Unnamed classes (was: Paving the on-ramp)

[Dan Heidinga]

On Wed, Oct 19, 2022 at 6:59 PM John Rose <john.r.rose at oracle.com> wrote:

> On 19 Oct 2022, at 13:35, Brian Goetz wrote:
>
> Let's pull on this string.
>
> When we say `javac Foo.java`, the compiler has to create a class file, and
> doesn't have the benefit of a declared class name. The logical output file
> is `Foo.class`, because otherwise the next thing the user is likely to do
> is `java Foo`, and the class loader is going to look for Foo.class.
>
> I guess what I’m suggesting here is that what’s new (or what *should* be
> new) is a full removal of all coupling between the file name (of the .class
> file) and the class’s bytecode name (in the ClassFile structure). VM
> anonymous classes (VMACs) are the precedent I’m looking at; IIRC only the
> package prefix of a VMAC is significant to any VM operation. (With minor
> exceptions: Reflection and stack traces probably report a string that
> depends on the bytecode name.)
>
> It seems reasonable to steer towards such a decoupling, because it would
> be (a) similar to what we have with VMACs, but with an accidental inclusion
> of a file system container *of an irrelevant name*; it would also be (b)
> useful to put zero constraints on the classfile name (other than a .class
> suffix perhaps) so that tools have fewer irrelevant details to worry about.
>
> A .class file has a ClassFile structure, which has a `this_class` field
> which names the current class.
>
> Names it for what purposes? For a nameless class, the only purposes are
> informative as with VMACs. I think that not even Class.forName should be
> able to recover the current class, only this.getClass(). Maybe that’s too
> much of a reach?
>
> We experimented with calling the class something like `$Foo` or $Unnamed,
> but this trick just garners a NoClassDefFoundError, with reason "wrong
> name".  This error comes from the native method `ClassLoader::defineClass1`.
>
> That’s a superficial problem. We are defining a new path for loading
> classes, so we have a right to adjust the rules.
>
> OTOH, and alternatively, and more in line with the gradual on-ramp: We
> could insist that, no, we are just defining an easier way to define a
> regular old class; we take care of issuing the name for you. Which is less
> surprising, that a named class you didn’t declare pops up somewhere and
> with a name vaguely like the filename, but is “just a class you could have
> coded”, or that, if there is no name, none of the name-related operations
> work.
>
> I guess part of the issue is the anonymous classes (of various sorts) are
> two things at once: 1. a really obscure power-user feature, and 2. a
> desirable default early on the onramp. Having it both ways causes tension.
>
> If we embrace “it is anonymous” I think we get the cleanest experience.
> People won’t be tempted to predict the name and refer to it somehow (with
> reflection or even source code).
>
> If we embrace “it has a name you didn’t pick” I think we get a simpler
> onramp story, but at the cost of dark corners in the UE. Users will
> immediately try to exploit the fact that the name is semi-predictable, and
> write code that works with the names of these classes.
>
After reading this (and the other thread), I'm more firmly convinced that
using the file name is the right answer due to that "simpler onramp
story".  It allows the unnamed class to have a stable name which enables
growing it to have proper constructors, referencing it from other classes,
etc.  Users trying to predict are learning more advanced features and will
be ready to upgrade their classes to have explicit names.  It becomes a
much smaller step to then add the opening "class Foo {"  & closing "}".

If the goal is an easier on ramp, guiding them to understand the connection
between Class names and file names (though not 100%) will help them
navigate other source bases as well.  This seems like a pure win.

Treating it as anonymous doesn't help users to take that next step.

--Dan

> With inner classes, we've taken the position that class names with $ in
> their name are likely to be unstable names not to be counted on.  So
> calling it $Foo sends that signal, good.  But we'd have to be willing to
> loosen the checking in the class loader to allow loading a class with a
> slightly mangled name such as $Unnamed (and then make the launcher more
> tolerant of that.)
>
> In the end, I’m totally willing to do this.
>
> Idea: Have the launcher not call defineClass at all, but rather take the
> byte image of the *.class file, and run it into the VM as a VMAC. That is a
> principled position that will prevent lots of nonsense about secret names.
>
> — John
>
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