Introduce null-safety of Spring Framework API

This commit introduces 2 new @Nullable and @NonNullApi
annotations that leverage JSR 305 (dormant but available via
Findbugs jsr305 dependency and already used by libraries
like OkHttp) meta-annotations to specify explicitly
null-safety of Spring Framework parameters and return values.

In order to avoid adding too much annotations, the
default is set at package level with @NonNullApi and
@Nullable annotations are added when needed at parameter or
return value level. These annotations are intended to be used
on Spring Framework itself but also by other Spring projects.

@Nullable annotations have been introduced based on Javadoc
and search of patterns like "return null;". It is expected that
nullability of Spring Framework API will be polished with
complementary commits.

In practice, this will make the whole Spring Framework API
null-safe for Kotlin projects (when KT-10942 will be fixed)
since Kotlin will be able to leverage these annotations to
know if a parameter or a return value is nullable or not. But
this is also useful for Java developers as well since IntelliJ
IDEA, for example, also understands these annotations to
generate warnings when unsafe nullable usages are detected.

Issue: SPR-15540

build.gradle

@@ -177,6 +177,7 @@ configure(allprojects) { project ->
 	}
 
 	dependencies {
+		provided("com.google.code.findbugs:jsr305:3.0.2")
 		testCompile("junit:junit:${junitVersion}") {
 			exclude group:'org.hamcrest', module:'hamcrest-core'
 		}

spring-aop/src/main/java/org/aopalliance/aop/AspectException.java

@@ -16,6 +16,8 @@
 
 package org.aopalliance.aop;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Superclass for all AOP infrastructure exceptions.
  * Unchecked, as such exceptions are fatal and end user
@@ -41,7 +43,7 @@ public class AspectException extends RuntimeException {
 	 * @param message the exception message
 	 * @param cause the root cause, if any
 	 */
-	public AspectException(String message, Throwable cause) {
+	public AspectException(String message, @Nullable Throwable cause) {
 		super(message, cause);
 	}
 

spring-aop/src/main/java/org/aopalliance/intercept/MethodInterceptor.java

@@ -16,6 +16,8 @@
 
 package org.aopalliance.intercept;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Intercepts calls on an interface on its way to the target. These
  * are nested "on top" of the target.
@@ -52,6 +54,7 @@ public interface MethodInterceptor extends Interceptor {
 	 * @throws Throwable if the interceptors or the target object
 	 * throws an exception
 	 */
+	@Nullable
 	Object invoke(MethodInvocation invocation) throws Throwable;
 
 }

spring-aop/src/main/java/org/springframework/aop/AfterReturningAdvice.java

@@ -18,6 +18,8 @@ package org.springframework.aop;
 
 import java.lang.reflect.Method;
 
+import org.springframework.lang.Nullable;
+
 /**
  * After returning advice is invoked only on normal method return, not if an
  * exception is thrown. Such advice can see the return value, but cannot change it.
@@ -39,6 +41,6 @@ public interface AfterReturningAdvice extends AfterAdvice {
 	 * allowed by the method signature. Otherwise the exception
 	 * will be wrapped as a runtime exception.
 	 */
-	void afterReturning(Object returnValue, Method method, Object[] args, Object target) throws Throwable;
+	void afterReturning(@Nullable Object returnValue, Method method, Object[] args, @Nullable Object target) throws Throwable;
 
 }

spring-aop/src/main/java/org/springframework/aop/IntroductionAwareMethodMatcher.java

@@ -18,6 +18,8 @@ package org.springframework.aop;
 
 import java.lang.reflect.Method;
 
+import org.springframework.lang.Nullable;
+
 /**
  * A specialized type of {@link MethodMatcher} that takes into account introductions
  * when matching methods. If there are no introductions on the target class,
@@ -39,6 +41,6 @@ public interface IntroductionAwareMethodMatcher extends MethodMatcher {
 	 * asking is the subject on one or more introductions; {@code false} otherwise
 	 * @return whether or not this method matches statically
 	 */
-	boolean matches(Method method, Class<?> targetClass, boolean hasIntroductions);
+	boolean matches(Method method, @Nullable Class<?> targetClass, boolean hasIntroductions);
 
 }

spring-aop/src/main/java/org/springframework/aop/MethodBeforeAdvice.java

@@ -18,6 +18,8 @@ package org.springframework.aop;
 
 import java.lang.reflect.Method;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Advice invoked before a method is invoked. Such advices cannot
  * prevent the method call proceeding, unless they throw a Throwable.
@@ -39,6 +41,6 @@ public interface MethodBeforeAdvice extends BeforeAdvice {
 	 * allowed by the method signature. Otherwise the exception
 	 * will be wrapped as a runtime exception.
 	 */
-	void before(Method method, Object[] args, Object target) throws Throwable;
+	void before(Method method, Object[] args, @Nullable Object target) throws Throwable;
 
 }

spring-aop/src/main/java/org/springframework/aop/MethodMatcher.java

@@ -18,6 +18,8 @@ package org.springframework.aop;
 
 import java.lang.reflect.Method;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Part of a {@link Pointcut}: Checks whether the target method is eligible for advice.
  *
@@ -57,7 +59,7 @@ public interface MethodMatcher {
 	 * the candidate class must be taken to be the method's declaring class)
 	 * @return whether or not this method matches statically
 	 */
-	boolean matches(Method method, Class<?> targetClass);
+	boolean matches(Method method, @Nullable Class<?> targetClass);
 
 	/**
 	 * Is this MethodMatcher dynamic, that is, must a final call be made on the
@@ -86,7 +88,7 @@ public interface MethodMatcher {
 	 * @return whether there's a runtime match
 	 * @see MethodMatcher#matches(Method, Class)
 	 */
-	boolean matches(Method method, Class<?> targetClass, Object... args);
+	boolean matches(Method method, @Nullable Class<?> targetClass, Object... args);
 
 
 	/**

spring-aop/src/main/java/org/springframework/aop/ProxyMethodInvocation.java

@@ -18,6 +18,8 @@ package org.springframework.aop;
 
 import org.aopalliance.intercept.MethodInvocation;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Extension of the AOP Alliance {@link org.aopalliance.intercept.MethodInvocation}
  * interface, allowing access to the proxy that the method invocation was made through.
@@ -73,7 +75,7 @@ public interface ProxyMethodInvocation extends MethodInvocation {
 	 * @param key the name of the attribute
 	 * @param value the value of the attribute, or {@code null} to reset it
 	 */
-	void setUserAttribute(String key, Object value);
+	void setUserAttribute(String key, @Nullable Object value);
 
 	/**
 	 * Return the value of the specified user attribute.
@@ -81,6 +83,7 @@ public interface ProxyMethodInvocation extends MethodInvocation {
 	 * @return the value of the attribute, or {@code null} if not set
 	 * @see #setUserAttribute
 	 */
+	@Nullable
 	Object getUserAttribute(String key);
 
 }

spring-aop/src/main/java/org/springframework/aop/TargetClassAware.java

@@ -16,6 +16,8 @@
 
 package org.springframework.aop;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Minimal interface for exposing the target class behind a proxy.
  *
@@ -34,6 +36,7 @@ public interface TargetClassAware {
 	 * (typically a proxy configuration or an actual proxy).
 	 * @return the target Class, or {@code null} if not known
 	 */
+	@Nullable
 	Class<?> getTargetClass();
 
 }

spring-aop/src/main/java/org/springframework/aop/TargetSource.java

@@ -16,6 +16,8 @@
 
 package org.springframework.aop;
 
+import org.springframework.lang.Nullable;
+
 /**
  * A {@code TargetSource} is used to obtain the current "target" of
  * an AOP invocation, which will be invoked via reflection if no around
@@ -58,6 +60,7 @@ public interface TargetSource extends TargetClassAware {
 	 * @return the target object, which contains the joinpoint
 	 * @throws Exception if the target object can't be resolved
 	 */
+	@Nullable
 	Object getTarget() throws Exception;
 
 	/**

spring-aop/src/main/java/org/springframework/aop/aspectj/AbstractAspectJAdvice.java

@@ -42,6 +42,7 @@ import org.springframework.aop.support.MethodMatchers;
 import org.springframework.aop.support.StaticMethodMatcher;
 import org.springframework.core.DefaultParameterNameDiscoverer;
 import org.springframework.core.ParameterNameDiscoverer;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
@@ -548,7 +549,7 @@ public abstract class AbstractAspectJAdvice implements Advice, AspectJPrecedence
 	 * @param ex the exception thrown by the method execution (may be null)
 	 * @return the empty array if there are no arguments
 	 */
-	protected Object[] argBinding(JoinPoint jp, JoinPointMatch jpMatch, Object returnValue, Throwable ex) {
+	protected Object[] argBinding(JoinPoint jp, JoinPointMatch jpMatch, @Nullable Object returnValue, @Nullable Throwable ex) {
 		calculateArgumentBindings();
 
 		// AMC start
@@ -607,7 +608,7 @@ public abstract class AbstractAspectJAdvice implements Advice, AspectJPrecedence
 	 * @return the invocation result
 	 * @throws Throwable in case of invocation failure
 	 */
-	protected Object invokeAdviceMethod(JoinPointMatch jpMatch, Object returnValue, Throwable ex) throws Throwable {
+	protected Object invokeAdviceMethod(JoinPointMatch jpMatch, @Nullable Object returnValue, @Nullable Throwable ex) throws Throwable {
 		return invokeAdviceMethodWithGivenArgs(argBinding(getJoinPoint(), jpMatch, returnValue, ex));
 	}
 

spring-aop/src/main/java/org/springframework/aop/aspectj/AspectJAdviceParameterNameDiscoverer.java

@@ -30,6 +30,7 @@ import org.aspectj.weaver.tools.PointcutParser;
 import org.aspectj.weaver.tools.PointcutPrimitive;
 
 import org.springframework.core.ParameterNameDiscoverer;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -473,6 +474,7 @@ public class AspectJAdviceParameterNameDiscoverer implements ParameterNameDiscov
 	/*
 	 * If the token starts meets Java identifier conventions, it's in.
 	 */
+	@Nullable
 	private String maybeExtractVariableName(String candidateToken) {
 		if (candidateToken == null || candidateToken.equals("")) {
 			return null;

spring-aop/src/main/java/org/springframework/aop/aspectj/AspectJAfterReturningAdvice.java

@@ -22,6 +22,7 @@ import java.lang.reflect.Type;
 
 import org.springframework.aop.AfterAdvice;
 import org.springframework.aop.AfterReturningAdvice;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.TypeUtils;
 
@@ -75,7 +76,7 @@ public class AspectJAfterReturningAdvice extends AbstractAspectJAdvice
 	 * @param returnValue the return value of the target method
 	 * @return whether to invoke the advice method for the given return value
 	 */
-	private boolean shouldInvokeOnReturnValueOf(Method method, Object returnValue) {
+	private boolean shouldInvokeOnReturnValueOf(Method method, @Nullable Object returnValue) {
 		Class<?> type = getDiscoveredReturningType();
 		Type genericType = getDiscoveredReturningGenericType();
 		// If we aren't dealing with a raw type, check if generic parameters are assignable.
@@ -94,7 +95,7 @@ public class AspectJAfterReturningAdvice extends AbstractAspectJAdvice
 	 * @param returnValue the return value of the target method
 	 * @return whether to invoke the advice method for the given return value and type
 	 */
-	private boolean matchesReturnValue(Class<?> type, Method method, Object returnValue) {
+	private boolean matchesReturnValue(Class<?> type, Method method, @Nullable Object returnValue) {
 		if (returnValue != null) {
 			return ClassUtils.isAssignableValue(type, returnValue);
 		}

spring-aop/src/main/java/org/springframework/aop/aspectj/AspectJAopUtils.java

@@ -21,6 +21,7 @@ import org.aopalliance.aop.Advice;
 import org.springframework.aop.Advisor;
 import org.springframework.aop.AfterAdvice;
 import org.springframework.aop.BeforeAdvice;
+import org.springframework.lang.Nullable;
 
 /**
  * Utility methods for dealing with AspectJ advisors.
@@ -58,6 +59,7 @@ public abstract class AspectJAopUtils {
 	 * If neither the advisor nor the advice have precedence information, this method
 	 * will return {@code null}.
 	 */
+	@Nullable
 	public static AspectJPrecedenceInformation getAspectJPrecedenceInformationFor(Advisor anAdvisor) {
 		if (anAdvisor instanceof AspectJPrecedenceInformation) {
 			return (AspectJPrecedenceInformation) anAdvisor;

spring-aop/src/main/java/org/springframework/aop/aspectj/AspectJExpressionPointcut.java

@@ -56,6 +56,7 @@ import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.annotation.BeanFactoryAnnotationUtils;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
@@ -382,6 +383,7 @@ public class AspectJExpressionPointcut extends AbstractExpressionPointcut
 	/**
 	 * Get a new pointcut expression based on a target class's loader rather than the default.
 	 */
+	@Nullable
 	private PointcutExpression getFallbackPointcutExpression(Class<?> targetClass) {
 		try {
 			ClassLoader classLoader = targetClass.getClassLoader();

spring-aop/src/main/java/org/springframework/aop/aspectj/MethodInvocationProceedingJoinPoint.java

@@ -29,6 +29,7 @@ import org.aspectj.runtime.internal.AroundClosure;
 import org.springframework.aop.ProxyMethodInvocation;
 import org.springframework.core.DefaultParameterNameDiscoverer;
 import org.springframework.core.ParameterNameDiscoverer;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -109,6 +110,7 @@ public class MethodInvocationProceedingJoinPoint implements ProceedingJoinPoint,
 	 * Returns the Spring AOP target. May be {@code null} if there is no target.
 	 */
 	@Override
+	@Nullable
 	public Object getTarget() {
 		return this.methodInvocation.getThis();
 	}

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/AbstractAspectJAdvisorFactory.java

@@ -41,6 +41,7 @@ import org.aspectj.lang.reflect.PerClauseKind;
 import org.springframework.aop.framework.AopConfigException;
 import org.springframework.core.ParameterNameDiscoverer;
 import org.springframework.core.annotation.AnnotationUtils;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -125,6 +126,7 @@ public abstract class AbstractAspectJAdvisorFactory implements AspectJAdvisorFac
 	 * (there <i>should</i> only be one anyway...)
 	 */
 	@SuppressWarnings("unchecked")
+	@Nullable
 	protected static AspectJAnnotation<?> findAspectJAnnotationOnMethod(Method method) {
 		Class<?>[] classesToLookFor = new Class<?>[] {
 				Before.class, Around.class, After.class, AfterReturning.class, AfterThrowing.class, Pointcut.class};
@@ -137,6 +139,7 @@ public abstract class AbstractAspectJAdvisorFactory implements AspectJAdvisorFac
 		return null;
 	}
 
+	@Nullable
 	private static <A extends Annotation> AspectJAnnotation<A> findAnnotation(Method method, Class<A> toLookFor) {
 		A result = AnnotationUtils.findAnnotation(method, toLookFor);
 		if (result != null) {

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/AspectJAdvisorFactory.java

@@ -24,6 +24,7 @@ import org.aopalliance.aop.Advice;
 import org.springframework.aop.Advisor;
 import org.springframework.aop.aspectj.AspectJExpressionPointcut;
 import org.springframework.aop.framework.AopConfigException;
+import org.springframework.lang.Nullable;
 
 /**
  * Interface for factories that can create Spring AOP Advisors from classes
@@ -79,6 +80,7 @@ public interface AspectJAdvisorFactory {
 	 * or if it is a pointcut that will be used by other advice but will not
 	 * create a Spring advice in its own right
 	 */
+	@Nullable
 	Advisor getAdvisor(Method candidateAdviceMethod, MetadataAwareAspectInstanceFactory aspectInstanceFactory,
 			int declarationOrder, String aspectName);
 
@@ -98,6 +100,7 @@ public interface AspectJAdvisorFactory {
 	 * @see org.springframework.aop.aspectj.AspectJAfterReturningAdvice
 	 * @see org.springframework.aop.aspectj.AspectJAfterThrowingAdvice
 	 */
+	@Nullable
 	Advice getAdvice(Method candidateAdviceMethod, AspectJExpressionPointcut expressionPointcut,
 			MetadataAwareAspectInstanceFactory aspectInstanceFactory, int declarationOrder, String aspectName);
 

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/MetadataAwareAspectInstanceFactory.java

@@ -17,6 +17,7 @@
 package org.springframework.aop.aspectj.annotation;
 
 import org.springframework.aop.aspectj.AspectInstanceFactory;
+import org.springframework.lang.Nullable;
 
 /**
  * Subinterface of {@link org.springframework.aop.aspectj.AspectInstanceFactory}
@@ -44,6 +45,7 @@ public interface MetadataAwareAspectInstanceFactory extends AspectInstanceFactor
 	 * @return the mutex object (may be {@code null} for no mutex to use)
 	 * @since 4.3
 	 */
+	@Nullable
 	Object getAspectCreationMutex();
 
 }

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/ReflectiveAspectJAdvisorFactory.java

@@ -50,6 +50,7 @@ import org.springframework.beans.factory.BeanFactory;
 import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConvertingComparator;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.util.comparator.InstanceComparator;
@@ -113,7 +114,7 @@ public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFacto
 	 * @see AspectJExpressionPointcut#setBeanFactory
 	 * @see org.springframework.beans.factory.config.ConfigurableBeanFactory#getBeanClassLoader()
 	 */
-	public ReflectiveAspectJAdvisorFactory(BeanFactory beanFactory) {
+	public ReflectiveAspectJAdvisorFactory(@Nullable BeanFactory beanFactory) {
 		this.beanFactory = beanFactory;
 	}
 
@@ -176,6 +177,7 @@ public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFacto
 	 * @param introductionField the field to introspect
 	 * @return {@code null} if not an Advisor
 	 */
+	@Nullable
 	private Advisor getDeclareParentsAdvisor(Field introductionField) {
 		DeclareParents declareParents = introductionField.getAnnotation(DeclareParents.class);
 		if (declareParents == null) {
@@ -208,6 +210,7 @@ public class ReflectiveAspectJAdvisorFactory extends AbstractAspectJAdvisorFacto
 				this, aspectInstanceFactory, declarationOrderInAspect, aspectName);
 	}
 
+	@Nullable
 	private AspectJExpressionPointcut getPointcut(Method candidateAdviceMethod, Class<?> candidateAspectClass) {
 		AspectJAnnotation<?> aspectJAnnotation =
 				AbstractAspectJAdvisorFactory.findAspectJAnnotationOnMethod(candidateAdviceMethod);

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/package-info.java

@@ -3,4 +3,7 @@
  *
  * <p>Normally to be used through an AspectJAutoProxyCreator rather than directly.
  */
+@NonNullApi
 package org.springframework.aop.aspectj.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/aspectj/autoproxy/package-info.java

@@ -2,4 +2,7 @@
  * Base classes enabling auto-proxying based on AspectJ.
  * Support for AspectJ annotation aspects resides in the "aspectj.annotation" package.
  */
+@NonNullApi
 package org.springframework.aop.aspectj.autoproxy;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/aspectj/package-info.java

@@ -8,4 +8,7 @@
  * or AspectJ load-time weaver. It is intended to enable the use of a valuable subset of AspectJ
  * functionality, with consistent semantics, with the proxy-based Spring AOP framework.
  */
+@NonNullApi
 package org.springframework.aop.aspectj;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/config/AopConfigUtils.java

@@ -26,6 +26,7 @@ import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.core.Ordered;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -104,7 +105,7 @@ public abstract class AopConfigUtils {
 		}
 	}
 
-
+	@Nullable
 	private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry, Object source) {
 		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
 		if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {

spring-aop/src/main/java/org/springframework/aop/config/ConfigBeanDefinitionParser.java

@@ -45,6 +45,7 @@ import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.RootBeanDefinition;
 import org.springframework.beans.factory.xml.BeanDefinitionParser;
 import org.springframework.beans.factory.xml.ParserContext;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 import org.springframework.util.xml.DomUtils;
 
@@ -466,6 +467,7 @@ class ConfigBeanDefinitionParser implements BeanDefinitionParser {
 	 * {@link org.springframework.beans.factory.config.BeanDefinition} for the pointcut if  necessary
 	 * and returns its bean name, otherwise returns the bean name of the referred pointcut.
 	 */
+	@Nullable
 	private Object parsePointcutProperty(Element element, ParserContext parserContext) {
 		if (element.hasAttribute(POINTCUT) && element.hasAttribute(POINTCUT_REF)) {
 			parserContext.getReaderContext().error(

spring-aop/src/main/java/org/springframework/aop/config/package-info.java

@@ -2,4 +2,7 @@
  * Support package for declarative AOP configuration,
  * with XML schema being the primary configuration format.
  */
+@NonNullApi
 package org.springframework.aop.config;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/framework/AdvisorChainFactory.java

@@ -19,6 +19,8 @@ package org.springframework.aop.framework;
 import java.lang.reflect.Method;
 import java.util.List;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Factory interface for advisor chains.
  *
@@ -36,6 +38,6 @@ public interface AdvisorChainFactory {
 	 * target object, in which case the method's declaring class is the next best option)
 	 * @return List of MethodInterceptors (may also include InterceptorAndDynamicMethodMatchers)
 	 */
-	List<Object> getInterceptorsAndDynamicInterceptionAdvice(Advised config, Method method, Class<?> targetClass);
+	List<Object> getInterceptorsAndDynamicInterceptionAdvice(Advised config, Method method, @Nullable Class<?> targetClass);
 
 }

spring-aop/src/main/java/org/springframework/aop/framework/AopContext.java

@@ -17,6 +17,7 @@
 package org.springframework.aop.framework;
 
 import org.springframework.core.NamedThreadLocal;
+import org.springframework.lang.Nullable;
 
 /**
  * Class containing static methods used to obtain information about the current AOP invocation.
@@ -74,7 +75,8 @@ public abstract class AopContext {
 	 * @return the old proxy, which may be {@code null} if none was bound
 	 * @see #currentProxy()
 	 */
-	static Object setCurrentProxy(Object proxy) {
+	@Nullable
+	static Object setCurrentProxy(@Nullable Object proxy) {
 		Object old = currentProxy.get();
 		if (proxy != null) {
 			currentProxy.set(proxy);

spring-aop/src/main/java/org/springframework/aop/framework/AopProxy.java

@@ -16,6 +16,8 @@
 
 package org.springframework.aop.framework;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Delegate interface for a configured AOP proxy, allowing for the creation
  * of actual proxy objects.
@@ -48,6 +50,6 @@ public interface AopProxy {
 	 * (or {@code null} for the low-level proxy facility's default)
 	 * @return the new proxy object (never {@code null})
 	 */
-	Object getProxy(ClassLoader classLoader);
+	Object getProxy(@Nullable ClassLoader classLoader);
 
 }

spring-aop/src/main/java/org/springframework/aop/framework/AopProxyUtils.java

@@ -27,6 +27,7 @@ import org.springframework.aop.TargetSource;
 import org.springframework.aop.support.AopUtils;
 import org.springframework.aop.target.SingletonTargetSource;
 import org.springframework.core.DecoratingProxy;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -52,6 +53,7 @@ public abstract class AopProxyUtils {
 	 * @see Advised#getTargetSource()
 	 * @see SingletonTargetSource#getTarget()
 	 */
+	@Nullable
 	public static Object getSingletonTarget(Object candidate) {
 		if (candidate instanceof Advised) {
 			TargetSource targetSource = ((Advised) candidate).getTargetSource();

spring-aop/src/main/java/org/springframework/aop/framework/CglibAopProxy.java

@@ -50,6 +50,7 @@ import org.springframework.cglib.proxy.MethodProxy;
 import org.springframework.cglib.proxy.NoOp;
 import org.springframework.cglib.transform.impl.UndeclaredThrowableStrategy;
 import org.springframework.core.SmartClassLoader;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
@@ -372,7 +373,8 @@ class CglibAopProxy implements AopProxy, Serializable {
 	 * Process a return value. Wraps a return of {@code this} if necessary to be the
 	 * {@code proxy} and also verifies that {@code null} is not returned as a primitive.
 	 */
-	private static Object processReturnType(Object proxy, Object target, Method method, Object retVal) {
+	@Nullable
+	private static Object processReturnType(Object proxy, Object target, Method method, @Nullable Object retVal) {
 		// Massage return value if necessary
 		if (retVal != null && retVal == target &&
 				!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {

spring-aop/src/main/java/org/springframework/aop/framework/ProxyFactory.java

@@ -19,6 +19,7 @@ package org.springframework.aop.framework;
 import org.aopalliance.intercept.Interceptor;
 
 import org.springframework.aop.TargetSource;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -105,7 +106,7 @@ public class ProxyFactory extends ProxyCreatorSupport {
 	 * (or {@code null} for the low-level proxy facility's default)
 	 * @return the proxy object
 	 */
-	public Object getProxy(ClassLoader classLoader) {
+	public Object getProxy(@Nullable ClassLoader classLoader) {
 		return createAopProxy().getProxy(classLoader);
 	}
 

spring-aop/src/main/java/org/springframework/aop/framework/adapter/package-info.java

@@ -9,4 +9,7 @@
  *
  * <p>These adapters do not depend on any other Spring framework classes to allow such usage.
  */
+@NonNullApi
 package org.springframework.aop.framework.adapter;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java

@@ -47,6 +47,7 @@ import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.config.SmartInstantiationAwareBeanPostProcessor;
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -207,6 +208,7 @@ public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
 	 * Return the owning {@link BeanFactory}.
 	 * May be {@code null}, as this post-processor doesn't need to belong to a bean factory.
 	 */
+	@Nullable
 	protected BeanFactory getBeanFactory() {
 		return this.beanFactory;
 	}
@@ -400,6 +402,7 @@ public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
 	 * @return a TargetSource for this bean
 	 * @see #setCustomTargetSourceCreators
 	 */
+	@Nullable
 	protected TargetSource getCustomTargetSource(Class<?> beanClass, String beanName) {
 		// We can't create fancy target sources for directly registered singletons.
 		if (this.customTargetSourceCreators != null &&
@@ -578,7 +581,8 @@ public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
 	 * @see #DO_NOT_PROXY
 	 * @see #PROXY_WITHOUT_ADDITIONAL_INTERCEPTORS
 	 */
+	@Nullable
 	protected abstract Object[] getAdvicesAndAdvisorsForBean(
-			Class<?> beanClass, String beanName, TargetSource customTargetSource) throws BeansException;
+			Class<?> beanClass, String beanName, @Nullable TargetSource customTargetSource) throws BeansException;
 
 }

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/AutoProxyUtils.java

@@ -19,6 +19,7 @@ package org.springframework.aop.framework.autoproxy;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.core.Conventions;
+import org.springframework.lang.Nullable;
 
 /**
  * Utilities for auto-proxy aware components.
@@ -79,6 +80,7 @@ public abstract class AutoProxyUtils {
 	 * @since 4.2.3
 	 * @see org.springframework.beans.factory.BeanFactory#getType(String)
 	 */
+	@Nullable
 	public static Class<?> determineTargetClass(ConfigurableListableBeanFactory beanFactory, String beanName) {
 		if (beanName == null) {
 			return null;

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/ProxyCreationContext.java

@@ -17,6 +17,7 @@
 package org.springframework.aop.framework.autoproxy;
 
 import org.springframework.core.NamedThreadLocal;
+import org.springframework.lang.Nullable;
 
 /**
  * Holder for the current proxy creation context, as exposed by auto-proxy creators
@@ -37,6 +38,7 @@ public class ProxyCreationContext {
 	 * Return the name of the currently proxied bean instance.
 	 * @return the name of the bean, or {@code null} if none available
 	 */
+	@Nullable
 	public static String getCurrentProxiedBeanName() {
 		return currentProxiedBeanName.get();
 	}
@@ -45,7 +47,7 @@ public class ProxyCreationContext {
 	 * Set the name of the currently proxied bean instance.
 	 * @param beanName the name of the bean, or {@code null} to reset it
 	 */
-	static void setCurrentProxiedBeanName(String beanName) {
+	static void setCurrentProxiedBeanName(@Nullable String beanName) {
 		if (beanName != null) {
 			currentProxiedBeanName.set(beanName);
 		}

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/TargetSourceCreator.java

@@ -17,6 +17,7 @@
 package org.springframework.aop.framework.autoproxy;
 
 import org.springframework.aop.TargetSource;
+import org.springframework.lang.Nullable;
 
 /**
  * Implementations can create special target sources, such as pooling target
@@ -39,6 +40,7 @@ public interface TargetSourceCreator {
 	 * @return a special TargetSource or {@code null} if this TargetSourceCreator isn't
 	 * interested in the particular bean
 	 */
+	@Nullable
 	TargetSource getTargetSource(Class<?> beanClass, String beanName);
 
 }

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/package-info.java

@@ -9,4 +9,7 @@
  * as post-processors beans are only automatically detected in application contexts.
  * Post-processors can be explicitly registered on a ConfigurableBeanFactory instead.
  */
+@NonNullApi
 package org.springframework.aop.framework.autoproxy;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/target/AbstractBeanFactoryBasedTargetSourceCreator.java

@@ -35,6 +35,7 @@ import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
 import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.beans.factory.support.GenericBeanDefinition;
+import org.springframework.lang.Nullable;
 
 /**
  * Convenient superclass for
@@ -196,6 +197,7 @@ public abstract class AbstractBeanFactoryBasedTargetSourceCreator
 	 * @param beanName the name of the bean
 	 * @return the AbstractPrototypeBasedTargetSource, or {@code null} if we don't match this
 	 */
+	@Nullable
 	protected abstract AbstractBeanFactoryBasedTargetSource createBeanFactoryBasedTargetSource(
 			Class<?> beanClass, String beanName);
 

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/target/package.html

@@ -1,7 +0,0 @@
-<html>
-<body>
-
-Generic support classes for target source creation.
-
-</body>
-</html>

spring-aop/src/main/java/org/springframework/aop/framework/package-info.java

@@ -12,4 +12,7 @@
  * or ApplicationContext. However, proxies can be created programmatically using the
  * ProxyFactory class.
  */
+@NonNullApi
 package org.springframework.aop.framework;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/interceptor/AsyncExecutionAspectSupport.java

@@ -38,6 +38,7 @@ import org.springframework.core.task.AsyncListenableTaskExecutor;
 import org.springframework.core.task.AsyncTaskExecutor;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.core.task.support.TaskExecutorAdapter;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.util.concurrent.ListenableFuture;
@@ -143,6 +144,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * Should preferably return an {@link AsyncListenableTaskExecutor} implementation.
 	 * @return the executor to use (or {@code null}, but just if no default executor is available)
 	 */
+	@Nullable
 	protected AsyncTaskExecutor determineAsyncExecutor(Method method) {
 		AsyncTaskExecutor executor = this.executors.get(method);
 		if (executor == null) {
@@ -183,6 +185,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * @see #determineAsyncExecutor(Method)
 	 * @see #findQualifiedExecutor(BeanFactory, String)
 	 */
+	@Nullable
 	protected abstract String getExecutorQualifier(Method method);
 
 	/**
@@ -192,6 +195,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * @since 4.2.6
 	 * @see #getExecutorQualifier(Method)
 	 */
+	@Nullable
 	protected Executor findQualifiedExecutor(BeanFactory beanFactory, String qualifier) {
 		if (beanFactory == null) {
 			throw new IllegalStateException("BeanFactory must be set on " + getClass().getSimpleName() +
@@ -212,6 +216,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * @see #findQualifiedExecutor(BeanFactory, String)
 	 * @see #DEFAULT_TASK_EXECUTOR_BEAN_NAME
 	 */
+	@Nullable
 	protected Executor getDefaultExecutor(BeanFactory beanFactory) {
 		if (beanFactory != null) {
 			try {
@@ -256,6 +261,7 @@ public abstract class AsyncExecutionAspectSupport implements BeanFactoryAware {
 	 * @param returnType the declared return type (potentially a {@link Future} variant)
 	 * @return the execution result (potentially a corresponding {@link Future} handle)
 	 */
+	@Nullable
 	protected Object doSubmit(Callable<Object> task, AsyncTaskExecutor executor, Class<?> returnType) {
 		if (CompletableFuture.class.isAssignableFrom(returnType)) {
 			return CompletableFuture.supplyAsync(new Supplier<Object>() {

spring-aop/src/main/java/org/springframework/aop/interceptor/CustomizableTraceInterceptor.java

@@ -24,6 +24,7 @@ import org.aopalliance.intercept.MethodInvocation;
 import org.apache.commons.logging.Log;
 
 import org.springframework.core.Constants;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StopWatch;
@@ -317,7 +318,7 @@ public class CustomizableTraceInterceptor extends AbstractTraceInterceptor {
 	 * @return the formatted output to write to the log
 	 */
 	protected String replacePlaceholders(String message, MethodInvocation methodInvocation,
-			Object returnValue, Throwable throwable, long invocationTime) {
+			@Nullable Object returnValue, @Nullable Throwable throwable, long invocationTime) {
 
 		Matcher matcher = PATTERN.matcher(message);
 
@@ -371,7 +372,7 @@ public class CustomizableTraceInterceptor extends AbstractTraceInterceptor {
 	 * @param returnValue the value returned by the method invocation.
 	 */
 	private void appendReturnValue(
-			MethodInvocation methodInvocation, Matcher matcher, StringBuffer output, Object returnValue) {
+			MethodInvocation methodInvocation, Matcher matcher, StringBuffer output, @Nullable Object returnValue) {
 
 		if (methodInvocation.getMethod().getReturnType() == void.class) {
 			matcher.appendReplacement(output, "void");

spring-aop/src/main/java/org/springframework/aop/interceptor/package-info.java

@@ -3,4 +3,7 @@
  * More specific interceptors can be found in corresponding
  * functionality packages, like "transaction" and "orm".
  */
+@NonNullApi
 package org.springframework.aop.interceptor;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/package-info.java

@@ -17,4 +17,7 @@
  * <p>Spring AOP can be used programmatically or (preferably)
  * integrated with the Spring IoC container.
  */
+@NonNullApi
 package org.springframework.aop;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/scope/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Support for AOP-based scoping of target objects, with configurable backend.
  */
+@NonNullApi
 package org.springframework.aop.scope;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/support/AbstractBeanFactoryPointcutAdvisor.java

@@ -24,6 +24,7 @@ import org.aopalliance.aop.Advice;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -66,6 +67,7 @@ public abstract class AbstractBeanFactoryPointcutAdvisor extends AbstractPointcu
 	/**
 	 * Return the name of the advice bean that this advisor refers to, if any.
 	 */
+	@Nullable
 	public String getAdviceBeanName() {
 		return this.adviceBeanName;
 	}

spring-aop/src/main/java/org/springframework/aop/support/AbstractExpressionPointcut.java

@@ -18,6 +18,8 @@ package org.springframework.aop.support;
 
 import java.io.Serializable;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Abstract superclass for expression pointcuts,
  * offering location and expression properties.
@@ -49,6 +51,7 @@ public abstract class AbstractExpressionPointcut implements ExpressionPointcut,
 	 * @return location information as a human-readable String,
 	 * or {@code null} if none is available
 	 */
+	@Nullable
 	public String getLocation() {
 		return this.location;
 	}

spring-aop/src/main/java/org/springframework/aop/support/AopUtils.java

@@ -36,6 +36,7 @@ import org.springframework.aop.SpringProxy;
 import org.springframework.aop.TargetClassAware;
 import org.springframework.core.BridgeMethodResolver;
 import org.springframework.core.MethodIntrospector;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
@@ -187,7 +188,7 @@ public abstract class AopUtils {
 	 * {@code targetClass} doesn't implement it or is {@code null}
 	 * @see org.springframework.util.ClassUtils#getMostSpecificMethod
 	 */
-	public static Method getMostSpecificMethod(Method method, Class<?> targetClass) {
+	public static Method getMostSpecificMethod(Method method, @Nullable Class<?> targetClass) {
 		Method resolvedMethod = ClassUtils.getMostSpecificMethod(method, targetClass);
 		// If we are dealing with method with generic parameters, find the original method.
 		return BridgeMethodResolver.findBridgedMethod(resolvedMethod);
@@ -324,6 +325,7 @@ public abstract class AopUtils {
 	 * @throws Throwable if thrown by the target method
 	 * @throws org.springframework.aop.AopInvocationException in case of a reflection error
 	 */
+	@Nullable
 	public static Object invokeJoinpointUsingReflection(Object target, Method method, Object[] args)
 			throws Throwable {
 

spring-aop/src/main/java/org/springframework/aop/support/ControlFlowPointcut.java

@@ -22,6 +22,7 @@ import java.lang.reflect.Method;
 import org.springframework.aop.ClassFilter;
 import org.springframework.aop.MethodMatcher;
 import org.springframework.aop.Pointcut;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -59,7 +60,7 @@ public class ControlFlowPointcut implements Pointcut, ClassFilter, MethodMatcher
 	 * @param clazz the clazz
 	 * @param methodName the name of the method (may be {@code null})
 	 */
-	public ControlFlowPointcut(Class<?> clazz, String methodName) {
+	public ControlFlowPointcut(Class<?> clazz, @Nullable String methodName) {
 		Assert.notNull(clazz, "Class must not be null");
 		this.clazz = clazz;
 		this.methodName = methodName;

spring-aop/src/main/java/org/springframework/aop/support/DefaultIntroductionAdvisor.java

@@ -27,6 +27,7 @@ import org.springframework.aop.DynamicIntroductionAdvice;
 import org.springframework.aop.IntroductionAdvisor;
 import org.springframework.aop.IntroductionInfo;
 import org.springframework.core.Ordered;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
@@ -64,7 +65,7 @@ public class DefaultIntroductionAdvisor implements IntroductionAdvisor, ClassFil
 	 * @param introductionInfo the IntroductionInfo that describes
 	 * the interface to introduce (may be {@code null})
 	 */
-	public DefaultIntroductionAdvisor(Advice advice, IntroductionInfo introductionInfo) {
+	public DefaultIntroductionAdvisor(Advice advice, @Nullable IntroductionInfo introductionInfo) {
 		Assert.notNull(advice, "Advice must not be null");
 		this.advice = advice;
 		if (introductionInfo != null) {

spring-aop/src/main/java/org/springframework/aop/support/MethodMatchers.java

@@ -22,6 +22,7 @@ import java.lang.reflect.Method;
 import org.springframework.aop.ClassFilter;
 import org.springframework.aop.IntroductionAwareMethodMatcher;
 import org.springframework.aop.MethodMatcher;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -87,7 +88,7 @@ public abstract class MethodMatchers {
 	 * asking is the subject on one or more introductions; {@code false} otherwise
 	 * @return whether or not this method matches statically
 	 */
-	public static boolean matches(MethodMatcher mm, Method method, Class<?> targetClass, boolean hasIntroductions) {
+	public static boolean matches(MethodMatcher mm, Method method, @Nullable Class<?> targetClass, boolean hasIntroductions) {
 		Assert.notNull(mm, "MethodMatcher must not be null");
 		return ((mm instanceof IntroductionAwareMethodMatcher &&
 				((IntroductionAwareMethodMatcher) mm).matches(method, targetClass, hasIntroductions)) ||

spring-aop/src/main/java/org/springframework/aop/support/RegexpMethodPointcutAdvisor.java

@@ -21,6 +21,7 @@ import java.io.Serializable;
 import org.aopalliance.aop.Advice;
 
 import org.springframework.aop.Pointcut;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -132,6 +133,7 @@ public class RegexpMethodPointcutAdvisor extends AbstractGenericPointcutAdvisor
 	 * will be used.
 	 * @return the Pointcut instance (never {@code null})
 	 */
+	@Nullable
 	protected AbstractRegexpMethodPointcut createPointcut() {
 		return new JdkRegexpMethodPointcut();
 	}

spring-aop/src/main/java/org/springframework/aop/support/annotation/AnnotationMatchingPointcut.java

@@ -21,6 +21,7 @@ import java.lang.annotation.Annotation;
 import org.springframework.aop.ClassFilter;
 import org.springframework.aop.MethodMatcher;
 import org.springframework.aop.Pointcut;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -69,7 +70,7 @@ public class AnnotationMatchingPointcut implements Pointcut {
 	 * (can be {@code null})
 	 */
 	public AnnotationMatchingPointcut(
-			Class<? extends Annotation> classAnnotationType, Class<? extends Annotation> methodAnnotationType) {
+			@Nullable Class<? extends Annotation> classAnnotationType, @Nullable Class<? extends Annotation> methodAnnotationType) {
 
 		this(classAnnotationType, methodAnnotationType, false);
 	}

spring-aop/src/main/java/org/springframework/aop/support/annotation/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Annotation support for AOP pointcuts.
  */
+@NonNullApi
 package org.springframework.aop.support.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/support/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Convenience classes for using Spring's AOP API.
  */
+@NonNullApi
 package org.springframework.aop.support;
+
+import org.springframework.lang.NonNullApi;

spring-aop/src/main/java/org/springframework/aop/target/EmptyTargetSource.java

@@ -19,6 +19,7 @@ package org.springframework.aop.target;
 import java.io.Serializable;
 
 import org.springframework.aop.TargetSource;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -50,7 +51,7 @@ public class EmptyTargetSource implements TargetSource, Serializable {
 	 * @param targetClass the target Class (may be {@code null})
 	 * @see #getTargetClass()
 	 */
-	public static EmptyTargetSource forClass(Class<?> targetClass) {
+	public static EmptyTargetSource forClass(@Nullable Class<?> targetClass) {
 		return forClass(targetClass, true);
 	}
 
@@ -60,7 +61,7 @@ public class EmptyTargetSource implements TargetSource, Serializable {
 	 * @param isStatic whether the TargetSource should be marked as static
 	 * @see #getTargetClass()
 	 */
-	public static EmptyTargetSource forClass(Class<?> targetClass, boolean isStatic) {
+	public static EmptyTargetSource forClass(@Nullable Class<?> targetClass, boolean isStatic) {
 		return (targetClass == null && isStatic ? INSTANCE : new EmptyTargetSource(targetClass, isStatic));
 	}
 
@@ -81,7 +82,7 @@ public class EmptyTargetSource implements TargetSource, Serializable {
 	 * @param targetClass the target class to expose (may be {@code null})
 	 * @param isStatic whether the TargetSource is marked as static
 	 */
-	private EmptyTargetSource(Class<?> targetClass, boolean isStatic) {
+	private EmptyTargetSource(@Nullable Class<?> targetClass, boolean isStatic) {
 		this.targetClass = targetClass;
 		this.isStatic = isStatic;
 	}

spring-aop/src/main/java/org/springframework/aop/target/dynamic/package.html

@@ -1,7 +0,0 @@
-<html>
-<body>
-
-Support for AOP-based refreshing of target objects.
-
-</body>
-</html>

spring-beans/src/main/java/org/springframework/beans/AbstractNestablePropertyAccessor.java

@@ -40,6 +40,7 @@ import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.ConversionException;
 import org.springframework.core.convert.ConverterNotFoundException;
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
@@ -715,6 +716,7 @@ public abstract class AbstractNestablePropertyAccessor extends AbstractPropertyA
 	 * or {@code null} if not found
 	 * @throws BeansException in case of introspection failure
 	 */
+	@Nullable
 	protected PropertyHandler getPropertyHandler(String propertyName) throws BeansException {
 		Assert.notNull(propertyName, "Property name must not be null");
 		AbstractNestablePropertyAccessor nestedPa = getPropertyAccessorForPropertyPath(propertyName);
@@ -727,6 +729,7 @@ public abstract class AbstractNestablePropertyAccessor extends AbstractPropertyA
 	 * @param propertyName the name of a local property
 	 * @return the handler for that property or {@code null} if it has not been found
 	 */
+	@Nullable
 	protected abstract PropertyHandler getLocalPropertyHandler(String propertyName);
 
 	/**
@@ -1016,6 +1019,7 @@ public abstract class AbstractNestablePropertyAccessor extends AbstractPropertyA
 
 		public abstract TypeDescriptor nested(int level);
 
+		@Nullable
 		public abstract Object getValue() throws Exception;
 
 		public abstract void setValue(Object object, Object value) throws Exception;

spring-beans/src/main/java/org/springframework/beans/BeanInfoFactory.java

@@ -19,6 +19,8 @@ package org.springframework.beans;
 import java.beans.BeanInfo;
 import java.beans.IntrospectionException;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Strategy interface for creating {@link BeanInfo} instances for Spring beans.
  * Can be used to plug in custom bean property resolution strategies (e.g. for other
@@ -52,6 +54,7 @@ public interface BeanInfoFactory {
 	 * @return the BeanInfo, or {@code null} if the given class is not supported
 	 * @throws IntrospectionException in case of exceptions
 	 */
+	@Nullable
 	BeanInfo getBeanInfo(Class<?> beanClass) throws IntrospectionException;
 
 }

spring-beans/src/main/java/org/springframework/beans/BeanInstantiationException.java

@@ -19,6 +19,8 @@ package org.springframework.beans;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Method;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Exception thrown when instantiation of a bean failed.
  * Carries the offending bean class.
@@ -98,6 +100,7 @@ public class BeanInstantiationException extends FatalBeanException {
 	 * factory method or in case of default instantiation
 	 * @since 4.3
 	 */
+	@Nullable
 	public Constructor<?> getConstructor() {
 		return this.constructor;
 	}
@@ -108,6 +111,7 @@ public class BeanInstantiationException extends FatalBeanException {
 	 * or {@code null} in case of constructor-based instantiation
 	 * @since 4.3
 	 */
+	@Nullable
 	public Method getConstructingMethod() {
 		return this.constructingMethod;
 	}

spring-beans/src/main/java/org/springframework/beans/BeanMetadataAttributeAccessor.java

@@ -17,6 +17,7 @@
 package org.springframework.beans;
 
 import org.springframework.core.AttributeAccessorSupport;
+import org.springframework.lang.Nullable;
 
 /**
  * Extension of {@link org.springframework.core.AttributeAccessorSupport},
@@ -60,6 +61,7 @@ public class BeanMetadataAttributeAccessor extends AttributeAccessorSupport impl
 	 * @return the corresponding BeanMetadataAttribute object,
 	 * or {@code null} if no such attribute defined
 	 */
+	@Nullable
 	public BeanMetadataAttribute getMetadataAttribute(String name) {
 		return (BeanMetadataAttribute) super.getAttribute(name);
 	}

spring-beans/src/main/java/org/springframework/beans/BeanMetadataElement.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Interface to be implemented by bean metadata elements
  * that carry a configuration source object.
@@ -29,6 +31,7 @@ public interface BeanMetadataElement {
 	 * Return the configuration source {@code Object} for this metadata element
 	 * (may be {@code null}).
 	 */
+	@Nullable
 	Object getSource();
 
 }

spring-beans/src/main/java/org/springframework/beans/BeanUtils.java

@@ -35,6 +35,7 @@ import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ConcurrentReferenceHashMap;
@@ -172,6 +173,7 @@ public abstract class BeanUtils {
 	 * @see Class#getMethod
 	 * @see #findDeclaredMethod
 	 */
+	@Nullable
 	public static Method findMethod(Class<?> clazz, String methodName, Class<?>... paramTypes) {
 		try {
 			return clazz.getMethod(methodName, paramTypes);
@@ -192,6 +194,7 @@ public abstract class BeanUtils {
 	 * @return the Method object, or {@code null} if not found
 	 * @see Class#getDeclaredMethod
 	 */
+	@Nullable
 	public static Method findDeclaredMethod(Class<?> clazz, String methodName, Class<?>... paramTypes) {
 		try {
 			return clazz.getDeclaredMethod(methodName, paramTypes);
@@ -219,6 +222,7 @@ public abstract class BeanUtils {
 	 * @see Class#getMethods
 	 * @see #findDeclaredMethodWithMinimalParameters
 	 */
+	@Nullable
 	public static Method findMethodWithMinimalParameters(Class<?> clazz, String methodName)
 			throws IllegalArgumentException {
 
@@ -241,6 +245,7 @@ public abstract class BeanUtils {
 	 * could not be resolved to a unique method with minimal parameters
 	 * @see Class#getDeclaredMethods
 	 */
+	@Nullable
 	public static Method findDeclaredMethodWithMinimalParameters(Class<?> clazz, String methodName)
 			throws IllegalArgumentException {
 
@@ -260,6 +265,7 @@ public abstract class BeanUtils {
 	 * @throws IllegalArgumentException if methods of the given name were found but
 	 * could not be resolved to a unique method with minimal parameters
 	 */
+	@Nullable
 	public static Method findMethodWithMinimalParameters(Method[] methods, String methodName)
 			throws IllegalArgumentException {
 
@@ -311,6 +317,7 @@ public abstract class BeanUtils {
 	 * @see #findMethod
 	 * @see #findMethodWithMinimalParameters
 	 */
+	@Nullable
 	public static Method resolveSignature(String signature, Class<?> clazz) {
 		Assert.hasText(signature, "'signature' must not be empty");
 		Assert.notNull(clazz, "Class must not be null");
@@ -365,6 +372,7 @@ public abstract class BeanUtils {
 	 * @return the corresponding PropertyDescriptor, or {@code null} if none
 	 * @throws BeansException if PropertyDescriptor lookup fails
 	 */
+	@Nullable
 	public static PropertyDescriptor getPropertyDescriptor(Class<?> clazz, String propertyName)
 			throws BeansException {
 
@@ -381,6 +389,7 @@ public abstract class BeanUtils {
 	 * @return the corresponding PropertyDescriptor, or {@code null} if none
 	 * @throws BeansException if PropertyDescriptor lookup fails
 	 */
+	@Nullable
 	public static PropertyDescriptor findPropertyForMethod(Method method) throws BeansException {
 		return findPropertyForMethod(method, method.getDeclaringClass());
 	}
@@ -395,6 +404,7 @@ public abstract class BeanUtils {
 	 * @throws BeansException if PropertyDescriptor lookup fails
 	 * @since 3.2.13
 	 */
+	@Nullable
 	public static PropertyDescriptor findPropertyForMethod(Method method, Class<?> clazz) throws BeansException {
 		Assert.notNull(method, "Method must not be null");
 		PropertyDescriptor[] pds = getPropertyDescriptors(clazz);
@@ -415,6 +425,7 @@ public abstract class BeanUtils {
 	 * @param targetType the type to find an editor for
 	 * @return the corresponding editor, or {@code null} if none found
 	 */
+	@Nullable
 	public static PropertyEditor findEditorByConvention(Class<?> targetType) {
 		if (targetType == null || targetType.isArray() || unknownEditorTypes.contains(targetType)) {
 			return null;

spring-beans/src/main/java/org/springframework/beans/BeanWrapper.java

@@ -18,6 +18,8 @@ package org.springframework.beans;
 
 import java.beans.PropertyDescriptor;
 
+import org.springframework.lang.Nullable;
+
 /**
  * The central interface of Spring's low-level JavaBeans infrastructure.
  *
@@ -65,6 +67,7 @@ public interface BeanWrapper extends ConfigurablePropertyAccessor {
 	 * Return the bean instance wrapped by this object, if any.
 	 * @return the bean instance, or {@code null} if none set
 	 */
+	@Nullable
 	Object getWrappedInstance();
 
 	/**
@@ -72,6 +75,7 @@ public interface BeanWrapper extends ConfigurablePropertyAccessor {
 	 * @return the type of the wrapped bean instance,
 	 * or {@code null} if no wrapped object has been set
 	 */
+	@Nullable
 	Class<?> getWrappedClass();
 
 	/**

spring-beans/src/main/java/org/springframework/beans/ConfigurablePropertyAccessor.java

@@ -17,6 +17,7 @@
 package org.springframework.beans;
 
 import org.springframework.core.convert.ConversionService;
+import org.springframework.lang.Nullable;
 
 /**
  * Interface that encapsulates configuration methods for a PropertyAccessor.
@@ -41,6 +42,7 @@ public interface ConfigurablePropertyAccessor extends PropertyAccessor, Property
 	/**
 	 * Return the associated ConversionService, if any.
 	 */
+	@Nullable
 	ConversionService getConversionService();
 
 	/**

spring-beans/src/main/java/org/springframework/beans/ConversionNotSupportedException.java

@@ -18,6 +18,8 @@ package org.springframework.beans;
 
 import java.beans.PropertyChangeEvent;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Exception thrown when no suitable editor or converter can be found for a bean property.
  *
@@ -35,7 +37,7 @@ public class ConversionNotSupportedException extends TypeMismatchException {
 	 * @param cause the root cause (may be {@code null})
 	 */
 	public ConversionNotSupportedException(PropertyChangeEvent propertyChangeEvent,
-			Class<?> requiredType, Throwable cause) {
+			@Nullable Class<?> requiredType, @Nullable Throwable cause) {
 		super(propertyChangeEvent, requiredType, cause);
 	}
 
@@ -45,7 +47,7 @@ public class ConversionNotSupportedException extends TypeMismatchException {
 	 * @param requiredType the required target type (or {@code null} if not known)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public ConversionNotSupportedException(Object value, Class<?> requiredType, Throwable cause) {
+	public ConversionNotSupportedException(@Nullable Object value, @Nullable Class<?> requiredType, @Nullable Throwable cause) {
 		super(value, requiredType, cause);
 	}
 

spring-beans/src/main/java/org/springframework/beans/ExtendedBeanInfo.java

@@ -37,6 +37,7 @@ import java.util.TreeSet;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.ObjectUtils;
 
 /**
@@ -188,6 +189,7 @@ class ExtendedBeanInfo implements BeanInfo {
 		}
 	}
 
+	@Nullable
 	private PropertyDescriptor findExistingPropertyDescriptor(String propertyName, Class<?> propertyType) {
 		for (PropertyDescriptor pd : this.propertyDescriptors) {
 			final Class<?> candidateType;

spring-beans/src/main/java/org/springframework/beans/Mergeable.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Interface representing an object whose value set can be merged with
  * that of a parent object.
@@ -44,6 +46,6 @@ public interface Mergeable {
 	 * @exception IllegalStateException if merging is not enabled for this instance
 	 * (i.e. {@code mergeEnabled} equals {@code false}).
 	 */
-	Object merge(Object parent);
+	Object merge(@Nullable Object parent);
 
 }

spring-beans/src/main/java/org/springframework/beans/MutablePropertyValues.java

@@ -23,6 +23,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -267,6 +268,7 @@ public class MutablePropertyValues implements PropertyValues, Serializable {
 	 * @see #getPropertyValue(String)
 	 * @see PropertyValue#getValue()
 	 */
+	@Nullable
 	public Object get(String propertyName) {
 		PropertyValue pv = getPropertyValue(propertyName);
 		return (pv != null ? pv.getValue() : null);

spring-beans/src/main/java/org/springframework/beans/NotWritablePropertyException.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Exception thrown on an attempt to set the value of a property that
  * is not writable (typically because there is no setter method).
@@ -80,6 +82,7 @@ public class NotWritablePropertyException extends InvalidPropertyException {
 	 * Return suggestions for actual bean property names that closely match
 	 * the invalid property name, if any.
 	 */
+	@Nullable
 	public String[] getPossibleMatches() {
 		return this.possibleMatches;
 	}

spring-beans/src/main/java/org/springframework/beans/PropertyAccessException.java

@@ -18,6 +18,8 @@ package org.springframework.beans;
 
 import java.beans.PropertyChangeEvent;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Superclass for exceptions related to a property access,
  * such as type mismatch or invocation target exception.
@@ -57,6 +59,7 @@ public abstract class PropertyAccessException extends BeansException {
 	 * <p>May be {@code null}; only available if an actual bean property
 	 * was affected.
 	 */
+	@Nullable
 	public PropertyChangeEvent getPropertyChangeEvent() {
 		return this.propertyChangeEvent;
 	}
@@ -71,6 +74,7 @@ public abstract class PropertyAccessException extends BeansException {
 	/**
 	 * Return the affected value that was about to be set, if any.
 	 */
+	@Nullable
 	public Object getValue() {
 		return (this.propertyChangeEvent != null ? this.propertyChangeEvent.getNewValue() : null);
 	}

spring-beans/src/main/java/org/springframework/beans/PropertyAccessor.java

@@ -19,6 +19,7 @@ package org.springframework.beans;
 import java.util.Map;
 
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.lang.Nullable;
 
 /**
  * Common interface for classes that can access named properties
@@ -86,6 +87,7 @@ public interface PropertyAccessor {
 	 * @throws PropertyAccessException if the property was valid but the
 	 * accessor method failed
 	 */
+	@Nullable
 	Class<?> getPropertyType(String propertyName) throws BeansException;
 
 	/**
@@ -98,6 +100,7 @@ public interface PropertyAccessor {
 	 * @throws InvalidPropertyException if there is no such property or
 	 * if the property isn't readable
 	 */
+	@Nullable
 	TypeDescriptor getPropertyTypeDescriptor(String propertyName) throws BeansException;
 
 	/**

spring-beans/src/main/java/org/springframework/beans/PropertyBatchUpdateException.java

@@ -19,6 +19,7 @@ package org.springframework.beans;
 import java.io.PrintStream;
 import java.io.PrintWriter;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -71,6 +72,7 @@ public class PropertyBatchUpdateException extends BeansException {
 	/**
 	 * Return the exception for this field, or {@code null} if there isn't any.
 	 */
+	@Nullable
 	public PropertyAccessException getPropertyAccessException(String propertyName) {
 		for (PropertyAccessException pae : this.propertyAccessExceptions) {
 			if (ObjectUtils.nullSafeEquals(propertyName, pae.getPropertyName())) {

spring-beans/src/main/java/org/springframework/beans/PropertyEditorRegistry.java

@@ -18,6 +18,8 @@ package org.springframework.beans;
 
 import java.beans.PropertyEditor;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Encapsulates methods for registering JavaBeans {@link PropertyEditor PropertyEditors}.
  * This is the central interface that a {@link PropertyEditorRegistrar} operates on.
@@ -64,7 +66,7 @@ public interface PropertyEditorRegistry {
 	 * {@code null} if registering an editor for all properties of the given type
 	 * @param propertyEditor editor to register
 	 */
-	void registerCustomEditor(Class<?> requiredType, String propertyPath, PropertyEditor propertyEditor);
+	void registerCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath, PropertyEditor propertyEditor);
 
 	/**
 	 * Find a custom property editor for the given type and property.
@@ -74,6 +76,7 @@ public interface PropertyEditorRegistry {
 	 * {@code null} if looking for an editor for all properties of the given type
 	 * @return the registered editor, or {@code null} if none
 	 */
-	PropertyEditor findCustomEditor(Class<?> requiredType, String propertyPath);
+	@Nullable
+	PropertyEditor findCustomEditor(@Nullable Class<?> requiredType, @Nullable String propertyPath);
 
 }

spring-beans/src/main/java/org/springframework/beans/PropertyEditorRegistrySupport.java

@@ -74,6 +74,7 @@ import org.springframework.beans.propertyeditors.ZoneIdEditor;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.support.ResourceArrayPropertyEditor;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -118,6 +119,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	/**
 	 * Return the associated ConversionService, if any.
 	 */
+	@Nullable
 	public ConversionService getConversionService() {
 		return this.conversionService;
 	}
@@ -169,6 +171,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * @return the default editor, or {@code null} if none found
 	 * @see #registerDefaultEditors
 	 */
+	@Nullable
 	public PropertyEditor getDefaultEditor(Class<?> requiredType) {
 		if (!this.defaultEditorsActive) {
 			return null;
@@ -336,7 +339,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * can be {@code null} if not known)
 	 * @return whether a matching custom editor has been found
 	 */
-	public boolean hasCustomEditorForElement(Class<?> elementType, String propertyPath) {
+	public boolean hasCustomEditorForElement(@Nullable Class<?> elementType, @Nullable String propertyPath) {
 		if (propertyPath != null && this.customEditorsForPath != null) {
 			for (Map.Entry<String, CustomEditorHolder> entry : this.customEditorsForPath.entrySet()) {
 				if (PropertyAccessorUtils.matchesProperty(entry.getKey(), propertyPath)) {
@@ -361,6 +364,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * @return the type of the property, or {@code null} if not determinable
 	 * @see BeanWrapper#getPropertyType(String)
 	 */
+	@Nullable
 	protected Class<?> getPropertyType(String propertyPath) {
 		return null;
 	}
@@ -371,6 +375,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * @param requiredType the type to look for
 	 * @return the custom editor, or {@code null} if none specific for this property
 	 */
+	@Nullable
 	private PropertyEditor getCustomEditor(String propertyName, Class<?> requiredType) {
 		CustomEditorHolder holder = this.customEditorsForPath.get(propertyName);
 		return (holder != null ? holder.getPropertyEditor(requiredType) : null);
@@ -384,6 +389,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * @return the custom editor, or {@code null} if none found for this type
 	 * @see java.beans.PropertyEditor#getAsText()
 	 */
+	@Nullable
 	private PropertyEditor getCustomEditor(Class<?> requiredType) {
 		if (requiredType == null || this.customEditors == null) {
 			return null;
@@ -420,6 +426,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * @param propertyName the name of the property
 	 * @return the property type, or {@code null} if not determinable
 	 */
+	@Nullable
 	protected Class<?> guessPropertyTypeFromEditors(String propertyName) {
 		if (this.customEditorsForPath != null) {
 			CustomEditorHolder editorHolder = this.customEditorsForPath.get(propertyName);
@@ -445,7 +452,7 @@ public class PropertyEditorRegistrySupport implements PropertyEditorRegistry {
 	 * If this is non-null, only editors registered for a path below this nested property
 	 * will be copied. If this is null, all editors will be copied.
 	 */
-	protected void copyCustomEditorsTo(PropertyEditorRegistry target, String nestedProperty) {
+	protected void copyCustomEditorsTo(PropertyEditorRegistry target, @Nullable String nestedProperty) {
 		String actualPropertyName =
 				(nestedProperty != null ? PropertyAccessorUtils.getPropertyName(nestedProperty) : null);
 		if (this.customEditors != null) {

spring-beans/src/main/java/org/springframework/beans/PropertyValues.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Holder containing one or more {@link PropertyValue} objects,
  * typically comprising one update for a specific target bean.
@@ -37,6 +39,7 @@ public interface PropertyValues {
 	 * @param propertyName the name to search for
 	 * @return the property value, or {@code null}
 	 */
+	@Nullable
 	PropertyValue getPropertyValue(String propertyName);
 
 	/**

spring-beans/src/main/java/org/springframework/beans/TypeConverter.java

@@ -19,6 +19,7 @@ package org.springframework.beans;
 import java.lang.reflect.Field;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.lang.Nullable;
 
 /**
  * Interface that defines type conversion methods. Typically (but not necessarily)
@@ -49,7 +50,7 @@ public interface TypeConverter {
 	 * @see org.springframework.core.convert.ConversionService
 	 * @see org.springframework.core.convert.converter.Converter
 	 */
-	<T> T convertIfNecessary(Object value, Class<T> requiredType) throws TypeMismatchException;
+	<T> T convertIfNecessary(Object value, @Nullable Class<T> requiredType) throws TypeMismatchException;
 
 	/**
 	 * Convert the value to the required type (if necessary from a String).
@@ -67,7 +68,7 @@ public interface TypeConverter {
 	 * @see org.springframework.core.convert.ConversionService
 	 * @see org.springframework.core.convert.converter.Converter
 	 */
-	<T> T convertIfNecessary(Object value, Class<T> requiredType, MethodParameter methodParam)
+	<T> T convertIfNecessary(Object value, @Nullable Class<T> requiredType, @Nullable MethodParameter methodParam)
 			throws TypeMismatchException;
 
 	/**
@@ -86,7 +87,7 @@ public interface TypeConverter {
 	 * @see org.springframework.core.convert.ConversionService
 	 * @see org.springframework.core.convert.converter.Converter
 	 */
-	<T> T convertIfNecessary(Object value, Class<T> requiredType, Field field)
+	<T> T convertIfNecessary(Object value, @Nullable Class<T> requiredType, @Nullable Field field)
 			throws TypeMismatchException;
 
 }

spring-beans/src/main/java/org/springframework/beans/TypeConverterDelegate.java

@@ -34,6 +34,7 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.convert.ConversionFailedException;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.NumberUtils;
 import org.springframework.util.ReflectionUtils;
@@ -90,7 +91,7 @@ class TypeConverterDelegate {
 	 * @return the new value, possibly the result of type conversion
 	 * @throws IllegalArgumentException if type conversion failed
 	 */
-	public <T> T convertIfNecessary(Object newValue, Class<T> requiredType, MethodParameter methodParam)
+	public <T> T convertIfNecessary(Object newValue, @Nullable Class<T> requiredType, @Nullable MethodParameter methodParam)
 			throws IllegalArgumentException {
 
 		return convertIfNecessary(null, null, newValue, requiredType,
@@ -107,7 +108,7 @@ class TypeConverterDelegate {
 	 * @return the new value, possibly the result of type conversion
 	 * @throws IllegalArgumentException if type conversion failed
 	 */
-	public <T> T convertIfNecessary(Object newValue, Class<T> requiredType, Field field)
+	public <T> T convertIfNecessary(Object newValue, @Nullable Class<T> requiredType, @Nullable Field field)
 			throws IllegalArgumentException {
 
 		return convertIfNecessary(null, null, newValue, requiredType,
@@ -125,7 +126,7 @@ class TypeConverterDelegate {
 	 * @throws IllegalArgumentException if type conversion failed
 	 */
 	public <T> T convertIfNecessary(
-			String propertyName, Object oldValue, Object newValue, Class<T> requiredType)
+			String propertyName, @Nullable Object oldValue, Object newValue, @Nullable Class<T> requiredType)
 			throws IllegalArgumentException {
 
 		return convertIfNecessary(propertyName, oldValue, newValue, requiredType, TypeDescriptor.valueOf(requiredType));
@@ -144,8 +145,8 @@ class TypeConverterDelegate {
 	 * @throws IllegalArgumentException if type conversion failed
 	 */
 	@SuppressWarnings("unchecked")
-	public <T> T convertIfNecessary(String propertyName, Object oldValue, Object newValue,
-			Class<T> requiredType, TypeDescriptor typeDescriptor) throws IllegalArgumentException {
+	public <T> T convertIfNecessary(String propertyName, @Nullable Object oldValue, Object newValue,
+			@Nullable Class<T> requiredType, TypeDescriptor typeDescriptor) throws IllegalArgumentException {
 
 		// Custom editor for this type?
 		PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName);
@@ -357,6 +358,7 @@ class TypeConverterDelegate {
 	 * @param requiredType the type to find an editor for
 	 * @return the corresponding editor, or {@code null} if none
 	 */
+	@Nullable
 	private PropertyEditor findDefaultEditor(Class<?> requiredType) {
 		PropertyEditor editor = null;
 		if (requiredType != null) {
@@ -381,7 +383,7 @@ class TypeConverterDelegate {
 	 * @return the new value, possibly the result of type conversion
 	 * @throws IllegalArgumentException if type conversion failed
 	 */
-	private Object doConvertValue(Object oldValue, Object newValue, Class<?> requiredType, PropertyEditor editor) {
+	private Object doConvertValue(@Nullable Object oldValue, Object newValue, @Nullable Class<?> requiredType, PropertyEditor editor) {
 		Object convertedValue = newValue;
 
 		if (editor != null && !(convertedValue instanceof String)) {
@@ -443,7 +445,7 @@ class TypeConverterDelegate {
 	 * @param editor the PropertyEditor to use
 	 * @return the converted value
 	 */
-	private Object doConvertTextValue(Object oldValue, String newTextValue, PropertyEditor editor) {
+	private Object doConvertTextValue(@Nullable Object oldValue, String newTextValue, PropertyEditor editor) {
 		try {
 			editor.setValue(oldValue);
 		}

spring-beans/src/main/java/org/springframework/beans/TypeMismatchException.java

@@ -18,6 +18,7 @@ package org.springframework.beans;
 
 import java.beans.PropertyChangeEvent;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 
 /**
@@ -55,7 +56,7 @@ public class TypeMismatchException extends PropertyAccessException {
 	 * @param requiredType the required target type (or {@code null} if not known)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public TypeMismatchException(PropertyChangeEvent propertyChangeEvent, Class<?> requiredType, Throwable cause) {
+	public TypeMismatchException(PropertyChangeEvent propertyChangeEvent, @Nullable Class<?> requiredType, @Nullable Throwable cause) {
 		super(propertyChangeEvent,
 				"Failed to convert property value of type '" +
 				ClassUtils.getDescriptiveType(propertyChangeEvent.getNewValue()) + "'" +
@@ -73,7 +74,7 @@ public class TypeMismatchException extends PropertyAccessException {
 	 * @param value the offending value that couldn't be converted (may be {@code null})
 	 * @param requiredType the required target type (or {@code null} if not known)
 	 */
-	public TypeMismatchException(Object value, Class<?> requiredType) {
+	public TypeMismatchException(@Nullable Object value, @Nullable Class<?> requiredType) {
 		this(value, requiredType, null);
 	}
 
@@ -83,7 +84,7 @@ public class TypeMismatchException extends PropertyAccessException {
 	 * @param requiredType the required target type (or {@code null} if not known)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public TypeMismatchException(Object value, Class<?> requiredType, Throwable cause) {
+	public TypeMismatchException(@Nullable Object value, @Nullable Class<?> requiredType, @Nullable Throwable cause) {
 		super("Failed to convert value of type '" + ClassUtils.getDescriptiveType(value) + "'" +
 				(requiredType != null ? " to required type '" + ClassUtils.getQualifiedName(requiredType) + "'" : ""),
 				cause);
@@ -103,6 +104,7 @@ public class TypeMismatchException extends PropertyAccessException {
 	/**
 	 * Return the required target type, if any.
 	 */
+	@Nullable
 	public Class<?> getRequiredType() {
 		return this.requiredType;
 	}

spring-beans/src/main/java/org/springframework/beans/annotation/AnnotationBeanUtils.java

@@ -24,6 +24,7 @@ import java.util.Set;
 
 import org.springframework.beans.BeanWrapper;
 import org.springframework.beans.PropertyAccessorFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StringValueResolver;
 
@@ -44,7 +45,7 @@ public abstract class AnnotationBeanUtils {
 	 * @param excludedProperties the names of excluded properties, if any
 	 * @see org.springframework.beans.BeanWrapper
 	 */
-	public static void copyPropertiesToBean(Annotation ann, Object bean, String... excludedProperties) {
+	public static void copyPropertiesToBean(Annotation ann, Object bean, @Nullable String... excludedProperties) {
 		copyPropertiesToBean(ann, bean, null, excludedProperties);
 	}
 
@@ -58,7 +59,7 @@ public abstract class AnnotationBeanUtils {
 	 * @param excludedProperties the names of excluded properties, if any
 	 * @see org.springframework.beans.BeanWrapper
 	 */
-	public static void copyPropertiesToBean(Annotation ann, Object bean, StringValueResolver valueResolver, String... excludedProperties) {
+	public static void copyPropertiesToBean(Annotation ann, Object bean, @Nullable StringValueResolver valueResolver, @Nullable String... excludedProperties) {
 		Set<String> excluded = new HashSet<>(Arrays.asList(excludedProperties));
 		Method[] annotationProperties = ann.annotationType().getDeclaredMethods();
 		BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(bean);

spring-beans/src/main/java/org/springframework/beans/annotation/package-info.java

@@ -1,4 +1,7 @@
 /**
  * Support package for beans-style handling of Java 5 annotations.
  */
+@NonNullApi
 package org.springframework.beans.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-beans/src/main/java/org/springframework/beans/factory/BeanClassLoaderAware.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans.factory;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Callback that allows a bean to be aware of the bean
  * {@link ClassLoader class loader}; that is, the class loader used by the
@@ -50,6 +52,6 @@ public interface BeanClassLoaderAware extends Aware {
 	 * the {@code ClassLoader} obtained via
 	 * {@link org.springframework.util.ClassUtils#getDefaultClassLoader()}
 	 */
-	void setBeanClassLoader(ClassLoader classLoader);
+	void setBeanClassLoader(@Nullable ClassLoader classLoader);
 
 }

spring-beans/src/main/java/org/springframework/beans/factory/BeanCreationException.java

@@ -23,6 +23,7 @@ import java.util.List;
 
 import org.springframework.beans.FatalBeanException;
 import org.springframework.core.NestedRuntimeException;
+import org.springframework.lang.Nullable;
 
 /**
  * Exception thrown when a BeanFactory encounters an error when
@@ -109,6 +110,7 @@ public class BeanCreationException extends FatalBeanException {
 	/**
 	 * Return the name of the bean requested, if any.
 	 */
+	@Nullable
 	public String getBeanName() {
 		return this.beanName;
 	}
@@ -117,6 +119,7 @@ public class BeanCreationException extends FatalBeanException {
 	 * Return the description of the resource that the bean
 	 * definition came from, if any.
 	 */
+	@Nullable
 	public String getResourceDescription() {
 		return this.resourceDescription;
 	}
@@ -138,6 +141,7 @@ public class BeanCreationException extends FatalBeanException {
 	 * Return the related causes, if any.
 	 * @return the array of related causes, or {@code null} if none
 	 */
+	@Nullable
 	public Throwable[] getRelatedCauses() {
 		if (this.relatedCauses == null) {
 			return null;

spring-beans/src/main/java/org/springframework/beans/factory/BeanDefinitionStoreException.java

@@ -17,6 +17,7 @@
 package org.springframework.beans.factory;
 
 import org.springframework.beans.FatalBeanException;
+import org.springframework.lang.Nullable;
 
 /**
  * Exception thrown when a BeanFactory encounters an invalid bean definition:
@@ -47,7 +48,7 @@ public class BeanDefinitionStoreException extends FatalBeanException {
 	 * @param msg the detail message (used as exception message as-is)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public BeanDefinitionStoreException(String msg, Throwable cause) {
+	public BeanDefinitionStoreException(String msg, @Nullable Throwable cause) {
 		super(msg, cause);
 	}
 
@@ -67,7 +68,7 @@ public class BeanDefinitionStoreException extends FatalBeanException {
 	 * @param msg the detail message (used as exception message as-is)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public BeanDefinitionStoreException(String resourceDescription, String msg, Throwable cause) {
+	public BeanDefinitionStoreException(String resourceDescription, String msg, @Nullable Throwable cause) {
 		super(msg, cause);
 		this.resourceDescription = resourceDescription;
 	}
@@ -91,7 +92,7 @@ public class BeanDefinitionStoreException extends FatalBeanException {
 	 * the resource and the name of the bean)
 	 * @param cause the root cause (may be {@code null})
 	 */
-	public BeanDefinitionStoreException(String resourceDescription, String beanName, String msg, Throwable cause) {
+	public BeanDefinitionStoreException(String resourceDescription, String beanName, String msg, @Nullable Throwable cause) {
 		super("Invalid bean definition with name '" + beanName + "' defined in " + resourceDescription + ": " + msg, cause);
 		this.resourceDescription = resourceDescription;
 		this.beanName = beanName;
@@ -102,6 +103,7 @@ public class BeanDefinitionStoreException extends FatalBeanException {
 	 * Return the description of the resource that the bean
 	 * definition came from, if any.
 	 */
+	@Nullable
 	public String getResourceDescription() {
 		return this.resourceDescription;
 	}
@@ -109,6 +111,7 @@ public class BeanDefinitionStoreException extends FatalBeanException {
 	/**
 	 * Return the name of the bean requested, if any.
 	 */
+	@Nullable
 	public String getBeanName() {
 		return this.beanName;
 	}

spring-beans/src/main/java/org/springframework/beans/factory/BeanFactory.java

@@ -18,6 +18,7 @@ package org.springframework.beans.factory;
 
 import org.springframework.beans.BeansException;
 import org.springframework.core.ResolvableType;
+import org.springframework.lang.Nullable;
 
 /**
  * The root interface for accessing a Spring bean container.
@@ -156,7 +157,7 @@ public interface BeanFactory {
 	 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
 	 * @throws BeansException if the bean could not be created
 	 */
-	<T> T getBean(String name, Class<T> requiredType) throws BeansException;
+	<T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException;
 
 	/**
 	 * Return the bean instance that uniquely matches the given object type, if any.
@@ -313,6 +314,7 @@ public interface BeanFactory {
 	 * @see #getBean
 	 * @see #isTypeMatch
 	 */
+	@Nullable
 	Class<?> getType(String name) throws NoSuchBeanDefinitionException;
 
 	/**

spring-beans/src/main/java/org/springframework/beans/factory/FactoryBean.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans.factory;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Interface to be implemented by objects used within a {@link BeanFactory} which
  * are themselves factories for individual objects. If a bean implements this
@@ -72,6 +74,7 @@ public interface FactoryBean<T> {
 	 * @throws Exception in case of creation errors
 	 * @see FactoryBeanNotInitializedException
 	 */
+	@Nullable
 	T getObject() throws Exception;
 
 	/**
@@ -93,6 +96,7 @@ public interface FactoryBean<T> {
 	 * or {@code null} if not known at the time of the call
 	 * @see ListableBeanFactory#getBeansOfType
 	 */
+	@Nullable
 	Class<?> getObjectType();
 
 	/**

spring-beans/src/main/java/org/springframework/beans/factory/HierarchicalBeanFactory.java

@@ -16,6 +16,8 @@
 
 package org.springframework.beans.factory;
 
+import org.springframework.lang.Nullable;
+
 /**
  * Sub-interface implemented by bean factories that can be part
  * of a hierarchy.
@@ -34,6 +36,7 @@ public interface HierarchicalBeanFactory extends BeanFactory {
 	/**
 	 * Return the parent bean factory, or {@code null} if there is none.
 	 */
+	@Nullable
 	BeanFactory getParentBeanFactory();
 
 	/**

spring-beans/src/main/java/org/springframework/beans/factory/InjectionPoint.java

@@ -22,6 +22,7 @@ import java.lang.reflect.Field;
 import java.lang.reflect.Member;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -83,6 +84,7 @@ public class InjectionPoint {
 	 * <p>Note: Either MethodParameter or Field is available.
 	 * @return the MethodParameter, or {@code null} if none
 	 */
+	@Nullable
 	public MethodParameter getMethodParameter() {
 		return this.methodParameter;
 	}
@@ -92,6 +94,7 @@ public class InjectionPoint {
 	 * <p>Note: Either MethodParameter or Field is available.
 	 * @return the Field, or {@code null} if none
 	 */
+	@Nullable
 	public Field getField() {
 		return this.field;
 	}
@@ -117,6 +120,7 @@ public class InjectionPoint {
 	 * @return the annotation instance, or {@code null} if none found
 	 * @since 4.3.9
 	 */
+	@Nullable
 	public <A extends Annotation> A getAnnotation(Class<A> annotationType) {
 		return (this.field != null ? this.field.getAnnotation(annotationType) :
 				this.methodParameter.getParameterAnnotation(annotationType));

spring-beans/src/main/java/org/springframework/beans/factory/ListableBeanFactory.java

@@ -21,6 +21,7 @@ import java.util.Map;
 
 import org.springframework.beans.BeansException;
 import org.springframework.core.ResolvableType;
+import org.springframework.lang.Nullable;
 
 /**
  * Extension of the {@link BeanFactory} interface to be implemented by bean factories
@@ -113,7 +114,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @see FactoryBean#getObjectType
 	 * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors(ListableBeanFactory, ResolvableType)
 	 */
-	String[] getBeanNamesForType(ResolvableType type);
+	String[] getBeanNamesForType(@Nullable ResolvableType type);
 
 	/**
 	 * Return the names of beans matching the given type (including subclasses),
@@ -140,7 +141,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @see FactoryBean#getObjectType
 	 * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors(ListableBeanFactory, Class)
 	 */
-	String[] getBeanNamesForType(Class<?> type);
+	String[] getBeanNamesForType(@Nullable Class<?> type);
 
 	/**
 	 * Return the names of beans matching the given type (including subclasses),
@@ -173,7 +174,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @see FactoryBean#getObjectType
 	 * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors(ListableBeanFactory, Class, boolean, boolean)
 	 */
-	String[] getBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit);
+	String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit);
 
 	/**
 	 * Return the bean instances that match the given object type (including
@@ -203,7 +204,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @see FactoryBean#getObjectType
 	 * @see BeanFactoryUtils#beansOfTypeIncludingAncestors(ListableBeanFactory, Class)
 	 */
-	<T> Map<String, T> getBeansOfType(Class<T> type) throws BeansException;
+	<T> Map<String, T> getBeansOfType(@Nullable Class<T> type) throws BeansException;
 
 	/**
 	 * Return the bean instances that match the given object type (including
@@ -238,7 +239,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @see FactoryBean#getObjectType
 	 * @see BeanFactoryUtils#beansOfTypeIncludingAncestors(ListableBeanFactory, Class, boolean, boolean)
 	 */
-	<T> Map<String, T> getBeansOfType(Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
+	<T> Map<String, T> getBeansOfType(@Nullable Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
 			throws BeansException;
 
 	/**
@@ -271,6 +272,7 @@ public interface ListableBeanFactory extends BeanFactory {
 	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
 	 * @since 3.0
 	 */
+	@Nullable
 	<A extends Annotation> A findAnnotationOnBean(String beanName, Class<A> annotationType)
 			throws NoSuchBeanDefinitionException;
 

spring-beans/src/main/java/org/springframework/beans/factory/NoUniqueBeanDefinitionException.java

@@ -19,6 +19,7 @@ package org.springframework.beans.factory;
 import java.util.Arrays;
 import java.util.Collection;
 
+import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
 /**
@@ -85,6 +86,7 @@ public class NoUniqueBeanDefinitionException extends NoSuchBeanDefinitionExcepti
 	 * @since 4.3
 	 * @see #getBeanType()
 	 */
+	@Nullable
 	public Collection<String> getBeanNamesFound() {
 		return this.beanNamesFound;
 	}

spring-beans/src/main/java/org/springframework/beans/factory/ObjectProvider.java

@@ -19,6 +19,7 @@ package org.springframework.beans.factory;
 import java.util.function.Supplier;
 
 import org.springframework.beans.BeansException;
+import org.springframework.lang.Nullable;
 
 /**
  * A variant of {@link ObjectFactory} designed specifically for injection points,
@@ -48,6 +49,7 @@ public interface ObjectProvider<T> extends ObjectFactory<T> {
 	 * @throws BeansException in case of creation errors
 	 * @see #getObject()
 	 */
+	@Nullable
 	T getIfAvailable() throws BeansException;
 
 	/**
@@ -74,6 +76,7 @@ public interface ObjectProvider<T> extends ObjectFactory<T> {
 	 * @throws BeansException in case of creation errors
 	 * @see #getObject()
 	 */
+	@Nullable
 	T getIfUnique() throws BeansException;
 
 	/**

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

@@ -19,6 +19,7 @@ package org.springframework.beans.factory.annotation;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.core.type.AnnotationMetadata;
 import org.springframework.core.type.MethodMetadata;
+import org.springframework.lang.Nullable;
 
 /**
  * Extended {@link org.springframework.beans.factory.config.BeanDefinition}
@@ -44,6 +45,7 @@ public interface AnnotatedBeanDefinition extends BeanDefinition {
 	 * @return the factory method metadata, or {@code null} if none
 	 * @since 4.1.1
 	 */
+	@Nullable
 	MethodMetadata getFactoryMethodMetadata();
 
 }

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

@@ -60,6 +60,7 @@ import org.springframework.core.Ordered;
 import org.springframework.core.PriorityOrdered;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.AnnotationAttributes;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
@@ -468,6 +469,7 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 		return new InjectionMetadata(clazz, elements);
 	}
 
+	@Nullable
 	private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
 		if (ao.getAnnotations().length > 0) {
 			for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
@@ -691,6 +693,7 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 			}
 		}
 
+		@Nullable
 		private Object[] resolveCachedArguments(String beanName) {
 			if (this.cachedMethodArguments == null) {
 				return null;

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

@@ -31,6 +31,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.beans.MutablePropertyValues;
 import org.springframework.beans.PropertyValues;
 import org.springframework.beans.factory.support.RootBeanDefinition;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -232,6 +233,7 @@ public class InjectionMetadata {
 		/**
 		 * Either this or {@link #inject} needs to be overridden.
 		 */
+		@Nullable
 		protected Object getResourceToInject(Object target, String requestingBeanName) {
 			return null;
 		}

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

@@ -36,6 +36,7 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.AnnotationAttributes;
 import org.springframework.core.annotation.AnnotationUtils;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
@@ -341,6 +342,7 @@ public class QualifierAnnotationAutowireCandidateResolver extends GenericTypeAwa
 	/**
 	 * Determine a suggested value from any of the given candidate annotations.
 	 */
+	@Nullable
 	protected Object findValue(Annotation[] annotationsToSearch) {
 		AnnotationAttributes attr = AnnotatedElementUtils.getMergedAnnotationAttributes(
 				AnnotatedElementUtils.forAnnotations(annotationsToSearch), this.valueAnnotationType);

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

@@ -1,4 +1,7 @@
 /**
  * Support package for annotation-driven bean configuration.
  */
+@NonNullApi
 package org.springframework.beans.factory.annotation;
+
+import org.springframework.lang.NonNullApi;

spring-beans/src/main/java/org/springframework/beans/factory/config/AbstractFactoryBean.java

@@ -33,6 +33,7 @@ import org.springframework.beans.factory.DisposableBean;
 import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.FactoryBeanNotInitializedException;
 import org.springframework.beans.factory.InitializingBean;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
@@ -225,6 +226,7 @@ public abstract class AbstractFactoryBean<T>
 	 * or {@code null} to indicate a FactoryBeanNotInitializedException
 	 * @see org.springframework.beans.factory.FactoryBeanNotInitializedException
 	 */
+	@Nullable
 	protected Class<?>[] getEarlySingletonInterfaces() {
 		Class<?> type = getObjectType();
 		return (type != null && type.isInterface() ? new Class<?>[] {type} : null);

spring-beans/src/main/java/org/springframework/beans/factory/config/AutowireCapableBeanFactory.java

@@ -23,6 +23,7 @@ import org.springframework.beans.TypeConverter;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.NoSuchBeanDefinitionException;
 import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
+import org.springframework.lang.Nullable;
 
 /**
  * Extension of the {@link org.springframework.beans.factory.BeanFactory}
@@ -337,6 +338,7 @@ public interface AutowireCapableBeanFactory extends BeanFactory {
 	 * @since 2.5
 	 * @see #resolveDependency(DependencyDescriptor, String, Set, TypeConverter)
 	 */
+	@Nullable
 	Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName) throws BeansException;
 
 	/**
@@ -353,6 +355,7 @@ public interface AutowireCapableBeanFactory extends BeanFactory {
 	 * @since 2.5
 	 * @see DependencyDescriptor
 	 */
+	@Nullable
 	Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
 			Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException;
 

spring-beans/src/main/java/org/springframework/beans/factory/config/BeanDefinition.java

@@ -19,6 +19,7 @@ package org.springframework.beans.factory.config;
 import org.springframework.beans.BeanMetadataElement;
 import org.springframework.beans.MutablePropertyValues;
 import org.springframework.core.AttributeAccessor;
+import org.springframework.lang.Nullable;
 
 /**
  * A BeanDefinition describes a bean instance, which has property values,
@@ -84,11 +85,12 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	/**
 	 * Set the name of the parent definition of this bean definition, if any.
 	 */
-	void setParentName(String parentName);
+	void setParentName(@Nullable String parentName);
 
 	/**
 	 * Return the name of the parent definition of this bean definition, if any.
 	 */
+	@Nullable
 	String getParentName();
 
 	/**
@@ -126,6 +128,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	 * Return the name of the current target scope for this bean,
 	 * or {@code null} if not known yet.
 	 */
+	@Nullable
 	String getScope();
 
 	/**
@@ -188,6 +191,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	/**
 	 * Return the factory bean name, if any.
 	 */
+	@Nullable
 	String getFactoryBeanName();
 
 	/**
@@ -203,6 +207,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	/**
 	 * Return a factory method, if any.
 	 */
+	@Nullable
 	String getFactoryMethodName();
 
 	/**
@@ -260,6 +265,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	 * Return a description of the resource that this bean definition
 	 * came from (for the purpose of showing context in case of errors).
 	 */
+	@Nullable
 	String getResourceDescription();
 
 	/**
@@ -268,6 +274,7 @@ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
 	 * <p>Note that this method returns the immediate originator. Iterate through the
 	 * originator chain to find the original BeanDefinition as defined by the user.
 	 */
+	@Nullable
 	BeanDefinition getOriginatingBeanDefinition();
 
 }