Allow imported auto-configuration to be missing

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

@@ -73,4 +73,13 @@
 	 */
 	Class<?>[] exclude() default {};
 
+	/**
+	 * Whether to ignore classes listed in the {@code .imports} file that are not on the
+	 * classpath.
+	 * @since 3.1.3
+	 * @return whether to ignore missing classes
+	 * @see #classes
+	 */
+	boolean ignoreMissingClasses() default false;
+
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2022 the original author or authors.
+ * Copyright 2012-2023 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.
@@ -32,6 +32,7 @@
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.AnnotationAttributes;
 import org.springframework.core.annotation.AnnotationUtils;
+import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.type.AnnotationMetadata;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.LinkedMultiValueMap;
@@ -92,13 +93,24 @@ private Collection<String> getConfigurationsForAnnotation(Class<?> source, Annot
 		if (classes.length > 0) {
 			return Arrays.asList(classes);
 		}
-		return loadFactoryNames(source);
+		Collection<String> factoryNames = loadFactoryNames(source);
+		boolean ignoreMissingClasses = (boolean) AnnotationUtils.getAnnotationAttributes(annotation, true)
+			.get("ignoreMissingClasses");
+		if (!ignoreMissingClasses) {
+			return factoryNames;
+		}
+		return factoryNames.stream().filter(this::present).toList();
 	}
 
 	protected Collection<String> loadFactoryNames(Class<?> source) {
 		return ImportCandidates.load(source, getBeanClassLoader()).getCandidates();
 	}
 
+	private boolean present(String className) {
+		String resourcePath = ClassUtils.convertClassNameToResourcePath(className) + ".class";
+		return new ClassPathResource(resourcePath).exists();
+	}
+
 	@Override
 	protected Set<String> getExclusions(AnnotationMetadata metadata, AnnotationAttributes attributes) {
 		Set<String> exclusions = new LinkedHashSet<>();

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

@@ -74,6 +74,23 @@ void importsAreSelectedUsingClassesAttribute() throws Exception {
 		assertThat(imports).containsExactly(FreeMarkerAutoConfiguration.class.getName());
 	}
 
+	@Test
+	void importsAreSelectedFromImportsFile() throws Exception {
+		AnnotationMetadata annotationMetadata = getAnnotationMetadata(FromImportsFile.class);
+		String[] imports = this.importSelector.selectImports(annotationMetadata);
+		assertThat(imports).containsExactly(
+				"org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration",
+				"org.springframework.boot.autoconfigure.missing.MissingAutoConfiguration");
+	}
+
+	@Test
+	void importsSelectedFromImportsFileCanIgnoreMissingClasses() throws Exception {
+		AnnotationMetadata annotationMetadata = getAnnotationMetadata(FromImportsFileIgnoringMissingClasses.class);
+		String[] imports = this.importSelector.selectImports(annotationMetadata);
+		assertThat(imports)
+			.containsExactly("org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration");
+	}
+
 	@Test
 	void propertyExclusionsAreApplied() throws IOException {
 		this.environment.setProperty("spring.autoconfigure.exclude", FreeMarkerAutoConfiguration.class.getName());
@@ -312,6 +329,18 @@ Class<?>[] excludeAutoConfiguration() default {
 
 	}
 
+	@Retention(RetentionPolicy.RUNTIME)
+	@ImportAutoConfiguration
+	@interface FromImportsFile {
+
+	}
+
+	@Retention(RetentionPolicy.RUNTIME)
+	@ImportAutoConfiguration(ignoreMissingClasses = true)
+	@interface FromImportsFileIgnoringMissingClasses {
+
+	}
+
 	static class TestImportAutoConfigurationImportSelector extends ImportAutoConfigurationImportSelector {
 
 		@Override

spring-boot-project/spring-boot-autoconfigure/src/test/resources/META-INF/spring/org.springframework.boot.autoconfigure.ImportAutoConfigurationImportSelectorTests$FromImportsFile.imports

@@ -0,0 +1,2 @@
+org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration
+org.springframework.boot.autoconfigure.missing.MissingAutoConfiguration

spring-boot-project/spring-boot-autoconfigure/src/test/resources/META-INF/spring/org.springframework.boot.autoconfigure.ImportAutoConfigurationImportSelectorTests$FromImportsFileIgnoringMissingClasses.imports

@@ -0,0 +1,2 @@
+com.example.ImportedByFile
+org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration