Add Gradle plugin mapping strategy

This commit adds a mapping strategy for Gradle plugins that don't have
a plugin marker artifact. This is necessary to avoid using a buildScript
section to declare the plugin implementation artifact.

Closes gh-1189

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradleBuildSettings.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 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.
@@ -16,7 +16,11 @@
 
 package io.spring.initializr.generator.buildsystem.gradle;
 
+import java.util.ArrayList;
+import java.util.List;
+
 import io.spring.initializr.generator.buildsystem.BuildSettings;
+import io.spring.initializr.generator.buildsystem.Dependency;
 
 /**
  * Gradle-specific {@linkplain BuildSettings build settings}.
@@ -27,9 +31,12 @@ public class GradleBuildSettings extends BuildSettings {
 
 	private final String sourceCompatibility;
 
+	private final List<PluginMapping> pluginMappings;
+
 	protected GradleBuildSettings(Builder builder) {
 		super(builder);
 		this.sourceCompatibility = builder.sourceCompatibility;
+		this.pluginMappings = new ArrayList<>(builder.pluginMappings);
 	}
 
 	/**
@@ -40,6 +47,14 @@ public class GradleBuildSettings extends BuildSettings {
 		return this.sourceCompatibility;
 	}
 
+	/**
+	 * Return the {@link PluginMapping plugin mappings}, if any.
+	 * @return the plugin mappings
+	 */
+	public List<PluginMapping> getPluginMappings() {
+		return this.pluginMappings;
+	}
+
 	/**
 	 * Builder for {@link GradleBuildSettings}.
 	 */
@@ -47,6 +62,8 @@ public class GradleBuildSettings extends BuildSettings {
 
 		private String sourceCompatibility;
 
+		private final List<PluginMapping> pluginMappings = new ArrayList<>();
+
 		/**
 		 * Set the java version compatibility to use when compiling Java source.
 		 * @param sourceCompatibility java version compatibility
@@ -57,6 +74,21 @@ public class GradleBuildSettings extends BuildSettings {
 			return self();
 		}
 
+		/**
+		 * Map the plugin with the specified id to the specified {@link Dependency}. This
+		 * is mandatory when a plugin does not have an appropriate plugin marker artifact.
+		 * @param id the id of a plugin
+		 * @param pluginDependency the dependency for that plugin
+		 * @return this for method chaining
+		 */
+		public Builder mapPlugin(String id, Dependency pluginDependency) {
+			if (pluginDependency.getVersion() == null || pluginDependency.getVersion().isProperty()) {
+				throw new IllegalArgumentException("Mapping for plugin '" + id + "' must have a version");
+			}
+			this.pluginMappings.add(new PluginMapping(id, pluginDependency));
+			return this;
+		}
+
 		/**
 		 * Build a {@link GradleBuildSettings} with the current state of this builder.
 		 * @return a {@link GradleBuildSettings}
@@ -67,4 +99,36 @@ public class GradleBuildSettings extends BuildSettings {
 
 	}
 
+	/**
+	 * Map a plugin identifier to a plugin implementation artifact.
+	 */
+	public static class PluginMapping {
+
+		private final String id;
+
+		private final Dependency dependency;
+
+		PluginMapping(String id, Dependency dependency) {
+			this.id = id;
+			this.dependency = dependency;
+		}
+
+		/**
+		 * Return the id of the plugin.
+		 * @return the plugin id
+		 */
+		public String getId() {
+			return this.id;
+		}
+
+		/**
+		 * Return the plugin implementation dependency.
+		 * @return the plugin implementation
+		 */
+		public Dependency getDependency() {
+			return this.dependency;
+		}
+
+	}
+
 }

initializr-generator/src/main/java/io/spring/initializr/generator/buildsystem/gradle/GradleSettingsWriter.java

@@ -16,7 +16,9 @@
 
 package io.spring.initializr.generator.buildsystem.gradle;
 
+import io.spring.initializr.generator.buildsystem.Dependency;
 import io.spring.initializr.generator.buildsystem.MavenRepository;
+import io.spring.initializr.generator.buildsystem.gradle.GradleBuildSettings.PluginMapping;
 import io.spring.initializr.generator.io.IndentingWriter;
 
 /**
@@ -41,15 +43,21 @@ public abstract class GradleSettingsWriter {
 	}
 
 	private void writePluginManagement(IndentingWriter writer, GradleBuild build) {
-		if (build.pluginRepositories().isEmpty()) {
+		if (build.pluginRepositories().isEmpty() && build.getSettings().getPluginMappings().isEmpty()) {
 			return;
 		}
 		writer.println("pluginManagement {");
-		writer.indented(() -> writeRepositories(writer, build));
+		writer.indented(() -> {
+			writeRepositories(writer, build);
+			writeResolutionStrategy(writer, build);
+		});
 		writer.println("}");
 	}
 
 	private void writeRepositories(IndentingWriter writer, GradleBuild build) {
+		if (build.pluginRepositories().isEmpty()) {
+			return;
+		}
 		writer.println("repositories {");
 		writer.indented(() -> {
 			build.pluginRepositories().items().map(this::repositoryAsString).forEach(writer::println);
@@ -58,6 +66,29 @@ public abstract class GradleSettingsWriter {
 		writer.println("}");
 	}
 
+	private void writeResolutionStrategy(IndentingWriter writer, GradleBuild build) {
+		if (build.getSettings().getPluginMappings().isEmpty()) {
+			return;
+		}
+		writer.println("resolutionStrategy {");
+		writer.indented(() -> {
+			writer.println("eachPlugin {");
+			writer.indented(() -> build.getSettings().getPluginMappings()
+					.forEach((pluginMapping) -> writePluginMapping(writer, pluginMapping)));
+			writer.println("}");
+		});
+		writer.println("}");
+	}
+
+	private void writePluginMapping(IndentingWriter writer, PluginMapping pluginMapping) {
+		writer.println("if (requested.id.id == " + wrapWithQuotes(pluginMapping.getId()) + ") {");
+		Dependency dependency = pluginMapping.getDependency();
+		String module = String.format("%s:%s:%s", dependency.getGroupId(), dependency.getArtifactId(),
+				dependency.getVersion().getValue());
+		writer.indented(() -> writer.println("useModule(" + wrapWithQuotes(module) + ")"));
+		writer.println("}");
+	}
+
 	private String repositoryAsString(MavenRepository repository) {
 		if (MavenRepository.MAVEN_CENTRAL.equals(repository)) {
 			return "mavenCentral()";

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/GradleBuildSettingsTests.java

@@ -0,0 +1,64 @@
+/*
+ * 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 io.spring.initializr.generator.buildsystem.gradle;
+
+import io.spring.initializr.generator.buildsystem.Dependency;
+import io.spring.initializr.generator.buildsystem.gradle.GradleBuildSettings.Builder;
+import io.spring.initializr.generator.version.VersionReference;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+
+/**
+ * Tests for {@link GradleBuildSettings}.
+ *
+ * @author Stephane Nicoll
+ */
+class GradleBuildSettingsTests {
+
+	@Test
+	void mapPluginWithoutVersionIsNotAllowed() {
+		Builder settingsBuilder = new Builder();
+		assertThatIllegalArgumentException().isThrownBy(
+				() -> settingsBuilder.mapPlugin("test", Dependency.withCoordinates("com.example", "plugin").build()))
+				.withMessage("Mapping for plugin 'test' must have a version");
+	}
+
+	@Test
+	void mapPluginWithVersionReferenceIsNotAllowed() {
+		Builder settingsBuilder = new Builder();
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> settingsBuilder.mapPlugin("test",
+						Dependency.withCoordinates("com.example", "plugin")
+								.version(VersionReference.ofProperty("test.version")).build()))
+				.withMessage("Mapping for plugin 'test' must have a version");
+	}
+
+	@Test
+	void settingsFromBuilderClonePluginMappings() {
+		Builder settingsBuilder = new Builder();
+		settingsBuilder.mapPlugin("test",
+				Dependency.withCoordinates("com.example", "plugin").version(VersionReference.ofValue("1.0.0")).build());
+		GradleBuildSettings firstSettings = settingsBuilder.build();
+		settingsBuilder.mapPlugin("another", Dependency.withCoordinates("com.example", "another")
+				.version(VersionReference.ofValue("2.0.0")).build());
+		assertThat(firstSettings.getPluginMappings()).singleElement()
+				.satisfies((pluginMapping) -> assertThat(pluginMapping.getId()).isEqualTo("test"));
+	}
+
+}

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/GroovyDslGradleSettingsWriterTests.java

@@ -20,8 +20,10 @@ import java.io.StringWriter;
 import java.util.Arrays;
 import java.util.List;
 
+import io.spring.initializr.generator.buildsystem.Dependency;
 import io.spring.initializr.generator.buildsystem.MavenRepository;
 import io.spring.initializr.generator.io.IndentingWriter;
+import io.spring.initializr.generator.version.VersionReference;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -73,6 +75,32 @@ class GroovyDslGradleSettingsWriterTests {
 				"}");
 	}
 
+	@Test
+	void gradleBuildWithPluginMappings() {
+		GradleBuild build = new GradleBuild();
+		build.settings()
+				.mapPlugin("com.example",
+						Dependency.withCoordinates("com.example", "gradle-plugin")
+								.version(VersionReference.ofValue("1.0.0")).build())
+				.mapPlugin("org.acme", Dependency.withCoordinates("org.acme.plugin", "gradle")
+						.version(VersionReference.ofValue("2.0.0")).build());
+		List<String> lines = generateSettings(build);
+		assertThat(lines)
+				.containsSequence(// @formatter:off
+				"pluginManagement {",
+				"    resolutionStrategy {",
+				"        eachPlugin {",
+				"            if (requested.id.id == 'com.example') {",
+				"                useModule('com.example:gradle-plugin:1.0.0')",
+				"            }",
+				"            if (requested.id.id == 'org.acme') {",
+				"                useModule('org.acme.plugin:gradle:2.0.0')",
+				"            }",
+				"        }",
+				"    }",
+				"}"); // @formatter:on
+	}
+
 	@Test
 	void artifactIdShouldBeUsedAsTheRootProjectName() {
 		GradleBuild build = new GradleBuild();

initializr-generator/src/test/java/io/spring/initializr/generator/buildsystem/gradle/KotlinDslGradleSettingsWriterTests.java

@@ -20,8 +20,10 @@ import java.io.StringWriter;
 import java.util.Arrays;
 import java.util.List;
 
+import io.spring.initializr.generator.buildsystem.Dependency;
 import io.spring.initializr.generator.buildsystem.MavenRepository;
 import io.spring.initializr.generator.io.IndentingWriter;
+import io.spring.initializr.generator.version.VersionReference;
 import org.junit.jupiter.api.Test;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -72,6 +74,32 @@ class KotlinDslGradleSettingsWriterTests {
 				"    }", "}");
 	}
 
+	@Test
+	void gradleBuildWithPluginMappings() {
+		GradleBuild build = new GradleBuild();
+		build.settings()
+				.mapPlugin("com.example",
+						Dependency.withCoordinates("com.example", "gradle-plugin")
+								.version(VersionReference.ofValue("1.0.0")).build())
+				.mapPlugin("org.acme", Dependency.withCoordinates("org.acme.plugin", "gradle")
+						.version(VersionReference.ofValue("2.0.0")).build());
+		List<String> lines = generateSettings(build);
+		assertThat(lines)
+				.containsSequence(// @formatter:off
+				"pluginManagement {",
+				"    resolutionStrategy {",
+				"        eachPlugin {",
+				"            if (requested.id.id == \"com.example\") {",
+				"                useModule(\"com.example:gradle-plugin:1.0.0\")",
+				"            }",
+				"            if (requested.id.id == \"org.acme\") {",
+				"                useModule(\"org.acme.plugin:gradle:2.0.0\")",
+				"            }",
+				"        }",
+				"    }",
+				"}"); // @formatter:on
+	}
+
 	@Test
 	void artifactIdShouldBeUsedAsTheRootProjectName() {
 		GradleBuild build = new GradleBuild();