Translate NullBean result to null for lookup method with bean name

Closes gh-25806

Author: jhoeller

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -238,8 +238,10 @@ public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp
 			Assert.state(lo != null, "LookupOverride not found");
 			Object[] argsToUse = (args.length > 0 ? args : null);  // if no-arg, don't insist on args at all
 			if (StringUtils.hasText(lo.getBeanName())) {
-				return (argsToUse != null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
+				Object bean = (argsToUse != null ? this.owner.getBean(lo.getBeanName(), argsToUse) :
 						this.owner.getBean(lo.getBeanName()));
+				// Detect package-protected NullBean instance through equals(null) check
+				return (bean.equals(null) ? null : bean);
 			}
 			else {
 				return (argsToUse != null ? this.owner.getBean(method.getReturnType(), argsToUse) :

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -108,10 +108,23 @@ public void testWithEarlyInjection() {
 		assertThat(beanFactory.getBean(BeanConsumer.class).abstractBean).isSameAs(bean);
 	}
 
+	@Test  // gh-25806
+	public void testWithNullBean() {
+		RootBeanDefinition tbd = new RootBeanDefinition(TestBean.class, () -> null);
+		tbd.setScope(BeanDefinition.SCOPE_PROTOTYPE);
+		beanFactory.registerBeanDefinition("testBean", tbd);
+
+		AbstractBean bean = beanFactory.getBean("beanConsumer", BeanConsumer.class).abstractBean;
+		assertThat(bean).isNotNull();
+		Object expected = bean.get();
+		assertThat(expected).isNull();
+		assertThat(beanFactory.getBean(BeanConsumer.class).abstractBean).isSameAs(bean);
+	}
+
 
 	public static abstract class AbstractBean {
 
-		@Lookup
+		@Lookup("testBean")
 		public abstract TestBean get();
 
 		@Lookup