Support for Java 8's java.util.Optional at injection points

Issue: SPR-11833

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

@@ -36,6 +36,7 @@ import java.util.IdentityHashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import javax.inject.Provider;
@@ -108,6 +109,8 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 
 	private static Class<?> javaxInjectProviderClass = null;
 
+	private static Class<?> javaUtilOptionalClass = null;
+
 	static {
 		try {
 			javaxInjectProviderClass =
@@ -116,6 +119,13 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		catch (ClassNotFoundException ex) {
 			// JSR-330 API not available - Provider interface simply not supported then.
 		}
+		try {
+			javaUtilOptionalClass =
+					ClassUtils.forName("java.util.Optional", DefaultListableBeanFactory.class.getClassLoader());
+		}
+		catch (ClassNotFoundException ex) {
+			// Java 8 not available - Optional references simply not supported then.
+		}
 	}
 
 
@@ -854,6 +864,9 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		else if (descriptor.getDependencyType().equals(javaxInjectProviderClass)) {
 			return new DependencyProviderFactory().createDependencyProvider(descriptor, beanName);
 		}
+		else if (descriptor.getDependencyType().equals(javaUtilOptionalClass)) {
+			return new OptionalDependencyFactory().createOptionalDependency(descriptor, beanName);
+		}
 		else {
 			Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, beanName);
 			if (result == null) {
@@ -1319,4 +1332,22 @@ public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFacto
 		}
 	}
 
+
+	/**
+	 * Separate inner class for avoiding a hard dependency on the {@code javax.inject} API.
+	 */
+	private class OptionalDependencyFactory {
+
+		public Object createOptionalDependency(DependencyDescriptor descriptor, String beanName) {
+			DependencyDescriptor descriptorToUse = new DependencyDescriptor(descriptor) {
+				@Override
+				public boolean isRequired() {
+					return false;
+				}
+			};
+			descriptorToUse.increaseNestingLevel();
+			return Optional.ofNullable(doResolveDependency(descriptorToUse, beanName, null, null));
+		}
+	}
+
 }