Include optional shade plugin in published pom

Update `MavenPublishingConventions` to add a new `mavenOptional` feature
that allows us to declare optional dependencies that are also published
in the generated POM.

This change allows us to include the maven-shade-plugin in the
spring-boot-maven-plugin POM which fixes an issue with Eclipse m2e.

Fixes gh-21992

buildSrc/src/main/java/org/springframework/boot/build/MavenPublishingConventions.java

 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -19,7 +19,10 @@ package org.springframework.boot.build;
 import org.apache.maven.artifact.repository.MavenArtifactRepository;
 import org.gradle.api.Project;
 import org.gradle.api.attributes.Usage;
+import org.gradle.api.component.AdhocComponentWithVariants;
+import org.gradle.api.component.ConfigurationVariantDetails;
 import org.gradle.api.plugins.JavaPlugin;
+import org.gradle.api.plugins.JavaPluginConvention;
 import org.gradle.api.plugins.JavaPluginExtension;
 import org.gradle.api.publish.PublishingExtension;
 import org.gradle.api.publish.maven.MavenPom;
@@ -49,8 +52,6 @@ import org.gradle.api.publish.maven.plugins.MavenPublishPlugin;
  * </ul>
  * </ul>
  *
- * <p/>
- *
  * @author Andy Wilkinson
  * @author Christoph Dreis
  * @author Mike Smithson
@@ -80,6 +81,7 @@ class MavenPublishingConventions {
 		customizePom(publication.getPom(), project);
 		project.getPlugins().withType(JavaPlugin.class)
 				.all((javaPlugin) -> customizeJavaMavenPublication(publication, project));
+		suppressMavenOptionalFeatureWarnings(publication);
 	}
 
 	private void customizePom(MavenPom pom, Project project) {
@@ -98,12 +100,35 @@ class MavenPublishingConventions {
 	}
 
 	private void customizeJavaMavenPublication(MavenPublication publication, Project project) {
+		addMavenOptionalFeature(project);
 		publication.versionMapping((strategy) -> strategy.usage(Usage.JAVA_API, (mappingStrategy) -> mappingStrategy
 				.fromResolutionOf(JavaPlugin.RUNTIME_CLASSPATH_CONFIGURATION_NAME)));
 		publication.versionMapping((strategy) -> strategy.usage(Usage.JAVA_RUNTIME,
 				(mappingStrategy) -> mappingStrategy.fromResolutionResult()));
 	}
 
+	/**
+	 * Add a feature that allows maven plugins to declare optional dependencies that
+	 * appear in the POM. This is required to make m2e in Eclipse happy.
+	 * @param project the project to add the feature to
+	 */
+	private void addMavenOptionalFeature(Project project) {
+		JavaPluginExtension extension = project.getExtensions().getByType(JavaPluginExtension.class);
+		JavaPluginConvention convention = project.getConvention().getPlugin(JavaPluginConvention.class);
+		extension.registerFeature("mavenOptional",
+				(feature) -> feature.usingSourceSet(convention.getSourceSets().getByName("main")));
+		AdhocComponentWithVariants javaComponent = (AdhocComponentWithVariants) project.getComponents()
+				.findByName("java");
+		javaComponent.addVariantsFromConfiguration(
+				project.getConfigurations().findByName("mavenOptionalRuntimeElements"),
+				ConfigurationVariantDetails::mapToOptional);
+	}
+
+	private void suppressMavenOptionalFeatureWarnings(MavenPublication publication) {
+		publication.suppressPomMetadataWarningsFor("mavenOptionalApiElements");
+		publication.suppressPomMetadataWarningsFor("mavenOptionalRuntimeElements");
+	}
+
 	private void customizeOrganization(MavenPomOrganization organization) {
 		organization.getName().set("Pivotal Software, Inc.");
 		organization.getUrl().set("https://spring.io");

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/build.gradle

@@ -32,7 +32,7 @@ dependencies {
 	intTestImplementation("org.junit.jupiter:junit-jupiter")
 	intTestImplementation("org.testcontainers:testcontainers")
 
-	optional("org.apache.maven.plugins:maven-shade-plugin")
+	mavenOptionalImplementation("org.apache.maven.plugins:maven-shade-plugin")
 
 	runtimeOnly("org.sonatype.plexus:plexus-build-api")
 

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/intTest/java/org/springframework/boot/maven/EclipseM2eIntegrationTests.java

+/*
+ * Copyright 2012-2021 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.maven;
+
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
+import java.util.Comparator;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.util.FileCopyUtils;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests to check that our plugin works well with Eclipse m2e.
+ *
+ * @author Phillip Webb
+ */
+public class EclipseM2eIntegrationTests {
+
+	@Test // gh-21992
+	void pluginPomIncludesOptionalShadeDependency() throws Exception {
+		SpringBootDependenciesBom bom = new SpringBootDependenciesBom();
+		String version = bom.get("version");
+		File repository = new File("build/int-test-maven-repository");
+		File pluginDirectory = new File(repository, "org/springframework/boot/spring-boot-maven-plugin/" + version);
+		File[] pomFiles = pluginDirectory.listFiles(this::isPomFile);
+		Arrays.sort(pomFiles, Comparator.comparing(File::getName));
+		File pomFile = pomFiles[pomFiles.length - 1];
+		String pomContent = new String(FileCopyUtils.copyToByteArray(pomFile), StandardCharsets.UTF_8);
+		assertThat(pomContent).contains("maven-shade-plugin");
+	}
+
+	private boolean isPomFile(File file) {
+		return file.getName().endsWith(".pom");
+	}
+
+}