Load auto-configurations from META-INF/spring-boot

Implements a new AutoConfigurationLoader, which loads
auto-configurations from a file in META-INF/spring-boot.

Adapts the AutoConfigurationImportSelector to use the new loader.

Adapts the ImportAutoConfigurationImportSelector to use the new loader.

Adapts the metadata plugin in the build to additionally load the
auto-configurations from the new file.

Updates the documentation for auto-configurations and test slices.

Closes gh-29872

buildSrc/src/main/java/org/springframework/boot/build/autoconfigure/AutoConfigurationMetadata.java

@@ -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.
@@ -16,14 +16,19 @@
 
 package org.springframework.boot.build.autoconfigure;
 
+import java.io.BufferedReader;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileReader;
 import java.io.FileWriter;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
 import java.io.Reader;
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Properties;
 import java.util.Set;
 import java.util.concurrent.Callable;
@@ -48,6 +53,8 @@ import org.springframework.util.StringUtils;
  */
 public class AutoConfigurationMetadata extends DefaultTask {
 
+	private static final String COMMENT_START = "#";
+
 	private SourceSet sourceSet;
 
 	private File outputFile;
@@ -57,6 +64,12 @@ public class AutoConfigurationMetadata extends DefaultTask {
 				.file((Callable<File>) () -> new File(this.sourceSet.getOutput().getResourcesDir(),
 						"META-INF/spring.factories"))
 				.withPathSensitivity(PathSensitivity.RELATIVE).withPropertyName("spring.factories");
+		getInputs()
+				.file((Callable<File>) () -> new File(this.sourceSet.getOutput().getResourcesDir(),
+						"META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration"))
+				.withPathSensitivity(PathSensitivity.RELATIVE)
+				.withPropertyName("org.springframework.boot.autoconfigure.AutoConfiguration");
+
 		dependsOn((Callable<String>) () -> this.sourceSet.getProcessResourcesTaskName());
 		getProject().getConfigurations()
 				.maybeCreate(AutoConfigurationPlugin.AUTO_CONFIGURATION_METADATA_CONFIGURATION_NAME);
@@ -86,11 +99,9 @@ public class AutoConfigurationMetadata extends DefaultTask {
 
 	private Properties readAutoConfiguration() throws IOException {
 		Properties autoConfiguration = CollectionFactory.createSortedProperties(true);
-		Properties springFactories = readSpringFactories(
-				new File(this.sourceSet.getOutput().getResourcesDir(), "META-INF/spring.factories"));
-		String enableAutoConfiguration = springFactories
-				.getProperty("org.springframework.boot.autoconfigure.EnableAutoConfiguration");
-		Set<String> classNames = StringUtils.commaDelimitedListToSet(enableAutoConfiguration);
+		Set<String> classNames = new LinkedHashSet<>();
+		classNames.addAll(readSpringFactories());
+		classNames.addAll(readAutoConfigurationsFile());
 		Set<String> publicClassNames = new LinkedHashSet<>();
 		for (String className : classNames) {
 			File classFile = findClassFile(className);
@@ -109,6 +120,57 @@ public class AutoConfigurationMetadata extends DefaultTask {
 		return autoConfiguration;
 	}
 
+	/**
+	 * Reads auto-configurations from META-INF/spring.factories.
+	 * @return auto-configurations
+	 */
+	private Set<String> readSpringFactories() throws IOException {
+		File file = new File(this.sourceSet.getOutput().getResourcesDir(), "META-INF/spring.factories");
+		if (!file.exists()) {
+			return Collections.emptySet();
+		}
+		Properties springFactories = readSpringFactories(file);
+		String enableAutoConfiguration = springFactories
+				.getProperty("org.springframework.boot.autoconfigure.EnableAutoConfiguration");
+		return StringUtils.commaDelimitedListToSet(enableAutoConfiguration);
+	}
+
+	/**
+	 * Reads auto-configurations from
+	 * META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration.
+	 * @return auto-configurations
+	 */
+	private List<String> readAutoConfigurationsFile() throws IOException {
+		File file = new File(this.sourceSet.getOutput().getResourcesDir(),
+				"META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration");
+		if (!file.exists()) {
+			return Collections.emptyList();
+		}
+		// Nearly identical copy of
+		// org.springframework.boot.autoconfigure.AutoConfigurationLoader.readAutoConfigurations
+		try (BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(file)))) {
+			List<String> autoConfigurations = new ArrayList<>();
+			String line;
+			while ((line = reader.readLine()) != null) {
+				line = stripComment(line);
+				line = line.trim();
+				if (line.isEmpty()) {
+					continue;
+				}
+				autoConfigurations.add(line);
+			}
+			return autoConfigurations;
+		}
+	}
+
+	private String stripComment(String line) {
+		int commentStart = line.indexOf(COMMENT_START);
+		if (commentStart == -1) {
+			return line;
+		}
+		return line.substring(0, commentStart);
+	}
+
 	private File findClassFile(String className) {
 		String classFileName = className.replace(".", "/") + ".class";
 		for (File classesDir : this.sourceSet.getOutput().getClassesDirs()) {

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfiguration.java

@@ -26,6 +26,7 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.context.annotation.Conditional;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.support.SpringFactoriesLoader;
 
 /**
  * Indicates that a class provides configuration that can be automatically applied by
@@ -33,6 +34,10 @@ import org.springframework.context.annotation.Configuration;
  * {@link Configuration @Configuration} with the exception that
  * {@literal Configuration#proxyBeanMethods() proxyBeanMethods} is always {@code false}.
  * <p>
+ * They are located using the {@link AutoConfigurationLoader} and the
+ * {@link SpringFactoriesLoader} mechanism (keyed against
+ * {@link EnableAutoConfiguration}).
+ * <p>
  * Generally auto-configuration classes are marked as {@link Conditional @Conditional}
  * (most often using {@link ConditionalOnClass @ConditionalOnClass} and
  * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfigurationExcludeFilter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 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.
@@ -17,6 +17,7 @@
 package org.springframework.boot.autoconfigure;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.List;
 
 import org.springframework.beans.factory.BeanClassLoaderAware;
@@ -54,13 +55,20 @@ public class AutoConfigurationExcludeFilter implements TypeFilter, BeanClassLoad
 	}
 
 	private boolean isAutoConfiguration(MetadataReader metadataReader) {
-		return getAutoConfigurations().contains(metadataReader.getClassMetadata().getClassName());
+		boolean annotatedWithAutoConfiguration = metadataReader.getAnnotationMetadata()
+				.isAnnotated(AutoConfiguration.class.getName());
+		return annotatedWithAutoConfiguration
+				|| getAutoConfigurations().contains(metadataReader.getClassMetadata().getClassName());
 	}
 
 	protected List<String> getAutoConfigurations() {
 		if (this.autoConfigurations == null) {
-			this.autoConfigurations = SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,
-					this.beanClassLoader);
+			List<String> autoConfigurations = new ArrayList<>();
+			autoConfigurations.addAll(
+					SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class, this.beanClassLoader));
+			autoConfigurations
+					.addAll(new AutoConfigurationLoader().loadNames(AutoConfiguration.class, this.beanClassLoader));
+			this.autoConfigurations = autoConfigurations;
 		}
 		return this.autoConfigurations;
 	}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfigurationImportSelector.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 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.
@@ -67,6 +67,7 @@ import org.springframework.util.StringUtils;
  * @author Andy Wilkinson
  * @author Stephane Nicoll
  * @author Madhura Bhave
+ * @author Moritz Halbritter
  * @since 1.3.0
  * @see EnableAutoConfiguration
  */
@@ -167,7 +168,9 @@ public class AutoConfigurationImportSelector implements DeferredImportSelector,
 
 	/**
 	 * Return the auto-configuration class names that should be considered. By default
-	 * this method will load candidates using {@link SpringFactoriesLoader} with
+	 * this method will load candidates using {@link AutoConfigurationLoader} with
+	 * {@link #getSpringFactoriesLoaderFactoryClass()}. For backward compatible reasons it
+	 * will also consider {@link SpringFactoriesLoader} with
 	 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 	 * @param metadata the source metadata
 	 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
@@ -175,10 +178,13 @@ public class AutoConfigurationImportSelector implements DeferredImportSelector,
 	 * @return a list of candidate configurations
 	 */
 	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
-		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
-				getBeanClassLoader());
-		Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
-				+ "are using a custom packaging, make sure that file is correct.");
+		List<String> configurations = new ArrayList<>();
+		configurations.addAll(
+				SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()));
+		configurations.addAll(new AutoConfigurationLoader().loadNames(AutoConfiguration.class, getBeanClassLoader()));
+		Assert.notEmpty(configurations,
+				"No auto configuration classes found in META-INF/spring.factories nor in META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration. If you "
+						+ "are using a custom packaging, make sure that file is correct.");
 		return configurations;
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/AutoConfigurationLoader.java

@@ -0,0 +1,113 @@
+/*
+ * 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.
+ * 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.autoconfigure;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Enumeration;
+import java.util.List;
+
+import org.springframework.core.io.UrlResource;
+import org.springframework.core.io.support.SpringFactoriesLoader;
+import org.springframework.util.Assert;
+
+/**
+ * Loads the names of annotated classes, usually @{@link AutoConfiguration}.
+ *
+ * The names of the classes are stored in files named META-INF/spring-boot/{full qualified
+ * name of the annotation}. Every line contains the full qualified class name of the
+ * annotated class. Comments are supported using the # character.
+ *
+ * @author Moritz Halbritter
+ * @see AutoConfiguration
+ * @see SpringFactoriesLoader
+ */
+class AutoConfigurationLoader {
+
+	private static final String LOCATION = "META-INF/spring-boot/";
+
+	private static final String COMMENT_START = "#";
+
+	/**
+	 * Loads the names of annotated classes.
+	 * @param annotation annotation to load
+	 * @param classLoader class loader to use for loading
+	 * @return list of names of annotated classes
+	 */
+	List<String> loadNames(Class<?> annotation, ClassLoader classLoader) {
+		Assert.notNull(annotation, "'annotation' must not be null");
+		ClassLoader classLoaderToUse = decideClassloader(classLoader);
+		String location = LOCATION + annotation.getName();
+		Enumeration<URL> urls = findUrlsInClasspath(classLoaderToUse, location);
+		List<String> autoConfigurations = new ArrayList<>();
+		while (urls.hasMoreElements()) {
+			URL url = urls.nextElement();
+			autoConfigurations.addAll(readAutoConfigurations(url));
+		}
+		return autoConfigurations;
+	}
+
+	private ClassLoader decideClassloader(ClassLoader classLoader) {
+		if (classLoader == null) {
+			return AutoConfigurationLoader.class.getClassLoader();
+		}
+		return classLoader;
+	}
+
+	private Enumeration<URL> findUrlsInClasspath(ClassLoader classLoader, String location) {
+		try {
+			return classLoader.getResources(location);
+		}
+		catch (IOException ex) {
+			throw new IllegalArgumentException("Failed to load autoconfigurations from location [" + location + "]",
+					ex);
+		}
+	}
+
+	private List<String> readAutoConfigurations(URL url) {
+		try (BufferedReader reader = new BufferedReader(
+				new InputStreamReader(new UrlResource(url).getInputStream(), StandardCharsets.UTF_8))) {
+			List<String> autoConfigurations = new ArrayList<>();
+			String line;
+			while ((line = reader.readLine()) != null) {
+				line = stripComment(line);
+				line = line.trim();
+				if (line.isEmpty()) {
+					continue;
+				}
+				autoConfigurations.add(line);
+			}
+			return autoConfigurations;
+		}
+		catch (IOException ex) {
+			throw new IllegalArgumentException("Unable to load autoconfigurations from location [" + url + "]", ex);
+		}
+	}
+
+	private String stripComment(String line) {
+		int commentStart = line.indexOf(COMMENT_START);
+		if (commentStart == -1) {
+			return line;
+		}
+		return line.substring(0, commentStart);
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/EnableAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 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.
@@ -60,9 +60,9 @@ import org.springframework.core.io.support.SpringFactoriesLoader;
  * and classes can be searched.
  * <p>
  * Auto-configuration classes are regular Spring {@link Configuration @Configuration}
- * beans. They are located using the {@link SpringFactoriesLoader} mechanism (keyed
- * against this class). Generally auto-configuration beans are
- * {@link Conditional @Conditional} beans (most often using
+ * beans. They are located using the {@link AutoConfigurationLoader} and the
+ * {@link SpringFactoriesLoader} mechanism (keyed against this class). Generally
+ * auto-configuration beans are {@link Conditional @Conditional} beans (most often using
  * {@link ConditionalOnClass @ConditionalOnClass} and
  * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
  *

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/ImportAutoConfigurationImportSelector.java

@@ -44,6 +44,7 @@ import org.springframework.util.ObjectUtils;
  *
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Moritz Halbritter
  */
 class ImportAutoConfigurationImportSelector extends AutoConfigurationImportSelector implements DeterminableImports {
 
@@ -94,7 +95,10 @@ class ImportAutoConfigurationImportSelector extends AutoConfigurationImportSelec
 	}
 
 	protected Collection<String> loadFactoryNames(Class<?> source) {
-		return SpringFactoriesLoader.loadFactoryNames(source, getBeanClassLoader());
+		List<String> factoryNames = new ArrayList<>();
+		factoryNames.addAll(SpringFactoriesLoader.loadFactoryNames(source, getBeanClassLoader()));
+		factoryNames.addAll(new AutoConfigurationLoader().loadNames(source, getBeanClassLoader()));
+		return factoryNames;
 	}
 
 	@Override

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/AutoConfigurationLoaderTests.java

@@ -0,0 +1,53 @@
+/*
+ * 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.
+ * 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.autoconfigure;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import java.util.List;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class AutoConfigurationLoaderTests {
+
+	private AutoConfigurationLoader sut;
+
+	@BeforeEach
+	void setUp() {
+		this.sut = new AutoConfigurationLoader();
+	}
+
+	@Test
+	void loadNames() {
+		List<String> classNames = this.sut.loadNames(TestAutoConfiguration.class, null);
+
+		assertThat(classNames).containsExactly("class1", "class2", "class3");
+	}
+
+	@AutoConfiguration
+	@Target(ElementType.TYPE)
+	@Retention(RetentionPolicy.RUNTIME)
+	public @interface TestAutoConfiguration {
+
+	}
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/resources/META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfigurationLoaderTests$TestAutoConfiguration

@@ -0,0 +1,6 @@
+# A comment spanning a complete line
+class1
+
+class2 # with comment at the end
+  # Comment with some whitespace in front
+class3

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/developing-auto-configuration.adoc

@@ -24,16 +24,18 @@ You can browse the source code of {spring-boot-autoconfigure-module-code}[`sprin
 
 [[features.developing-auto-configuration.locating-auto-configuration-candidates]]
 === Locating Auto-configuration Candidates
-Spring Boot checks for the presence of a `META-INF/spring.factories` file within your published jar.
-The file should list your configuration classes under the `EnableAutoConfiguration` key, as shown in the following example:
+
+Spring Boot checks for the presence of a `META-INF/spring-boot/org.springframework.boot.autoconfigure.AutoConfiguration` file within your published jar.
+The file should list your configuration classes, as shown in the following example:
 
 [indent=0]
 ----
-	org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
-	com.mycorp.libx.autoconfigure.LibXAutoConfiguration,\
-	com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration
+    com.mycorp.libx.autoconfigure.LibXAutoConfiguration
+    com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration
 ----
 
+TIP: You can use comments via `#` in this file.
+
 NOTE: Auto-configurations must be loaded that way _only_.
 Make sure that they are defined in a specific package space and that they are never the target of component scanning.
 Furthermore, auto-configuration classes should not enable component scanning to find additional components.

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

@@ -769,13 +769,18 @@ include::code:MyJdbcTests[]
 
 NOTE: Make sure to not use the regular `@Import` annotation to import auto-configurations as they are handled in a specific way by Spring Boot.
 
-Alternatively, additional auto-configurations can be added for any use of a slice annotation by registering them in `META-INF/spring.factories` as shown in the following example:
+Alternatively, additional auto-configurations can be added for any use of a slice annotation by registering them in a file stored in `META-INF/spring-boot` as shown in the following example:
 
+.META-INF/spring-boot/org.springframework.boot.test.autoconfigure.jdbc.JdbcTest
 [indent=0]
 ----
-	org.springframework.boot.test.autoconfigure.jdbc.JdbcTest=com.example.IntegrationAutoConfiguration
+	com.example.IntegrationAutoConfiguration
 ----
 
+In this example, the `com.example.IntegrationAutoConfiguration` is enabled on every test annotated with `@JdbcTest`.
+
+TIP: You can use comments via `#` in this file.
+
 TIP: A slice or `@AutoConfigure...` annotation can be customized this way as long as it is meta-annotated with `@ImportAutoConfiguration`.