The code below extracts a file named mydb.mdb from a JAR file. Its useful when you have a JAR file with lot of resources and you want to extract just few files out of it.

Java Code:
public class ExtractFromJAR {

  public void extractMyMDBromJAR(String dest){
     try {
      String home = getClass().getProtectionDomain().
                    getCodeSource().getLocation().toString().
                    substring(6);
      JarFile jar = new JarFile(home);
      ZipEntry entry = jar.getEntry("mydb.mdb");
      File efile = new File(dest, entry.getName());

      InputStream in = 
         new BufferedInputStream(jar.getInputStream(entry));
      OutputStream out = 
         new BufferedOutputStream(new FileOutputStream(efile));
      byte[] buffer = new byte[2048];
      for (;;)  {
        int nBytes = in.read(buffer);
        if (nBytes <= 0) break;
        out.write(buffer, 0, nBytes);
      }
      out.flush();
      out.close();
      in.close();
     }
     catch (Exception e) {
      e.printStackTrace();
    }
  }

 public static void main(String args []){
     new ExtractFromJAR().extractMyMDBFromJAR(".");
 }

}