Consistently resolve unique default candidate bean

Closes gh-34432

Author: jhoeller

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

@@ -35,6 +35,7 @@
 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.lang.Nullable;
@@ -279,6 +280,25 @@ public static boolean isAutowireCandidate(ConfigurableBeanFactory beanFactory, S
 		}
 	}
 
+	/**
+	 * 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

@@ -1497,6 +1497,9 @@ else if (candidateNames.length > 1) {
 			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) {
@@ -1967,7 +1970,12 @@ protected String determineAutowireCandidate(Map<String, Object> candidates, Depe
 		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();
@@ -2128,6 +2136,28 @@ protected Integer getPriority(Object beanInstance) {
 		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/test/java/org/springframework/beans/factory/DefaultListableBeanFactoryTests.java

@@ -1658,12 +1658,40 @@ void getBeanByTypeWithPrimary() {
 		bd2.setPrimary(true);
 		lbf.registerBeanDefinition("bd1", bd1);
 		lbf.registerBeanDefinition("bd2", bd2);
+		lbf.registerSingleton("bd3", new TestBean());
 
 		TestBean bean = lbf.getBean(TestBean.class);
 		assertThat(bean.getBeanName()).isEqualTo("bd2");
 		assertThat(lbf.containsSingleton("bd1")).isFalse();
 	}
 
+	@Test
+	void getBeanByTypeWithUniqueNonDefaultDefinition() {
+		RootBeanDefinition bd1 = new RootBeanDefinition(TestBean.class);
+		bd1.setDefaultCandidate(false);
+		bd1.setLazyInit(true);
+		RootBeanDefinition bd2 = new RootBeanDefinition(TestBean.class);
+		lbf.registerBeanDefinition("bd1", bd1);
+		lbf.registerBeanDefinition("bd2", bd2);
+
+		TestBean bean = lbf.getBean(TestBean.class);
+		assertThat(bean.getBeanName()).isEqualTo("bd2");
+		assertThat(lbf.containsSingleton("bd1")).isFalse();
+	}
+
+	@Test
+	void getBeanByTypeWithUniqueNonDefaultSingleton() {
+		RootBeanDefinition bd1 = new RootBeanDefinition(TestBean.class);
+		bd1.setDefaultCandidate(false);
+		bd1.setLazyInit(true);
+		lbf.registerBeanDefinition("bd1", bd1);
+		lbf.registerSingleton("bd2", new TestBean());
+
+		TestBean bean = lbf.getBean(TestBean.class);
+		assertThat(bean.getBeanName()).isNull();
+		assertThat(lbf.containsSingleton("bd1")).isFalse();
+	}
+
 	@Test
 	@SuppressWarnings("rawtypes")
 	void getFactoryBeanByTypeWithPrimary() {