Model that a Maven dependency is optional

Closes gh-913

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/Dependency.java

@@ -45,7 +45,7 @@ public class Dependency {
 
 	private final Set<Exclusion> exclusions;
 
-	Dependency(Builder builder) {
+	protected Dependency(Builder<?> builder) {
 		this.groupId = builder.groupId;
 		this.artifactId = builder.artifactId;
 		this.version = builder.version;
@@ -54,7 +54,7 @@ public class Dependency {
 		this.exclusions = new LinkedHashSet<>(builder.exclusions);
 	}
 
-	public static Builder withCoordinates(String groupId, String artifactId) {
+	public static Builder<?> withCoordinates(String groupId, String artifactId) {
 		return new Builder(groupId, artifactId);
 	}
 
@@ -111,9 +111,10 @@ public class Dependency {
 	/**
 	 * Builder for a dependency.
 	 *
+	 * @param <B> builder type
 	 * @see Dependency#withCoordinates(String, String)
 	 */
-	public static final class Builder {
+	public static class Builder<B extends Builder> {
 
 		private String groupId;
 
@@ -127,39 +128,44 @@ public class Dependency {
 
 		private Set<Exclusion> exclusions = new LinkedHashSet<>();
 
-		private Builder(String groupId, String artifactId) {
+		protected Builder(String groupId, String artifactId) {
 			this.groupId = groupId;
 			this.artifactId = artifactId;
 		}
 
-		public Builder groupId(String groupId) {
+		public B groupId(String groupId) {
 			this.groupId = groupId;
-			return this;
+			return self();
 		}
 
-		public Builder artifactId(String artifactId) {
+		public B artifactId(String artifactId) {
 			this.artifactId = artifactId;
-			return this;
+			return self();
 		}
 
-		public Builder version(VersionReference version) {
+		public B version(VersionReference version) {
 			this.version = version;
-			return this;
+			return self();
 		}
 
-		public Builder scope(DependencyScope scope) {
+		public B scope(DependencyScope scope) {
 			this.scope = scope;
-			return this;
+			return self();
 		}
 
-		public Builder type(String type) {
+		public B type(String type) {
 			this.type = type;
-			return this;
+			return self();
 		}
 
-		public Builder exclusions(Exclusion... exclusions) {
+		public B exclusions(Exclusion... exclusions) {
 			this.exclusions = new LinkedHashSet<>(Arrays.asList(exclusions));
-			return this;
+			return self();
+		}
+
+		@SuppressWarnings("unchecked")
+		protected B self() {
+			return (B) this;
 		}
 
 		public Dependency build() {

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/DependencyContainer.java

@@ -47,7 +47,7 @@ public class DependencyContainer extends BuildItemContainer<String, Dependency>
 	 * @param id the id of the dependency
 	 * @param builder the state of the dependency
 	 */
-	public void add(String id, Dependency.Builder builder) {
+	public void add(String id, Dependency.Builder<?> builder) {
 		add(id, builder.build());
 	}
 

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/maven/MavenBuildWriter.java

@@ -155,7 +155,7 @@ public class MavenBuildWriter {
 			writeSingleElement(writer, "version",
 					determineVersion(dependency.getVersion()));
 			writeSingleElement(writer, "scope", scopeForType(dependency.getScope()));
-			if (isOptional(dependency.getScope())) {
+			if (isOptional(dependency)) {
 				writeSingleElement(writer, "optional", Boolean.toString(true));
 			}
 			writeSingleElement(writer, "type", dependency.getType());
@@ -202,9 +202,13 @@ public class MavenBuildWriter {
 		}
 	}
 
-	private boolean isOptional(DependencyScope type) {
-		return (type == DependencyScope.ANNOTATION_PROCESSOR
-				|| type == DependencyScope.COMPILE_ONLY);
+	private boolean isOptional(Dependency dependency) {
+		if (dependency instanceof MavenDependency
+				&& ((MavenDependency) dependency).isOptional()) {
+			return true;
+		}
+		return (dependency.getScope() == DependencyScope.ANNOTATION_PROCESSOR
+				|| dependency.getScope() == DependencyScope.COMPILE_ONLY);
 	}
 
 	private void writeDependencyManagement(IndentingWriter writer, MavenBuild build) {

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/maven/MavenDependency.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2012-2019 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 io.spring.initializr.generator.buildsystem.maven;
+
+import io.spring.initializr.generator.buildsystem.Dependency;
+
+/**
+ * A {@link Dependency} with specific settings for the Maven build system.
+ *
+ * @author Stephane Nicoll
+ */
+public class MavenDependency extends Dependency {
+
+	protected final boolean optional;
+
+	protected MavenDependency(Builder builder) {
+		super(builder);
+		this.optional = builder.optional;
+	}
+
+	public static Builder withCoordinates(String groupId, String artifactId) {
+		return new Builder(groupId, artifactId);
+	}
+
+	public boolean isOptional() {
+		return this.optional;
+	}
+
+	/**
+	 * Builder for a Maven dependency.
+	 *
+	 * @see MavenDependency#withCoordinates(String, String)
+	 */
+	public static class Builder extends Dependency.Builder<Builder> {
+
+		private boolean optional;
+
+		protected Builder(String groupId, String artifactId) {
+			super(groupId, artifactId);
+		}
+
+		public Builder optional(boolean optional) {
+			this.optional = optional;
+			return self();
+		}
+
+		@Override
+		public Dependency build() {
+			return new MavenDependency(this);
+		}
+
+	}
+
+}

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/maven/MavenBuildWriterTests.java

@@ -287,6 +287,28 @@ class MavenBuildWriterTests {
 		});
 	}
 
+	@Test
+	void pomWithOptionalDependency() throws Exception {
+		MavenBuild build = new MavenBuild();
+		build.setGroup("com.example.demo");
+		build.setArtifact("demo");
+		build.dependencies().add("annotation-processor",
+				MavenDependency
+						.withCoordinates("org.springframework.boot",
+								"spring-boot-configuration-processor")
+						.scope(DependencyScope.COMPILE).optional(true));
+		generatePom(build, (pom) -> {
+			NodeAssert dependency = pom.nodeAtPath("/project/dependencies/dependency");
+			assertThat(dependency).textAtPath("groupId")
+					.isEqualTo("org.springframework.boot");
+			assertThat(dependency).textAtPath("artifactId")
+					.isEqualTo("spring-boot-configuration-processor");
+			assertThat(dependency).textAtPath("version").isNullOrEmpty();
+			assertThat(dependency).textAtPath("scope").isNullOrEmpty();
+			assertThat(dependency).textAtPath("optional").isEqualTo("true");
+		});
+	}
+
 	@Test
 	void pomWithNonNullArtifactTypeDependency() throws Exception {
 		MavenBuild build = new MavenBuild();