Merge branch '3.5.x'

The issue had already been fixed in 4.0 as part of the nullability
work. As such, this commit doesn't fix anything. It does, however,
merge forward a test and makes some stylistic changes to the code so
that it's aligned with the same code in 3.5.x and 3.4.x

Author: wilkinsona

core/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/OnBeanCondition.java

@@ -472,13 +472,19 @@ private void appendMessageForMatches(StringBuilder reason, Map<String, Collectio
 	}
 
 	private List<String> getPrimaryBeans(Map<String, @Nullable BeanDefinition> beanDefinitions) {
-		return getMatchingBeans(beanDefinitions,
-				(beanDefinition) -> beanDefinition != null && beanDefinition.isPrimary());
+		return getMatchingBeans(beanDefinitions, this::isPrimary);
+	}
+
+	private boolean isPrimary(@Nullable BeanDefinition beanDefinition) {
+		return (beanDefinition != null) && beanDefinition.isPrimary();
 	}
 
 	private List<String> getNonFallbackBeans(Map<String, @Nullable BeanDefinition> beanDefinitions) {
-		return getMatchingBeans(beanDefinitions,
-				Predicate.not((beanDefinition) -> beanDefinition != null && beanDefinition.isFallback()));
+		return getMatchingBeans(beanDefinitions, this::isNotFallback);
+	}
+
+	private boolean isNotFallback(@Nullable BeanDefinition beanDefinition) {
+		return (beanDefinition == null) || !beanDefinition.isFallback();
 	}
 
 	private List<String> getMatchingBeans(Map<String, @Nullable BeanDefinition> beanDefinitions,

core/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionalOnSingleCandidateTests.java

@@ -195,6 +195,16 @@ void singleCandidateMultipleCandidatesOneDefaultCandidate() {
 			});
 	}
 
+	@Test
+	void singleCandidateDoesNotMatchWhenMultipleRegisteredAsSingletonCandidates() {
+		this.contextRunner.withInitializer((context) -> {
+			context.getBeanFactory().registerSingleton("alpha", "alpha");
+			context.getBeanFactory().registerSingleton("bravo", "bravo");
+		})
+			.withUserConfiguration(OnBeanSingleCandidateConfiguration.class)
+			.run((context) -> assertThat(context).doesNotHaveBean("consumer"));
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@ConditionalOnSingleCandidate(String.class)
 	static class OnBeanSingleCandidateConfiguration {