Support running Kotlin apps without kotlin-reflect

This commit includes an optimization of BeansUtils#instantiateClass
that favors Java reflection for default constructors before leveraging
Kotlin one for finding primary constructors and avoids Kotlin related
conditions when running in Java.

Issue: SPR-17069

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

@@ -16,17 +16,16 @@
 
 package org.springframework.core;
 
-import org.springframework.util.ClassUtils;
-
 /**
  * Default implementation of the {@link ParameterNameDiscoverer} strategy interface,
  * using the Java 8 standard reflection mechanism (if available), and falling back
  * to the ASM-based {@link LocalVariableTableParameterNameDiscoverer} for checking
  * debug information in the class file.
  *
- * <p>If Kotlin is present, {@link KotlinReflectionParameterNameDiscoverer} is added first
- * in the list and used for Kotlin classes and interfaces. When compiling or running as
- * a Graal native image, no {@link ParameterNameDiscoverer} is used.
+ * <p>If a Kotlin reflection implementation is present,
+ * {@link KotlinReflectionParameterNameDiscoverer} is added first in the list and used
+ * for Kotlin classes and interfaces. When compiling or running as a Graal native image,
+ * no {@link ParameterNameDiscoverer} is used.
  *
  * <p>Further discoverers may be added through {@link #addDiscoverer(ParameterNameDiscoverer)}.
  *
@@ -39,16 +38,13 @@ import org.springframework.util.ClassUtils;
  */
 public class DefaultParameterNameDiscoverer extends PrioritizedParameterNameDiscoverer {
 
-	private static final boolean kotlinPresent =
-			ClassUtils.isPresent("kotlin.Unit", DefaultParameterNameDiscoverer.class.getClassLoader());
-
 	// See https://github.com/oracle/graal/blob/master/sdk/src/org.graalvm.nativeimage/src/org/graalvm/nativeimage/ImageInfo.java
 	private static final boolean inImageCode = (System.getProperty("org.graalvm.nativeimage.imagecode") != null);
 
 
 	public DefaultParameterNameDiscoverer() {
 		if (!inImageCode) {
-			if (kotlinPresent) {
+			if (KotlinDetector.isKotlinReflectPresent()) {
 				addDiscoverer(new KotlinReflectionParameterNameDiscoverer());
 			}
 			addDiscoverer(new StandardReflectionParameterNameDiscoverer());

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,9 @@ package org.springframework.core;
 
 import java.lang.annotation.Annotation;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
 
@@ -31,19 +34,28 @@ import org.springframework.util.ClassUtils;
 @SuppressWarnings("unchecked")
 public abstract class KotlinDetector {
 
+	private static final Log logger = LogFactory.getLog(KotlinDetector.class);
+
 	@Nullable
 	private static final Class<? extends Annotation> kotlinMetadata;
 
+	private static final boolean kotlinReflectPresent;
+
 	static {
 		Class<?> metadata;
+		ClassLoader classLoader = KotlinDetector.class.getClassLoader();
 		try {
-			metadata = ClassUtils.forName("kotlin.Metadata", KotlinDetector.class.getClassLoader());
+			metadata = ClassUtils.forName("kotlin.Metadata", classLoader);
 		}
 		catch (ClassNotFoundException ex) {
 			// Kotlin API not available - no Kotlin support
 			metadata = null;
 		}
 		kotlinMetadata = (Class<? extends Annotation>) metadata;
+		kotlinReflectPresent = ClassUtils.isPresent("kotlin.reflect.full.KClasses", classLoader);
+		if (kotlinMetadata != null && !kotlinReflectPresent) {
+			logger.info("Kotlin reflection implementation not found at runtime, related features won't be available.");
+		}
 	}
 
 
@@ -54,6 +66,14 @@ public abstract class KotlinDetector {
 		return (kotlinMetadata != null);
 	}
 
+	/**
+	 * Determine whether Kotlin reflection is present.
+	 * @since 5.1
+	 */
+	public static boolean isKotlinReflectPresent() {
+		return kotlinReflectPresent;
+	}
+
 	/**
 	 * Determine whether the given {@code Class} is a Kotlin type
 	 * (with Kotlin metadata present on it).

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

@@ -342,7 +342,9 @@ public class MethodParameter {
 	 */
 	public boolean isOptional() {
 		return (getParameterType() == Optional.class || hasNullableAnnotation() ||
-				(KotlinDetector.isKotlinType(getContainingClass()) && KotlinDelegate.isOptional(this)));
+				(KotlinDetector.isKotlinReflectPresent() &&
+						KotlinDetector.isKotlinType(getContainingClass()) &&
+						KotlinDelegate.isOptional(this)));
 	}
 
 	/**