Autodetect Kotlin nullability for optional injection points (analogous to java.util.Optional)

Built-in support in MethodParameter and DependencyDescriptor supersedes our separate KotlinUtils helper.

Issue: SPR-14951

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

@@ -23,6 +23,11 @@ import java.lang.reflect.Field;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.Map;
+import java.util.Optional;
+
+import kotlin.Metadata;
+import kotlin.reflect.KProperty;
+import kotlin.reflect.jvm.ReflectJvmMapping;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
@@ -33,6 +38,7 @@ import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ParameterNameDiscoverer;
 import org.springframework.core.ResolvableType;
+import org.springframework.util.ClassUtils;
 
 /**
  * Descriptor for a specific dependency that is about to be injected.
@@ -45,6 +51,10 @@ import org.springframework.core.ResolvableType;
 @SuppressWarnings("serial")
 public class DependencyDescriptor extends InjectionPoint implements Serializable {
 
+	private static final boolean kotlinPresent =
+			ClassUtils.isPresent("kotlin.Unit", DependencyDescriptor.class.getClassLoader());
+
+
 	private final Class<?> declaringClass;
 
 	private String methodName;
@@ -83,6 +93,7 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 	 */
 	public DependencyDescriptor(MethodParameter methodParameter, boolean required, boolean eager) {
 		super(methodParameter);
+
 		this.declaringClass = methodParameter.getDeclaringClass();
 		if (this.methodParameter.getMethod() != null) {
 			this.methodName = methodParameter.getMethod().getName();
@@ -116,6 +127,7 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 	 */
 	public DependencyDescriptor(Field field, boolean required, boolean eager) {
 		super(field);
+
 		this.declaringClass = field.getDeclaringClass();
 		this.fieldName = field.getName();
 		this.required = required;
@@ -128,6 +140,7 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 	 */
 	public DependencyDescriptor(DependencyDescriptor original) {
 		super(original);
+
 		this.declaringClass = original.declaringClass;
 		this.methodName = original.methodName;
 		this.parameterTypes = original.parameterTypes;
@@ -144,7 +157,17 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 	 * Return whether this dependency is required.
 	 */
 	public boolean isRequired() {
-		return this.required;
+		if (!this.required) {
+			return false;
+		}
+
+		if (this.field != null) {
+			return !(this.field.getType() == Optional.class ||
+					(kotlinPresent && KotlinDelegate.isNullable(this.field)));
+		}
+		else {
+			return !this.methodParameter.isOptional();
+		}
 	}
 
 	/**
@@ -398,4 +421,22 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 		}
 	}
 
+
+	/**
+	 * Inner class to avoid a hard dependency on Kotlin at runtime.
+	 */
+	private static class KotlinDelegate {
+
+		/**
+		 * Check whether the specified {@link Field} represents a nullable Kotlin type or not.
+		 */
+		public static boolean isNullable(Field field) {
+			if (field.getDeclaringClass().isAnnotationPresent(Metadata.class)) {
+				KProperty<?> property = ReflectJvmMapping.getKotlinProperty(field);
+				return (property != null && property.getReturnType().isMarkedNullable());
+			}
+			return false;
+		}
+	}
+
 }