Efficient Kotlin metadata detection

Follow-up of 3991ab4a23.

Issue: SPR-15673

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

@@ -71,16 +71,18 @@ public abstract class BeanUtils {
 			Collections.newSetFromMap(new ConcurrentReferenceHashMap<>(64));
 
 	@Nullable
-	private static Class<?> kotlinMetadata;
+	private static final Class<?> kotlinMetadata;
 
 	static {
+		Class<?> metadata;
 		try {
-			kotlinMetadata = ClassUtils.forName("kotlin.Metadata", BeanUtils.class.getClassLoader());
+			metadata = ClassUtils.forName("kotlin.Metadata", BeanUtils.class.getClassLoader());
 		}
 		catch (ClassNotFoundException ex) {
 			// Kotlin API not available - no special support for Kotlin class instantiation
-			kotlinMetadata = null;
+			metadata = null;
 		}
+		kotlinMetadata = metadata;
 	}
 
 
@@ -125,7 +127,7 @@ public abstract class BeanUtils {
 			throw new BeanInstantiationException(clazz, "Specified class is an interface");
 		}
 		try {
-			Constructor<T> ctor = (isKotlinClass(clazz) ?
+			Constructor<T> ctor = (useKotlinSupport(clazz) ?
 					KotlinDelegate.findPrimaryConstructor(clazz) : clazz.getDeclaredConstructor());
 			if (ctor == null) {
 				throw new BeanInstantiationException(clazz, "No default constructor found");
@@ -172,7 +174,7 @@ public abstract class BeanUtils {
 		Assert.notNull(ctor, "Constructor must not be null");
 		try {
 			ReflectionUtils.makeAccessible(ctor);
-			return (isKotlinClass(ctor.getDeclaringClass()) ?
+			return (useKotlinSupport(ctor.getDeclaringClass()) ?
 					KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
 		}
 		catch (InstantiationException ex) {
@@ -340,7 +342,7 @@ public abstract class BeanUtils {
 	@Nullable
 	public static <T> Constructor<T> findPrimaryConstructor(Class<T> clazz) {
 		Assert.notNull(clazz, "Class must not be null");
-		if (isKotlinClass(clazz)) {
+		if (useKotlinSupport(clazz)) {
 			return KotlinDelegate.findPrimaryConstructor(clazz);
 		}
 		else {
@@ -707,10 +709,10 @@ public abstract class BeanUtils {
 	}
 
 	/**
-	 * Return true if the specified class is a Kotlin one.
+	 * Return true if Kotlin is present and if the specified class is a Kotlin one.
 	 */
 	@SuppressWarnings("unchecked")
-	private static boolean isKotlinClass(Class<?> clazz) {
+	private static boolean useKotlinSupport(Class<?> clazz) {
 		return (kotlinMetadata != null &&
 				clazz.getDeclaredAnnotation((Class<? extends Annotation>) kotlinMetadata) != null);
 	}

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

@@ -26,7 +26,6 @@ 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;
 
@@ -52,8 +51,20 @@ import org.springframework.util.ClassUtils;
 @SuppressWarnings("serial")
 public class DependencyDescriptor extends InjectionPoint implements Serializable {
 
-	private static final boolean kotlinPresent =
-			ClassUtils.isPresent("kotlin.Metadata", DependencyDescriptor.class.getClassLoader());
+	@Nullable
+	private static final Class<?> kotlinMetadata;
+
+	static {
+		Class<?> metadata;
+		try {
+			metadata = ClassUtils.forName("kotlin.Metadata", DependencyDescriptor.class.getClassLoader());
+		}
+		catch (ClassNotFoundException ex) {
+			// Kotlin API not available - no Kotlin support
+			metadata = null;
+		}
+		kotlinMetadata = metadata;
+	}
 
 
 	private final Class<?> declaringClass;
@@ -172,13 +183,22 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 
 		if (this.field != null) {
 			return !(this.field.getType() == Optional.class || hasNullableAnnotation() ||
-					(kotlinPresent && KotlinDelegate.isNullable(this.field)));
+					(useKotlinSupport(this.field.getDeclaringClass()) && KotlinDelegate.isNullable(this.field)));
 		}
 		else {
 			return !obtainMethodParameter().isOptional();
 		}
 	}
 
+	/**
+	 * Return true if Kotlin is present and if the specified class is a Kotlin one.
+	 */
+	@SuppressWarnings("unchecked")
+	private static boolean useKotlinSupport(Class<?> clazz) {
+		return (kotlinMetadata != null &&
+				clazz.getDeclaredAnnotation((Class<? extends Annotation>) kotlinMetadata) != null);
+	}
+
 	/**
 	 * Check whether the underlying field is annotated with any variant of a
 	 * {@code Nullable} annotation, e.g. {@code javax.annotation.Nullable} or
@@ -435,11 +455,8 @@ public class DependencyDescriptor extends InjectionPoint implements Serializable
 		 * 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;
+			KProperty<?> property = ReflectJvmMapping.getKotlinProperty(field);
+			return (property != null && property.getReturnType().isMarkedNullable());
 		}
 	}
 

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

@@ -58,8 +58,20 @@ import org.springframework.util.ClassUtils;
  */
 public class MethodParameter {
 
-	private static final boolean kotlinPresent =
-			ClassUtils.isPresent("kotlin.Metadata", MethodParameter.class.getClassLoader());
+	@Nullable
+	private static final Class<?> kotlinMetadata;
+
+	static {
+		Class<?> metadata;
+		try {
+			metadata = ClassUtils.forName("kotlin.Metadata", MethodParameter.class.getClassLoader());
+		}
+		catch (ClassNotFoundException ex) {
+			// Kotlin API not available - no Kotlin support
+			metadata = null;
+		}
+		kotlinMetadata = metadata;
+	}
 
 
 	private final Executable executable;
@@ -341,7 +353,16 @@ public class MethodParameter {
 	 */
 	public boolean isOptional() {
 		return (getParameterType() == Optional.class || hasNullableAnnotation() ||
-				(kotlinPresent && KotlinDelegate.isNullable(this)));
+				(useKotlinSupport(this.getContainingClass()) && KotlinDelegate.isNullable(this)));
+	}
+
+	/**
+	 * Return true if Kotlin is present and if the specified class is a Kotlin one.
+	 */
+	@SuppressWarnings("unchecked")
+	private static boolean useKotlinSupport(Class<?> clazz) {
+		return (kotlinMetadata != null &&
+				clazz.getDeclaredAnnotation((Class<? extends Annotation>) kotlinMetadata) != null);
 	}
 
 	/**
@@ -736,41 +757,30 @@ public class MethodParameter {
 		 * Check whether the specified {@link MethodParameter} represents a nullable Kotlin type or not.
 		 */
 		public static boolean isNullable(MethodParameter param) {
-			if (isKotlinClass(param.getContainingClass())) {
-				Method method = param.getMethod();
-				Constructor<?> ctor = param.getConstructor();
-				int index = param.getParameterIndex();
-				if (method != null && index == -1) {
-					KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
-					return (function != null && function.getReturnType().isMarkedNullable());
+			Method method = param.getMethod();
+			Constructor<?> ctor = param.getConstructor();
+			int index = param.getParameterIndex();
+			if (method != null && index == -1) {
+				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+				return (function != null && function.getReturnType().isMarkedNullable());
+			}
+			else {
+				KFunction<?> function = null;
+				if (method != null) {
+					function = ReflectJvmMapping.getKotlinFunction(method);
 				}
-				else {
-					KFunction<?> function = null;
-					if (method != null) {
-						function = ReflectJvmMapping.getKotlinFunction(method);
-					}
-					else if (ctor != null) {
-						function = ReflectJvmMapping.getKotlinFunction(ctor);
-					}
-					if (function != null) {
-						List<KParameter> parameters = function.getParameters();
-						return parameters
-								.stream()
-								.filter(p -> KParameter.Kind.VALUE.equals(p.getKind()))
-								.collect(Collectors.toList())
-								.get(index)
-								.getType()
-								.isMarkedNullable();
-					}
+				else if (ctor != null) {
+					function = ReflectJvmMapping.getKotlinFunction(ctor);
 				}
-			}
-			return false;
-		}
-
-		private static boolean isKotlinClass(Class<?> clazz) {
-			for (Annotation annotation : clazz.getDeclaredAnnotations()) {
-				if (annotation.annotationType().getName().equals("kotlin.Metadata")) {
-					return true;
+				if (function != null) {
+					List<KParameter> parameters = function.getParameters();
+					return parameters
+							.stream()
+							.filter(p -> KParameter.Kind.VALUE.equals(p.getKind()))
+							.collect(Collectors.toList())
+							.get(index)
+							.getType()
+							.isMarkedNullable();
 				}
 			}
 			return false;