Improving compiler messages for preview API

Jonathan Gibbons jonathan.gibbons at
Wed Aug 7 20:30:55 UTC 2019

On 8/7/19 1:22 PM, Alex Buckley wrote:
> On 8/7/2019 11:52 AM, Joe Darcy wrote:
>> To be precise on terminology, if compiling with --enable-preview, I 
>> think the warnings generated for using preview languages feature and 
>> associated API elements should be a little-w warnings. That is, 
>> messages from the compiler that still allows the compile to exist 
>> with a zero exit code. Operationally, such messages would be 
>> Diagnostic.Kind.NOTE rather than Diagnostic.Kind.{WARNING, 
> Sure, that is a decision for javac.
>>> 3. At run time -- If you compile without --enable-preview, and the 
>>> source code refers to an API element associated with a preview 
>>> feature, then javac could give a (non-suppressible) warning and 
>>> _mark the class file as depending on preview features_. It is 
>>> important to handle this scenario firmly because the API element 
>>> might be gone in a later release. Annotating the type/method gives 
>>> grounds for javac to explain what's going on: "This method is marked 
>>> @Preview; your class file is now preview-feature dependent."
>> As I understand it, currently the call file target to use in javac is 
>> determined solely from the command line arguments at the start of the 
>> compile. It may be awkward to change this during the compile. (And 
>> would it change only for the for the types using the preview feature 
>> or fall all types in the compile?)
> Does "call file target" mean "class file target", e.g. 57.0 versus 
> 57.65535? I suspect it's moot anyway -- this thread has already moved 
> on from "give a warning, and mark the class file as depending on 
> preview features", to "give an error, so no class file to worry about."
>>> The annotation type that you defined in the webrev 
>>> (j.l.a.PreviewFeature) looks good. However, I don't think the 
>>> `release` element is needed. The API in SE $N is what it is; API 
>>> elements associated with a preview feature are there because a JEP 
>>> put them there. The `release` element would always be the current 
>>> JDK release.
>> It isn't necessarily the case an API associated with a preview 
>> feature will have been introduced in the most recent JDK. For 
>> example, treating JEP 325: "Switch Expressions (Preview)" in 12 and 
>> JEP 354: "Switch Expressions (Preview)" in 13 as iterations of the 
>> same feature, the tree API has methods to support switch expression 
>> support from both JDK 12 and 13:
>> SwitchExpressionTree introduced in 12, still in use in 13:
>> YieldTree introduced in 13:
>> >
>> This situation hasn't occurred yet in the Java SE APIs, but if the 
>> iterations of a feature are previewed over multiple releases, it 
>> certainly could.
>> The "release" element was modeled after the "since" element in 
>> java.lang.Deprecated. It isn't essential information, but I think it 
>> is useful to have it presented.
> Sorry, I am not concerned with how to mark compiler-internal APIs that 
> support preview features, whether in javac or in any other compiler. 
> The audience of developers who might accidentally over-rely on such 
> APIs is tiny. I am concerned about how to mark Java SE APIs as being 
> associated with preview features, because the audience of developers 
> who might accidentally over-rely on them is enormous. In that vein, if 
> an SE 12 preview feature re-previews in SE 13, then any associated API 
> in 13 should NOT say @PreviewFeature(release="12") -- even if the API 
> is unchanged between 12's preview feature and 13's preview feature. 
> The JEP which does the re-previewing is responsible for saying how 
> associated APIs from the first round evolve in the second round, so by 
> definition, any associated API present in 13 is the 13 version, and 
> `release="13"` is unnecessary.
> Alex

While I accept that you may not be concerned with non-SE API, I note 
that the APIs I'm talking about are public documented supported JDK 
APIs, and not just compiler-internal APIs.

-- Jon

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