Merge branch '6.2.x'

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

@@ -37,6 +37,7 @@ import org.jspecify.annotations.Nullable;
 import org.springframework.beans.BeanMetadataElement;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
 import org.springframework.beans.factory.ObjectFactory;
+import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
 import org.springframework.beans.factory.config.TypedStringValue;
 import org.springframework.util.Assert;
@@ -280,6 +281,25 @@ abstract class AutowireUtils {
 		}
 	}
 
+	/**
+	 * Check the default-candidate status for the specified bean.
+	 * @param beanFactory the bean factory
+	 * @param beanName the name of the bean to check
+	 * @return whether the specified bean qualifies as a default candidate
+	 * @since 6.2.4
+	 * @see AbstractBeanDefinition#isDefaultCandidate()
+	 */
+	public static boolean isDefaultCandidate(ConfigurableBeanFactory beanFactory, String beanName) {
+		try {
+			BeanDefinition mbd = beanFactory.getMergedBeanDefinition(beanName);
+			return (!(mbd instanceof AbstractBeanDefinition abd) || abd.isDefaultCandidate());
+		}
+		catch (NoSuchBeanDefinitionException ex) {
+			// A manually registered singleton instance not backed by a BeanDefinition.
+			return true;
+		}
+	}
+
 
 	/**
 	 * Reflective {@link InvocationHandler} for lazy access to the current target object.

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

@@ -1479,6 +1479,9 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 			if (candidateName == null) {
 				candidateName = determineHighestPriorityCandidate(candidates, requiredType.toClass());
 			}
+			if (candidateName == null) {
+				candidateName = determineDefaultCandidate(candidates);
+			}
 			if (candidateName != null) {
 				Object beanInstance = candidates.get(candidateName);
 				if (beanInstance == null) {
@@ -1939,7 +1942,12 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		if (priorityCandidate != null) {
 			return priorityCandidate;
 		}
-		// Step 4: pick directly registered dependency
+		// Step 4: pick unique default-candidate
+		String defaultCandidate = determineDefaultCandidate(candidates);
+		if (defaultCandidate != null) {
+			return defaultCandidate;
+		}
+		// Step 5: pick directly registered dependency
 		for (Map.Entry<String, Object> entry : candidates.entrySet()) {
 			String candidateName = entry.getKey();
 			Object beanInstance = entry.getValue();
@@ -2097,6 +2105,28 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		return null;
 	}
 
+	/**
+	 * Return a unique "default-candidate" among remaining non-default candidates.
+	 * @param candidates a Map of candidate names and candidate instances
+	 * (or candidate classes if not created yet) that match the required type
+	 * @return the name of the default candidate, or {@code null} if none found
+	 * @since 6.2.4
+	 * @see AbstractBeanDefinition#isDefaultCandidate()
+	 */
+	@Nullable
+	private String determineDefaultCandidate(Map<String, Object> candidates) {
+		String defaultBeanName = null;
+		for (String candidateBeanName : candidates.keySet()) {
+			if (AutowireUtils.isDefaultCandidate(this, candidateBeanName)) {
+				if (defaultBeanName != null) {
+					return null;
+				}
+				defaultBeanName = candidateBeanName;
+			}
+		}
+		return defaultBeanName;
+	}
+
 	/**
 	 * Determine whether the given candidate name matches the bean name or the aliases
 	 * stored in this bean definition.

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

@@ -366,8 +366,18 @@ public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
 					}
 					afterSingletonCreation(beanName);
 				}
+
 				if (newSingleton) {
-					addSingleton(beanName, singletonObject);
+					try {
+						addSingleton(beanName, singletonObject);
+					}
+					catch (IllegalStateException ex) {
+						// Leniently accept same instance if implicitly appeared.
+						Object object = this.singletonObjects.get(beanName);
+						if (singletonObject != object) {
+							throw ex;
+						}
+					}
 				}
 			}
 			return singletonObject;