Avoid Autowired shortcut resolution for NullBean values

Includes getBean documentation against NullBean values.

Closes spring-projectsgh-30485

spring-beans/src/main/java/org/springframework/beans/factory/BeanFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -136,7 +136,10 @@ public interface BeanFactory {
 	 * <p>Translates aliases back to the corresponding canonical bean name.
 	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
 	 * @param name the name of the bean to retrieve
-	 * @return an instance of the bean
+	 * @return an instance of the bean.
+	 * Note that the return value will never be {@code null} but possibly a stub for
+	 * {@code null} returned from a factory method, to be checked via {@code equals(null)}.
+	 * Consider using {@link #getBeanProvider(Class)} for resolving optional dependencies.
 	 * @throws NoSuchBeanDefinitionException if there is no bean with the specified name
 	 * @throws BeansException if the bean could not be obtained
 	 */
@@ -152,7 +155,11 @@ public interface BeanFactory {
 	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
 	 * @param name the name of the bean to retrieve
 	 * @param requiredType type the bean must match; can be an interface or superclass
-	 * @return an instance of the bean
+	 * @return an instance of the bean.
+	 * Note that the return value will never be {@code null}. In case of a stub for
+	 * {@code null} from a factory method having been resolved for the requested bean, a
+	 * {@code BeanNotOfRequiredTypeException} against the NullBean stub will be raised.
+	 * Consider using {@link #getBeanProvider(Class)} for resolving optional dependencies.
 	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
 	 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
 	 * @throws BeansException if the bean could not be created

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -719,7 +719,7 @@ private Object resolveFieldValue(Field field, Object bean, @Nullable String bean
 					if (value != null || this.required) {
 						cachedFieldValue = desc;
 						registerDependentBeans(beanName, autowiredBeanNames);
-						if (autowiredBeanNames.size() == 1) {
+						if (value != null && autowiredBeanNames.size() == 1) {
 							String autowiredBeanName = autowiredBeanNames.iterator().next();
 							if (beanFactory.containsBean(autowiredBeanName) &&
 									beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
@@ -829,7 +829,7 @@ private Object[] resolveMethodArguments(Method method, Object bean, @Nullable St
 							Class<?>[] paramTypes = method.getParameterTypes();
 							for (int i = 0; i < paramTypes.length; i++) {
 								String autowiredBeanName = it.next();
-								if (beanFactory.containsBean(autowiredBeanName) &&
+								if (arguments[i] != null && beanFactory.containsBean(autowiredBeanName) &&
 										beanFactory.isTypeMatch(autowiredBeanName, paramTypes[i])) {
 									cachedMethodArguments[i] = new ShortcutDependencyDescriptor(
 											descriptors[i], autowiredBeanName, paramTypes[i]);

spring-beans/src/test/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessorTests.java

@@ -130,6 +130,26 @@ void resourceInjection() {
 		assertThat(bean.getTestBean2()).isSameAs(tb);
 	}
 
+	@Test
+	void resourceInjectionWithNullBean() {
+		RootBeanDefinition bd = new RootBeanDefinition(NonPublicResourceInjectionBean.class);
+		bd.setScope(BeanDefinition.SCOPE_PROTOTYPE);
+		bf.registerBeanDefinition("annotatedBean", bd);
+		RootBeanDefinition tb = new RootBeanDefinition(NullFactoryMethods.class);
+		tb.setFactoryMethodName("createTestBean");
+		bf.registerBeanDefinition("testBean", tb);
+
+		NonPublicResourceInjectionBean bean = (NonPublicResourceInjectionBean) bf.getBean("annotatedBean");
+		assertThat(bean.getTestBean()).isNull();
+		assertThat(bean.getTestBean2()).isNull();
+		assertThat(bean.getTestBean3()).isNull();
+
+		bean = (NonPublicResourceInjectionBean) bf.getBean("annotatedBean");
+		assertThat(bean.getTestBean()).isNull();
+		assertThat(bean.getTestBean2()).isNull();
+		assertThat(bean.getTestBean3()).isNull();
+	}
+
 	@Test
 	void extendedResourceInjection() {
 		RootBeanDefinition bd = new RootBeanDefinition(TypedExtendedResourceInjectionBean.class);