Allow arbitrary code snippets to be added in a Gradle build

This commit provides an escape hatch for the DSL that we currently don't
support in a gradle build. Snippets can be added with a callback to an
IndentingWriter. For convenience, imports can be added as well.

Snippets are rendered at the end of the file.

Closes gh-1079

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2022 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.
@@ -38,6 +38,8 @@ public class GradleBuild extends Build {
 
 	private final GradleTaskContainer tasks = new GradleTaskContainer();
 
+	private final GradleSnippetContainer snippets = new GradleSnippetContainer();
+
 	private final GradleBuildscript.Builder buildscript = new GradleBuildscript.Builder();
 
 	/**
@@ -92,6 +94,15 @@ public GradleTaskContainer tasks() {
 		return this.tasks;
 	}
 
+	/**
+	 * Return the {@linkplain GradleSnippetContainer snippet container} to use to apply
+	 * snippets.
+	 * @return the {@link GradleSnippetContainer}
+	 */
+	public GradleSnippetContainer snippets() {
+		return this.snippets;
+	}
+
 	/**
 	 * Customize the {@code buildscript} of the build using the specified consumer.
 	 * @param buildscript a consumer of the current buildscript

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

@@ -29,6 +29,7 @@
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import io.spring.initializr.generator.buildsystem.BillOfMaterials;
 import io.spring.initializr.generator.buildsystem.Dependency;
@@ -59,7 +60,7 @@ public abstract class GradleBuildWriter {
 	 */
 	public final void writeTo(IndentingWriter writer, GradleBuild build) {
 		GradleBuildSettings settings = build.getSettings();
-		writeImports(writer, build.tasks());
+		writeImports(writer, build.tasks(), build.snippets());
 		writeBuildscript(writer, build);
 		writePlugins(writer, build);
 		writeProperty(writer, "group", settings.getGroup());
@@ -72,10 +73,11 @@ public final void writeTo(IndentingWriter writer, GradleBuild build) {
 		writeDependencies(writer, build);
 		writeBoms(writer, build);
 		writeTasks(writer, build.tasks());
+		writeSnippets(writer, build.snippets());
 	}
 
-	private void writeImports(IndentingWriter writer, GradleTaskContainer tasks) {
-		List<String> imports = tasks.importedTypes().sorted().toList();
+	private void writeImports(IndentingWriter writer, GradleTaskContainer tasks, GradleSnippetContainer snippets) {
+		List<String> imports = Stream.concat(tasks.importedTypes(), snippets.importedTypes()).sorted().toList();
 		imports.forEach((importedType) -> writer.println("import " + importedType));
 		if (!imports.isEmpty()) {
 			writer.println();
@@ -202,6 +204,16 @@ protected final void writeTaskCustomization(IndentingWriter writer, GradleTask t
 
 	protected abstract String invocationAsString(GradleTask.Invocation invocation);
 
+	private void writeSnippets(IndentingWriter writer, GradleSnippetContainer snippets) {
+		if (!snippets.isEmpty()) {
+			writer.println();
+		}
+		snippets.values().forEach((snippet) -> {
+			snippet.apply(writer);
+			writer.println();
+		});
+	}
+
 	protected final <T> void writeNestedCollection(IndentingWriter writer, String name, Collection<T> collection,
 			Function<T, String> itemToStringConverter) {
 		this.writeNestedCollection(writer, name, collection, itemToStringConverter, null);

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

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2022 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 io.spring.initializr.generator.buildsystem.gradle;
+
+import java.util.Set;
+import java.util.function.Consumer;
+
+import io.spring.initializr.generator.io.IndentingWriter;
+
+/**
+ * A free-form {@code snippet} to add to a Gradle build.
+ *
+ * @author Stephane Nicoll
+ */
+public class GradleSnippet {
+
+	private final Set<String> importedTypes;
+
+	private final Consumer<IndentingWriter> writer;
+
+	GradleSnippet(Set<String> importedTypes, Consumer<IndentingWriter> writer) {
+		this.importedTypes = Set.copyOf(importedTypes);
+		this.writer = writer;
+	}
+
+	Set<String> getImportedTypes() {
+		return this.importedTypes;
+	}
+
+	/**
+	 * Apply the snippet using the specified {@link IndentingWriter}.
+	 * @param indentingWriter the writer to use
+	 */
+	public void apply(IndentingWriter indentingWriter) {
+		this.writer.accept(indentingWriter);
+	}
+
+}

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

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2012-2022 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 io.spring.initializr.generator.buildsystem.gradle;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.Set;
+import java.util.function.Consumer;
+import java.util.stream.Stream;
+
+import io.spring.initializr.generator.io.IndentingWriter;
+
+/**
+ * A container for {@linkplain GradleSnippet Gradle snippets}.
+ *
+ * @author Stephane Nicoll
+ */
+public class GradleSnippetContainer {
+
+	private final List<GradleSnippet> snippets = new ArrayList<>();
+
+	/**
+	 * Specify if this container is empty.
+	 * @return {@code true} if no snippet is registered
+	 */
+	public boolean isEmpty() {
+		return this.snippets.isEmpty();
+	}
+
+	/**
+	 * Return the {@link GradleSnippet Gradle snippets} to apply.
+	 * @return the gradle snippets
+	 */
+	public Stream<GradleSnippet> values() {
+		return this.snippets.stream();
+	}
+
+	/**
+	 * Return the fully qualified name of types to import.
+	 * @return the imported types
+	 */
+	public Stream<String> importedTypes() {
+		return this.snippets.stream().map(GradleSnippet::getImportedTypes).flatMap(Collection::stream);
+	}
+
+	/**
+	 * Register a {@code snippet} with the specified types to import and writer.
+	 * @param importedTypes the types to import
+	 * @param writer the writer to use.
+	 */
+	public void add(Set<String> importedTypes, Consumer<IndentingWriter> writer) {
+		this.snippets.add(new GradleSnippet(importedTypes, writer));
+	}
+
+	/**
+	 * Register a {@code snippet} with no import.
+	 * @param writer the writer to use.
+	 */
+	public void add(Consumer<IndentingWriter> writer) {
+		add(Collections.emptySet(), writer);
+	}
+
+}

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

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2012-2022 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 io.spring.initializr.generator.buildsystem.gradle;
+
+import java.util.Set;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Common tests for {@link GradleBuildWriter} implementations.
+ *
+ * @author Stephane Nicoll
+ */
+public abstract class GradleBuildWriterTests {
+
+	@Test
+	void gradleBuildWithSnippet() {
+		GradleBuild build = new GradleBuild();
+		build.snippets().add((writer) -> {
+			writer.println("custom {");
+			writer.indented(() -> {
+				writer.println("first = 1");
+				writer.println("second = 2");
+			});
+			writer.println("}");
+		});
+		assertThat(write(build)).contains("""
+				custom {
+					first = 1
+					second = 2
+				}
+				""");
+	}
+
+	@Test
+	void gradleBuildWithSnippetsAreSeparated() {
+		GradleBuild build = new GradleBuild();
+		build.snippets().add((writer) -> {
+			writer.println("custom {");
+			writer.indented(() -> {
+				writer.println("first = 1");
+				writer.println("second = 2");
+			});
+			writer.println("}");
+		});
+		build.snippets().add((writer) -> {
+			writer.println("another {");
+			writer.indented(() -> {
+				writer.println("third = 3");
+				writer.println("fourth = 4");
+			});
+			writer.println("}");
+		});
+
+		assertThat(write(build)).contains("""
+				custom {
+					first = 1
+					second = 2
+				}
+
+				another {
+					third = 3
+					fourth = 4
+				}
+				""");
+	}
+
+	@Test
+	void gradleBuildWithSnippetAndImports() {
+		GradleBuild build = new GradleBuild();
+		build.snippets().add(Set.of("com.example.CustomTask"), (writer) -> writer.println("custom { }"));
+		assertThat(write(build)).containsOnlyOnce("import com.example.CustomTask");
+	}
+
+	protected abstract String write(GradleBuild build);
+
+}

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

@@ -39,7 +39,7 @@
  * @author Jean-Baptiste Nizet
  * @author Stephane Nicoll
  */
-class GroovyDslGradleBuildWriterTests {
+class GroovyDslGradleBuildWriterTests extends GradleBuildWriterTests {
 
 	@Test
 	void gradleBuildWithCoordinates() {
@@ -556,7 +556,7 @@ void gradleBuildWithCustomVersion() {
 		assertThat(write(build)).contains("version = '1.2.4.RELEASE'");
 	}
 
-	private String write(GradleBuild build) {
+	protected String write(GradleBuild build) {
 		return write(new GroovyDslGradleBuildWriter(), build);
 	}