Preview APIs for preview features -- JDK 14+

Alex Buckley alex.buckley at
Mon Aug 5 19:11:31 UTC 2019

On 8/5/2019 12:00 PM, Alan Bateman wrote:
> On 05/08/2019 11:23, Alex Buckley wrote:
>> To be clear: this proposal means there will be APIs in the Java SE 
>> Platform that can only be used when preview features are enabled -- 
>> just as there are already language features in the Platform that can 
>> only be used when preview features are enabled.
> Just to double check my understanding: There is no run-time enforcement 
> and also no run-time filtering of methods for preview features when not 
> running with --enable-preview. The methods for preview features will be 
> discoverable (and usable) by reflection but that an untypical usage that 
> we shouldn't be concerned about.

Correct. This proposal is about strengthening the developer's awareness 
of incautious usage of associated APIs, by strengthening the 
compile-time story from a removal warning to an error. At run time, we 
do NOT propose to reject a class file referencing String::stripIndent 
when preview features are disabled; the checking would be heavyweight 
and could never account for reflective use. Again, the goal here is 
developer awareness when they read javadoc and write code.


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