Source file launcher - Handling of pre-compiled classes of the source file

[Jaikiran Pai]

Please consider this trivial code C.java:

public class C {
    public static void main(String[] args) throws Exception {
        System.out.println("main() execution started");
    }
}


> ls
C.java

Similar to a previous discussion[1] while doing random testing, I ended
up compiling C.java explicitly using javac:

> javac C.java
> ls
C.java C.class

and then at a later date tried to use the source file launcher feature
of Java 11 (without realizing C.class was already present in the dir):

> java C.java

This threw the error:

error: class found on application class path: C

Although the error isn't that clear for the reason I note in [2], having
run into this before, I was aware what this meant and deleted the
C.class and moved on. The important part here is that the source
launcher noticed this condition and aborted even before it auto
compiled(?) and launched and executed the main() of the program.

Now consider a slight modification to that source file:

public class C {
    public static void main(String[] args) throws Exception {
        System.out.println("main() execution started");
        final B b = new B();
        System.out.println("Done");
    }

    private static class B {

    }
}

Again at some point I compiled this explicitly using javac, so my
directory is (among other things):

> ls
C$B.class    C.class        C.java

Then ran the source file launcher feature:

> java C.java
error: class found on application class path: C

As expected, ran into the same previous error. As before, in order to
move on, deleted C.class:

> rm C.class

but forgot to delete the nested static class that belonged to it. So the
directory now contained:

> ls
C$B.class    C.java

Now used the source launcher feature again:

> java C.java

This time it failed with:

main() execution started
Exception in thread "main" java.lang.IllegalAccessError: failed to
access class C$B from class C (C$B is in unnamed module of loader 'app';
C is in unnamed module of loader
com.sun.tools.javac.launcher.Main$MemoryClassLoader @1b1473ab)
    at C.main(C.java:4)

The error message isn't clear to pinpoint the issue, but at least the
reference to MemoryClassLoader was a hint that was enough for me to
understand where to look. It did take me a few minutes to realize that
C$B.class was lying around which I needed to remove too.

However, IMO, the important part here is that unlike in the first case
where the program itself wasn't launched and instead was aborted early,
in this case the program did get executed (notice the System.out.println
"main() execution started" message that got printed) and failed at runtime.

Would it be possible to make these two behaviours consistent and detect
such cases and abort early here too? Or would that add too much
complexity to this feature?

Finally, any thoughts on the error messages for this feature to make it
a bit easier in terms of debugging (classloading) issues like these?

[1] http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001425.html
[2] http://mail.openjdk.java.net/pipermail/jdk-dev/2018-June/001438.html

-Jaikiran