Reinstate Bean Override support for replacing a manually registered singleton

Closes gh-33678

spring-test/src/main/java/org/springframework/test/context/bean/override/BeanOverrideBeanFactoryPostProcessor.java

@@ -128,7 +128,11 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 			if (beanName != null) {
 				// We are overriding an existing bean by-type.
 				beanName = BeanFactoryUtils.transformedBeanName(beanName);
-				existingBeanDefinition = beanFactory.getBeanDefinition(beanName);
+				// If we are overriding a manually registered singleton, we won't find
+				// an existing bean definition.
+				if (beanFactory.containsBeanDefinition(beanName)) {
+					existingBeanDefinition = beanFactory.getBeanDefinition(beanName);
+				}
 			}
 			else {
 				// We will later generate a name for the nonexistent bean, but since NullAway
@@ -150,6 +154,12 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 			}
 		}
 
+		// Ensure we don't have any manually registered singletons registered, since we
+		// register a bean override instance as a manual singleton at the end of this method.
+		if (beanFactory.containsSingleton(beanName)) {
+			destroySingleton(beanFactory, beanName);
+		}
+
 		if (existingBeanDefinition != null) {
 			// Validate the existing bean definition.
 			//
@@ -333,6 +343,10 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 		// Since the isSingleton() check above may have registered a singleton as a side
 		// effect -- for example, for a FactoryBean -- we need to destroy the singleton,
 		// because we later manually register a bean override instance as a singleton.
+		destroySingleton(beanFactory, beanName);
+	}
+
+	private static void destroySingleton(ConfigurableListableBeanFactory beanFactory, String beanName) {
 		if (beanFactory instanceof DefaultListableBeanFactory dlbf) {
 			dlbf.destroySingleton(beanName);
 		}

spring-test/src/test/java/org/springframework/test/context/bean/override/mockito/MockitoBeanManuallyRegisteredSingletonTests.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2002-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.context.bean.override.mockito;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.context.ApplicationContextInitializer;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.test.context.bean.override.mockito.MockitoBeanManuallyRegisteredSingletonTests.SingletonRegistrar;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.when;
+
+/**
+ * Verifies support for overriding a manually registered singleton bean with
+ * {@link MockitoBean @MockitoBean}.
+ *
+ * @author Andy Wilkinson
+ * @author Sam Brannen
+ * @since 6.2
+ */
+@SpringJUnitConfig(initializers = SingletonRegistrar.class)
+class MockitoBeanManuallyRegisteredSingletonTests {
+
+	@MockitoBean
+	MessageService messageService;
+
+	@Test
+	void test() {
+		when(messageService.getMessage()).thenReturn("override");
+		assertThat(messageService.getMessage()).isEqualTo("override");
+	}
+
+	static class SingletonRegistrar implements ApplicationContextInitializer<ConfigurableApplicationContext> {
+
+		@Override
+		public void initialize(ConfigurableApplicationContext applicationContext) {
+			applicationContext.getBeanFactory().registerSingleton("messageService", new MessageService());
+		}
+	}
+
+	static class MessageService {
+
+		String getMessage() {
+			return "production";
+		}
+	}
+
+}