group == null) {
+				unclaimed.add(handler);
+			}
+			else {
+				grouped.add(group, handler);
+			}
 		}
 		for (ObservationHandlerGroup group : this.groups) {
 			List<ObservationHandler<?>> members = grouped.get(group);
@@ -61,7 +69,6 @@ class ObservationHandlerGroups {
 				group.registerMembers(config, members);
 			}
 		}
-		List<ObservationHandler<?>> unclaimed = grouped.get(null);
 		if (!CollectionUtils.isEmpty(unclaimed)) {
 			for (ObservationHandler<?> handler : unclaimed) {
 				config.observationHandler(handler);
@@ -69,7 +76,7 @@ class ObservationHandlerGroups {
 		}
 	}
 
-	private ObservationHandlerGroup findGroup(ObservationHandler<?> handler) {
+	private @Nullable ObservationHandlerGroup findGroup(ObservationHandler<?> handler) {
 		for (ObservationHandlerGroup group : this.groups) {
 			if (group.isMember(handler)) {
 				return group;

module/spring-boot-observation/src/main/java/org/springframework/boot/observation/autoconfigure/ObservationRegistryPostProcessor.java

@@ -21,6 +21,7 @@ import io.micrometer.observation.ObservationFilter;
 import io.micrometer.observation.ObservationHandler;
 import io.micrometer.observation.ObservationPredicate;
 import io.micrometer.observation.ObservationRegistry;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.ObjectProvider;
@@ -47,7 +48,7 @@ class ObservationRegistryPostProcessor implements BeanPostProcessor {
 
 	private final ObjectProvider<ObservationFilter> observationFilters;
 
-	private volatile ObservationRegistryConfigurer configurer;
+	private volatile @Nullable ObservationRegistryConfigurer configurer;
 
 	ObservationRegistryPostProcessor(ObjectProvider<ObservationRegistryCustomizer<?>> observationRegistryCustomizers,
 			ObjectProvider<ObservationPredicate> observationPredicates,

module/spring-boot-observation/src/main/java/org/springframework/boot/observation/autoconfigure/SpelValueExpressionResolver.java

@@ -17,11 +17,13 @@
 package org.springframework.boot.observation.autoconfigure;
 
 import io.micrometer.common.annotation.ValueExpressionResolver;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.expression.Expression;
 import org.springframework.expression.ExpressionParser;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
 import org.springframework.expression.spel.support.SimpleEvaluationContext;
+import org.springframework.util.Assert;
 
 /**
  * A {@link Expression SpEL}-based {@link ValueExpressionResolver}.
@@ -33,11 +35,13 @@ class SpelValueExpressionResolver implements ValueExpressionResolver {
 	private final ExpressionParser expressionParser = new SpelExpressionParser();
 
 	@Override
-	public String resolve(String expression, Object parameter) {
+	public String resolve(String expression, @Nullable Object parameter) {
 		try {
 			SimpleEvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
 			Expression expressionToEvaluate = this.expressionParser.parseExpression(expression);
-			return expressionToEvaluate.getValue(context, parameter, String.class);
+			String value = expressionToEvaluate.getValue(context, parameter, String.class);
+			Assert.state(value != null, "'value' must not be null");
+			return value;
 		}
 		catch (Exception ex) {
 			throw new IllegalStateException("Unable to evaluate SpEL expression '%s'".formatted(expression), ex);

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

@@ -17,4 +17,7 @@
 /**
  * Auto-configuration for Micrometer Observation.
  */
+@NullMarked
 package org.springframework.boot.observation.autoconfigure;
+
+import org.jspecify.annotations.NullMarked;