Allow non-public Kotlin classes/ctors to be instantiated

Closes gh-24712

Author: Tom van den Berge

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

@@ -38,6 +38,7 @@
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
 import kotlin.reflect.full.KClasses;
+import kotlin.reflect.jvm.KCallablesJvm;
 import kotlin.reflect.jvm.ReflectJvmMapping;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -780,6 +781,11 @@ public static <T> T instantiateClass(Constructor<T> ctor, Object... args)
 			if (kotlinConstructor == null) {
 				return ctor.newInstance(args);
 			}
+
+			if ((!Modifier.isPublic(ctor.getModifiers()) || !Modifier.isPublic(ctor.getDeclaringClass().getModifiers()))) {
+				KCallablesJvm.setAccessible(kotlinConstructor, true);
+			}
+
 			List<KParameter> parameters = kotlinConstructor.getParameters();
 			Map<KParameter, Object> argParameters = new HashMap<>(parameters.size());
 			Assert.isTrue(args.length <= parameters.size(),

spring-beans/src/test/java/org/springframework/beans/BeanUtilsTests.java

@@ -95,6 +95,12 @@ void testInstantiateClassWithMoreArgsThanParameters() throws NoSuchMethodExcepti
 				BeanUtils.instantiateClass(ctor, null, null, "foo", null));
 	}
 
+	@Test
+	void testInstantiatePrivateClassWithPrivateConstructor() throws NoSuchMethodException {
+		Constructor<PrivateBeanWithPrivateConstructor> ctor = PrivateBeanWithPrivateConstructor.class.getDeclaredConstructor();
+		BeanUtils.instantiateClass(ctor);
+	}
+
 	@Test
 	void testGetPropertyDescriptors() throws Exception {
 		PropertyDescriptor[] actual = Introspector.getBeanInfo(TestBean.class).getPropertyDescriptors();
@@ -555,4 +561,10 @@ public String getValue() {
 		}
 	}
 
+	private static class PrivateBeanWithPrivateConstructor {
+
+		private PrivateBeanWithPrivateConstructor() {
+		}
+	}
+
 }

spring-beans/src/test/kotlin/org/springframework/beans/KotlinBeanUtilsTests.kt

@@ -74,6 +74,22 @@ class KotlinBeanUtilsTests {
 		assertThat(baz.param2).isEqualTo(12)
 	}
 
+	@Test
+	@Suppress("UsePropertyAccessSyntax")
+	fun `Instantiate class with private constructor`() {
+		BeanUtils.instantiateClass(PrivateConstructor::class.java.getDeclaredConstructor())
+	}
+
+	@Test
+	fun `Instantiate class with protected constructor`() {
+		BeanUtils.instantiateClass(ProtectedConstructor::class.java.getDeclaredConstructor())
+	}
+
+	@Test
+	fun `Instantiate private class`() {
+		BeanUtils.instantiateClass(PrivateClass::class.java.getDeclaredConstructor())
+	}
+
 	class Foo(val param1: String, val param2: Int)
 
 	class Bar(val param1: String, val param2: Int = 12)
@@ -106,4 +122,10 @@ class KotlinBeanUtilsTests {
 		constructor(param1: String)
 	}
 
+	class PrivateConstructor private constructor()
+
+	open class ProtectedConstructor protected constructor()
+
+	private class PrivateClass
+
 }