"Cannot find main class" error when creating a .jar!

**davetheant** · 11-07-2011, 02:27 AM

Hello. I have a class called GUI.java and (of course) it contains a main method and I would like to make it a jar file for execution. I've tried 100 times with different options, tried using eclipse to do it for me, tried using the command line and making my own manifest file, everything. Whenever I double click the jar I created, it says "Cannot find main class: GUI. Program will now exit". What can I do??

**davetheant** · 11-08-2011, 01:20 PM

bump.. no responses yet??

**FallenBlade** · 11-08-2011, 02:13 PM

Hi, have you tried calling the Jar from command line?

Java Code:

java -jar FileName.jar

**tnrh1** · 11-08-2011, 03:26 PM

When you export your project as a jar in the last step it asks you to specify the main class.
Do that!

**Norm** · 11-08-2011, 03:40 PM

Check that the jar file contains a manifest file with a Main-Class: entry that has the correct path to your class.

**davetheant** · 11-08-2011, 06:42 PM

I specified the Main class in Eclipse and my manifest file (from when I was trying to create the jar myself) contained Main-Class: GUI with two carriage returns (enter key) afterwards.

**Norm** · 11-08-2011, 06:56 PM

Is the correctly formatted manifest file in the jar file?

**davetheant** · 11-09-2011, 12:32 AM

Yes it is in the jar file when I open it with 7-zip.

**Norm** · 11-09-2011, 12:48 AM

If the manifest is in the jar and is in correct format, then the java command should start the class named by Main-Class.
If it does not, then something is wrong and you need to check again.

Open a command prompt, change to the folder with the jar file and enter:
java -jar <YOURJARFILENAMEHERE>.jar
Copy the console contents here.
To copy the contents of the command prompt window:
Click on Icon in upper left corner
Select Edit
Select 'Select All' - The selection will show
Click in upper left again
Select Edit and click 'Copy'

Paste here.

**UJJAL DHAR** · 11-09-2011, 04:20 AM

OP,You can try this.

Originally Posted by Norm

If the manifest is in the jar and is in correct format, then the java command should start the class named by Main-Class.
If it does not, then something is wrong and you need to check again.

Open a command prompt, change to the folder with the jar file and enter:
java -jar <YOURJARFILENAMEHERE>.jar
Copy the console contents here.
To copy the contents of the command prompt window:
Click on Icon in upper left corner
Select Edit
Select 'Select All' - The selection will show
Click in upper left again
Select Edit and click 'Copy'

Paste here.

Or, you can try this.
JAVA [creating Jar files] « Me, when programming

As you have created JAR file you should just try the

Ways to update a jar file with manifest……

portion.

**davetheant** · 11-09-2011, 02:35 PM

The strangest thing: when I opened a command window, went to my directory and typed java -jar G.jar (G.jar is the name of my jar file), it ran my jar! But whenever I double click it I get the "Cannot find Main Class GUI.java"... Anything?

**Norm** · 11-09-2011, 02:42 PM

I get the "Cannot find Main Class GUI.java".

That message is different from the one you posted earlier.

"Cannot find main class: GUI. Program will now exit".

Are you sure it is complete and correct?

Can you look in the Windows registry and see what the command line is for the jar files?

**UJJAL DHAR** · 11-09-2011, 04:21 PM

Originally Posted by Norm

That message is different from the one you posted earlier.

Are you sure it is complete and correct?

Can you look in the Windows registry and see what the command line is for the jar files?

@OP
I have faced the same problem. I use XP,Windows7 and ubuntu. My jar files works on xp and ubuntu fine. But in W7 I got the message "Cannot find Main Class". Try your JAR files on another pc. And You can change OS's default file type.Make sure that when double clicking It is opened by javaw.exe

**Norm** · 11-09-2011, 04:30 PM

I have several different versions of java on my machine. I get different results when I try to open a jar file that does not have a manifest file with different versions of java.
One gives me this error window:

**davetheant** · 11-09-2011, 05:22 PM

The first error I posted was correct. I wrote the second one from memory, sorry. I'm running windows 7 x64 so this must be the issue. Thanks everyone

**Norm** · 11-09-2011, 05:27 PM

What is the full text of the error message you get?
Here's what I get when I leave out the -jar option:

Microsoft Windows XP [Version 5.1.2600]

D:\JavaDevelopment>java acme.jar
Exception in thread "main" java.lang.NoClassDefFoundError: acme/jar
Caused by: java.lang.ClassNotFoundException: acme.jar
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: acme.jar. Program will exit.

**davetheant** · 11-10-2011, 02:43 AM

It's nothing like that. A window comes up saying "Cannot find main class: GUI. Program will now exit" and that's all. It works from the command line and works on my windows vista desktop.

**Norm** · 11-10-2011, 12:20 PM

Strange that the message wording is different:
Could not vs Cannot
will exit vs will now exit

Can you screen print the message window and post it here?