Re: 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