Main class can't be loaded

Main class can't be loaded

[phiroc]

Hello,

Let's say a file called mod1.scm contains the following definition:

(define (fun1 x y)
 (list x y))

...and has been compiled to a Java class, thus:

kawa -C mod1.scm

Why does the following message appear when you run the class?

 "the main class can't be loaded"

java -cp . mod1

Many thanks.

Phil

Re: Main class can't be loaded

[spellcard199]

Hello. First of all the java classpath needs both:
- kawa.jar
- your classes' directory

So the java command becomes:

  java -cp "/path/to/kawa.jar:." mod1

If you try to run the above command it should give another error:

  Error: Main method not found in class mod1, please define the main method as:
     public static void main(String[] args)
  or a JavaFX application class must extend javafx.application.Application

To solve this recompile using the --main flag
(https://www.gnu.org/software/kawa/Compiling.html):

  kawa --main -C mod1.scm

Now it should work.


P.S. Why we need kawa.jar in classpath:

On my OS the full error was:

  Error: Could not find or load main class mod1
  Caused by: java.lang.NoClassDefFoundError: gnu/expr/RunnableModule

gnu.expr.RunnableModule is a class in the Kawa codebase.

The output of javap on mod1.class is:

  public class mod1 extends gnu.expr.ModuleBody implements java.lang.Runnable,gnu.expr.RunnableModule {
    public static mod1 $instance;
    public final void run(gnu.mapping.CallContext);
    public static {};
    public mod1();
    public static void main(java.lang.String[]);
  }

You can see that this java code depends on several classes defined in Kawa:
- gnu.expr.ModuleBody
- gnu.expr.RunnableModule
- gnu.mapping.CallContext

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Tuesday, September 7th, 2021 at 10:23 AM, phiroc--- via Kawa <kawa@sourceware.org> wrote:

> Hello,
>
> Let's say a file called mod1.scm contains the following definition:
>
> (define (fun1 x y)
>
> (list x y))
>
> ...and has been compiled to a Java class, thus:
>
> kawa -C mod1.scm
>
> Why does the following message appear when you run the class?
>
> "the main class can't be loaded"
>
> java -cp . mod1
>
> Many thanks.
>
> Phil

Re: Main class can't be loaded

[phiroc]

Very thorough explanation, spellcard199. Many thanks.

Philippe

----- Mail original -----
De: "spellcard199" <spellcard199@protonmail.com>
À: phiroc@free.fr
Cc: kawa@sourceware.org
Envoyé: Mardi 7 Septembre 2021 12:16:08
Objet: Re: Main class can't be loaded

Hello. First of all the java classpath needs both:
- kawa.jar
- your classes' directory

So the java command becomes:

  java -cp "/path/to/kawa.jar:." mod1

If you try to run the above command it should give another error:

  Error: Main method not found in class mod1, please define the main method as:
     public static void main(String[] args)
  or a JavaFX application class must extend javafx.application.Application

To solve this recompile using the --main flag
(https://www.gnu.org/software/kawa/Compiling.html):

  kawa --main -C mod1.scm

Now it should work.


P.S. Why we need kawa.jar in classpath:

On my OS the full error was:

  Error: Could not find or load main class mod1
  Caused by: java.lang.NoClassDefFoundError: gnu/expr/RunnableModule

gnu.expr.RunnableModule is a class in the Kawa codebase.

The output of javap on mod1.class is:

  public class mod1 extends gnu.expr.ModuleBody implements java.lang.Runnable,gnu.expr.RunnableModule {
    public static mod1 $instance;
    public final void run(gnu.mapping.CallContext);
    public static {};
    public mod1();
    public static void main(java.lang.String[]);
  }

You can see that this java code depends on several classes defined in Kawa:
- gnu.expr.ModuleBody
- gnu.expr.RunnableModule
- gnu.mapping.CallContext

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Tuesday, September 7th, 2021 at 10:23 AM, phiroc--- via Kawa <kawa@sourceware.org> wrote:

> Hello,
>
> Let's say a file called mod1.scm contains the following definition:
>
> (define (fun1 x y)
>
> (list x y))
>
> ...and has been compiled to a Java class, thus:
>
> kawa -C mod1.scm
>
> Why does the following message appear when you run the class?
>
> "the main class can't be loaded"
>
> java -cp . mod1
>
> Many thanks.
>
> Phil