RFR: 8335989: Implement Module Import Declarations (Second Preview)

[Alan Bateman]

On Wed, 9 Oct 2024 19:22:01 GMT, Jan Lahoda <jlahoda at openjdk.org> wrote:

> This is a current patch for module imports declarations, second preview. At least the JEP number and preview revision will need to be updated in `jdk.internal.javac.PreviewFeature.Feature`, but otherwise I believe this is ready to receive feedback.
> 
> The main changes are:
> - `requires transitive java.base;` is permitted, under the preview flag. Both javac and the runtime module system are updated to accept this directive when preview is enabled.
> - the `java.se` module is using `requires transitive java.base;`, and is deemed to be participating in preview, so its classfile version is not tainted. Runtime is updated to access `requires transitive java.base;` in any `java.*`, considering all of them to be participating in preview. Can be tighten up to only `java.se` is desired.
> - the types imported through module imports can be shadowed using on-demand imports. So, for example, having:
> 
> import module java.base;
> import module java.desktop;
> ...
> List l;//ambigous
> 
> but:
> 
> import module java.base;
> import module java.desktop;
> import java.util.*;
> ...
> List l;//not ambigous, reference to java.util.List

Can you check if test/jdk/java/lang/module/ClassFileVersionsTest.java should be updated too?

src/java.base/share/classes/jdk/internal/module/ModuleInfo.java line 193:

> 191:         int minor_version = in.readUnsignedShort();
> 192:         int major_version = in.readUnsignedShort();
> 193:         boolean previewClassfile = minor_version == ClassFile.PREVIEW_MINOR_VERSION;

If you rename this to isPreview then it would make the uses further down easier to read.

src/java.base/share/classes/jdk/internal/module/ModuleInfo.java line 417:

> 415:                          //java.* modules are deemed participating in preview
> 416:                          //and are allowed to use requires transitive java.base:
> 417:                          !mn.startsWith("java."))

A block comment before the `if` statement would be a lot clearer here.

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PR Comment: https://git.openjdk.org/jdk/pull/21431#issuecomment-2404032481
PR Review Comment: https://git.openjdk.org/jdk/pull/21431#discussion_r1794661012
PR Review Comment: https://git.openjdk.org/jdk/pull/21431#discussion_r1794662049