Add YAML support for configuration file format

initializr-actuator/src/main/java/io/spring/initializr/actuate/stat/ProjectRequestDocument.java

@@ -48,6 +48,8 @@ public class ProjectRequestDocument {
 
 	private String language;
 
+	private String configurationFileFormat;
+
 	private String packaging;
 
 	private String packageName;
@@ -116,6 +118,14 @@ public void setLanguage(String language) {
 		this.language = language;
 	}
 
+	public String getConfigurationFileFormat() {
+		return this.configurationFileFormat;
+	}
+
+	public void setConfigurationFileFormat(String configurationFileFormat) {
+		this.configurationFileFormat = configurationFileFormat;
+	}
+
 	public String getPackaging() {
 		return this.packaging;
 	}
@@ -177,6 +187,7 @@ public String toString() {
 			.add("artifactId='" + this.artifactId + "'")
 			.add("javaVersion='" + this.javaVersion + "'")
 			.add("language='" + this.language + "'")
+			.add("configurationFileFormat='" + this.configurationFileFormat + "'")
 			.add("packaging='" + this.packaging + "'")
 			.add("packageName='" + this.packageName + "'")
 			.add("version=" + this.version)
@@ -331,6 +342,8 @@ public static class ErrorStateInformation {
 
 		private Boolean language;
 
+		private Boolean configurationFileFormat;
+
 		private Boolean packaging;
 
 		private Boolean type;
@@ -359,6 +372,14 @@ public void setLanguage(Boolean language) {
 			this.language = language;
 		}
 
+		public Boolean getConfigurationFileFormat() {
+			return this.configurationFileFormat;
+		}
+
+		public void setConfigurationFileFormat(Boolean configurationFileFormat) {
+			this.configurationFileFormat = configurationFileFormat;
+		}
+
 		public Boolean getPackaging() {
 			return this.packaging;
 		}
@@ -396,6 +417,7 @@ public String toString() {
 			return new StringJoiner(", ", "{", "}").add("invalid=" + this.invalid)
 				.add("javaVersion=" + this.javaVersion)
 				.add("language=" + this.language)
+				.add("configurationFileFormat=" + this.configurationFileFormat)
 				.add("packaging=" + this.packaging)
 				.add("type=" + this.type)
 				.add("dependencies=" + this.dependencies)

initializr-actuator/src/main/java/io/spring/initializr/actuate/stat/ProjectRequestDocumentFactory.java

@@ -68,6 +68,12 @@ public ProjectRequestDocument createDocument(ProjectRequestEvent event) {
 			document.triggerError().setLanguage(true);
 		}
 
+		document.setConfigurationFileFormat(request.getConfigurationFileFormat());
+		if (StringUtils.hasText(request.getConfigurationFileFormat())
+				&& metadata.getConfigurationFileFormats().get(request.getConfigurationFileFormat()) == null) {
+			document.triggerError().setConfigurationFileFormat(true);
+		}
+
 		document.setPackaging(request.getPackaging());
 		if (StringUtils.hasText(request.getPackaging())
 				&& metadata.getPackagings().get(request.getPackaging()) == null) {

initializr-actuator/src/test/resources/stat/request-invalid-dependencies.json

@@ -6,6 +6,7 @@
   "artifactId": "test",
   "javaVersion": "1.8",
   "language": "java",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.test",
   "version": {

initializr-actuator/src/test/resources/stat/request-invalid-java-version.json

@@ -6,6 +6,7 @@
   "artifactId": "test",
   "javaVersion": "1.2",
   "language": "java",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.test",
   "version": {

initializr-actuator/src/test/resources/stat/request-invalid-language.json

@@ -6,6 +6,7 @@
   "artifactId": "test",
   "javaVersion": "1.8",
   "language": "c",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.test",
   "version": {

initializr-actuator/src/test/resources/stat/request-invalid-type.json

@@ -5,6 +5,7 @@
   "artifactId": "test",
   "javaVersion": "1.8",
   "language": "java",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.test",
   "version": {

initializr-actuator/src/test/resources/stat/request-no-client.json

@@ -6,6 +6,7 @@
   "artifactId": "test",
   "javaVersion": "1.8",
   "language": "java",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.test",
   "version": {

initializr-actuator/src/test/resources/stat/request-simple.json

@@ -6,6 +6,7 @@
   "artifactId": "project",
   "javaVersion": "1.8",
   "language": "java",
+  "configurationFileFormat": "properties",
   "packaging": "jar",
   "packageName": "com.example.acme.project",
   "version": {

initializr-generator-spring/src/main/java/io/spring/initializr/generator/spring/properties/ApplicationProperties.java

@@ -29,6 +29,8 @@
  */
 public class ApplicationProperties {
 
+	private static final String YAML_SPACE = "  ";
+
 	private final Map<String, Object> properties = new HashMap<>();
 
 	/**
@@ -73,6 +75,52 @@ void writeTo(PrintWriter writer) {
 		}
 	}
 
+	void writeYaml(PrintWriter writer) {
+		Map<String, Object> nested = flattenToNestedMap(this.properties);
+		writeYamlRecursive(nested, writer, 0);
+	}
+
+	private static Map<String, Object> flattenToNestedMap(Map<String, Object> flatMap) {
+		Map<String, Object> nested = new HashMap<>();
+		flatMap.forEach((key, value) -> {
+			String[] path = parseKeyPath(key);
+			insertValueAtPath(nested, path, value);
+		});
+		return nested;
+	}
+
+	private static String[] parseKeyPath(String key) {
+		return key.split("\\.");
+	}
+
+	@SuppressWarnings("unchecked")
+	private static void insertValueAtPath(Map<String, Object> map, String[] path, Object value) {
+		Map<String, Object> current = map;
+		for (int i = 0; i < path.length - 1; i++) {
+			String segment = path[i];
+			current = (Map<String, Object>) current.computeIfAbsent(segment, (k) -> new HashMap<>());
+		}
+		current.put(path[path.length - 1], value);
+	}
+
+	private static void writeYamlRecursive(Map<String, Object> map, PrintWriter writer, int indent) {
+		map.entrySet().forEach((entry) -> writeEntry(entry, writer, indent));
+	}
+
+	@SuppressWarnings("unchecked")
+	private static void writeEntry(Map.Entry<String, Object> entry, PrintWriter writer, int indent) {
+		String indentStr = YAML_SPACE.repeat(indent);
+		Object value = entry.getValue();
+
+		if (value instanceof Map<?, ?> nestedMap) {
+			writer.printf("%s%s:%n", indentStr, entry.getKey());
+			writeYamlRecursive((Map<String, Object>) nestedMap, writer, indent + 1);
+		}
+		else {
+			writer.printf("%s%s: %s%n", indentStr, entry.getKey(), value);
+		}
+	}
+
 	private void add(String key, Object value) {
 		Assert.state(!this.properties.containsKey(key), () -> "Property '%s' already exists".formatted(key));
 		this.properties.put(key, value);

initializr-generator-spring/src/main/java/io/spring/initializr/generator/spring/properties/ApplicationPropertiesProjectGenerationConfiguration.java

@@ -16,6 +16,9 @@
 
 package io.spring.initializr.generator.spring.properties;
 
+import io.spring.initializr.generator.condition.ConditionalOnConfigurationFileFormat;
+import io.spring.initializr.generator.configuration.format.properties.PropertiesFormat;
+import io.spring.initializr.generator.configuration.format.yaml.YamlFormat;
 import io.spring.initializr.generator.project.ProjectGenerationConfiguration;
 
 import org.springframework.beans.factory.ObjectProvider;
@@ -37,8 +40,16 @@ ApplicationProperties applicationProperties(ObjectProvider<ApplicationProperties
 	}
 
 	@Bean
+	@ConditionalOnConfigurationFileFormat(PropertiesFormat.ID)
 	ApplicationPropertiesContributor applicationPropertiesContributor(ApplicationProperties applicationProperties) {
 		return new ApplicationPropertiesContributor(applicationProperties);
 	}
 
+	@Bean
+	@ConditionalOnConfigurationFileFormat(YamlFormat.ID)
+	ApplicationYamlPropertiesContributor applicationYamlPropertiesContributor(
+			ApplicationProperties applicationProperties) {
+		return new ApplicationYamlPropertiesContributor(applicationProperties);
+	}
+
 }

initializr-generator-spring/src/main/java/io/spring/initializr/generator/spring/properties/ApplicationYamlPropertiesContributor.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2012 - present 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 io.spring.initializr.generator.spring.properties;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
+
+import io.spring.initializr.generator.project.contributor.ProjectContributor;
+
+/**
+ * A {@link ProjectContributor} that contributes a {@code application.yml} file to a
+ * project.
+ *
+ * @author Sijun Yang
+ */
+public class ApplicationYamlPropertiesContributor implements ProjectContributor {
+
+	private static final String FILE = "src/main/resources/application.yml";
+
+	private final ApplicationProperties properties;
+
+	public ApplicationYamlPropertiesContributor(ApplicationProperties properties) {
+		this.properties = properties;
+	}
+
+	@Override
+	public void contribute(Path projectRoot) throws IOException {
+		Path output = projectRoot.resolve(FILE);
+		if (!Files.exists(output)) {
+			Files.createDirectories(output.getParent());
+			Files.createFile(output);
+		}
+		try (PrintWriter writer = new PrintWriter(Files.newOutputStream(output, StandardOpenOption.APPEND), false,
+				StandardCharsets.UTF_8)) {
+			this.properties.writeYaml(writer);
+		}
+	}
+
+}

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/ProjectGeneratorIntegrationTests.java

@@ -23,6 +23,8 @@
 import io.spring.initializr.generator.buildsystem.BuildSystem;
 import io.spring.initializr.generator.buildsystem.gradle.GradleBuildSystem;
 import io.spring.initializr.generator.buildsystem.maven.MavenBuildSystem;
+import io.spring.initializr.generator.configuration.format.properties.PropertiesFormat;
+import io.spring.initializr.generator.configuration.format.yaml.YamlFormat;
 import io.spring.initializr.generator.language.java.JavaLanguage;
 import io.spring.initializr.generator.project.MutableProjectDescription;
 import io.spring.initializr.generator.project.ProjectGenerationConfiguration;
@@ -80,10 +82,27 @@ void customBaseDirectoryIsUsedWhenGeneratingProject() {
 					"test/demo-app/src/test/java/com/example/demo/DemoApplicationTests.java", "test/demo-app/HELP.md");
 	}
 
+	@Test
+	void yaml() {
+		MutableProjectDescription description = initProjectDescription();
+		description.setBuildSystem(new MavenBuildSystem());
+		description.setBaseDirectory("test/demo-app");
+		description.setConfigurationFileFormat(new YamlFormat());
+		ProjectStructure project = this.projectTester.generate(description);
+		assertThat(project).filePaths()
+			.containsOnly("test/demo-app/.gitignore", "test/demo-app/.gitattributes", "test/demo-app/pom.xml",
+					"test/demo-app/mvnw", "test/demo-app/mvnw.cmd",
+					"test/demo-app/.mvn/wrapper/maven-wrapper.properties",
+					"test/demo-app/src/main/java/com/example/demo/DemoApplication.java",
+					"test/demo-app/src/main/resources/application.yml",
+					"test/demo-app/src/test/java/com/example/demo/DemoApplicationTests.java", "test/demo-app/HELP.md");
+	}
+
 	@Test
 	void generatedMavenProjectBuilds(@TempDir Path mavenHome) throws Exception {
 		MutableProjectDescription description = initProjectDescription();
 		description.setBuildSystem(new MavenBuildSystem());
+		description.setConfigurationFileFormat(new YamlFormat());
 		ProjectStructure project = this.projectTester.generate(description);
 		Path projectDirectory = project.getProjectDirectory();
 		runBuild(mavenHome, projectDirectory, description);
@@ -146,6 +165,7 @@ private MutableProjectDescription initProjectDescription() {
 		description.setApplicationName("DemoApplication");
 		description.setPlatformVersion(Version.parse(SpringBootVersion.getVersion()));
 		description.setLanguage(new JavaLanguage(JAVA_VERSION));
+		description.setConfigurationFileFormat(new PropertiesFormat());
 		description.setGroupId("com.example");
 		return description;
 	}

initializr-generator-spring/src/test/java/io/spring/initializr/generator/spring/code/CodeComplianceTests.java

@@ -59,8 +59,7 @@ void currentGenerationJar(Language language) {
 					String.format("src/main/%s/com/example/demo/DemoApplication.%s", language.id(),
 							language.sourceFileExtension()),
 					String.format("src/test/%s/com/example/demo/DemoApplicationTests.%s", language.id(),
-							language.sourceFileExtension()),
-					"src/main/resources/application.properties");
+							language.sourceFileExtension()));
 	}
 
 	@ParameterizedTest
@@ -73,8 +72,7 @@ void currentGenerationWar(Language language) {
 					String.format("src/main/%s/com/example/demo/DemoApplication.%s", language.id(),
 							language.sourceFileExtension()),
 					String.format("src/test/%s/com/example/demo/DemoApplicationTests.%s", language.id(),
-							language.sourceFileExtension()),
-					"src/main/resources/application.properties");
+							language.sourceFileExtension()));
 	}
 
 	@ParameterizedTest