Relax @ConstructorBinding member class requirement

Update `@ConfigurationProperties` so that `@ConstructorBinding` classes
no longer need to repeat the annotation for their members.

Closes gh-18481

Author: philwebb

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -881,7 +881,6 @@ The example in the previous section can be rewritten in an immutable fashion as
 
 			private final List<String> roles;
 
-			@ConstructorBinding
 			public Security(String username, String password,
 					@DefaultValue("USER") List<String> roles) {
 				this.username = username;
@@ -903,16 +902,16 @@ The example in the previous section can be rewritten in an immutable fashion as
 In this setup, the `@ImmutableConfigurationProperties` 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.
 
-Nested classes that also require constructor binding (such as `Security` in the example above) should use the `@ConstructorBinding` annotation.
+Nested members of a `@ImmutableConfigurationProperties` class (such as `Security` in the example above) will also be bound via their constructor.
 
 Default values can be specified using `@DefaultValue` and the same conversion service will be applied to coerce the `String` value to the target type of a missing property.
 
-TIP: You can also use `@ConstructorBinding` on the actual constructor that should be bound.
-This is required if you have more than one constructor for your class.
-
 NOTE: To use constructor binding the class must be enabled using `@EnableConfigurationProperties` or configuration property scanning.
 You cannot use constructor binding with beans that are created by the regular Spring mechanisms (e.g. `@Component` beans, beans created via `@Bean` methods or beans loaded using `@Import`)
 
+TIP: `@ImmutableConfigurationProperties` is actually a meta-annotation composed of `@ConfigurationProperties` and `@ConstructorBinding`.
+If you  have more than one constructor for your class you can also use `@ConstructorBinding` directly on actual constructor that should be bound.
+
 
 
 [[boot-features-external-config-enabling]]

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/PropertyDescriptorResolver.java

@@ -23,6 +23,7 @@
 import java.util.stream.Stream;
 
 import javax.lang.model.element.ExecutableElement;
+import javax.lang.model.element.NestingKind;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.element.VariableElement;
 import javax.lang.model.type.TypeMirror;
@@ -169,13 +170,24 @@ private ExecutableElement findBoundConstructor() {
 		}
 
 		static ConfigurationPropertiesTypeElement of(TypeElement type, MetadataGenerationEnvironment env) {
-			boolean constructorBoundType = env.hasConstructorBindingAnnotation(type);
+			boolean constructorBoundType = isConstructorBoundType(type, env);
 			List<ExecutableElement> constructors = ElementFilter.constructorsIn(type.getEnclosedElements());
 			List<ExecutableElement> boundConstructors = constructors.stream()
 					.filter(env::hasConstructorBindingAnnotation).collect(Collectors.toList());
 			return new ConfigurationPropertiesTypeElement(type, constructorBoundType, constructors, boundConstructors);
 		}
 
+		private static boolean isConstructorBoundType(TypeElement type, MetadataGenerationEnvironment env) {
+			if (env.hasConstructorBindingAnnotation(type)) {
+				return true;
+			}
+			if (type.getNestingKind() == NestingKind.MEMBER) {
+				return isConstructorBoundType((TypeElement) type.getEnclosingElement(), env);
+			}
+			return false;
+
+		}
+
 	}
 
 }

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/DeducedImmutablePropertiesMetadataGenerationTests.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2012-2019 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
+ *
+ *      https://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.boot.configurationprocessor;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.configurationprocessor.metadata.ConfigurationMetadata;
+import org.springframework.boot.configurationprocessor.metadata.Metadata;
+import org.springframework.boot.configurationsample.immutable.DeducedImmutableClassProperties;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Metadata generation tests for immutable properties deduced because they're nested.
+ *
+ * @author Phillip Webb
+ */
+class DeducedImmutablePropertiesMetadataGenerationTests extends AbstractMetadataGenerationTests {
+
+	@Test
+	void immutableSimpleProperties() {
+		ConfigurationMetadata metadata = compile(DeducedImmutableClassProperties.class);
+		assertThat(metadata).has(Metadata.withGroup("test").fromSource(DeducedImmutableClassProperties.class));
+		assertThat(metadata).has(Metadata.withProperty("test.nested.name", String.class));
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationsample/immutable/DeducedImmutableClassProperties.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2012-2019 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
+ *
+ *      https://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.boot.configurationsample.immutable;
+
+import org.springframework.boot.configurationsample.ConfigurationProperties;
+import org.springframework.boot.configurationsample.ConstructorBinding;
+
+/**
+ * Inner properties, in immutable format.
+ *
+ * @author Phillip Webb
+ */
+@ConfigurationProperties("test")
+@ConstructorBinding
+public class DeducedImmutableClassProperties {
+
+	private final Nested nested;
+
+	public DeducedImmutableClassProperties(Nested nested) {
+		this.nested = nested;
+	}
+
+	public Nested getNested() {
+		return this.nested;
+	}
+
+	public static class Nested {
+
+		private String name;
+
+		public Nested(String name) {
+			this.name = name;
+		}
+
+		public String getName() {
+			return this.name;
+		}
+
+	}
+
+}

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

@@ -265,13 +265,17 @@ public enum BindMethod {
 		VALUE_OBJECT;
 
 		static BindMethod forClass(Class<?> type) {
-			if (MergedAnnotations.from(type, SearchStrategy.TYPE_HIERARCHY).isPresent(ConstructorBinding.class)
-					|| findBindConstructor(type) != null) {
+			if (isConstructorBindingType(type) || findBindConstructor(type) != null) {
 				return VALUE_OBJECT;
 			}
 			return JAVA_BEAN;
 		}
 
+		private static boolean isConstructorBindingType(Class<?> type) {
+			return MergedAnnotations.from(type, SearchStrategy.TYPE_HIERARCHY_AND_ENCLOSING_CLASSES)
+					.isPresent(ConstructorBinding.class);
+		}
+
 		static Constructor<?> findBindConstructor(Class<?> type) {
 			if (KotlinDetector.isKotlinPresent() && KotlinDetector.isKotlinType(type)) {
 				Constructor<?> constructor = BeanUtils.findPrimaryConstructor(type);

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

@@ -836,6 +836,19 @@ void loadWhenBindingToMultiConstructorConfigurationPropertiesUsingShortcutSyntax
 		assertThat(nested.getAge()).isEqualTo(0);
 	}
 
+	@Test // gh-18481
+	void loadWhenBindingToNestedConstructorPropertiesWithDeducedNestedShouldBind() {
+		MutablePropertySources sources = this.context.getEnvironment().getPropertySources();
+		Map<String, Object> source = new HashMap<>();
+		source.put("test.name", "spring");
+		source.put("test.nested.age", "5");
+		sources.addLast(new MapPropertySource("test", source));
+		load(DeducedNestedConstructorPropertiesConfiguration.class);
+		DeducedNestedConstructorProperties bean = this.context.getBean(DeducedNestedConstructorProperties.class);
+		assertThat(bean.getName()).isEqualTo("spring");
+		assertThat(bean.getNested().getAge()).isEqualTo(5);
+	}
+
 	private AnnotationConfigApplicationContext load(Class<?> configuration, String... inlinedProperties) {
 		return load(new Class<?>[] { configuration }, inlinedProperties);
 	}
@@ -2014,4 +2027,46 @@ static class MultiConstructorConfigurationPropertiesConfiguration {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@EnableConfigurationProperties(DeducedNestedConstructorProperties.class)
+	static class DeducedNestedConstructorPropertiesConfiguration {
+
+	}
+
+	@ImmutableConfigurationProperties("test")
+	static class DeducedNestedConstructorProperties {
+
+		private final String name;
+
+		private final Nested nested;
+
+		DeducedNestedConstructorProperties(String name, Nested nested) {
+			this.name = name;
+			this.nested = nested;
+		}
+
+		String getName() {
+			return this.name;
+		}
+
+		Nested getNested() {
+			return this.nested;
+		}
+
+		static class Nested {
+
+			private final int age;
+
+			Nested(int age) {
+				this.age = age;
+			}
+
+			int getAge() {
+				return this.age;
+			}
+
+		}
+
+	}
+
 }