Reinstate Bean Override support for replacing a manually registered singleton

Closes gh-33678

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";
+		}
+	}
+
+}