Write classpath index file from Gradle plugin

Closes gh-19847

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

@@ -20,7 +20,9 @@ import java.io.BufferedWriter;
 import java.io.File;
 import java.io.IOException;
 import java.io.StringWriter;
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.List;
 import java.util.concurrent.Callable;
 
 import org.gradle.api.Action;
@@ -59,7 +61,9 @@ public class BootJar extends Jar implements BootArchive {
 
 	private Layers layers;
 
-	private static final String BOOT_INF_LAYERS = "BOOT-INF/layers/";
+	private static final String BOOT_INF_LAYERS = "BOOT-INF/layers";
+
+	private List<String> dependencies = new ArrayList<>();
 
 	/**
 	 * Creates a new {@code BootJar} task.
@@ -68,7 +72,10 @@ public class BootJar extends Jar implements BootArchive {
 		this.bootInf = getProject().copySpec().into("BOOT-INF");
 		getMainSpec().with(this.bootInf);
 		this.bootInf.into("classes", classpathFiles(File::isDirectory));
-		this.bootInf.into("lib", classpathFiles(File::isFile));
+		this.bootInf.into("lib", classpathFiles(File::isFile))
+				.eachFile((details) -> BootJar.this.dependencies.add(details.getPath()));
+		this.bootInf.into("",
+				(spec) -> spec.from((Callable<File>) () -> createClasspathIndex(BootJar.this.dependencies)));
 		this.bootInf.filesMatching("module-info.class",
 				(details) -> details.setRelativePath(details.getRelativeSourcePath()));
 		getRootSpec().eachFile((details) -> {
@@ -93,9 +100,29 @@ public class BootJar extends Jar implements BootArchive {
 			attributes.remove("Spring-Boot-Lib");
 			attributes.putIfAbsent("Spring-Boot-Layers-Index", "BOOT-INF/layers.idx");
 		}
+		attributes.putIfAbsent("Spring-Boot-Classpath-Index", "BOOT-INF/classpath.idx");
 		super.copy();
 	}
 
+	private File createClasspathIndex(List<String> dependencies) {
+		try {
+			StringWriter content = new StringWriter();
+			BufferedWriter writer = new BufferedWriter(content);
+			for (String dependency : dependencies) {
+				writer.write(dependency);
+				writer.write("\n");
+			}
+			writer.flush();
+			File source = getProject().getResources().getText().fromString(content.toString()).asFile();
+			File indexFile = new File(source.getParentFile(), "classpath.idx");
+			source.renameTo(indexFile);
+			return indexFile;
+		}
+		catch (IOException ex) {
+			throw new RuntimeException("Failed to create classpath.idx", ex);
+		}
+	}
+
 	@Override
 	protected CopyAction createCopyAction() {
 		return this.support.createCopyAction(this);

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

@@ -96,9 +96,37 @@ class BootJarTests extends AbstractBootArchiveTests<BootJar> {
 			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Lib")).isEqualTo(null);
 			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Layers-Index"))
 					.isEqualTo("BOOT-INF/layers.idx");
+			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Classpath-Index"))
+					.isEqualTo("BOOT-INF/classpath.idx");
 			try (InputStream input = jarFile.getInputStream(jarFile.getEntry("BOOT-INF/layers.idx"))) {
 				assertThat(input).hasContent("dependencies\nsnapshot-dependencies\nresources\napplication\n");
 			}
+			try (InputStream input = jarFile.getInputStream(jarFile.getEntry("BOOT-INF/classpath.idx"))) {
+				assertThat(input).hasContent(
+						"BOOT-INF/layers/dependencies/lib/first-library.jar\nBOOT-INF/layers/dependencies/lib/second-library.jar\nBOOT-INF/layers/snapshot-dependencies/lib/third-library-SNAPSHOT.jar\n");
+			}
+		}
+	}
+
+	@Test
+	void classpathIndex() throws IOException {
+		BootJar bootJar = getTask();
+		bootJar.setMainClassName("com.example.Main");
+		File classesJavaMain = new File(this.temp, "classes/java/main");
+		File applicationClass = new File(classesJavaMain, "com/example/Application.class");
+		applicationClass.getParentFile().mkdirs();
+		applicationClass.createNewFile();
+		bootJar.classpath(classesJavaMain, jarFile("first-library.jar"), jarFile("second-library.jar"),
+				jarFile("third-library-SNAPSHOT.jar"));
+		bootJar.requiresUnpack("second-library.jar");
+		executeTask();
+		try (JarFile jarFile = new JarFile(bootJar.getArchiveFile().get().getAsFile())) {
+			assertThat(jarFile.getManifest().getMainAttributes().getValue("Spring-Boot-Classpath-Index"))
+					.isEqualTo("BOOT-INF/classpath.idx");
+			try (InputStream input = jarFile.getInputStream(jarFile.getEntry("BOOT-INF/classpath.idx"))) {
+				assertThat(input).hasContent(
+						"BOOT-INF/lib/first-library.jar\nBOOT-INF/lib/second-library.jar\nBOOT-INF/lib/third-library-SNAPSHOT.jar\n");
+			}
 		}
 	}