Allow beans without public constructors to load

Allow `BeanDefinitionLoader` to load classes that don't have public
constructors. The constraint was first introduced in d82c5080 to
solve an issue with anonymous Groovy classes but causes particular
problems with `@SpringBootTest`.

See gh-20929

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -31,8 +31,6 @@ import org.springframework.beans.factory.support.BeanNameGenerator;
 import org.springframework.beans.factory.xml.XmlBeanDefinitionReader;
 import org.springframework.context.annotation.AnnotatedBeanDefinitionReader;
 import org.springframework.context.annotation.ClassPathBeanDefinitionScanner;
-import org.springframework.core.annotation.MergedAnnotations;
-import org.springframework.core.annotation.MergedAnnotations.SearchStrategy;
 import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
@@ -41,7 +39,6 @@ import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
 import org.springframework.core.io.support.ResourcePatternResolver;
 import org.springframework.core.type.filter.AbstractTypeHierarchyTraversingFilter;
 import org.springframework.core.type.filter.TypeFilter;
-import org.springframework.stereotype.Component;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
@@ -53,6 +50,7 @@ import org.springframework.util.StringUtils;
  * {@link SpringApplication} for the types of sources that are supported.
  *
  * @author Phillip Webb
+ * @author Vladislav Kisel
  * @see #setBeanNameGenerator(BeanNameGenerator)
  */
 class BeanDefinitionLoader {
@@ -273,16 +271,14 @@ class BeanDefinitionLoader {
 		return Package.getPackage(source.toString());
 	}
 
+	/**
+	 * Check whether the bean is eligible for registration.
+	 * @param type candidate bean type
+	 * @return true if the given bean type is eligible for registration, i.e. not a groovy
+	 * closure nor an anonymous class
+	 */
 	private boolean isComponent(Class<?> type) {
-		// This has to be a bit of a guess. The only way to be sure that this type is
-		// eligible is to make a bean definition out of it and try to instantiate it.
-		if (MergedAnnotations.from(type, SearchStrategy.TYPE_HIERARCHY).isPresent(Component.class)) {
-			return true;
-		}
-		// Nested anonymous classes are not eligible for registration, nor are groovy
-		// closures
-		return !type.getName().matches(".*\\$_.*closure.*") && !type.isAnonymousClass()
-				&& type.getConstructors() != null && type.getConstructors().length != 0;
+		return !type.getName().matches(".*\\$_.*closure.*") && !type.isAnonymousClass();
 	}
 
 	/**

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/BeanDefinitionLoaderTests.java

@@ -22,6 +22,7 @@ import org.junit.jupiter.api.Test;
 import sampleconfig.MyComponentInPackageWithoutDot;
 
 import org.springframework.boot.sampleconfig.MyComponent;
+import org.springframework.boot.sampleconfig.MyNamedComponent;
 import org.springframework.context.support.StaticApplicationContext;
 import org.springframework.core.io.ClassPathResource;
 
@@ -31,6 +32,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link BeanDefinitionLoader}.
  *
  * @author Phillip Webb
+ * @author Vladislav Kisel
  */
 class BeanDefinitionLoaderTests {
 
@@ -53,6 +55,21 @@ class BeanDefinitionLoaderTests {
 		assertThat(this.registry.containsBean("myComponent")).isTrue();
 	}
 
+	@Test
+	void anonymousClassNotLoaded() {
+		MyComponent myComponent = new MyComponent() {
+		};
+		BeanDefinitionLoader loader = new BeanDefinitionLoader(this.registry, myComponent.getClass());
+		assertThat(loader.load()).isEqualTo(0);
+	}
+
+	@Test
+	void loadJsr330Class() {
+		BeanDefinitionLoader loader = new BeanDefinitionLoader(this.registry, MyNamedComponent.class);
+		assertThat(loader.load()).isEqualTo(1);
+		assertThat(this.registry.containsBean("myNamedComponent")).isTrue();
+	}
+
 	@Test
 	void loadXmlResource() {
 		ClassPathResource resource = new ClassPathResource("sample-beans.xml", getClass());
@@ -83,8 +100,9 @@ class BeanDefinitionLoaderTests {
 	@Test
 	void loadPackage() {
 		BeanDefinitionLoader loader = new BeanDefinitionLoader(this.registry, MyComponent.class.getPackage());
-		assertThat(loader.load()).isEqualTo(1);
+		assertThat(loader.load()).isEqualTo(2);
 		assertThat(this.registry.containsBean("myComponent")).isTrue();
+		assertThat(this.registry.containsBean("myNamedComponent")).isTrue();
 	}
 
 	@Test
@@ -113,8 +131,9 @@ class BeanDefinitionLoaderTests {
 	@Test
 	void loadPackageName() {
 		BeanDefinitionLoader loader = new BeanDefinitionLoader(this.registry, MyComponent.class.getPackage().getName());
-		assertThat(loader.load()).isEqualTo(1);
+		assertThat(loader.load()).isEqualTo(2);
 		assertThat(this.registry.containsBean("myComponent")).isTrue();
+		assertThat(this.registry.containsBean("myNamedComponent")).isTrue();
 	}
 
 	@Test
@@ -131,8 +150,9 @@ class BeanDefinitionLoaderTests {
 	void loadPackageAndClassDoesNotDoubleAdd() {
 		BeanDefinitionLoader loader = new BeanDefinitionLoader(this.registry, MyComponent.class.getPackage(),
 				MyComponent.class);
-		assertThat(loader.load()).isEqualTo(1);
+		assertThat(loader.load()).isEqualTo(2);
 		assertThat(this.registry.containsBean("myComponent")).isTrue();
+		assertThat(this.registry.containsBean("myNamedComponent")).isTrue();
 	}
 
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/sampleconfig/MyNamedComponent.java

+/*
+ * Copyright 2012-2020 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.sampleconfig;
+
+import javax.inject.Named;
+
+@Named
+public class MyNamedComponent {
+
+	MyNamedComponent() {
+
+	}
+
+}