Generate the AutoConfiguration.imports file from annotations

This commit adds the `AutoConfigurationImportsAnnotationProcessor` to
the `spring-boot-autoconfigure-processor` annotation processor
module. When added to a project build, the annotation processor will
generate the
`org.springframework.boot.autoconfigure.AutoConfiguration.imports`
file automatically from `@AutoConfiguration`-annotated classes. It
also applies the annotation processor to the Spring Boot build.

Closes gh-31228

spring-boot-project/spring-boot-tools/spring-boot-autoconfigure-processor/src/main/java/org/springframework/boot/autoconfigureprocessor/AutoConfigurationImportsAnnotationProcessor.java

@@ -0,0 +1,89 @@
+/*
+ * 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.autoconfigureprocessor;
+
+import java.io.IOException;
+import java.io.OutputStreamWriter;
+import java.io.Writer;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
+
+import javax.annotation.processing.AbstractProcessor;
+import javax.annotation.processing.Filer;
+import javax.annotation.processing.RoundEnvironment;
+import javax.annotation.processing.SupportedAnnotationTypes;
+import javax.lang.model.SourceVersion;
+import javax.lang.model.element.Element;
+import javax.lang.model.element.TypeElement;
+import javax.tools.FileObject;
+import javax.tools.StandardLocation;
+
+/**
+ * Annotation processor to generate a
+ * {@code META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports}
+ * file from {@code @AutoConfiguration} annotations.
+ *
+ * @author Scott Frederick
+ * @since 3.0.0
+ */
+@SupportedAnnotationTypes({ "org.springframework.boot.autoconfigure.AutoConfiguration" })
+public class AutoConfigurationImportsAnnotationProcessor extends AbstractProcessor {
+
+	static final String IMPORTS_FILE_PATH = "META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports";
+
+	private final List<String> qualifiedClassNames = new ArrayList<>();
+
+	@Override
+	public SourceVersion getSupportedSourceVersion() {
+		return SourceVersion.latestSupported();
+	}
+
+	@Override
+	public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
+		for (TypeElement annotation : annotations) {
+			Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(annotation);
+			for (Element element : elements) {
+				this.qualifiedClassNames.add(element.asType().toString());
+			}
+		}
+		if (roundEnv.processingOver()) {
+			try {
+				writeImportsFile();
+			}
+			catch (IOException ex) {
+				throw new IllegalStateException("Failed to write auto-configuration imports file", ex);
+			}
+		}
+		return false;
+	}
+
+	private void writeImportsFile() throws IOException {
+		if (!this.qualifiedClassNames.isEmpty()) {
+			Filer filer = this.processingEnv.getFiler();
+			FileObject file = filer.createResource(StandardLocation.CLASS_OUTPUT, "", IMPORTS_FILE_PATH);
+			try (Writer writer = new OutputStreamWriter(file.openOutputStream(), StandardCharsets.UTF_8)) {
+				for (String className : this.qualifiedClassNames) {
+					writer.append(className);
+					writer.append(System.lineSeparator());
+				}
+			}
+		}
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-autoconfigure-processor/src/main/resources/META-INF/gradle/incremental.annotation.processors

@@ -1 +1,2 @@
-org.springframework.boot.autoconfigureprocessor.AutoConfigureAnnotationProcessor,aggregating
+org.springframework.boot.autoconfigureprocessor.AutoConfigureAnnotationProcessor,aggregating
+org.springframework.boot.autoconfigureprocessor.AutoConfigurationImportsAnnotationProcessor,aggregating

spring-boot-project/spring-boot-tools/spring-boot-autoconfigure-processor/src/main/resources/META-INF/services/javax.annotation.processing.Processor

@@ -1 +1,2 @@
 org.springframework.boot.autoconfigureprocessor.AutoConfigureAnnotationProcessor
+org.springframework.boot.autoconfigureprocessor.AutoConfigurationImportsAnnotationProcessor