On Thu, 5 Mar 2026 10:39:02 GMT, Jan Lahoda <[email protected]> wrote:
> There's an enum
> `src/java.base/share/classes/jdk/internal/javac/PreviewFeature.java#PreviewFeature.Feature`
> that defines the current preview features, in particular those that have an
> API. The API is then marked with the appropriate enum constant.
>
> There's a problem with this enum: javac's APIs may also need to use constants
> from the enum, and as javac is built twice, once with the bootstrap JDK to
> get the so called "interim javac" (which is then used to compile nearly all
> the rest of the JDK), and second time to get the final version of javac, this
> enum needs to be dragged into the interim compilation as well.
>
> As a consequence, the bootstrap JDK classfiles are resolved against this
> up-to-date feature enum. And if any of the bootstrap JDK classfiles would
> refer to an enum constant that is not part of this up-to-date feature enum,
> javac would report a warning, failing the build.
>
> As a consequence, the guidance is that when a preview feature ceases to be
> preview (either by maturing to a final and permanent feature, or by removal),
> we keep the appropriate enum constant as long as the bootstrap JDK may use it.
>
> This works, but causes constant confusion, and unnecessary work, as
> finalizing a preview feature requires another action in the next release to
> cleanup the enum constant.
>
> This PR is exploring an alternative solution: it takes the up-to-date
> `Feature` enum, and auto-generates a hybrid enum containing constants both
> from the up-to-date and bootstrap `Feature` enum. As a consequence, the
> up-to-date enum does not need to contain any legacy constants, as those are
> automatically injected for the interim build. This hybrid enum should never
> be used outside of the interim javac build.
>
> As a consequence, when a feature ceases to be a preview feature, we can clean
> up the feature enum immediately, leading to less confusion and less
> downstream work.
make/langtools/tools/previewfeature/SetupPreviewFeature.java line 85:
> 83:
> "/*compatibility constants:*/ ",
> 84:
> ",\n")) +
> 85: sourceCode.substring(insertPosition));
It might be better to use [`BufferedWriter.write(String s, int off, int
len)`] here:
Suggestion:
out.write(sourceCode, 0, insertPosition);
out.write(constantsToAdd.stream()
.collect(Collectors.joining(", ",
"/*compatibility constants:*/ ",
",\n")));
out.write(sourceCode, insertPosition, sourceCode.length());
And also handle the case when `constantsToAdd` is empty without producing a
syntax error due to doubled `,`s, but that needs to be done elsewhere in this
method.
[`BufferedWriter.write(String s, int off, int len)`]:
https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/io/BufferedWriter.html#write(java.lang.String,int,int)
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/30072#discussion_r2892630855
