Consistently override (and prevent overriding for) aliases with bean definitions

Closes gh-27866

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,7 +49,7 @@ public class BeanDefinitionOverrideException extends BeanDefinitionStoreExceptio
 
 		super(beanDefinition.getResourceDescription(), beanName,
 				"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
-				"': There is already [" + existingDefinition + "] bound.");
+				"' since there is already [" + existingDefinition + "] bound.");
 		this.beanDefinition = beanDefinition;
 		this.existingDefinition = existingDefinition;
 	}

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

@@ -994,6 +994,23 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 			this.beanDefinitionMap.put(beanName, beanDefinition);
 		}
 		else {
+			if (isAlias(beanName)) {
+				if (!isAllowBeanDefinitionOverriding()) {
+					String aliasedName = canonicalName(beanName);
+					if (containsBeanDefinition(aliasedName)) {  // alias for existing bean definition
+						throw new BeanDefinitionOverrideException(
+								beanName, beanDefinition, getBeanDefinition(aliasedName));
+					}
+					else {  // alias pointing to non-existing bean definition
+						throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
+								"Cannot register bean definition for bean '" + beanName +
+								"' since there is already an alias for bean '" + aliasedName + "' bound.");
+					}
+				}
+				else {
+					removeAlias(beanName);
+				}
+			}
 			if (hasBeanCreationStarted()) {
 				// Cannot modify startup-time collection elements anymore (for stable iteration)
 				synchronized (this.beanDefinitionMap) {

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

@@ -854,8 +854,11 @@ class DefaultListableBeanFactoryTests {
 		lbf.registerBeanDefinition("test", new RootBeanDefinition(NestedTestBean.class));
 		lbf.registerAlias("otherTest", "test2");
 		lbf.registerAlias("test", "test2");
+		lbf.registerAlias("test", "testX");
+		lbf.registerBeanDefinition("testX", new RootBeanDefinition(TestBean.class));
 		assertThat(lbf.getBean("test")).isInstanceOf(NestedTestBean.class);
 		assertThat(lbf.getBean("test2")).isInstanceOf(NestedTestBean.class);
+		assertThat(lbf.getBean("testX")).isInstanceOf(TestBean.class);
 	}
 
 	@Test
@@ -864,6 +867,7 @@ class DefaultListableBeanFactoryTests {
 		BeanDefinition oldDef = new RootBeanDefinition(TestBean.class);
 		BeanDefinition newDef = new RootBeanDefinition(NestedTestBean.class);
 		lbf.registerBeanDefinition("test", oldDef);
+		lbf.registerAlias("test", "testX");
 		assertThatExceptionOfType(BeanDefinitionOverrideException.class).isThrownBy(() ->
 				lbf.registerBeanDefinition("test", newDef))
 				.satisfies(ex -> {
@@ -871,6 +875,13 @@ class DefaultListableBeanFactoryTests {
 					assertThat(ex.getBeanDefinition()).isEqualTo(newDef);
 					assertThat(ex.getExistingDefinition()).isEqualTo(oldDef);
 				});
+		assertThatExceptionOfType(BeanDefinitionOverrideException.class).isThrownBy(() ->
+						lbf.registerBeanDefinition("testX", newDef))
+				.satisfies(ex -> {
+					assertThat(ex.getBeanName()).isEqualTo("testX");
+					assertThat(ex.getBeanDefinition()).isEqualTo(newDef);
+					assertThat(ex.getExistingDefinition()).isEqualTo(oldDef);
+				});
 	}
 
 	@Test