Support @Nullable annotations as indicators for optional injection points

Issue: SPR-15028

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

@@ -19,6 +19,7 @@ package org.springframework.beans.factory.config;
 import java.io.IOException;
 import java.io.ObjectInputStream;
 import java.io.Serializable;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Field;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
@@ -155,6 +156,10 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 
 	/**
 	 * Return whether this dependency is required.
+	 * <p>Optional semantics are derived from Java 8's {@link java.util.Optional},
+	 * any variant of a parameter-level {@code Nullable} annotation (such as from
+	 * JSR-305 or the FindBugs set of annotations), or a language-level nullable
+	 * type declaration in Kotlin.
 	 */
 	public boolean isRequired() {
 		if (!this.required) {
@@ -162,7 +167,7 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 		}
 
 		if (this.field != null) {
-			return !(this.field.getType() == Optional.class ||
+			return !(this.field.getType() == Optional.class || hasNullableAnnotation() ||
 					(kotlinPresent && KotlinDelegate.isNullable(this.field)));
 		}
 		else {
@@ -170,6 +175,20 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 		}
 	}
 
+	/**
+	 * Check whether the underlying field is annotated with any variant of a
+	 * {@code Nullable} annotation, e.g. {@code javax.annotation.Nullable} or
+	 * {@code edu.umd.cs.findbugs.annotations.Nullable}.
+	 */
+	private boolean hasNullableAnnotation() {
+		for (Annotation ann : getAnnotations()) {
+			if ("Nullable".equals(ann.annotationType().getSimpleName())) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Return whether this dependency is 'eager' in the sense of
 	 * eagerly resolving potential target beans for type matching.

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

@@ -17,6 +17,8 @@
 package org.springframework.beans.factory.annotation;
 
 import java.io.Serializable;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -574,6 +576,60 @@ public class InjectAnnotationBeanPostProcessorTests {
 		bf.destroySingletons();
 	}
 
+	@Test
+	public void testNullableFieldInjectionWithBeanAvailable() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(NullableFieldInjectionBean.class));
+		bf.registerBeanDefinition("testBean", new RootBeanDefinition(TestBean.class));
+
+		NullableFieldInjectionBean bean = (NullableFieldInjectionBean) bf.getBean("annotatedBean");
+		assertSame(bf.getBean("testBean"), bean.getTestBean());
+		bf.destroySingletons();
+	}
+
+	@Test
+	public void testNullableFieldInjectionWithBeanNotAvailable() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(NullableFieldInjectionBean.class));
+
+		NullableFieldInjectionBean bean = (NullableFieldInjectionBean) bf.getBean("annotatedBean");
+		assertNull(bean.getTestBean());
+		bf.destroySingletons();
+	}
+
+	@Test
+	public void testNullableMethodInjectionWithBeanAvailable() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(NullableMethodInjectionBean.class));
+		bf.registerBeanDefinition("testBean", new RootBeanDefinition(TestBean.class));
+
+		NullableMethodInjectionBean bean = (NullableMethodInjectionBean) bf.getBean("annotatedBean");
+		assertSame(bf.getBean("testBean"), bean.getTestBean());
+		bf.destroySingletons();
+	}
+
+	@Test
+	public void testNullableMethodInjectionWithBeanNotAvailable() {
+		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
+		AutowiredAnnotationBeanPostProcessor bpp = new AutowiredAnnotationBeanPostProcessor();
+		bpp.setBeanFactory(bf);
+		bf.addBeanPostProcessor(bpp);
+		bf.registerBeanDefinition("annotatedBean", new RootBeanDefinition(NullableMethodInjectionBean.class));
+
+		NullableMethodInjectionBean bean = (NullableMethodInjectionBean) bf.getBean("annotatedBean");
+		assertNull(bean.getTestBean());
+		bf.destroySingletons();
+	}
+
 	@Test
 	public void testOptionalFieldInjectionWithBeanAvailable() {
 		DefaultListableBeanFactory bf = new DefaultListableBeanFactory();
@@ -1275,6 +1331,36 @@ public class InjectAnnotationBeanPostProcessorTests {
 	}
 
 
+	@Retention(RetentionPolicy.RUNTIME)
+	public @interface Nullable {}
+
+
+	public static class NullableFieldInjectionBean {
+
+		@Inject @Nullable
+		private TestBean testBean;
+
+		public TestBean getTestBean() {
+			return this.testBean;
+		}
+	}
+
+
+	public static class NullableMethodInjectionBean {
+
+		private TestBean testBean;
+
+		@Inject
+		public void setTestBean(@Nullable TestBean testBean) {
+			this.testBean = testBean;
+		}
+
+		public TestBean getTestBean() {
+			return this.testBean;
+		}
+	}
+
+
 	public static class OptionalFieldInjectionBean {
 
 		@Inject
@@ -1291,8 +1377,8 @@ public class InjectAnnotationBeanPostProcessorTests {
 		private Optional<TestBean> testBean;
 
 		@Inject
-		public void setTestBean(Optional<TestBean> testBeanFactory) {
+		public void setTestBean(Optional<TestBean> testBean) {
-			this.testBean = testBeanFactory;
+			this.testBean = testBean;
 		}
 
 		public Optional<TestBean> getTestBean() {
@@ -1317,8 +1403,8 @@ public class InjectAnnotationBeanPostProcessorTests {
 		private Optional<List<TestBean>> testBean;
 
 		@Inject
-		public void setTestBean(Optional<List<TestBean>> testBeanFactory) {
+		public void setTestBean(Optional<List<TestBean>> testBean) {
-			this.testBean = testBeanFactory;
+			this.testBean = testBean;
 		}
 
 		public Optional<List<TestBean>> getTestBean() {
@@ -1343,8 +1429,8 @@ public class InjectAnnotationBeanPostProcessorTests {
 		private Provider<Optional<TestBean>> testBean;
 
 		@Inject
-		public void setTestBean(Provider<Optional<TestBean>> testBeanFactory) {
+		public void setTestBean(Provider<Optional<TestBean>> testBean) {
-			this.testBean = testBeanFactory;
+			this.testBean = testBean;
 		}
 
 		public Optional<TestBean> getTestBean() {

spring-core/src/main/java/org/springframework/core/MethodParameter.java

@@ -322,16 +322,32 @@ public class MethodParameter {
 	}
 
 	/**
-	 * Return whether this method indicates a parameter which is not required
+	 * Return whether this method indicates a parameter which is not required:
-	 * (either in the form of Java 8's {@link java.util.Optional} or Kotlin's
+	 * either in the form of Java 8's {@link java.util.Optional}, any variant
-	 * nullable type).
+	 * of a parameter-level {@code Nullable} annotation (such as from JSR-305
+	 * or the FindBugs set of annotations), or a language-level nullable type
+	 * declaration in Kotlin.
 	 * @since 4.3
 	 */
 	public boolean isOptional() {
-		return (getParameterType() == Optional.class ||
+		return (getParameterType() == Optional.class || hasNullableAnnotation() ||
 				(kotlinPresent && KotlinDelegate.isNullable(this)));
 	}
 
+	/**
+	 * Check whether this method parameter is annotated with any variant of a
+	 * {@code Nullable} annotation, e.g. {@code javax.annotation.Nullable} or
+	 * {@code edu.umd.cs.findbugs.annotations.Nullable}.
+	 */
+	private boolean hasNullableAnnotation() {
+		for (Annotation ann : getParameterAnnotations()) {
+			if ("Nullable".equals(ann.annotationType().getSimpleName())) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Return a variant of this {@code MethodParameter} which points to
 	 * the same parameter but one nesting level deeper in case of a