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

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

@@ -123,4 +123,10 @@ class CentralDirectoryEndRecord {
 		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 @@ public class JarFile extends java.util.jar.JarFile {
 
 			@Override
 			public void visitStart(CentralDirectoryEndRecord endRecord, RandomAccessData centralDirectoryData) {
+				JarFile.this.comment = endRecord.getComment();
 			}
 
 			@Override
@@ -290,6 +293,11 @@ public class JarFile extends java.util.jar.JarFile {
 				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 abstract class TestJarCreator {
 	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 @@ public abstract class TestJarCreator {
 	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 class JarFileTests {
 	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 class JarFileTests {
 		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 class JarFileTests {
 	@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/");