Thread: Cannot find class in .class or .jar file

Dear community,

One of the frustrating problems I find with programming is when the compiler or runtime environment can’t find stuff when the location to me is blatantly obvious.

I have followed an introductory course on setting up a Java development environment at Pluralsight, using IntelliJ. At the end was explained in two ways how to create a jar file that is supposed to work stand-alone. The exercise project contains a class called ‘Main’.

The first method was using the command prompt, but it fails because the runtime environment can’t find or load the class called ‘Main’.
The second way was using IntelliJ, but it fails for the same reason.

So I created a new, simpler project, called HelloWorld, with only two files, with also a class called ‘Main’. It fails for the same reason.

Then I tried some exercise on webeducator (that I can't link to for some reason). It contains only a single file and it actually works, despite the erroneous image in step 4.

In my HelloWorld project I have two classes :

package com.example.helloworld;

public class Main {
 
public static void main(String[] args) {
    HelloWorldCreator hello = new HelloWorldCreator() ;
 
    hello.giveGreeting() ;
}
}

and

package com.example.helloworld;
 
public class HelloWorldCreator {
    String greeting = "Greetings" ;
    String addressee = "World";
 
    public void setGreeting(String greeting) {
        this.greeting = greeting ;
    }
 
    public void setAddressee(String addressee) {
        this.addressee = addressee ;
    }
 
    public void giveGreeting() {
        System.out.printf("%s, %s!", greeting, addressee) ;
    }
}

I managed to convert these to .class files using the instruction
javac Main.java HelloWorldCreator.java
in the command prompt.

Issuing the instruction
java Main
in the pertinent folder returns
Error: Could not find or load main class Main
Caused by: java.lang.NoClassDefFoundError: com/example/helloworld/Main (wrong name: Main)

How can explain to the JRE where to find that class ?

Thanks for your help.

The folder in which I tried to run the classes is :
C:\Users\Knarf\IdeaProjects\HelloWorld\src\com\exa mple\helloworld
That is where they are located.

Indeed. Moving 3 levels up and instructing
java com.example.helloworld.Main
works.

So the JRE is too stupid to look in the folder where it is being executed.
However, in order to turn those class files into jar files, the JRE needs to run in the helloworld folder.

Running the resulting jar file must again be done from src folder.


I tried the same exercise again, the HelloWorld2 project, with the same code, except for omitting the line
package com.example.helloworld;
from each class and not putting the classes in a package.

That way the JRE can find the Main class (created with IntelliJ) without the extra hassle.

I make jar files with the commands
jar -cvf HelloWorldCreator.jar .
and
jar -cvf Main.jar .

These jar files can simply be run from the pertinent folder. No package gymnastics required.

I am correct in understanding that for such simple programs packaging is merely a nuisance, but that in complicated programs the benefits outway the drawbacks?

Apparently my JRE is not installed correctly because when I double-click a jar file Windows asks me what I would like to open it with.

The Pluralsight course also explained how one is supposed to tell the JRE where the main class is within the jar file.

The location in command prompt is
C:\Users\Knarf\Documents\School\Informatica\Softwa re Testing\IdeaProjects\HelloWorld2\out\production\He lloWorld2\MyPackage>

So I create a file MANIFEST.MF with
Main-Class: Mypackage.Main
(Allegedly one needs to end that line with a new line for it to work.)

Then I instruct
jar -cvmf MANIFEST.MF HelloWorldCreator.jar .

Running HelloWorldCreator.jar that yields
Error: Could not find or load main class Mypackage.Main
Caused by: java.lang.ClassNotFoundException: Mypackage.Main

The content of my jar file is
META-INF/
META-INF/MANIFEST.MF
HelloWorldCreator.class
MANIFEST.MF
Main.class

How can I explain inside the jar file where to find the main class ?

This is the code from the Main class :

package MyPackage;
 
public class Main {
 
    public static void main(String[] args) {
        HelloWorldCreator hello = new HelloWorldCreator();
 
        hello.giveGreeting();
    }
}

I corrected the typo in the MANIFEST.MF file. It now contains
Main-Class: MyPackage.Main

Instructing
java -jar HelloWorldCreator.jar

again results in
Error: Could not find or load main class MyPackage.Main
Caused by: java.lang.ClassNotFoundException: MyPackage.Main

I still don’t know how to create a working standalone jar-file from my simple program using the JDK alone. Apparently that is overly complicated if you are not doing that frequently.

So, I try it with IntelliJ. I try to follow the instructions on http://www.jetbrains.com/help/idea/c...n_packaged_jar and lo and behold, the HelloWorld2.jar created that way actually works.

The content of the jar is
META-INF/MANIFEST.MF
META-INF/
MyPackage/
MyPackage/HelloWorldCreator.class
MyPackage/Main.class

That is different than what was created by the JDK (see post 5). There is probably a way to convince the JDK to create such jar file, but I don’t know how.

I have still the commend prompt window open, so I can copy the instruction I gave :
C:\Users\Knarf\Documents\School\Informatica\Softwa re Testing\IdeaProjects\HelloWorld2\out\production\He lloWorld2>java jar HelloWorld.jar

Response :
Error: Could not find or load main class jar
Caused by: java.lang.ClassNotFoundException: jar

The contents of the HelloWorld2 folder is
29/08/2022 19:02 <DIR> .
29/08/2022 19:02 <DIR> ..
29/08/2022 19:02 1.456 HelloWorld.jar
29/08/2022 18:12 26 MANIFEST.MF
29/08/2022 11:09 <DIR> META-INF
29/08/2022 19:02 <DIR> MyPackage

OK, so I instructed
C:\Users\Knarf\Documents\School\Informatica\Softwa re Testing\IdeaProjects\HelloWorld2>java -jar HelloWorld2.jar

Response :
Error: Unable to access jarfile HelloWorld2.jar

I tried adding MyPackage\ and MyPackage. in front of HelloWorld2.jar, to no avail

HelloWorld2.jar is in C:\Users\Knarf\Documents\School\Informatica\Softwa re Testing\IdeaProjects\HelloWorld2\out\artifacts\Hel loWorld2_jar

Running it there works, but that is the version produced by IntelliJ

The version in C:\Users\Knarf\Documents\School\Informatica\Softwa re Testing\IdeaProjects\HelloWorld2\out\production\He lloWorld2 doesn’t work :
no main manifest attribute, in HelloWorld.jar

That folder contains
29/08/2022 19:02 <DIR> .
29/08/2022 19:02 <DIR> ..
29/08/2022 19:02 1.456 HelloWorld.jar
29/08/2022 18:12 26 MANIFEST.MF
29/08/2022 11:09 <DIR> META-INF
29/08/2022 19:02 <DIR> MyPackage

HelloWorld.jar contains
META-INF/
META-INF/MANIFEST.MF
MyPackage/HelloWorldCreator.class
MyPackage/Main.class

OK. Thanks for your help.