On Tue, 30 Jun 2026 12:49:20 GMT, Jan Lahoda <[email protected]> wrote:
> Consider source files like this:
>
> $ cat /tmp/ValueClass.java
> value class ValueClass {}
> $ cat /tmp/TestValue.java
> public class TestValue {
> ValueClass c;
> }
>
>
> When compiled like this:
>
> $ javac --enable-preview --release 28 -Xlint:preview /tmp/ValueClass.java
> /tmp/ValueClass.java:1: warning: [preview] value classes are a preview
> feature and may be removed in a future release.
> value class ValueClass {}
> ^
> 1 warning
> $ javac --enable-preview --release 28 -Xlint:preview -classpath /tmp
> /tmp/TestValue.java
> warning: [preview] class file for /tmp/ValueClass.class uses preview features
> of Java SE 28.
> 1 warning
>
>
> Note that while the `TestValue.class` is marked as preview, there's no
> warning on the use of `ValueClass`. This is not aligned with the JEP 12
> wording:
>
>> When compiling with preview features enabled, any source code reference to
>> (i) a class or interface declared using a preview language feature, ...,
>> causes a preview warning.
>
> Currently javac marks the classfile preview using an explicit test in the
> `ClassWriter`, but that does not seem quite right. This PR proposes to mark
> value classes as `Preview.declaredUsingPreviewFeature == true`, which should
> automatically produce a warning, and mark the appropriate classfile as
> preview.
>
> For the example above:
>
> $ javac --enable-preview --release 28 -Xlint:preview -classpath /tmp
> /tmp/TestValue.java
> warning: [preview] class file for /tmp/ValueClass.class uses preview features
> of Java SE 28.
> /tmp/TestValue.java:2: warning: [preview] class ValueClass is declared using
> a preview feature, which may be removed in a future release.
> ValueClass c;
> ^
> 2 warnings
>
>
> Note this will also report the warning for `Integer`, when preview is enabled:
>
> $ cat /tmp/Test.java
> public class Test {
> Integer i;
> }
> $ javac /tmp/Test.java
> $ javac --enable-preview --release 28 -Xlint:preview /tmp/Test.java
> warning: [preview] class file for /@/java.base/java/lang/Integer.sig uses
> preview features of Java SE 28.
> /tmp/Test.java:2: warning: [preview] class Integer is declared using a
> preview feature, which may be removed in a future release.
> Integer i;
> ^
> 2 warnings
>
>
> ---------
> - [x] I confirm that I make this contribution in accordance with the [OpenJDK
> Interim AI Policy](https://openjdk.org/legal/ai).
Nice improvement!
-------------
Marked as reviewed by mcimadamore (Committer).
PR Review:
https://git.openjdk.org/valhalla/pull/2601#pullrequestreview-4600478700
