Creating JAR Files Using JarOutputStream

To generate a JAR file programmatically, JarOutputStream is often employed. However, it is essential to avoid certain undocumented quirks within JarOutputStream:

1. Directories Ending with a Slash:

Directory names in JAR files must end with a '/' slash.

2. Paths Using Forward Slashes:

Use forward slashes '/' in paths, as opposed to backslashes '\'.

3. No Leading Slashes in Entry Names:

Entry names should not start with a '/' slash.

Example Code with Corrections:

The following corrected code constructs a valid JAR file with a manifest file:

public void run() throws IOException {
    Manifest manifest = new Manifest();
    manifest.getMainAttributes().put(Attributes.Name.MANIFEST_VERSION, "1.0");
    JarOutputStream target = new JarOutputStream(new FileOutputStream("output.jar"), manifest);
    add(new File("inputDirectory"), target);
    target.close();
}

private void add(File source, JarOutputStream target) throws IOException {
    String name = source.getPath().replace("\\", "/");
    if (source.isDirectory()) {
        if (!name.endsWith("/")) {
            name  = "/";
        }
        JarEntry entry = new JarEntry(name);
        entry.setTime(source.lastModified());
        target.putNextEntry(entry);
        target.closeEntry();
        for (File nestedFile : source.listFiles()) {
            add(nestedFile, target);
        }
    } else {
        JarEntry entry = new JarEntry(name);
        entry.setTime(source.lastModified());
        target.putNextEntry(entry);
        try (BufferedInputStream in = new BufferedInputStream(new FileInputStream(source))) {
            byte[] buffer = new byte[1024];
            while (true) {
                int count = in.read(buffer);
                if (count == -1) {
                    break;
                }
                target.write(buffer, 0, count);
            }
            target.closeEntry();
        }
    }
}