This JAR is driving me crazy! Dukes to come...

Question

Hi all,
I can create an executable jar, both programmatically and from the command line and the internal directory structure comes out fine, just as expected. However, If I double click on it, I get the dreaded "Cannot find main class, program will exit" error.
I have checked and double checked my manifest and it seems fine too. This is what is created inside the jar:
Directory called "jai" containing class files
Directory called "META-INF" containing one file, "manifest.mf"
Manifest looks like this:
Manifest-Version: 1.0
Main-Class: jai.Run
including the empty line at the end. Run.class does exist inside jai. I have also tried changing the main class to jai/Run with the same result. I just have no more ideas on what could be wrong even after trawling through the many postings on this topic. Any suggestions would be reaaaallly appreciated. I am so desperate that I will definitely add Dukes to this topic when I next get some more, but at the moment I'm all out!
Thanks in advance!

Answer 1

buckman1:
you may be on to something. There IS a jar inside the jar. The purpose of the main class is to extract the files from the outside jar, and run this one. (Frustratingly, the jar inside executes perfectly when double clicked!). Can you expand on what I have to do in the manifes? However, I don't think that's my problem as the error message is "Can't find main class", so it leads me to believe that the main class doesn't even get a chance to run.
Here's the directory structure inside the jar:
/META-INF/MANIFEST.MF (on checking, this always gets changed to upper case even if you add it in lower case).
/jai/Run.class
/jai/ua.jar
/jai/data/installConfig.txt
and that's it - only small.
Below is the Run code (obviously untested as I can't get the jar to run!):
package jai;
import java.lang.Runtime;
import java.io.*;
import java.net.URL;
import java.util.*;
import java.util.jar.*;
import java.util.zip.*;
import java.security.*;
public class Run {
  public Run() {
  public static void main(String[] args) throws IOException {
    extract();
  private static URL getJar() {
    Class theClass = new Run().getClass();
    ProtectionDomain pDomain = theClass.getProtectionDomain();
    CodeSource codeSource = pDomain.getCodeSource();
    URL loc = null;
    try {
      loc = codeSource.getLocation();
    catch (Exception ex) {
      ex.printStackTrace();
    return loc;
  private static void extract() throws IOException {
    String strJarFileName = getJar().toString();
    JarFile theJar = new JarFile(strJarFileName);
    Enumeration enuJarEntries = theJar.entries();
    while (enuJarEntries.hasMoreElements()) {
      JarEntry jeFile = (JarEntry) enuJarEntries.nextElement();
      //strTempDir is the Dir where the files are to be unjarred
      File oneFile = new File(System.getProperty("java.io.tmpdir") + "/" +
                              "JAITEMP" + "/" + jeFile.toString());
      if (jeFile.isDirectory()) {
        oneFile.mkdir();
      else {
        CheckedInputStream cis = new CheckedInputStream
            (theJar.getInputStream(jeFile), new CRC32());
        FileWriter fw = new FileWriter(oneFile);
        BufferedWriter bw = new BufferedWriter(fw);
        for (int i = 0; i < jeFile.getSize(); i++) {
          bw.write(cis.read());
        bw.flush();
        bw.close();
        fw.close();
}