Package, import and type declarations are allowed now in module-info.java by spec

[Alex Buckley]

On 2/26/2016 8:37 AM, Georgiy Rakov wrote:
> current spec [1] now contains following assertions related to grammar:
>
>     A compilation unit (JLS 7.3) may contain a module declaration, in
>     which case the filename of the compilation unit is typically
>     |module-info.java|.
>
>     CompilationUnit:
>        [PackageDeclaration] {ImportDeclaration} {TypeDeclaration}
>        ModuleDeclaration
>
> These assertions allows to specify any of import, package or type
> declarations in any compilation unit, for instance module-info.java is
> allowed to contain any of the mentioned declarations. However currently
> javac in the latest jigsaw build [2] reports an error on such cases
> provided they are compiled in module mode. For example if we have
> following directory structure:
>
>     mod\module-info.java:
>     module mod {
>          exports pkg;
>     }
>
>     mod\pkg\module-info.java:
>     package pkg;
>
>     class C {
>     }
>
> then compiling it by following command line with javac from [2]:
>
>     javac -modulesourcepath . mod\module-info.java mod\pkg\module-info.java
>
> causes following output:
>
>     mod\pkg\module-info.java:1: error: expected 'module'
>     package pkg;
>     ^
>     1 error

javac is merely choosing to implement the rule at the end of JLS 7.6 
that a type declaration (optionally preceded by package/import 
declarations) must be provided in a suitably named file.

Perhaps I should say "a variant of the rule" because 7.6 as written 
concerns a public type and your example has a package-access type. 
Still, bottom line, javac is free to require that a compilation unit 
which starts with a package declaration _must not_ be in a file called 
foo-bar.java -- the hyphen indicates a name that can't possibly align 
with the type declared in the compilation unit.

The error message for mod\pkg\module-info.java could be a bit more 
helpful, but that's a quality-of-implementation detail.

Conversely, a compilation unit that contains a module declaration _may_ 
be in a file called module-info.java, or in a file called foo-bar.java, 
or in a file called mod_decl.JAV. The "typically" in [1] is meant to 
indicate that the sub-clause on filename is non-normative. This is akin 
to how a compilation unit that contains a package-access type 
declaration for class C _may_ be in a file D.java.

Alex