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

It turns out that JarOutputStream has three undocumented quirks:

1. Directory names must end with a '/' slash.
2. Paths must use '/' slashes, not '\'
3. Entries may not begin with a '/' slash.

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 {     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();         }     } } 

Answer 2

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.