Make @ConstructorBinding implict for config prop records

Closes gh-27216

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/external-config.adoc

@@ -706,6 +706,8 @@ include::{docs-java}/features/externalconfig/typesafeconfigurationproperties/con
 
 In this setup, the `@ConstructorBinding` annotation is used to indicate that constructor binding should be used.
 This means that the binder will expect to find a constructor with the parameters that you wish to have bound.
+If you are using Java 16 or later, constructor binding can be used with records.
+In this case, unless your record has multiple constructors, there is no need to use `@ConstructorBinding`.
 
 Nested members of a `@ConstructorBinding` class (such as `Security` in the example above) will also be bound via their constructor.
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -46,7 +46,7 @@ Constructor<?> getBindConstructor(Class<?> type, boolean isNestedConstructorBind
 			return null;
 		}
 		Constructor<?> constructor = findConstructorBindingAnnotatedConstructor(type);
-		if (constructor == null && (isConstructorBindingAnnotatedType(type) || isNestedConstructorBinding)) {
+		if (constructor == null && (isConstructorBindingType(type) || isNestedConstructorBinding)) {
 			constructor = deduceBindConstructor(type);
 		}
 		return constructor;
@@ -76,6 +76,15 @@ private Constructor<?> findAnnotatedConstructor(Class<?> type, Constructor<?>...
 		return constructor;
 	}
 
+	private boolean isConstructorBindingType(Class<?> type) {
+		return isImplicitConstructorBindingType(type) || isConstructorBindingAnnotatedType(type);
+	}
+
+	private boolean isImplicitConstructorBindingType(Class<?> type) {
+		Class<?> superclass = type.getSuperclass();
+		return (superclass != null) && "java.lang.Record".equals(superclass.getName());
+	}
+
 	private boolean isConstructorBindingAnnotatedType(Class<?> type) {
 		return MergedAnnotations.from(type, MergedAnnotations.SearchStrategy.TYPE_HIERARCHY_AND_ENCLOSING_CLASSES)
 				.isPresent(ConstructorBinding.class);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/ConfigurationPropertiesBeanTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -16,9 +16,15 @@
 
 package org.springframework.boot.context.properties;
 
+import java.lang.reflect.Constructor;
 import java.util.Map;
 
+import net.bytebuddy.ByteBuddy;
+import net.bytebuddy.ClassFileVersion;
+import net.bytebuddy.description.annotation.AnnotationDescription;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledForJreRange;
+import org.junit.jupiter.api.condition.JRE;
 import org.junit.jupiter.api.function.ThrowingConsumer;
 
 import org.springframework.boot.context.properties.ConfigurationPropertiesBean.BindMethod;
@@ -201,7 +207,7 @@ void getWhenHasValidatedBeanAndFactoryMethodBindsWithFactoryMethodAnnotation() t
 	}
 
 	@Test
-	void forValueObjectReturnsBean() {
+	void forValueObjectWithConstructorBindingAnnotatedClassReturnsBean() {
 		ConfigurationPropertiesBean propertiesBean = ConfigurationPropertiesBean
 				.forValueObject(ConstructorBindingOnConstructor.class, "valueObjectBean");
 		assertThat(propertiesBean.getName()).isEqualTo("valueObjectBean");
@@ -216,6 +222,31 @@ void forValueObjectReturnsBean() {
 				.getBindConstructor(ConstructorBindingOnConstructor.class, false)).isNotNull();
 	}
 
+	@Test
+	@EnabledForJreRange(min = JRE.JAVA_16)
+	void forValueObjectWithUnannotatedRecordReturnsBean() {
+		Class<?> implicitConstructorBinding = new ByteBuddy(ClassFileVersion.JAVA_V16).makeRecord()
+				.name("org.springframework.boot.context.properties.ImplicitConstructorBinding")
+				.annotateType(AnnotationDescription.Builder.ofType(ConfigurationProperties.class)
+						.define("prefix", "implicit").build())
+				.defineRecordComponent("someString", String.class).defineRecordComponent("someInteger", Integer.class)
+				.make().load(getClass().getClassLoader()).getLoaded();
+		ConfigurationPropertiesBean propertiesBean = ConfigurationPropertiesBean
+				.forValueObject(implicitConstructorBinding, "implicitBindingRecord");
+		assertThat(propertiesBean.getName()).isEqualTo("implicitBindingRecord");
+		assertThat(propertiesBean.getInstance()).isNull();
+		assertThat(propertiesBean.getType()).isEqualTo(implicitConstructorBinding);
+		assertThat(propertiesBean.getBindMethod()).isEqualTo(BindMethod.VALUE_OBJECT);
+		assertThat(propertiesBean.getAnnotation()).isNotNull();
+		Bindable<?> target = propertiesBean.asBindTarget();
+		assertThat(target.getType()).isEqualTo(ResolvableType.forClass(implicitConstructorBinding));
+		assertThat(target.getValue()).isNull();
+		Constructor<?> bindConstructor = ConfigurationPropertiesBindConstructorProvider.INSTANCE
+				.getBindConstructor(implicitConstructorBinding, false);
+		assertThat(bindConstructor).isNotNull();
+		assertThat(bindConstructor.getParameterTypes()).containsExactly(String.class, Integer.class);
+	}
+
 	@Test
 	void forValueObjectWhenJavaBeanBindTypeThrowsException() {
 		assertThatIllegalStateException()