Allow @component on mutable @ConfigurationProperties

Update configuration properties support to allow the `@Component`
annotation to be used on `@ConfigurationProperties` beans as long
as they are mutable.

This restores the behavior of Spring Boot 2.1 for mutable beans whilst
still allowing us to enforce the stricter rules for immutable value
object configuration properties.

Closes gh-18138

Author: wilkinsona

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

@@ -903,6 +903,8 @@ In this setup one, and only one constructor must be defined with the list of pro
 
 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.
 
+NOTE: To use constructor binding the class must not be annotated with `@Component` and must be enabled using `@EnableConfigurationProperties` or configuration property scanning instead.
+
 
 
 [[boot-features-external-config-enabling]]
@@ -933,20 +935,15 @@ In these cases, you can specify the list of types to process on any `@Configurat
 
 [NOTE]
 ====
-When the `@ConfigurationProperties` bean is registered using scanning or via `@EnableConfigurationProperties`, the bean has a conventional name: `<prefix>-<fqn>`, where `<prefix>` is the environment key prefix specified in the `@ConfigurationProperties` annotation and `<fqn>` is the fully qualified name of the bean.
+When the `@ConfigurationProperties` bean is registered using configuration property scanning or via `@EnableConfigurationProperties`, the bean has a conventional name: `<prefix>-<fqn>`, where `<prefix>` is the environment key prefix specified in the `@ConfigurationProperties` annotation and `<fqn>` is the fully qualified name of the bean.
 If the annotation does not provide any prefix, only the fully qualified name of the bean is used.
 
 The bean name in the example above is `acme-com.example.AcmeProperties`.
 ====
 
 We recommend that `@ConfigurationProperties` only deal with the environment and, in particular, does not inject other beans from the context.
-In particular, it is not possible to inject other beans using the constructor as this would trigger the constructor binder that only deals with the environment.
-
-For corner cases, setter injection can be used or any of the `*Aware` interfaces provided
-by the framework (such as `EnvironmentAware` if you need access to the `Environment`).
-
-NOTE: Annotating a `@ConfigurationProperties` type with `@Component` will result in two beans of the same type if the type is also scanned as part of classpath scanning.
-If you want to register the bean yourself using `@Component`, consider disabling scanning of `@ConfigurationProperties`.
+For corner cases, setter injection can be used or any of the `*Aware` interfaces provided by the framework (such as `EnvironmentAware` if you need access to the `Environment`).
+If you still want to inject other beans using the constructor, the configuration properties bean must be annotated with `@Component` and use JavaBean-based property binding.
 
 
 

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

@@ -28,8 +28,8 @@
 import org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider;
 import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
 import org.springframework.core.annotation.AnnotationAttributes;
-import org.springframework.core.annotation.MergedAnnotation;
 import org.springframework.core.annotation.MergedAnnotations;
+import org.springframework.core.annotation.MergedAnnotations.SearchStrategy;
 import org.springframework.core.env.Environment;
 import org.springframework.core.io.ResourceLoader;
 import org.springframework.core.type.AnnotationMetadata;
@@ -99,21 +99,18 @@ private void scan(ConfigurableListableBeanFactory beanFactory, BeanDefinitionReg
 			String beanClassName = candidate.getBeanClassName();
 			try {
 				Class<?> type = ClassUtils.forName(beanClassName, null);
-				validateScanConfiguration(type);
-				ConfigurationPropertiesBeanDefinitionRegistrar.register(registry, beanFactory, type);
+				if (!isComponent(type)) {
+					ConfigurationPropertiesBeanDefinitionRegistrar.register(registry, beanFactory, type);
+				}
 			}
 			catch (ClassNotFoundException ex) {
 				// Ignore
 			}
 		}
 	}
 
-	private void validateScanConfiguration(Class<?> type) {
-		MergedAnnotation<Component> component = MergedAnnotations
-				.from(type, MergedAnnotations.SearchStrategy.TYPE_HIERARCHY).get(Component.class);
-		if (component.isPresent()) {
-			throw new InvalidConfigurationPropertiesException(type, component.getRoot().getType());
-		}
+	private boolean isComponent(Class<?> type) {
+		return MergedAnnotations.from(type, SearchStrategy.TYPE_HIERARCHY).isPresent(Component.class);
 	}
 
 	@Override

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

@@ -23,15 +23,14 @@
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.support.DefaultListableBeanFactory;
 import org.springframework.beans.factory.support.GenericBeanDefinition;
-import org.springframework.boot.context.properties.scan.invalid.c.InvalidConfiguration;
-import org.springframework.boot.context.properties.scan.invalid.d.OtherInvalidConfiguration;
+import org.springframework.boot.context.properties.scan.combined.c.CombinedConfiguration;
+import org.springframework.boot.context.properties.scan.combined.d.OtherCombinedConfiguration;
 import org.springframework.boot.context.properties.scan.valid.ConfigurationPropertiesScanConfiguration;
 import org.springframework.core.type.AnnotationMetadata;
 import org.springframework.core.type.classreading.SimpleMetadataReaderFactory;
 import org.springframework.mock.env.MockEnvironment;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
 /**
  * Tests for {@link ConfigurationPropertiesScanRegistrar}.
@@ -99,38 +98,33 @@ void scanWhenBasePackagesAndBasePackcageClassesProvidedShouldUseThat() throws IO
 	}
 
 	@Test
-	void scanWhenComponentAnnotationPresentShouldThrowException() {
+	void scanWhenComponentAnnotationPresentShouldSkipType() throws IOException {
 		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
 		beanFactory.setAllowBeanDefinitionOverriding(false);
-		assertThatExceptionOfType(InvalidConfigurationPropertiesException.class)
-				.isThrownBy(() -> this.registrar
-						.registerBeanDefinitions(getAnnotationMetadata(InvalidScanConfiguration.class), beanFactory))
-				.withMessageContaining(
-						"Found @Component and @ConfigurationProperties on org.springframework.boot.context.properties.scan.invalid.c.InvalidConfiguration$MyProperties.");
+		this.registrar.registerBeanDefinitions(getAnnotationMetadata(CombinedScanConfiguration.class), beanFactory);
+		assertThat(beanFactory.getBeanDefinitionCount()).isEqualTo(0);
 	}
 
 	@Test
-	void scanWhenOtherComponentAnnotationPresentShouldThrowException() {
+	void scanWhenOtherComponentAnnotationPresentShouldSkipType() throws IOException {
 		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
 		beanFactory.setAllowBeanDefinitionOverriding(false);
-		assertThatExceptionOfType(InvalidConfigurationPropertiesException.class)
-				.isThrownBy(() -> this.registrar.registerBeanDefinitions(
-						getAnnotationMetadata(OtherInvalidScanConfiguration.class), beanFactory))
-				.withMessageContaining(
-						"Found @RestController and @ConfigurationProperties on org.springframework.boot.context.properties.scan.invalid.d.OtherInvalidConfiguration$MyControllerProperties.");
+		this.registrar.registerBeanDefinitions(getAnnotationMetadata(OtherCombinedScanConfiguration.class),
+				beanFactory);
+		assertThat(beanFactory.getBeanDefinitionCount()).isEqualTo(0);
 	}
 
 	private AnnotationMetadata getAnnotationMetadata(Class<?> source) throws IOException {
 		return new SimpleMetadataReaderFactory().getMetadataReader(source.getName()).getAnnotationMetadata();
 	}
 
-	@ConfigurationPropertiesScan(basePackageClasses = InvalidConfiguration.class)
-	static class InvalidScanConfiguration {
+	@ConfigurationPropertiesScan(basePackageClasses = CombinedConfiguration.class)
+	static class CombinedScanConfiguration {
 
 	}
 
-	@ConfigurationPropertiesScan(basePackageClasses = OtherInvalidConfiguration.class)
-	static class OtherInvalidScanConfiguration {
+	@ConfigurationPropertiesScan(basePackageClasses = OtherCombinedConfiguration.class)
+	static class OtherCombinedScanConfiguration {
 
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/diagnostics/analyzer/InvalidConfigurationPropertiesFailureAnalyzer.java

@@ -13,15 +13,15 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package org.springframework.boot.context.properties.scan.invalid.c;
+package org.springframework.boot.context.properties.scan.combined.c;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.stereotype.Component;
 
 /**
  * @author Madhura Bhave
  */
-public class InvalidConfiguration {
+public class CombinedConfiguration {
 
 	@Component
 	@ConfigurationProperties(prefix = "b")

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

@@ -14,15 +14,15 @@
  * limitations under the License.
  */
 
-package org.springframework.boot.context.properties.scan.invalid.d;
+package org.springframework.boot.context.properties.scan.combined.d;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.web.bind.annotation.RestController;
 
 /**
  * @author Madhura Bhave
  */
-public class OtherInvalidConfiguration {
+public class OtherCombinedConfiguration {
 
 	@RestController
 	@ConfigurationProperties(prefix = "c")