review comments for JEP 12: Preview Language and VM Features JDK-8195734

Karen Kinnear karen.kinnear at
Mon Dec 10 15:54:34 UTC 2018

1. As discussed in <>
the following paragraph is not-implementable: 
If a class file's use of a preview VM feature of Java SE $N causes an exception during loading, linking, or execution, then the JVM implementation must indicate that the exception is due to an preview VM feature of Java SE $N.

As discussed in the bug, we have added off-by-default logging when loading a class that supports a preview version. I don’t believe we want to mandate the details of diagnostic information. In particular, the JVM does not have always have the information to detect that the root cause of an exception lies in using a preview VM feature. So I would recommend removing this.

2. Relationship to Java SE APIs
2nd paragraph: A preview language or VM feature must not rely on an incubating API, since the API is not part of the Java SE Platform.

4th paragraph: The implementation of a preview language or VM feature may rely on incubating APIs.
(and similarly 5th paragraph).

Unless I misread this, this is inconsistent, unless there is a subtle distinction between the feature
and a specific implementation.


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