Support getComment() on a nested JarFile

Previously, calling getComment() on a nested jar file would result
in the outer jar file's comment being returned.

This commit updates the loader's JarFile to read the file's comment
from the central directory end record and return it from getComment().

Fixes gh-18128

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/CentralDirectoryEndRecord.java

@@ -123,4 +123,10 @@ public int getNumberOfRecords() {
 		return (int) numberOfRecords;
 	}
 
+	String getComment() {
+		int commentLength = (int) Bytes.littleEndianValue(this.block, this.offset + COMMENT_LENGTH_OFFSET, 2);
+		AsciiBytes comment = new AsciiBytes(this.block, this.offset + COMMENT_LENGTH_OFFSET + 2, commentLength);
+		return comment.toString();
+	}
+
 }

spring-boot-project/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java

@@ -80,6 +80,8 @@ public class JarFile extends java.util.jar.JarFile {
 
 	private boolean signed;
 
+	private String comment;
+
 	/**
 	 * Create a new {@link JarFile} backed by the specified file.
 	 * @param file the root jar file
@@ -146,6 +148,7 @@ private CentralDirectoryVisitor centralDirectoryVisitor() {
 
 			@Override
 			public void visitStart(CentralDirectoryEndRecord endRecord, RandomAccessData centralDirectoryData) {
+				JarFile.this.comment = endRecord.getComment();
 			}
 
 			@Override
@@ -290,6 +293,11 @@ private JarFile createJarFileFromFileEntry(JarEntry entry) throws IOException {
 				JarFileType.NESTED_JAR);
 	}
 
+	@Override
+	public String getComment() {
+		return this.comment;
+	}
+
 	@Override
 	public int size() {
 		return this.entries.getSize();

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/TestJarCreator.java

@@ -41,6 +41,7 @@ public static void createTestJar(File file) throws Exception {
 	public static void createTestJar(File file, boolean unpackNested) throws Exception {
 		FileOutputStream fileOutputStream = new FileOutputStream(file);
 		try (JarOutputStream jarOutputStream = new JarOutputStream(fileOutputStream)) {
+			jarOutputStream.setComment("outer");
 			writeManifest(jarOutputStream, "j1");
 			writeEntry(jarOutputStream, "1.dat", 1);
 			writeEntry(jarOutputStream, "2.dat", 2);
@@ -88,6 +89,7 @@ private static void writeNestedEntry(String name, boolean unpackNested, JarOutpu
 	private static byte[] getNestedJarData(boolean multiRelease) throws Exception {
 		ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
 		JarOutputStream jarOutputStream = new JarOutputStream(byteArrayOutputStream);
+		jarOutputStream.setComment("nested");
 		writeManifest(jarOutputStream, "j2", multiRelease);
 		if (multiRelease) {
 			writeEntry(jarOutputStream, "multi-release.dat", 8);

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java

@@ -79,6 +79,7 @@ public void setup() throws Exception {
 	public void jdkJarFile() throws Exception {
 		// Sanity checks to see how the default jar file operates
 		java.util.jar.JarFile jarFile = new java.util.jar.JarFile(this.rootJarFile);
+		assertThat(jarFile.getComment()).isEqualTo("outer");
 		Enumeration<java.util.jar.JarEntry> entries = jarFile.entries();
 		assertThat(entries.nextElement().getName()).isEqualTo("META-INF/");
 		assertThat(entries.nextElement().getName()).isEqualTo("META-INF/MANIFEST.MF");
@@ -152,6 +153,11 @@ public void getJarEntry() {
 		assertThat(entry.getName()).isEqualTo("1.dat");
 	}
 
+	@Test
+	public void getComment() {
+		assertThat(this.jarFile.getComment()).isEqualTo("outer");
+	}
+
 	@Test
 	public void getInputStream() throws Exception {
 		InputStream inputStream = this.jarFile.getInputStream(this.jarFile.getEntry("1.dat"));
@@ -245,6 +251,7 @@ public void getEntryUrlStream() throws Exception {
 	@Test
 	public void getNestedJarFile() throws Exception {
 		JarFile nestedJarFile = this.jarFile.getNestedJarFile(this.jarFile.getEntry("nested.jar"));
+		assertThat(nestedJarFile.getComment()).isEqualTo("nested");
 
 		Enumeration<java.util.jar.JarEntry> entries = nestedJarFile.entries();
 		assertThat(entries.nextElement().getName()).isEqualTo("META-INF/");