Log warning for single optional constructor when no default constructor to fall back to

Issue: SPR-12161

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

@@ -299,9 +299,17 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 					}
 					if (!candidates.isEmpty()) {
 						// Add default constructor to list of optional constructors, as fallback.
-						if (requiredConstructor == null && defaultConstructor != null) {
+						if (requiredConstructor == null) {
+							if (defaultConstructor != null) {
 								candidates.add(defaultConstructor);
 							}
+							else if (candidates.size() == 1 && logger.isWarnEnabled()) {
+								logger.warn("Inconsistent constructor declaration on bean with name '" + beanName +
+										"': single autowire-marked constructor flagged as optional - this constructor " +
+										"is effectively required since there is no default constructor to fall back to: " +
+										candidates.get(0));
+							}
+						}
 						candidateConstructors = candidates.toArray(new Constructor<?>[candidates.size()]);
 					}
 					else {

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

@@ -35,6 +35,7 @@ import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.ObjectFactory;
+import org.springframework.beans.factory.UnsatisfiedDependencyException;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.config.TypedStringValue;
@@ -52,7 +53,6 @@ import org.springframework.tests.sample.beans.TestBean;
 import org.springframework.util.SerializationTestUtils;
 
 import static org.junit.Assert.*;
-import static org.mockito.Mockito.*;
 
 /**
  * @author Juergen Hoeller
@@ -552,6 +552,23 @@ public class AutowiredAnnotationBeanPostProcessorTests {
 		bf.destroySingletons();
 	}
 
+	@Test
+	public void testConstructorResourceInjectionWithNoCandidatesAndNoFallback() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(ConstructorWithoutFallbackBean.class));
+
+		try {
+			bf.getBean("annotatedBean");
+			fail("Should have thrown UnsatisfiedDependencyException");
+		}
+		catch (UnsatisfiedDependencyException ex) {
+			// expected
+		}
+	}
+
 	@Test
 	public void testConstructorResourceInjectionWithMultipleCandidatesAsCollection() {
 		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
@@ -2048,6 +2065,21 @@ public class AutowiredAnnotationBeanPostProcessorTests {
 	}
 
 
+	public static class ConstructorWithoutFallbackBean {
+
+		protected ITestBean testBean3;
+
+		@Autowired(required = false)
+		public ConstructorWithoutFallbackBean(ITestBean testBean3) {
+			this.testBean3 = testBean3;
+		}
+
+		public ITestBean getTestBean3() {
+			return this.testBean3;
+		}
+	}
+
+
 	public static class ConstructorsCollectionResourceInjectionBean {
 
 		protected ITestBean testBean3;
@@ -2113,7 +2145,6 @@ public class AutowiredAnnotationBeanPostProcessorTests {
 		@Autowired
 		private Map<String, TestBean> testBeanMap;
 
-
 		public Map<String, TestBean> getTestBeanMap() {
 			return this.testBeanMap;
 		}