Add OptionalDependenciesPlugin

.gitignore

@@ -7,7 +7,9 @@ build.sh
 integration-repo
 ivy-cache
 .gradle
-build
+/build
+buildSrc/build
+/spring-*/build
 target
 
 # Eclipse metadata

buildSrc/src/main/java/org/springframework/build/optional/OptionalDependenciesPlugin.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2002-2019 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.build.optional;
+
+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.JavaPluginConvention;
+import org.gradle.api.tasks.SourceSetContainer;
+import org.gradle.plugins.ide.eclipse.EclipsePlugin;
+import org.gradle.plugins.ide.eclipse.model.EclipseModel;
+
+/**
+ * A {@code Plugin} that adds support for Maven-style optional dependencies. Creates a new
+ * {@code optional} configuration. The {@code optional} configuration is part of the
+ * project's compile and runtime classpath's but does not affect the classpath of
+ * dependent projects.
+ *
+ * @author Andy Wilkinson
+ */
+public class OptionalDependenciesPlugin implements Plugin<Project> {
+
+	/**
+	 * Name of the {@code optional} configuration.
+	 */
+	public static final String OPTIONAL_CONFIGURATION_NAME = "optional";
+
+	@Override
+	public void apply(Project project) {
+		Configuration optional = project.getConfigurations().create("optional");
+		project.getPlugins().withType(JavaPlugin.class, (javaPlugin) -> {
+			SourceSetContainer sourceSets = project.getConvention()
+					.getPlugin(JavaPluginConvention.class).getSourceSets();
+			sourceSets.all((sourceSet) -> {
+				sourceSet.setCompileClasspath(
+						sourceSet.getCompileClasspath().plus(optional));
+				sourceSet.setRuntimeClasspath(
+						sourceSet.getRuntimeClasspath().plus(optional));
+			});
+		});
+		project.getPlugins().withType(EclipsePlugin.class, (eclipePlugin) -> {
+			project.getExtensions().getByType(EclipseModel.class)
+					.classpath((classpath) -> {
+						classpath.getPlusConfigurations().add(optional);
+					});
+		});
+	}
+
+}