Ignore duplicate @⁠Priority values when determining highest priority

Prior to this commit, DefaultListableBeanFactory's
determineHighestPriorityCandidate() method sometimes failed to
determine the highest priority candidate if duplicate priority
candidates were detected whose priority was not the highest priority in
the candidate set. In addition, the bean registration order affected
the outcome of the algorithm: if the highest priority was detected
before other duplicate priorities were detected, the algorithm
succeeded in determining the highest priority candidate.

This commit addresses those shortcomings by ignoring duplicate
@⁠Priority values unless the duplication is for the highest priority
encountered, in which case a NoUniqueBeanDefinitionException is still
thrown to signal that multiple beans were found with the same "highest
priority".

Closes gh-33733

spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java

@@ -1776,12 +1776,15 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 	 * @param requiredType the target dependency type to match against
 	 * @return the name of the candidate with the highest priority,
 	 * or {@code null} if none found
+	 * @throws NoUniqueBeanDefinitionException if multiple beans are detected with
+	 * the same highest priority value
 	 * @see #getPriority(Object)
 	 */
 	@Nullable
 	protected String determineHighestPriorityCandidate(Map<String, Object> candidates, Class<?> requiredType) {
 		String highestPriorityBeanName = null;
 		Integer highestPriority = null;
+		boolean highestPriorityConflictDetected = false;
 		for (Map.Entry<String, Object> entry : candidates.entrySet()) {
 			String candidateBeanName = entry.getKey();
 			Object beanInstance = entry.getValue();
@@ -1790,13 +1793,12 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 				if (candidatePriority != null) {
 					if (highestPriority != null) {
 						if (candidatePriority.equals(highestPriority)) {
-							throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
-									"Multiple beans found with the same priority ('" + highestPriority +
-									"') among candidates: " + candidates.keySet());
+							highestPriorityConflictDetected = true;
 						}
 						else if (candidatePriority < highestPriority) {
 							highestPriorityBeanName = candidateBeanName;
 							highestPriority = candidatePriority;
+							highestPriorityConflictDetected = false;
 						}
 					}
 					else {
@@ -1806,6 +1808,13 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 				}
 			}
 		}
+
+		if (highestPriorityConflictDetected) {
+			throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
+					"Multiple beans found with the same highest priority (" + highestPriority +
+					") among candidates: " + candidates.keySet());
+
+		}
 		return highestPriorityBeanName;
 	}
 

spring-beans/src/test/java/org/springframework/beans/factory/DefaultListableBeanFactoryTests.java

@@ -1729,18 +1729,73 @@ class DefaultListableBeanFactoryTests {
 		assertThat(bean.getBeanName()).isEqualTo("bd1");
 	}
 
+	/**
+	 * {@code determineHighestPriorityCandidate()} should reject duplicate
+	 * priorities for the highest priority detected.
+	 *
+	 * @see #getBeanByTypeWithMultipleNonHighestPriorityCandidates()
+	 */
 	@Test
-	void getBeanByTypeWithMultiplePriority() {
+	void getBeanByTypeWithMultipleHighestPriorityCandidates() {
 		lbf.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
 		RootBeanDefinition bd1 = new RootBeanDefinition(HighPriorityTestBean.class);
-		RootBeanDefinition bd2 = new RootBeanDefinition(HighPriorityTestBean.class);
+		RootBeanDefinition bd2 = new RootBeanDefinition(LowPriorityTestBean.class);
+		RootBeanDefinition bd3 = new RootBeanDefinition(HighPriorityTestBean.class);
 		lbf.registerBeanDefinition("bd1", bd1);
 		lbf.registerBeanDefinition("bd2", bd2);
+		lbf.registerBeanDefinition("bd3", bd3);
 
 		assertThatExceptionOfType(NoUniqueBeanDefinitionException.class)
 				.isThrownBy(() -> lbf.getBean(TestBean.class))
-				.withMessageContaining("Multiple beans found with the same priority")
-				.withMessageContaining("5"); // conflicting priority
+				.withMessageContaining("Multiple beans found with the same highest priority (5) among candidates: ");
+	}
+
+	/**
+	 * {@code determineHighestPriorityCandidate()} should ignore duplicate
+	 * priorities for any priority other than the highest, and the order in
+	 * which beans is declared should not affect the outcome.
+	 *
+	 * @see #getBeanByTypeWithMultipleHighestPriorityCandidates()
+	 */
+	@Test  // gh-33733
+	void getBeanByTypeWithMultipleNonHighestPriorityCandidates() {
+		getBeanByTypeWithMultipleNonHighestPriorityCandidates(
+				PriorityService1.class,
+				PriorityService2A.class,
+				PriorityService2B.class,
+				PriorityService3.class
+			);
+
+		getBeanByTypeWithMultipleNonHighestPriorityCandidates(
+				PriorityService3.class,
+				PriorityService2B.class,
+				PriorityService2A.class,
+				PriorityService1.class
+			);
+
+		getBeanByTypeWithMultipleNonHighestPriorityCandidates(
+				PriorityService2A.class,
+				PriorityService1.class,
+				PriorityService2B.class,
+				PriorityService3.class
+			);
+
+		getBeanByTypeWithMultipleNonHighestPriorityCandidates(
+				PriorityService2A.class,
+				PriorityService3.class,
+				PriorityService1.class,
+				PriorityService2B.class
+			);
+	}
+
+	private void getBeanByTypeWithMultipleNonHighestPriorityCandidates(Class<?>... classes) {
+		lbf.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
+		for (Class<?> clazz : classes) {
+			lbf.registerBeanDefinition(clazz.getSimpleName(), new RootBeanDefinition(clazz));
+		}
+
+		PriorityService bean = lbf.getBean(PriorityService.class);
+		assertThat(bean).isExactlyInstanceOf(PriorityService1.class);
 	}
 
 	@Test
@@ -3500,6 +3555,26 @@ class DefaultListableBeanFactoryTests {
 	}
 
 
+	interface PriorityService {
+	}
+
+	@Priority(1)
+	static class PriorityService1 implements PriorityService {
+	}
+
+	@Priority(2)
+	static class PriorityService2A implements PriorityService {
+	}
+
+	@Priority(2)
+	static class PriorityService2B implements PriorityService {
+	}
+
+	@Priority(3)
+	static class PriorityService3 implements PriorityService {
+	}
+
+
 	@Priority(5)
 	private static class HighPriorityTestBean extends TestBean {
 	}