Allow MockitoBean to create a mock for a bean that does not exist

This commit aligns the by-type lookup for bean overrides with what was
done when a bean name is present. It now correctly generate a bean
name rather than failing because no bean of that type exists.

Closes gh-32990

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

@@ -34,6 +34,8 @@ import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.config.DependencyDescriptor;
 import org.springframework.beans.factory.config.SmartInstantiationAwareBeanPostProcessor;
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
+import org.springframework.beans.factory.support.BeanNameGenerator;
+import org.springframework.beans.factory.support.DefaultBeanNameGenerator;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.core.Ordered;
 import org.springframework.core.PriorityOrdered;
@@ -63,6 +65,8 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 
 	private final BeanOverrideRegistrar overrideRegistrar;
 
+	private final BeanNameGenerator beanNameGenerator = new DefaultBeanNameGenerator();
+
 
 	/**
 	 * Create a new {@code BeanOverrideBeanFactoryPostProcessor} instance with
@@ -133,18 +137,11 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 		String beanNameIncludingFactory;
 		BeanDefinition existingBeanDefinition = null;
 		if (beanName == null) {
-			Set<String> candidateNames = getExistingBeanNamesByType(beanFactory, overrideMetadata, true);
-			int candidateCount = candidateNames.size();
-			if (candidateCount != 1) {
-				Field field = overrideMetadata.getField();
-				throw new IllegalStateException("Unable to select a bean definition to override: found " +
-						candidateCount + " bean definitions of type " + overrideMetadata.getBeanType() +
-						" (as required by annotated field '" + field.getDeclaringClass().getSimpleName() +
-						"." + field.getName() + "')" + (candidateCount > 0 ? ": " + candidateNames : ""));
-			}
-			beanNameIncludingFactory = candidateNames.iterator().next();
+			beanNameIncludingFactory = getBeanNameForType(beanFactory, registry, overrideMetadata, beanDefinition, enforceExistingDefinition);
 			beanName = BeanFactoryUtils.transformedBeanName(beanNameIncludingFactory);
-			existingBeanDefinition = beanFactory.getBeanDefinition(beanName);
+			if (registry.containsBeanDefinition(beanName)) {
+				existingBeanDefinition = beanFactory.getBeanDefinition(beanName);
+			}
 		}
 		else {
 			Set<String> candidates = getExistingBeanNamesByType(beanFactory, overrideMetadata, false);
@@ -176,6 +173,30 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 		this.overrideRegistrar.registerNameForMetadata(overrideMetadata, beanNameIncludingFactory);
 	}
 
+	private String getBeanNameForType(ConfigurableListableBeanFactory beanFactory, BeanDefinitionRegistry registry,
+			OverrideMetadata overrideMetadata, RootBeanDefinition beanDefinition, boolean enforceExistingDefinition) {
+		Set<String> candidateNames = getExistingBeanNamesByType(beanFactory, overrideMetadata, true);
+		int candidateCount = candidateNames.size();
+		if (candidateCount == 1) {
+			return candidateNames.iterator().next();
+		}
+		else if (candidateCount == 0) {
+			if (enforceExistingDefinition) {
+				Field field = overrideMetadata.getField();
+				throw new IllegalStateException(
+						"Unable to override bean: no bean definitions of type %s (as required by annotated field '%s.%s')"
+								.formatted(overrideMetadata.getBeanType(), field.getDeclaringClass().getSimpleName(), field.getName()));
+			}
+			return this.beanNameGenerator.generateBeanName(beanDefinition, registry);
+		}
+		Field field = overrideMetadata.getField();
+		throw new IllegalStateException(String.format(
+				"Unable to select a bean definition to override: found %s bean definitions of type %s " +
+						"(as required by annotated field '%s.%s'): %s",
+				candidateCount, overrideMetadata.getBeanType(), field.getDeclaringClass().getSimpleName(),
+				field.getName(), candidateNames));
+	}
+
 	/**
 	 * Check that the expected bean name is registered and matches the type to override.
 	 * <p>If so, put the override metadata in the early tracking map.
@@ -209,7 +230,7 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 	}
 
 	RootBeanDefinition createBeanDefinition(OverrideMetadata metadata) {
-		RootBeanDefinition definition = new RootBeanDefinition();
+		RootBeanDefinition definition = new RootBeanDefinition(metadata.getBeanType().resolve());
 		definition.setTargetType(metadata.getBeanType());
 		definition.setQualifiedElement(metadata.getField());
 		return definition;

spring-test/src/test/java/org/springframework/test/context/bean/override/convention/FailingTestBeanByTypeIntegrationTests.java

@@ -51,8 +51,8 @@ class FailingTestBeanByTypeIntegrationTests {
 				cause(
 					instanceOf(IllegalStateException.class),
 					message("""
-						Unable to select a bean definition to override: found 0 bean definitions \
-						of type %s (as required by annotated field '%s.example')"""
+						Unable to override bean: no bean definitions of type \
+						%s (as required by annotated field '%s.example')"""
 							.formatted(ExampleService.class.getName(), testClass.getSimpleName())))));
 	}
 

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

@@ -43,19 +43,6 @@ import static org.junit.platform.testkit.engine.TestExecutionResultConditions.me
  */
 class FailingMockitoBeanByTypeIntegrationTests {
 
-	@Test
-	void zeroCandidates() {
-		Class<?> testClass = ZeroCandidatesTestCase.class;
-		EngineTestKitUtils.executeTestsForClass(testClass).assertThatEvents().haveExactly(1,
-			finishedWithFailure(
-				cause(
-					instanceOf(IllegalStateException.class),
-					message("""
-						Unable to select a bean definition to override: found 0 bean definitions \
-						of type %s (as required by annotated field '%s.example')"""
-							.formatted(ExampleService.class.getName(), testClass.getSimpleName())))));
-	}
-
 	@Test
 	void tooManyCandidates() {
 		Class<?> testClass = TooManyCandidatesTestCase.class;
@@ -70,22 +57,6 @@ class FailingMockitoBeanByTypeIntegrationTests {
 	}
 
 
-	@SpringJUnitConfig
-	static class ZeroCandidatesTestCase {
-
-		@MockitoBean
-		ExampleService example;
-
-		@Test
-		void test() {
-			assertThat(example).isNotNull();
-		}
-
-		@Configuration
-		static class Config {
-		}
-	}
-
 	@SpringJUnitConfig
 	static class TooManyCandidatesTestCase {
 

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

@@ -49,6 +49,9 @@ import static org.mockito.Mockito.verifyNoMoreInteractions;
 @SpringJUnitConfig
 public class MockitoBeanByTypeIntegrationTests {
 
+	@MockitoBean
+	AnotherService serviceIsNotABean;
+
 	@MockitoBean
 	ExampleService anyNameForService;
 
@@ -60,6 +63,17 @@ public class MockitoBeanByTypeIntegrationTests {
 	@CustomQualifier
 	StringBuilder ambiguousMeta;
 
+	@Test
+	void mockIsCreatedWhenNoCandidateIsFound() {
+		assertThat(this.serviceIsNotABean)
+				.satisfies(o -> assertThat(Mockito.mockingDetails(o).isMock()).as("isMock").isTrue());
+
+		when(this.serviceIsNotABean.hello()).thenReturn("Mocked hello");
+
+		assertThat(this.serviceIsNotABean.hello()).isEqualTo("Mocked hello");
+		verify(this.serviceIsNotABean, times(1)).hello();
+		verifyNoMoreInteractions(this.serviceIsNotABean);
+	}
 
 	@Test
 	void overrideIsFoundByType(ApplicationContext ctx) {
@@ -109,6 +123,11 @@ public class MockitoBeanByTypeIntegrationTests {
 		verifyNoMoreInteractions(this.ambiguousMeta);
 	}
 
+	interface AnotherService {
+
+		String hello();
+
+	}
 
 	@Configuration
 	static class Config {