Add support for layered jars in gradle plugin

Closes gh-19792

Co-authored-by: Andy Wilkinson <awilkinson@pivotal.io>

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging.adoc

@@ -272,3 +272,29 @@ include::../gradle/packaging/boot-war-properties-launcher.gradle[tags=properties
 include::../gradle/packaging/boot-war-properties-launcher.gradle.kts[tags=properties-launcher]
 ----
 
+
+[[packaging-layered-jars]]
+==== Packaging layered jars
+
+By default, the `bootJar` tasks builds an archive that contains the application's classes and dependencies in `BOOT-INF/classes` and `BOOT-INF/lib` respectively.
+For cases where a docker image needs to be built from the contents of the jar, the jar format can be enhanced to support layer folders.
+To use this feature, the layering feature must be enabled:
+
+[source,groovy,indent=0,subs="verbatim,attributes",role="primary"]
+.Groovy
+----
+include::../gradle/packaging/boot-jar-layered.gradle[tags=layered]
+----
+
+[source,kotlin,indent=0,subs="verbatim,attributes",role="secondary"]
+.Kotlin
+----
+include::../gradle/packaging/boot-jar-layered.gradle.kts[tags=layered]
+----
+
+The jar will then be split into layer folders which may include:
+
+* `application`
+* `resources`
+* `snapshots-dependencies`
+* `dependencies`

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered.gradle

@@ -0,0 +1,14 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClassName 'com.example.ExampleApplication'
+}
+
+// tag::layered[]
+bootJar {
+	layered()
+}
+// end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-jar-layered.gradle.kts

@@ -0,0 +1,16 @@
+import org.springframework.boot.gradle.tasks.bundling.BootJar
+
+plugins {
+	java
+	id("org.springframework.boot") version "{version}"
+}
+
+tasks.getByName<BootJar>("bootJar") {
+	mainClassName = "com.example.ExampleApplication"
+}
+
+// tag::layered[]
+tasks.getByName<BootJar>("bootJar") {
+	layered()
+}
+// end::layered[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/tasks/bundling/BootJar.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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 +16,10 @@
 
 package org.springframework.boot.gradle.tasks.bundling;
 
+import java.io.BufferedWriter;
 import java.io.File;
+import java.io.IOException;
+import java.io.StringWriter;
 import java.util.Collections;
 import java.util.concurrent.Callable;
 
@@ -26,14 +29,21 @@ import org.gradle.api.file.FileCollection;
 import org.gradle.api.file.FileCopyDetails;
 import org.gradle.api.file.FileTreeElement;
 import org.gradle.api.internal.file.copy.CopyAction;
+import org.gradle.api.java.archives.Attributes;
 import org.gradle.api.specs.Spec;
+import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.Internal;
 import org.gradle.api.tasks.bundling.Jar;
 
+import org.springframework.boot.loader.tools.Layer;
+import org.springframework.boot.loader.tools.Layers;
+import org.springframework.boot.loader.tools.Library;
+
 /**
  * A custom {@link Jar} task that produces a Spring Boot executable jar.
  *
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  * @since 2.0.0
  */
 public class BootJar extends Jar implements BootArchive {
@@ -47,6 +57,10 @@ public class BootJar extends Jar implements BootArchive {
 
 	private FileCollection classpath;
 
+	private Layers layers;
+
+	private static final String BOOT_INF_LAYERS = "BOOT-INF/layers/";
+
 	/**
 	 * Creates a new {@code BootJar} task.
 	 */
@@ -73,6 +87,12 @@ public class BootJar extends Jar implements BootArchive {
 	@Override
 	public void copy() {
 		this.support.configureManifest(this, getMainClassName(), "BOOT-INF/classes/", "BOOT-INF/lib/");
+		Attributes attributes = this.getManifest().getAttributes();
+		if (this.layers != null) {
+			attributes.remove("Spring-Boot-Classes");
+			attributes.remove("Spring-Boot-Lib");
+			attributes.putIfAbsent("Spring-Boot-Layers-Index", "BOOT-INF/layers.idx");
+		}
 		super.copy();
 	}
 
@@ -122,6 +142,56 @@ public class BootJar extends Jar implements BootArchive {
 		action.execute(enableLaunchScriptIfNecessary());
 	}
 
+	/**
+	 * Configures the archive to have layers.
+	 */
+	public void layered() {
+		this.layers = Layers.IMPLICIT;
+		this.bootInf.eachFile((details) -> {
+			Layer layer = layerForFileDetails(details);
+			if (layer != null) {
+				details.setPath(
+						BOOT_INF_LAYERS + "/" + layer + "/" + details.getPath().substring("BOOT-INF/".length()));
+			}
+		}).setIncludeEmptyDirs(false);
+		this.bootInf.into("", (spec) -> spec.from(createLayersIndex()));
+	}
+
+	private Layer layerForFileDetails(FileCopyDetails details) {
+		String path = details.getPath();
+		if (path.startsWith("BOOT-INF/lib/")) {
+			return this.layers.getLayer(new Library(details.getFile(), null));
+		}
+		if (path.startsWith("BOOT-INF/classes/")) {
+			return this.layers.getLayer(details.getSourcePath());
+		}
+		return null;
+	}
+
+	private File createLayersIndex() {
+		try {
+			StringWriter content = new StringWriter();
+			BufferedWriter writer = new BufferedWriter(content);
+			for (Layer layer : this.layers) {
+				writer.write(layer.toString());
+				writer.write("\n");
+			}
+			writer.flush();
+			File source = getProject().getResources().getText().fromString(content.toString()).asFile();
+			File indexFile = new File(source.getParentFile(), "layers.idx");
+			source.renameTo(indexFile);
+			return indexFile;
+		}
+		catch (IOException ex) {
+			throw new RuntimeException("Failed to create layers.idx", ex);
+		}
+	}
+
+	@Input
+	boolean isLayered() {
+		return this.layers != null;
+	}
+
 	@Override
 	public FileCollection getClasspath() {
 		return this.classpath;

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/docs/PackagingDocumentationTests.java

@@ -178,6 +178,18 @@ class PackagingDocumentationTests {
 		assertThat(bootJar).isFile();
 	}
 
+	@TestTemplate
+	void bootJarLayered() throws IOException {
+		this.gradleBuild.script("src/docs/gradle/packaging/boot-jar-layered").build("bootJar");
+		File file = new File(this.gradleBuild.getProjectDir(),
+				"build/libs/" + this.gradleBuild.getProjectDir().getName() + ".jar");
+		assertThat(file).isFile();
+		try (JarFile jar = new JarFile(file)) {
+			JarEntry entry = jar.getJarEntry("BOOT-INF/layers.idx");
+			assertThat(entry).isNotNull();
+		}
+	}
+
 	protected void jarFile(File file) throws IOException {
 		try (JarOutputStream jar = new JarOutputStream(new FileOutputStream(file))) {
 			jar.putNextEntry(new ZipEntry("META-INF/MANIFEST.MF"));

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -16,6 +16,15 @@
 
 package org.springframework.boot.gradle.tasks.bundling;
 
+import java.io.IOException;
+
+import org.gradle.testkit.runner.InvalidRunnerConfigurationException;
+import org.gradle.testkit.runner.TaskOutcome;
+import org.gradle.testkit.runner.UnexpectedBuildFailure;
+import org.junit.jupiter.api.TestTemplate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * Integration tests for {@link BootJar}.
  *
@@ -27,4 +36,21 @@ class BootJarIntegrationTests extends AbstractBootArchiveIntegrationTests {
 		super("bootJar");
 	}
 
+	@TestTemplate
+	void upToDateWhenBuiltTwiceWithLayers()
+			throws InvalidRunnerConfigurationException, UnexpectedBuildFailure, IOException {
+		assertThat(this.gradleBuild.build("-Playered=true", "bootJar").task(":bootJar").getOutcome())
+				.isEqualTo(TaskOutcome.SUCCESS);
+		assertThat(this.gradleBuild.build("-Playered=true", "bootJar").task(":bootJar").getOutcome())
+				.isEqualTo(TaskOutcome.UP_TO_DATE);
+	}
+
+	@TestTemplate
+	void notUpToDateWhenBuiltWithoutLayersAndThenWithLayers()
+			throws InvalidRunnerConfigurationException, UnexpectedBuildFailure, IOException {
+		assertThat(this.gradleBuild.build("bootJar").task(":bootJar").getOutcome()).isEqualTo(TaskOutcome.SUCCESS);
+		assertThat(this.gradleBuild.build("-Playered=true", "bootJar").task(":bootJar").getOutcome())
+				.isEqualTo(TaskOutcome.SUCCESS);
+	}
+
 }

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/tasks/bundling/BootJarTests.java

@@ -18,6 +18,8 @@ package org.springframework.boot.gradle.tasks.bundling;
 
 import java.io.File;
 import java.io.IOException;
+import java.io.InputStream;
+import java.util.List;
 import java.util.jar.JarFile;
 
 import org.junit.jupiter.api.Test;
@@ -28,6 +30,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link BootJar}.
  *
  * @author Andy Wilkinson
+ * @author Madhura Bhave
  */
 class BootJarTests extends AbstractBootArchiveTests<BootJar> {
 
@@ -57,6 +60,48 @@ class BootJarTests extends AbstractBootArchiveTests<BootJar> {
 		}
 	}
 
+	@Test
+	void layers() throws IOException {
+		BootJar bootJar = getTask();
+		bootJar.setMainClassName("com.example.Main");
+		bootJar.layered();
+		File classesJavaMain = new File(this.temp, "classes/java/main");
+		File applicationClass = new File(classesJavaMain, "com/example/Application.class");
+		applicationClass.getParentFile().mkdirs();
+		applicationClass.createNewFile();
+		File resourcesMain = new File(this.temp, "resources/main");
+		File applicationProperties = new File(resourcesMain, "application.properties");
+		applicationProperties.getParentFile().mkdirs();
+		applicationProperties.createNewFile();
+		File staticResources = new File(resourcesMain, "static");
+		staticResources.mkdir();
+		File css = new File(staticResources, "test.css");
+		css.createNewFile();
+		bootJar.classpath(classesJavaMain, resourcesMain, jarFile("first-library.jar"), jarFile("second-library.jar"),
+				jarFile("third-library-SNAPSHOT.jar"));
+		bootJar.requiresUnpack("second-library.jar");
+		executeTask();
+		List<String> entryNames = getEntryNames(bootJar.getArchiveFile().get().getAsFile());
+		assertThat(entryNames).containsSubsequence("org/springframework/boot/loader/",
+				"BOOT-INF/layers/application/classes/com/example/Application.class",
+				"BOOT-INF/layers/resources/classes/static/test.css",
+				"BOOT-INF/layers/application/classes/application.properties",
+				"BOOT-INF/layers/dependencies/lib/first-library.jar",
+				"BOOT-INF/layers/dependencies/lib/second-library.jar",
+				"BOOT-INF/layers/snapshot-dependencies/lib/third-library-SNAPSHOT.jar");
+		assertThat(entryNames).doesNotContain("BOOT-INF/classes").doesNotContain("BOOT-INF/lib")
+				.doesNotContain("BOOT-INF/com/");
+		try (JarFile jarFile = new JarFile(bootJar.getArchiveFile().get().getAsFile())) {
+			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Classes")).isEqualTo(null);
+			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Lib")).isEqualTo(null);
+			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Layers-Index"))
+					.isEqualTo("BOOT-INF/layers.idx");
+			try (InputStream input = jarFile.getInputStream(jarFile.getEntry("BOOT-INF/layers.idx"))) {
+				assertThat(input).hasContent("dependencies\nsnapshot-dependencies\nresources\napplication\n");
+			}
+		}
+	}
+
 	@Override
 	protected void executeTask() {
 		getTask().copy();

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests.gradle

@@ -10,4 +10,7 @@ bootJar {
 			properties 'prop' : project.hasProperty('launchScriptProperty') ? launchScriptProperty : 'default'
 		}
 	}
+	if (project.hasProperty('layered') && project.getProperty('layered')) {
+		layered()
+	}
 }