Fix constructor binding to Kotlin data class with default values

Closes gh-32416

Author: wilkinsona

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/ConfigurationPropertiesBindConstructorProvider.java

@@ -19,9 +19,11 @@
 import java.lang.reflect.Constructor;
 import java.util.Arrays;
 
+import org.springframework.beans.BeanUtils;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.context.properties.bind.BindConstructorProvider;
 import org.springframework.boot.context.properties.bind.Bindable;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.annotation.MergedAnnotations;
 import org.springframework.util.Assert;
 
@@ -94,6 +96,9 @@ static Constructors getConstructors(Class<?> type) {
 				}
 				bind = findAnnotatedConstructor(type, bind, candidate);
 			}
+			if (bind == null && !hasAutowiredConstructor && isKotlinType(type)) {
+				bind = deduceKotlinBindConstructor(type);
+			}
 			return new Constructors(hasAutowiredConstructor, bind);
 		}
 
@@ -141,6 +146,18 @@ private static Constructor<?> findAnnotatedConstructor(Class<?> type, Constructo
 			return constructor;
 		}
 
+		private static boolean isKotlinType(Class<?> type) {
+			return KotlinDetector.isKotlinPresent() && KotlinDetector.isKotlinType(type);
+		}
+
+		private static Constructor<?> deduceKotlinBindConstructor(Class<?> type) {
+			Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(type);
+			if (primaryConstructor != null && primaryConstructor.getParameterCount() > 0) {
+				return primaryConstructor;
+			}
+			return null;
+		}
+
 	}
 
 }

spring-boot-project/spring-boot/src/test/kotlin/org/springframework/boot/context/properties/ConfigurationPropertiesBindConstructorProviderTests.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -113,6 +113,12 @@ class ConfigurationPropertiesBindConstructorProviderTests {
 		}
 	}
 
+	@Test
+	fun `data class with default values should use constructor binding`() {
+		val bindConstructor = this.constructorProvider.getBindConstructor(ConstructorBindingDataClassWithDefaultValues::class.java, false)
+		assertThat(bindConstructor).isNotNull();
+	}
+
 	@ConfigurationProperties(prefix = "foo")
 	class FooProperties
 
@@ -210,4 +216,7 @@ class ConfigurationPropertiesBindConstructorProviderTests {
 
 	}
 
+	@ConfigurationProperties(prefix = "bing")
+	data class ConstructorBindingDataClassWithDefaultValues(val name: String = "Joan", val counter: Int = 42)
+
 }