How to use JarOutputStream to create a JAR file?

Question

How does one create a JAR file programmatically using java.util.jar.JarOutputStream? The JAR file produced by my program looks correct (it extracts fine) but when I try loading a library from it Java complains that it cannot find files which are clearly stored inside it. If I extract the JAR file and use Sun's jar command-line tool to re-compress it the resulting library works fine. In short, something is wrong with my JAR file.

Please explain how to create a JAR file programmatically, complete with a manifest file.

Answer 1

Here's some sample code for creating a JAR file using the JarOutputStream:

• http://www.java2s.com/Code/Java/File-Input-Output/CreateJarfile.htm

Answer 2

Unfreaking believable...

It turns out that JarOutputStream has two undocumented quirks:

1. Directory names must end with a slash '/'
2. All paths must use '/' style slashes, not '\'

Here is the correct way to create a Jar 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
{
  BufferedInputStream in = null;
  try
  {
    if (source.isDirectory())
    {
      String name = source.getPath().replace("\\", "/");
      if (!name.isEmpty())
      {
        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);
      return;
    }

    JarEntry entry = new JarEntry(source.getPath().replace("\\", "/"));
    entry.setTime(source.lastModified());
    target.putNextEntry(entry);
    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();
  }
  finally
  {
    if (in != null)
      in.close();
  }
}

Answer 3

There's another "quirk" to pay attention: All JarEntry's names should NOT begin with "/".

For example: The jar entry name for the manifest file is "META-INF/MANIFEST.MF" and not "/META-INF/MANIFEST.MF".

The same rule should be followed for all jar entries.