error trying to execute a modular java jar file

Issue

I have a Java 11 Modular program that runs fine from the Command prompt. However, when I package it in a Jar file (Jar 11) and try to run it again I am getting "*Error: Could not find or load main class farm.vet.VetMain in module farm.vet*".

I have enclosed an image showing the various commands I have executed from the command line. The first line shows the program running normally (no jar used). There are a few modules involved, farm.vet is the starting point with farm.vet.VetMain containing `main()`.

I create the Jar file - it includes the module-info files for the various modules. However when I try to run it afterwards - that's when I get the error. I then describe the Jar file and am concerned about "No module descriptor". I believe I should have something at the top of the Modular Jar file - is that the reason for my error in running the Jar?

I have also included my directory structure ('out' is the parent directory for all my class files).

Any help greatly appreciated,
Seán.

java -p out -m farm.vet/farm.vet.VetMain

jar -cvf mods/farm.vet.jar -C out/ .
added manifest
adding: farm.animals/(in = 0) (out= 0)(stored 0%)
adding: farm.animals/farm/(in = 0) (out= 0)(stored 0%)
adding: farm.animals/farm/Animal.class(in = 360) (out= 246)(deflated 31%)
adding: farm.animals/farm/cattle/(in = 0) (out= 0)(stored 0%)
adding: farm.animals/farm/cattle/Cattle.class(in = 829) (out= 475)(deflated 42%)
adding: farm.animals/farm/cattle/Cow.class(in = 999) (out= 548)(deflated 45%)
adding: farm.animals/module-info.class(in = 195) (out= 149)(deflated 23%)
adding: farm.owner/(in = 0) (out= 0)(stored 0%)
adding: farm.owner/farm/(in = 0) (out= 0)(stored 0%)
adding: farm.owner/farm/owner/(in = 0) (out= 0)(stored 0%)
adding: farm.owner/farm/owner/Owner.class(in = 1569) (out= 853)(deflated 45%)
adding: farm.owner/module-info.class(in = 200) (out= 148)(deflated 26%)
adding: farm.vet/(in = 0) (out= 0)(stored 0%)
adding: farm.vet/farm/(in = 0) (out= 0)(stored 0%)
adding: farm.vet/farm/vet/(in = 0) (out= 0)(stored 0%)
adding: farm.vet/farm/vet/VetMain.class(in = 710) (out= 448)(deflated 36%)
adding: farm.vet/module-info.class(in = 194) (out= 143)(deflated 26%)

java -p mods -m farm.vet/farm.vet.VetMain
Error: Could not find or load main class farm.vet.VetMain in module farm.vet

jar --describe-module --file mods/farm.vet.jar
No module descriptor found. Derived automatic module.
farm.vet automatic
requires java.base mandated
contains farm.animals
contains farm.animals.farm
contains farm.animals.farm.cattle
contains farm.owner
contains farm.owner.farm.owner
contains farm.vet
contains farm.vet.farm.vet

Solution

There can only be one module per JAR file, since module-info.class must be in the root of the JAR file.

Try:

jar -cvf mods/farm.vet.jar -C out/farm.vet/ .
jar -cvf mods/farm.owner.jar -C out/farm.owner/ .
jar -cvf mods/farm.animals.jar -C out/farm.animals/ .

Based on Project Jigsaw: Module System Quick-Start Guide:

A modular JAR is a regular JAR file that has a module-info.class in its top-level directory.

and The jar Command description:

An archive becomes a modular JAR when you include a module descriptor, module-info.class, in the root of the given directories or in the root of the .jar archive.

^{I got a bit confused by "root of the given diretories" being mentioned above}

Answered By - user16320675