public static  void main(String[] args) throws Throwable 
{
    String usage = "RunJar jarFile [mainClass] args...";
    if (args.length <  1) {
      System.err.println(usage);
      System.exit(-1);
    }
    int firstArg = 0;
    String fileName = args[firstArg++];
    File file = new File(fileName);
    String mainClassName = null;
    JarFile jarFile;
    try {
      jarFile = new JarFile(fileName);
    } catch(IOException io) {
      throw new IOException("Error opening job jar: " + fileName)
        .initCause(io);
    }
    Manifest manifest = jarFile.getManifest();
    if (manifest != null) {
      mainClassName = manifest.getMainAttributes().getValue("Main-Class");
    }
    jarFile.close();
    if (mainClassName == null) {
      if (args.length <  2) {
        System.err.println(usage);
        System.exit(-1);
      }
      mainClassName = args[firstArg++];
    }
    mainClassName = mainClassName.replaceAll("/", ".");
    File tmpDir = new File(new Configuration().get("hadoop.tmp.dir"));
    tmpDir.mkdirs();
    if (!tmpDir.isDirectory()) { 
      System.err.println("Mkdirs failed to create " + tmpDir);
      System.exit(-1);
    }
    final File workDir = File.createTempFile("hadoop-unjar", "", tmpDir);
    workDir.delete();
    workDir.mkdirs();
    if (!workDir.isDirectory()) {
      System.err.println("Mkdirs failed to create " + workDir);
      System.exit(-1);
    }
    Runtime.getRuntime().addShutdownHook(new Thread() {
        public void run() {
          try {
            FileUtil.fullyDelete(workDir);
          } catch (IOException e) {
          }
        }
      });
    unJar(file, workDir);
    ArrayList< URL > classPath = new ArrayList< URL >();
    classPath.add(new File(workDir+"/").toURL());
    classPath.add(file.toURL());
    classPath.add(new File(workDir, "classes/").toURL());
    File[] libs = new File(workDir, "lib").listFiles();
    if (libs != null) {
      for (int i = 0; i <  libs.length; i++) {
        classPath.add(libs[i].toURL());
      }
    }
    ClassLoader loader =
      new URLClassLoader(classPath.toArray(new URL[0]));
    Thread.currentThread().setContextClassLoader(loader);
    Class< ? > mainClass = Class.forName(mainClassName, true, loader);
    Method main = mainClass.getMethod("main", new Class[] {
      Array.newInstance(String.class, 0).getClass()
    });
    String[] newArgs = (String[])Arrays.asList(args)
      .subList(firstArg, args.length).toArray(new String[0]);
    try {
      main.invoke(null, new Object[] { newArgs });
    } catch (InvocationTargetException e) {
      throw e.getTargetException();
    }
}

Run a Hadoop job jar. If the main class is not in the jar's manifest, then it must be provided on the command line.

 public static  void unJar(File jarFile,
    File toDir) throws IOException 
{
    JarFile jar = new JarFile(jarFile);
    try {
      Enumeration entries = jar.entries();
      while (entries.hasMoreElements()) {
        JarEntry entry = (JarEntry)entries.nextElement();
        if (!entry.isDirectory()) {
          InputStream in = jar.getInputStream(entry);
          try {
            File file = new File(toDir, entry.getName());
            if (!file.getParentFile().mkdirs()) {
              if (!file.getParentFile().isDirectory()) {
                throw new IOException("Mkdirs failed to create " + 
                                      file.getParentFile().toString());
              }
            }
            OutputStream out = new FileOutputStream(file);
            try {
              byte[] buffer = new byte[8192];
              int i;
              while ((i = in.read(buffer)) != -1) {
                out.write(buffer, 0, i);
              }
            } finally {
              out.close();
            }
          } finally {
            in.close();
          }
        }
      }
    } finally {
      jar.close();
    }
}

Unpack a jar file into a directory.