Relax singleton enforcement for Bean Overrides in the TestContext framework

In gh-33602, we introduced strict singleton enforcement for bean
overrides -- for example, for @⁠MockitoBean, @⁠TestBean, etc. However,
the use of BeanFactory#isSingleton(beanName) can result in a
BeanCreationException for certain beans, such as a Spring Data JPA
FactoryBean for a JpaRepository.

In light of that, this commit relaxes the singleton enforcement in
BeanOverrideBeanFactoryPostProcessor by only checking the result of
BeanDefinition#isSingleton() for existing bean definitions.

This commit also updates the Javadoc and reference documentation to
reflect the status quo.

See gh-33602
Closes gh-33800

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

@@ -197,8 +197,7 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 		// Now we have an instance (the override) that we can manually register as a singleton.
 		//
 		// However, we need to remove any existing singleton instance -- for example, a
-		// manually registered singleton or a singleton that was registered as a side effect
-		// of the isSingleton() check in validateBeanDefinition().
+		// manually registered singleton.
 		//
 		// As a bonus, by manually registering a singleton during "AOT processing", we allow
 		// GenericApplicationContext's preDetermineBeanType() method to transparently register
@@ -334,10 +333,18 @@ class BeanOverrideBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
 	/**
 	 * Validate that the {@link BeanDefinition} for the supplied bean name is suitable
 	 * for being replaced by a bean override.
+	 * <p>If there is no registered {@code BeanDefinition} for the supplied bean name,
+	 * no validation is performed.
 	 */
 	private static void validateBeanDefinition(ConfigurableListableBeanFactory beanFactory, String beanName) {
-		Assert.state(beanFactory.isSingleton(beanName),
-				() -> "Unable to override bean '" + beanName + "': only singleton beans can be overridden.");
+		// Due to https://github.com/spring-projects/spring-framework/issues/33800, we do NOT invoke
+		// beanFactory.isSingleton(beanName), since doing so can result in a BeanCreationException for
+		// certain beans -- for example, a Spring Data FactoryBean for a JpaRepository.
+		if (beanFactory.containsBeanDefinition(beanName)) {
+			BeanDefinition beanDefinition = beanFactory.getBeanDefinition(beanName);
+			Assert.state(beanDefinition.isSingleton(),
+					() -> "Unable to override bean '" + beanName + "': only singleton beans can be overridden.");
+		}
 	}
 
 	private static void destroySingleton(ConfigurableListableBeanFactory beanFactory, String beanName) {

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

@@ -98,7 +98,10 @@ import org.springframework.test.context.bean.override.BeanOverride;
  * }</code></pre>
  *
  * <p><strong>NOTE</strong>: Only <em>singleton</em> beans can be overridden.
- * Any attempt to override a non-singleton bean will result in an exception.
+ * Any attempt to override a non-singleton bean will result in an exception. When
+ * overriding a bean created by a {@link org.springframework.beans.factory.FactoryBean
+ * FactoryBean}, the {@code FactoryBean} will be replaced with a singleton bean
+ * corresponding to the value returned from the {@code @TestBean} factory method.
  *
  * @author Simon Baslé
  * @author Stephane Nicoll

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

@@ -52,8 +52,11 @@ import org.springframework.test.context.bean.override.BeanOverride;
  * registered directly}) will not be found, and a mocked bean will be added to
  * the context alongside the existing dependency.
  *
- * <p><strong>NOTE</strong>: Only <em>singleton</em> beans can be overridden.
- * Any attempt to mock a non-singleton bean will result in an exception.
+ * <p><strong>NOTE</strong>: Only <em>singleton</em> beans can be mocked.
+ * Any attempt to mock a non-singleton bean will result in an exception. When
+ * mocking a bean created by a {@link org.springframework.beans.factory.FactoryBean
+ * FactoryBean}, the {@code FactoryBean} will be replaced with a singleton mock
+ * of the type of object created by the {@code FactoryBean}.
  *
  * @author Simon Baslé
  * @author Sam Brannen

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

@@ -43,8 +43,11 @@ import org.springframework.test.context.bean.override.BeanOverride;
  * {@link org.springframework.beans.factory.config.ConfigurableListableBeanFactory#registerResolvableDependency(Class, Object)
  * registered directly} as resolvable dependencies.
  *
- * <p><strong>NOTE</strong>: Only <em>singleton</em> beans can be spied.
- * Any attempt to create a spy for a non-singleton bean will result in an exception.
+ * <p><strong>NOTE</strong>: Only <em>singleton</em> beans can be spied. Any attempt
+ * to create a spy for a non-singleton bean will result in an exception. When
+ * creating a spy for a {@link org.springframework.beans.factory.FactoryBean FactoryBean},
+ * a spy will be created for the object created by the {@code FactoryBean}, not
+ * for the {@code FactoryBean} itself.
  *
  * @author Simon Baslé
  * @author Sam Brannen