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Can't figure out how to run my jar file "could not find or load main class"

I'm trying to create a jar file and run it using java -cp main.jar com.test.Foo.Main but I keep getting:
Error: Could not find or load main class com.test.Foo.Main
Caused by: java.lang.ClassNotFoundException: com.test.Foo.Main
This is my file structure. So I'm thinking the line in my Main.java should be package com.test.Foo correct?
I'm compiling my Main.java with javac Main.java which outputs a Main.class file. Afterward, I create a jar file using jar cfm main.jar META-INF/MANIFEST.MF Main.class and finally while I'm in the same directory as the jar file <root>/src/com/test/Foo/ I run java -cp main.jar com.test.Foo.Main and that's when I run into the above error. Any idea how I can run this file like this (and yes I need it to run with this command specifically)?
Main.java
package com.test.Foo;
public class Main {
public static void main (String args[]) {
System.out.println("I am com.test.Foo.Main");
}
}
META-INF/MANIFEST.MF
Manifest-Version: 1.0
Main-Class: com.test.Foo.Main
I tried using some of the options given in this popular SO question and nothing helped.

The picture you're showing in your question is your project structure not your jar structure.
When you create a jar file, the structure for that jar file might be
different with your source code folder structure.
Every IDE (such as eclipse, netbeans, IntelliJ) has a mechanism for creating JAR files. In your case when you open the created jar file (using zip apps like winrar) you should see something like this :
com
|
test
|
Foo
|
Main
META-INF
|
MANIFEST.MF
This should be the ordering of your files and folders, otherwise Java can not find your main class from MANIFEST.MF
Now to solve this problem:
Open your jar file using a zip application like winrar
check the folder structure residing inside your jar file as I draw
Fix it right away within the winrar or try to correct your project structure to produce the structure I mentioned.

The class is called com.test.Foo.Main you need to specify the full name in the command:
java -cp main.jar com.test.Foo.Main
or you can use the simpler
java -jar main.jar

Check your META-INF/MANIFEST.MF for the attribute of Manifest-Version: 1.0
This attribute must be there.
Edit:
You need to move to the source root src/ and issue below command to create a valid jar.
javac com/test/Foo/*.java
and, create the jar using,
jar cmf com/test/Foo/MANIFEST.MF main.jar com/test/Foo/*.class
The thing is, package structure should match with the folder structure apparently.

Generated JAR throws ClassNotFoundException for main class

I'm using IntelliJ IDEA to create a JAR. I selected "JAR from modules with dependencies" and "extract to the target JAR" for library JARs - the generated JAR looks just fine:
myJar.jar
|
+- META-INF
| +- MANIFEST.MF
+- com
| +- my
| +- package
| +- Main.class
+- some dependencies...
I checked twice: all the needed dependencies are present. The Main-Class field in MANIFEST.MF points to the correct main class (com.my.package.Main in this example). I've opened the file in my archive tool and inspected it using
jar tf myJar.jar
and both show that the classes needed are available (also Main.class). I've left the Classpath field in IntelliJ's wizard empty, as I do not need any external libraries. However, when I issue
java -jar myJar.jar
it throws the following exception:
Error: could not find or load main class com.my.package.Main
Caused by: java.lang.ClassNotFoundException: com.my.package.Main
I've followed this guide by JetBrains
, and IntelliJ automatically added all dependencies from the Maven pom.xml to be extracted into the JAR, which is correct.
Am I missing something I should configure here and how comes that this does not work?
(Note: Generating a JAR from that project also does not work when I choose "copy to output folder and link via manifest" for the library JARs.)
(Note 2: Using maven-assembly-plugin is sadly not an option since I referenced other IntelliJ workspace modules which would then not get included.)
Update: That it should work as-is shows also the following phenominum: When I unzip my JAR and do
java -cp . com.my.package.Main
in the created directory, it works without any problems, which is strange given that Java refuses to load it...

In my case, my JAR did not work as expected due to some signed JAR dependencies.
These JARs bring a signature and key file, which also gets extracted when you embed the signed JARs. There is basically no risk in removing them, but they are one of the many possible reasons your JAR may not work.
How to remove such signatures from a JAR?
I'm describing the following steps for Linux/Darwin, but I think that there is a similar way for Windows.
Unpack you JAR.
Since JARs are nothing more than simple ZIP archives, you can use unzip:
mkdir temporaryDirectory
unzip myJar.jar -d temporaryDirectory/
The -d option is optional, but it helps keeping your directory structure clean because it sets the target directory.
Locate the signature files.
The signature files (or keys) are located in the META-INF/ directory, so change there:
cd temporaryDirectory/META-INF/
Next, we need to find the troubling files. They have the file extensions .SF (for the signature) and .DSA for the key file:
ll | grep '.DSA\|.SF'
Delete (or rename) the signature and key files.
Renaming these files bares the benefit that you can restore them later (for whatever reason), deleting them also does the trick but is a bit more risky:
Deleting:
rm signature.DSA signature.SF
# Either enter the name of the files
# instead of 'signature' or use * to delete any:
# rm *.DSA *.SF
Renaming:
rename 's/\.DSA$/.DSA.old/' * # Append ".old" to all .DSA files
rename 's/\.SF$/.SF.old/' * # Append ".old" to all .SF files
Repacking your JAR.
Still in temporaryDirectory/, we can repack our JARs to make them work:
cd ../ # If you're still in temporaryDirectory/META-INF
jar cfm ../myWorkingJar.jar ./META-INF/MANIFEST.MF -C ./ .
Explanation:
jar cfm
jar is Java's built-in JAR builder. Invoking it with c means that we want to create a JAR, f stands for the output file (which we specify next) and m is for the MANIFEST.MF we want to use. If you omit m, jar will write an empty MANIFEST.MF to the JAR.
../myWorkingJar.jar
This is the path we want to output our JAR to. It belongs to f which we specified earlier.
./META-INF/MANIFEST.MF
This is the manifest file we want to use. It belongs to the m of cfm.
-C ./
This means that our .class files are located in this directory (.).
. (last argument)
This specifies that we want to add this directory to the JAR.
If you want a detailed description of what jar is doing, you can issue the command from above with cvfm instead of cfm (v stands for verbose).
Verify that it works.
All set, now you can check that your JAR is working as intended by issuing
java -jar myWorkingJar.jar
Removing the temporary directory.
Since you're finished repairing your JAR, you can safely delete the temporary directory we created (and/or the "broken" JAR).
I've created a simple BASH script which "automates" this process a little bit:
#!/bin/bash
JARNAME=myJar.jar # enter your JAR's name here
OUT_JARNAME=myJar_out.jar # enter your output JAR's name here
# Creating the directory, unpacking the JAR, entering META-INF/
mkdir temp
unzip $JARNAME -d temp
cd temp/META-INF
# Renaming the troublemakers.
rename 's/\.DSA$/.DSA.old/' *
rename 's/\.SF$/.SF.old/' *
# Reassembling the JAR
cd ../
jar cfm ../$OUT_JARNAME ./META-INF/MANIFEST.MF -C ./ .
cd ../
# Uncomment this line if you wish to delete the temp directory upon finish.
# rm -r temp
I hope this helps people also encountering this issue.

Unable to load the jar files which are on the classpath

I am trying to run my jar file by executing java -jar myJarFile.jar arg1 but it throws ClassNotFoundException ! for the classfiles which are referred in my class files. FYI, these class files are present in some external jars which i have placed on my classpath by setting the env CLASSPATH variable pointing to the directory where all the external jars are placed. And my MANIFEST.MF would look like
Manifest-Version: 1.0
Ant-Version: Apache Ant 1.7.1
Created-By: 20.14-b01 (Sun Microsystems Inc.)
Main-Class: com.abc.test.FileGenerator
BuildDate: 01/10/2014 12:52:36
Class-Path: C:\libraries\commons-beanutils-1.8.3.jar C:\libraries\commons-collections.jar
C:\libraries\commons-discovery-0.2.jar C:\libraries\commons-lang-2.5.jar
C:\libraries\commons-logging-1.1.1.jar C:\libraries\commons-logging-1.0.4.jar
My main class is available in my jar and I am running from where my jar is getting created by ant script like dist folder. I even changed the absolute path to relative path.. It is still throwing the same error. Here is my jar structure -
myFile.jar
|- com
| |
| abc
| |
| test
| |-package1
| |-package2
| |-FileGenerator.class
|
|-META-INF | |- MANIFEST.MF
Kindly help me on this.
Thanks in advance!

You need to set the paths to all your jar files in the CLASSPATH, putting only a directory will cause jvm to look for .class files of which the packages start from that directory not .jar files.
So set your classpath something like this (windows):
set CLASSPATH=%CLASSPATH%;C:\path\to\libs\somelib1.jar;path2\to\libs\somelib2.jar
and so on.
You have then to run your program like this:
java your.main.Klass arg1
Alternatively you could add a Calss-path entry in your manifest and put all your dependencies in it:
Class-Path: libs/somelib1.jar libs/somelib3.jar
Then you can run it using the -jar option
java -jar myJarFile.jar arg1

Using -jar tells the JVM to ignore any other classpath settings (include CLASSPATH and -cp). You have to give a classpath in your manifest file at that point.

Please add the classpaths of your external jar in your manifest file like
Class-Path: abc.jar def.jar folder/ghi.jar
and then run your command java -jar myJarFile.jar arg1

Reference jars inside a jar

I have a jar whose content looks as shown below,
Below is my manifest file
Manifest-Version: 1.0
Ant-Version: Apache Ant 1.8.3
Created-By: 1.7.0_06-b24 (Oracle Corporation)
Main-Class: org.sai.com.DerbyDemo
Class-Path: derby.jar derbyclient.jar derbynet.jar derbytools.jar
When i try to run the jar, it has thrown a ClassNotFoundExcception meaning it isn't referencing the jars inside the outer jar.
In the Class-Path attribute, how can I reference jars (derby.jar, etc) inside the actual jar?

You will need a custom class loader for this, have a look at One Jar.
One-JAR lets you package a Java application together with its dependency Jars into a single executable Jar file.
It has an ant task which can simplify the building of it as well.
REFERENCE (from background)
Most developers reasonably assume that putting a dependency Jar file into their own Jar file, and adding a Class-Path attribute to the META-INF/MANIFEST will do the trick:
jarname.jar
| /META-INF
| | MANIFEST.MF
| | Main-Class: com.mydomain.mypackage.Main
| | Class-Path: commons-logging.jar
| /com/mydomain/mypackage
| | Main.class
| commons-logging.jar
Unfortunately this is does not work. The Java Launcher$AppClassLoader does not know how to load classes from a Jar inside a Jar with this kind of Class-Path. Trying to use jar:file:jarname.jar!/commons-logging.jar also leads down a dead-end. This approach will only work if you install (i.e. scatter) the supporting Jar files into the directory where the jarname.jar file is installed.

You can't. From the official tutorial:
By using the Class-Path header in the manifest, you can avoid having
to specify a long -classpath flag when invoking Java to run the your
application.
Note: The Class-Path header points to classes or JAR files on the
local network, not JAR files within the JAR file or classes accessible
over internet protocols. To load classes in JAR files within a JAR
file into the class path, you must write custom code to load those
classes. For example, if MyJar.jar contains another JAR file called
MyUtils.jar, you cannot use the Class-Path header in MyJar.jar's
manifest to load classes in MyUtils.jar into the class path.

In Eclipse you have option to export executable jar.
You have an option to package all project related jars into generated jar and in this way eclipse add custom class loader which will refer to you integrated jars within new jar.

Default implementations of the classloader cannot load from a jar-within-a-jar: in order to do so, the entire 'sub-jar' would have to be loaded into memory, which defeats the random-access benefits of the jar format (reference pending - I'll make an edit once I find the documentation supporting this).
I recommend using a program such as JarSplice to bundle everything for you into one clean executable jar.
Edit: Couldn't find the source reference, but here's an un-resolved RFE off the Sun website describing this exact 'problem': http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4648386
Also, you could 'test' that your program works by placing the library jar files in a \lib sub-directory of your classes directory, then running from the command line. In other words, with the following directory structure:
classes/org/sai/com/DerbyDemo.class
classes/org/sai/com/OtherClassFiles.class
classes/lib/derby.jar
classes/lib/derbyclient.jar
From the command line, navigate to the above-mentioned 'classes' directory, and type:
java -cp .:lib/* org.sai.com.DerbyDemo

if you do not want to create a custom class loader. You can read the jar file stream. And transfer it to a File object. Then you can get the url of the File. Send it to the URLClassLoader, you can load the jar file as you want.
sample:
InputStream resourceAsStream = this.getClass().getClassLoader().getResourceAsStream("example"+ ".jar");
final File tempFile = File.createTempFile("temp", ".jar");
tempFile.deleteOnExit(); // you can delete the temp file or not
try (FileOutputStream out = new FileOutputStream(tempFile)) {
IOUtils.copy(resourceAsStream, out);
}
IOUtils.closeQuietly(resourceAsStream);
URL url = tempFile.toURI().toURL();
URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{url});
urlClassLoader.loadClass()
...

Add the jar files to your library(if using netbeans) and modify your manifest's file classpath as follows:
Class-Path: lib/derby.jar lib/derbyclient.jar lib/derbynet.jar lib/derbytools.jar
a similar answer exists here

in eclipse, right click project, select RunAs -> Run Configuration and save your run configuration, this will be used when you next export as Runnable JARs

Cannot find class even when jar file is in working directory

I am struggling to get my Java program to run on AIX. I used Eclipse on Windows to create a runnable Jar file, jRams.jar below. I kept on getting a class not found error, until finally I put all the external libraries in the same directory.
$ ls
JAXB2_20081030.jar
JAXB2_20110601.jar
activation.jar
asjava.jar
commons-beanutils-1.8.3.jar
commons-beanutils-bean-collections-1.8.3.jar
commons-beanutils-core-1.8.3.jar
commons-codec-1.5.jar
commons-collections-3.2.1.jar
commons-configuration-1.6.jar
commons-digester-2.1.jar
commons-jxpath-1.3.jar
commons-lang-2.6.jar
commons-logging-1.1.1.jar
commons-logging-adapters-1.1.1.jar
commons-logging-api-1.1.1.jar
jRams.jar
jaxb-api.jar
jaxb-impl.jar
jaxb-xjc.jar
jaxb1-impl.jar
jremote.jar
jsr173_1.0_api.jar
log4j-1.2.16.jar
netty-3.2.4.Final.jar
$
Still, I get the class not found error.
$ java -jar jRams.jar
The java class is not found: com.jbase.jremote.JRemoteException
jremote.jar definitely contains JRemoteException. Why isn't this working?
UPDATE
Thank you for your straight-to-the-point answers. I now understand the nature of a java application and a manifest file far better.
Turns out my ftp client was transferring in ASCII mode and not Binary, so the jar files were corrupt. I have learned a great deal, nonetheless.

When using the -jar option, you need to specify which jar-files should be on your class path in the manifest file. Just having the required jar files in the same directory won't do it.
Add a line in your manifest that says:
Class-Path: JAXB2_20081030.jar:JAXB2_20110601.jar:....:netty-3.2.4.Final.jar
or skip the -jar option and launch using
java -cp JAXB2_20081030.jar:....:netty-3.2.4.Final.jar:jRams.jar pkg.JRamsMain
and it should work fine.
(Note that on *nix systems, as opposed to Windows machines, the jar files in the class paths should be separated using : instead of ;.)
Further reading:
The Java Tutorials: Adding Classes to the JAR File's Classpath

You need to add all those JARs to the runtime CLASSPATH by adding the -classpath parameter. AIX requires you to separate the JARs using :

You will have to specify the full path(if libraries not in the same directory as jRams) or just the names of the jar file in a manifest file (If all dependency jars are in the same folder). Alternative specify the path to all the dependent jars using -cp argument.
Example (This assume every dependency is in the same directory you are executing java command from):
java -cp commons-collections-3.2.1.jar; jaxb-impl.jar; etc.. ;jRams.jar package_to_class.MyMainClass.java
Where package_to_class is example: com.myproj.example.
EDITED.

Follow these steps to add "Class-Path" to existing jar file -
Create "newmanifest" file with following entry
Class-Path: additional/jars
Update existing jar file e.g. "classes.jar"
jar --update --manifest=newmanifest --file classes.jar
Inflate jar file
jar -xvf classes.jar
created: META-INF/
inflated: META-INF/MANIFEST.MF
Verify "Class-Path" is added to MANIFEST.MF
cat META-INF/MANIFEST.MF
Manifest-Version: 1.0
main-class: CLASSNAME
Created-By: 15.0.2 (Oracle Corporation)
Class-Path: additional/jars