Support instantiating Kotlin classes with optional parameters

This commit updates BeanUtils class in order to add Kotlin optional
parameters with default values support to the immutable data classes
support introduced by SPR-15199.

Issue: SPR-15673

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

@@ -18,6 +18,7 @@ package org.springframework.beans;
 
 import java.beans.Introspector;
 import java.beans.PropertyDescriptor;
+import java.lang.reflect.Constructor;
 import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.List;
@@ -274,6 +275,23 @@ public class BeanUtilsTests {
 			}
 		}
 	}
+	
+	@Test 
+	public void testFindDefaultConstructorAndInstantiate() {
+		Constructor<Bean> ctor = BeanUtils.findPrimaryConstructor(Bean.class);
+		assertNotNull(ctor);
+		Bean bean = BeanUtils.instantiateClass(ctor);
+		assertNotNull(bean);
+	}
+
+	@Test
+	public void testFindSingleNonDefaultConstructorAndInstantiate() {
+		Constructor<BeanWithSingleNonDefaultConstructor> ctor = BeanUtils.findPrimaryConstructor(BeanWithSingleNonDefaultConstructor.class);
+		assertNotNull(ctor);
+		BeanWithSingleNonDefaultConstructor bean = BeanUtils.instantiateClass(ctor, "foo");
+		assertNotNull(bean);
+		assertEquals("foo", bean.getName());
+	}
 
 	private void assertSignatureEquals(Method desiredMethod, String signature) {
 		assertEquals(desiredMethod, BeanUtils.resolveSignature(signature, MethodSignatureBean.class));
@@ -444,5 +462,18 @@ public class BeanUtilsTests {
 			value = aValue;
 		}
 	}
+	
+	private static class BeanWithSingleNonDefaultConstructor {
+		
+		private final String name;
+
+		public BeanWithSingleNonDefaultConstructor(String name) {
+			this.name = name;
+		}
+
+		public String getName() {
+			return name;
+		}
+	}
 
 }

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

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2002-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.beans
+
+import org.junit.Assert.*
+import org.junit.Test
+
+/**
+ * Kotlin tests for {@link BeanUtils}
+ * 
+ * @author Sebastien Deleuze
+ */
+class BeanUtilsKotlinTests {
+
+	@Test
+	fun `Instantiate immutable class`() {
+		val constructor = BeanUtils.findPrimaryConstructor(Foo::class.java)
+		val foo = BeanUtils.instantiateClass(constructor, "bar", 3) as Foo
+		assertEquals("bar", foo.param1)
+		assertEquals(3, foo.param2)
+	}
+
+	@Test
+	fun `Instantiate immutable class with optional parameter and all parameters specified`() {
+		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)
+		val bar = BeanUtils.instantiateClass(constructor, "baz", 8) as Bar
+		assertEquals("baz", bar.param1)
+		assertEquals(8, bar.param2)
+	}
+
+	@Test
+	fun `Instantiate immutable class with optional parameter and only mandatory parameters specified by position`() {
+		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)
+		val bar = BeanUtils.instantiateClass(constructor, "baz") as Bar
+		assertEquals("baz", bar.param1)
+		assertEquals(12, bar.param2)
+	}
+
+	@Test
+	fun `Instantiate immutable class with optional parameter specified with null value`() {
+		val constructor = BeanUtils.findPrimaryConstructor(Bar::class.java)
+		val bar = BeanUtils.instantiateClass(constructor, "baz", null) as Bar
+		assertEquals("baz", bar.param1)
+		assertEquals(12, bar.param2)
+	}
+
+	class Foo(val param1: String, val param2: Int)
+
+	class Bar(val param1: String, val param2: Int = 12)
+	
+}