Properly resolve null candidates for collection injection

Issue: SPR-16033

Author: jhoeller

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

@@ -1317,7 +1317,8 @@ private void addCandidateEntry(Map<String, Object> candidates, String candidateN
 			DependencyDescriptor descriptor, Class<?> requiredType) {
 
 		if (descriptor instanceof MultiElementDescriptor || containsSingleton(candidateName)) {
-			candidates.put(candidateName, descriptor.resolveCandidate(candidateName, requiredType, this));
+			Object beanInstance = descriptor.resolveCandidate(candidateName, requiredType, this);
+			candidates.put(candidateName, (beanInstance instanceof NullBean ? null : beanInstance));
 		}
 		else {
 			candidates.put(candidateName, getType(candidateName));

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

@@ -681,6 +681,29 @@ public void testConstructorResourceInjectionWithNoCandidatesAndNoFallback() {
 		}
 	}
 
+	@Test
+	public void testConstructorResourceInjectionWithCollectionAndNullFromFactoryBean() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(
+				ConstructorsCollectionResourceInjectionBean.class));
+		TestBean tb = new TestBean();
+		bf.registerSingleton("testBean", tb);
+		bf.registerBeanDefinition("nestedTestBean1", new RootBeanDefinition(NullNestedTestBeanFactoryBean.class));
+		NestedTestBean ntb2 = new NestedTestBean();
+		bf.registerSingleton("nestedTestBean2", ntb2);
+
+		ConstructorsCollectionResourceInjectionBean bean = (ConstructorsCollectionResourceInjectionBean) bf.getBean("annotatedBean");
+		assertNull(bean.getTestBean3());
+		assertSame(tb, bean.getTestBean4());
+		assertEquals(2, bean.getNestedTestBeans().size());
+		assertNull(bean.getNestedTestBeans().get(0));
+		assertSame(ntb2, bean.getNestedTestBeans().get(1));
+		bf.destroySingletons();
+	}
+
 	@Test
 	public void testConstructorResourceInjectionWithMultipleCandidatesAsCollection() {
 		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
@@ -860,23 +883,20 @@ public void testConstructorInjectionWithMap() {
 		bd.setScope(RootBeanDefinition.SCOPE_PROTOTYPE);
 		bf.registerBeanDefinition("annotatedBean", bd);
 		TestBean tb1 = new TestBean("tb1");
-		TestBean tb2 = new TestBean("tb2");
 		bf.registerSingleton("testBean1", tb1);
-		bf.registerSingleton("testBean2", tb2);
+		RootBeanDefinition tb2 = new RootBeanDefinition(NullFactoryMethods.class);
+		tb2.setFactoryMethodName("createTestBean");
+		bf.registerBeanDefinition("testBean2", tb2);
 
 		MapConstructorInjectionBean bean = (MapConstructorInjectionBean) bf.getBean("annotatedBean");
 		assertEquals(2, bean.getTestBeanMap().size());
-		assertTrue(bean.getTestBeanMap().keySet().contains("testBean1"));
-		assertTrue(bean.getTestBeanMap().keySet().contains("testBean2"));
-		assertTrue(bean.getTestBeanMap().values().contains(tb1));
-		assertTrue(bean.getTestBeanMap().values().contains(tb2));
+		assertSame(tb1, bean.getTestBeanMap().get("testBean1"));
+		assertNull(bean.getTestBeanMap().get("testBean2"));
 
 		bean = (MapConstructorInjectionBean) bf.getBean("annotatedBean");
 		assertEquals(2, bean.getTestBeanMap().size());
-		assertTrue(bean.getTestBeanMap().keySet().contains("testBean1"));
-		assertTrue(bean.getTestBeanMap().keySet().contains("testBean2"));
-		assertTrue(bean.getTestBeanMap().values().contains(tb1));
-		assertTrue(bean.getTestBeanMap().values().contains(tb2));
+		assertSame(tb1, bean.getTestBeanMap().get("testBean1"));
+		assertNull(bean.getTestBeanMap().get("testBean2"));
 	}
 
 	@Test