Make assertj dependency optional

- Fixes #561

buildSrc/src/main/java/org/springframework/shell/gradle/ManagementConfigurationPlugin.java

@@ -52,6 +52,8 @@ public class ManagementConfigurationPlugin implements Plugin<Project> {
 				configurations.getByName("testFixturesCompileClasspath").extendsFrom(management);
 				configurations.getByName("testFixturesRuntimeClasspath").extendsFrom(management);
 			});
+			plugins.withType(OptionalDependenciesPlugin.class, (optionalDependencies) -> configurations
+				.getByName(OptionalDependenciesPlugin.OPTIONAL_CONFIGURATION_NAME).extendsFrom(management));
 			plugins.withType(MavenPublishPlugin.class, (mavenPublish) -> {
 				PublishingExtension publishing = project.getExtensions().getByType(PublishingExtension.class);
 				publishing.getPublications().withType(MavenPublication.class, (mavenPublication -> {

buildSrc/src/main/java/org/springframework/shell/gradle/ModulePlugin.java

@@ -30,6 +30,7 @@ class ModulePlugin implements Plugin<Project> {
 	public final void apply(Project project) {
 		PluginManager pluginManager = project.getPluginManager();
 		pluginManager.apply(JavaPlugin.class);
+		pluginManager.apply(OptionalDependenciesPlugin.class);
 		pluginManager.apply(ManagementConfigurationPlugin.class);
 		pluginManager.apply(JavaLibraryPlugin.class);
 		pluginManager.apply(SpringMavenPlugin.class);

buildSrc/src/main/java/org/springframework/shell/gradle/OptionalDependenciesPlugin.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 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.shell.gradle;
+
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.Configuration;
+import org.gradle.api.plugins.JavaPlugin;
+import org.gradle.api.plugins.JavaPluginExtension;
+import org.gradle.api.tasks.SourceSetContainer;
+
+public class OptionalDependenciesPlugin implements Plugin<Project> {
+
+	public static final String OPTIONAL_CONFIGURATION_NAME = "optional";
+
+	@Override
+	public void apply(Project project) {
+		Configuration optional = project.getConfigurations().create(OPTIONAL_CONFIGURATION_NAME);
+		optional.setCanBeConsumed(false);
+		optional.setCanBeResolved(false);
+		project.getPlugins().withType(JavaPlugin.class, (javaPlugin) -> {
+			SourceSetContainer sourceSets = project.getExtensions().getByType(JavaPluginExtension.class)
+					.getSourceSets();
+			sourceSets.all((sourceSet) -> {
+				project.getConfigurations().getByName(sourceSet.getCompileClasspathConfigurationName())
+						.extendsFrom(optional);
+				project.getConfigurations().getByName(sourceSet.getRuntimeClasspathConfigurationName())
+						.extendsFrom(optional);
+			});
+		});
+	}
+}

buildSrc/src/main/java/org/springframework/shell/gradle/PublishAllJavaComponentsPlugin.java

@@ -18,7 +18,6 @@ package org.springframework.shell.gradle;
 import org.gradle.api.Action;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
-import org.gradle.api.component.SoftwareComponent;
 import org.gradle.api.plugins.JavaPlatformPlugin;
 import org.gradle.api.plugins.JavaPlugin;
 import org.gradle.api.publish.PublishingExtension;

buildSrc/src/main/java/org/springframework/shell/gradle/SpringMavenPlugin.java

@@ -21,6 +21,8 @@ import java.util.ListIterator;
 import org.gradle.api.Plugin;
 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.JavaPluginExtension;
 import org.gradle.api.plugins.PluginManager;
@@ -60,6 +62,35 @@ public class SpringMavenPlugin implements Plugin<Project> {
 
 	private void customizeMavenPublication(MavenPublication publication, Project project) {
 		customizePom(publication.getPom(), project);
+		project.getPlugins().withType(JavaPlugin.class)
+				.all((javaPlugin) -> customizeJavaMavenPublication(publication, project));
+		suppressMavenOptionalFeatureWarnings(publication);
+	}
+
+	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, VariantVersionMappingStrategy::fromResolutionResult));
+	}
+
+	private void suppressMavenOptionalFeatureWarnings(MavenPublication publication) {
+		publication.suppressPomMetadataWarningsFor("mavenOptionalApiElements");
+		publication.suppressPomMetadataWarningsFor("mavenOptionalRuntimeElements");
+	}
+
+	private void addMavenOptionalFeature(Project project) {
+		JavaPluginExtension extension = project.getExtensions().getByType(JavaPluginExtension.class);
+		extension.registerFeature("mavenOptional",
+				(feature) -> feature.usingSourceSet(extension.getSourceSets().getByName("main")));
+		AdhocComponentWithVariants javaComponent = (AdhocComponentWithVariants) project.getComponents()
+				.findByName("java");
+		if (javaComponent != null) {
+			javaComponent.addVariantsFromConfiguration(
+				project.getConfigurations().findByName("mavenOptionalRuntimeElements"),
+				ConfigurationVariantDetails::mapToOptional);
+		}
 	}
 
 	private void customizePom(MavenPom pom, Project project) {

spring-shell-core-test-support/spring-shell-core-test-support.gradle

@@ -7,5 +7,5 @@ description = 'Spring Shell Core Test Support'
 dependencies {
 	management platform(project(":spring-shell-management"))
 	implementation project(':spring-shell-core')
-	api('org.assertj:assertj-core')
+	optional('org.assertj:assertj-core')
 }