Enforce consistent null check on injection points for pre-existing singleton beans

Includes consistent JSpecify nullness check on fields as optional injection points

Closes gh-34952
See gh-34261

Author: jhoeller

spring-beans/src/main/java/org/springframework/beans/factory/config/DependencyDescriptor.java

@@ -34,6 +34,7 @@
 import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
 import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
+import org.springframework.core.Nullness;
 import org.springframework.core.ParameterNameDiscoverer;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.TypeDescriptor;
@@ -162,7 +163,7 @@ public boolean isRequired() {
 		}
 
 		if (this.field != null) {
-			return !(this.field.getType() == Optional.class || hasNullableAnnotation() ||
+			return !(this.field.getType() == Optional.class || Nullness.forField(this.field) == Nullness.NULLABLE ||
 					(KotlinDetector.isKotlinType(this.field.getDeclaringClass()) && KotlinDelegate.isNullable(this.field)));
 		}
 		else {

spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java

@@ -1656,12 +1656,7 @@ else if (descriptor.supportsLazyResolution()) {
 						if (autowiredBeanNames != null) {
 							autowiredBeanNames.add(dependencyName);
 						}
-						boolean preExisting = containsSingleton(dependencyName);
 						Object dependencyBean = getBean(dependencyName);
-						if (preExisting && dependencyBean instanceof NullBean) {
-							// for backwards compatibility with addCandidateEntry in the regular code path
-							dependencyBean = null;
-						}
 						return resolveInstance(dependencyBean, descriptor, type, dependencyName);
 					}
 				}
@@ -1972,7 +1967,7 @@ private void addCandidateEntry(Map<String, Object> candidates, String candidateN
 		else if (containsSingleton(candidateName) ||
 				(descriptor instanceof StreamDependencyDescriptor streamDescriptor && streamDescriptor.isOrdered())) {
 			Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);
-			candidates.put(candidateName, (beanInstance instanceof NullBean ? null : beanInstance));
+			candidates.put(candidateName, beanInstance);
 		}
 		else {
 			candidates.put(candidateName, getType(candidateName));

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

@@ -974,7 +974,9 @@ void constructorResourceInjectionWithCandidateAndNoFallback() {
 		bf.registerBeanDefinition("testBean", tb);
 
 		bf.getBean("testBean");
-		assertThat(bf.getBean("annotatedBean", ConstructorWithoutFallbackBean.class).getTestBean3()).isNull();
+		assertThatExceptionOfType(UnsatisfiedDependencyException.class)
+				.isThrownBy(() -> bf.getBean("annotatedBean"))
+				.satisfies(methodParameterDeclaredOn(ConstructorWithoutFallbackBean.class));
 	}
 
 	@Test
@@ -985,7 +987,9 @@ void constructorResourceInjectionWithNameMatchingCandidateAndNoFallback() {
 		bf.registerBeanDefinition("testBean3", tb);
 
 		bf.getBean("testBean3");
-		assertThat(bf.getBean("annotatedBean", ConstructorWithoutFallbackBean.class).getTestBean3()).isNull();
+		assertThatExceptionOfType(UnsatisfiedDependencyException.class)
+				.isThrownBy(() -> bf.getBean("annotatedBean"))
+				.satisfies(methodParameterDeclaredOn(ConstructorWithoutFallbackBean.class));
 	}
 
 	@Test
@@ -2733,22 +2737,22 @@ private void testBeanQualifierProvider() {}
 	public static class ResourceInjectionBean {
 
 		@Autowired(required = false)
-		private TestBean testBean;
+		private @Nullable TestBean testBean;
 
-		TestBean testBean2;
+		@Nullable TestBean testBean2;
 
 		@Autowired
-		public void setTestBean2(TestBean testBean2) {
+		public void setTestBean2(@Nullable TestBean testBean2) {
 			Assert.state(this.testBean != null, "Wrong initialization order");
 			Assert.state(this.testBean2 == null, "Already called");
 			this.testBean2 = testBean2;
 		}
 
-		public TestBean getTestBean() {
+		public @Nullable TestBean getTestBean() {
 			return this.testBean;
 		}
 
-		public TestBean getTestBean2() {
+		public @Nullable TestBean getTestBean2() {
 			return this.testBean2;
 		}
 	}
@@ -2757,13 +2761,13 @@ public TestBean getTestBean2() {
 	static class NonPublicResourceInjectionBean<T> extends ResourceInjectionBean {
 
 		@Autowired
-		public final ITestBean testBean3 = null;
+		public final @Nullable ITestBean testBean3 = null;
 
-		private T nestedTestBean;
+		private @Nullable T nestedTestBean;
 
-		private ITestBean testBean4;
+		private @Nullable ITestBean testBean4;
 
-		protected BeanFactory beanFactory;
+		protected @Nullable BeanFactory beanFactory;
 
 		public boolean baseInjected = false;
 
@@ -2772,18 +2776,18 @@ public NonPublicResourceInjectionBean() {
 
 		@Override
 		@Autowired
-		public void setTestBean2(TestBean testBean2) {
+		public void setTestBean2(@Nullable TestBean testBean2) {
 			this.testBean2 = testBean2;
 		}
 
 		@Autowired
-		private void inject(ITestBean testBean4, T nestedTestBean) {
+		private void inject(@Nullable ITestBean testBean4, @Nullable T nestedTestBean) {
 			this.testBean4 = testBean4;
 			this.nestedTestBean = nestedTestBean;
 		}
 
 		@Autowired
-		private void inject(ITestBean testBean4) {
+		private void inject(@Nullable ITestBean testBean4) {
 			this.baseInjected = true;
 		}
 
@@ -2793,11 +2797,11 @@ protected void initBeanFactory(BeanFactory beanFactory) {
 			this.beanFactory = beanFactory;
 		}
 
-		public ITestBean getTestBean3() {
+		public @Nullable ITestBean getTestBean3() {
 			return this.testBean3;
 		}
 
-		public ITestBean getTestBean4() {
+		public @Nullable ITestBean getTestBean4() {
 			return this.testBean4;
 		}