java eclipse jar executable-jar runnable-jar

Why does my manually built runnable jar respond with "Error: Could not find or load main class" when the class exists?

This is one of those error messages that comes up a lot, but none of the question/answers address this particular situation.

I am creating a runnable jar from the contents of an Eclipse project.
When I use the Eclipse IDE to create a runnable jar, it works.
When I use java.util.zip.* functions to create the runnable jar, it does not work.
When I compare the two files in Windows explorer they are identical.
When I extract the zip files and compare the resulting contents on disk (Using WinMerge), they are identical.

Why would one file work, and the other, seemingly identical file not work?

Here's a snip of the code that I'm using. Not the full implementation, just the interesting bit:

    ZipInputStream zin = new ZipInputStream(new FileInputStream(tempFile));
    ZipOutputStream out = new ZipOutputStream(new FileOutputStream(zipFile));
    for (int i = 0; i < fileList.length; i++) {
        InputStream in = new FileInputStream(fileList[i]);
        String absoluteFileNameString = fileList[i].getAbsolutePath();
        if (baseDirectory != null && baseDirectory.length() > 0 && absoluteFileNameString.length() > (baseDirectory.length() + 1) ) {
            absoluteFileNameString = absoluteFileNameString.substring(baseDirectory.length() + 1);
        }
        out.putNextEntry(new ZipEntry(absoluteFileNameString));
        int len;
        while ((len = in.read(buf)) > 0) {
            out.write(buf, 0, len);
        }
        out.closeEntry();
        in.close();
    }
    out.close();

Here is the output showing that one works and the other doesn't:

C:\Users\Owner\git\runnablejarbuilder\RunnableJarBuilder>java -jar RunnableJarBuilder.zip
Error: Could not find or load main class com.rjb.test.HelloRun
Caused by: java.lang.ClassNotFoundException: com.rjb.test.HelloRun

C:\Users\Owner\git\runnablejarbuilder\RunnableJarBuilder>java -jar RunnableJarBuilderIDE.zip
Hello Runnable Jar!

Below is a screen shot (the only way to capture what Windows explorer was telling me: the files were exactly duplicates of each other):

Solution

After analyzing the zip files using zipdump, I realized my code was using backslashes (\) as a path separator, but ZipEntry expected a forward slash (/). This was my non-working file:

C:\Users\Owner\git\runnablejarbuilder\RunnableJarBuilder\zipdump>C:\Users\Owner\AppData\Local\Programs\Python\Python38-32\python.exe zipdump.py RunnableJarBuilder.zip
        75 ( 97.3%)  2024-11-18 12:27:10  106e3e9c [NOLFH] META-INF/MANIFEST.MF
      7831 ( 48.9%)  2024-11-18 12:27:10  a30be143 [NOLFH] com\rjb\test\BuildJarFromClassFiles.class
      4601 ( 53.4%)  2024-11-18 12:27:10  0742ac06 [NOLFH] com\rjb\test\ClasspathReader.class
       561 ( 62.2%)  2024-11-18 12:27:10  d38480b3 [NOLFH] com\rjb\test\HelloRun.class
      6525 ( 50.6%)  2024-11-18 12:27:10  364411d0 [NOLFH] com\rjb\test\JarUpdater.class

And this was the working one, which the IDE created:

C:\Users\Owner\git\runnablejarbuilder\RunnableJarBuilder\zipdump>C:\Users\Owner\AppData\Local\Programs\Python\Python38-32\python.exe zipdump.py RunnableJarBuilderIDE.zip
        75 ( 97.3%)  2024-11-18 12:25:54  106e3e9c [NOLFH] META-INF/MANIFEST.MF
      4601 ( 53.4%)  2024-11-18 10:44:42  0742ac06 [NOLFH] com/rjb/test/ClasspathReader.class
      7831 ( 48.9%)  2024-11-18 12:23:00  a30be143 [NOLFH] com/rjb/test/BuildJarFromClassFiles.class
       561 ( 62.2%)  2024-11-18 09:02:48  d38480b3 [NOLFH] com/rjb/test/HelloRun.class
      6525 ( 50.6%)  2024-11-18 09:46:12  364411d0 [NOLFH] com/rjb/test/JarUpdater.class

I fixed the problem by replacing all the backslashes with forward slashes in the path name:

//REPLACE THIS LINE
//String absoluteFileNameString = fileList[i].getAbsolutePath();

//WITH THIS LINE - Never use backslashes in zip file entries!!
//String absoluteFileNameString = fileList[i].getAbsolutePath().replace("\\","/");
// or better, suggested by DuncG
String absoluteFileNameString = fileList[i].getAbsolutePath().replace(File.separatorChar,"/");

This might come up on Windows especially, and may be less of a problem on Linux.