Troubleshooting JarOutputStream for JAR File Creation

Creating JAR files programmatically using java.util.jar.JarOutputStream can seem straightforward, but certain nuances can lead to unexpected issues. This article explores these undocumented quirks and provides a comprehensive solution for creating valid JAR files.

Understanding the Quirks

When using JarOutputStream, it's crucial to adhere to the following undocumented rules:

• Directory names must terminate with a '/' slash.
• Paths should employ '/' slashes, not '\'.
• Entries cannot commence with a '/' slash.

Implementing the Correct Approach

Here's a detailed example of how to create a JAR file with a manifest file, addressing the aforementioned quirks:

public void run() throws IOException {
    // Prepare the manifest file
    Manifest manifest = new Manifest();
    manifest.getMainAttributes().put(Attributes.Name.MANIFEST_VERSION, "1.0");

    // Create a new JAROutputStream with the manifest
    JarOutputStream target = new JarOutputStream(new FileOutputStream("output.jar"), manifest);

    // Iterate over the source directory and add files to the JAR
    add(new File("inputDirectory"), target);

    // Close the JAROutputStream
    target.close();
}

private void add(File source, JarOutputStream target) throws IOException {
    // Prepare the entry path
    String name = source.getPath().replace("\\", "/");

    // Handle directories
    if (source.isDirectory()) {
        if (!name.endsWith("/")) {
            name  = "/";
        }
        // Create a directory entry with appropriate timestamps
        JarEntry entry = new JarEntry(name);
        entry.setTime(source.lastModified());
        target.putNextEntry(entry);
        target.closeEntry();

        // Recursively add files within the directory
        for (File nestedFile : source.listFiles()) {
            add(nestedFile, target);
        }
    } 
    // Handle files
    else {
        // Create a file entry with appropriate timestamps
        JarEntry entry = new JarEntry(name);
        entry.setTime(source.lastModified());
        target.putNextEntry(entry);

        // Read and write the file contents to the JAR
        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();
        }
    }
}

By following these guidelines, you can now confidently create valid JAR files programmatically, ensuring that libraries and other resources included within them are accessible as intended.