Includes efficient check for same ClassLoader in ClassUtils.isVisible, efficient MethodMatchers check for IntroductionAwareMethodMatcher, and supertype method resolution in MethodMapTransactionAttributeSource.
Issue: SPR-16723
Spring's CGLIB fork is patched with local copies of affected files here, introducing the notion of a "contextClass" (e.g. the proxy superclass) which gets passed through to ReflectUtils.defineClass for delegating to MethodHandles.Lookup.defineClass eventually, against a privateLookupIn(contextClass) lookup context on JDK 9/10/11.
Issue: SPR-15859
This commit allows several DeferredImportSelector instances to be
grouped and managed in a centralized fashion. This typically allows
different instances to provide a consistent ordered set of imports to
apply.
Issue: SPR-16589
Bean-derived null values may still get passed into bean properties and injection points but only if those are declared as non-required. Note that getBean will never return null; a manual bean.equals(null) / "null".equals(bean.toString()) check identifies expected null values now. This will only ever happen with custom FactoryBeans or factory methods returning null - and since all common cases are handled by autowiring or bean property values in bean definitions, there should be no need to ever manually check for such a null value received from getBean.
Issue: SPR-15829
This commit also removes nullability from two common spots: ResolvableType.getType() and TargetSource.getTarget(), both of which are never effectively null with any regular implementation. For such scenarios, a non-null empty type/target is the cleaner contract.
Issue: SPR-15540
Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.
Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.
Issue: SPR-15540
