Preview features to include Java SE APIs

[Alex Buckley]

In 2018, Oracle introduced the idea of "preview language features" as a 
way to have high-quality, fully-specified, but non-final technology in a 
mainline Java SE release [1]. This was well received by the Java 
community, so we are now proposing the idea of "preview APIs" [2]. The 
non-final technology in this case consists of modules, packages, types, 
methods, and fields in the Java SE API.

It's already normal for preview language features to motivate new types 
and methods in the Java SE API [3]. For example, the preview of Records 
introduced `java.lang.Record`, while the preview of Text Blocks added 
methods to `java.lang.String`. Such types and methods are available to 
programmers only when preview features are enabled overall. Going 
forward, any new API point, regardless of connection to the Java 
language, could be specified as a preview API; it would then be 
available to programmers only when preview features are enabled overall.

In the Java SE Platform Specification, the rules for preview features 
[4] would be extended to cover preview APIs too. The family of preview 
features in a given Java SE release would consist of the preview 
language features, preview VM features, and preview APIs specified for 
that release. All represent Java SE functionality that must be supported 
in all implementations, disabled by default, and enabled all at once.

We're confident that the successful outcomes seen with preview language 
features -- for example, the graduation of switch expressions from 
preview status in Java SE 12 and 13 to final permanent status in 14 -- 
will be repeated for preview APIs.

Without making any commitments, two possible sources of preview APIs are 
Projects Loom and Panama in OpenJDK. Loom's Early Access release 
enhances `java.lang.Thread` with support for "virtual threads" [5], 
while Panama offered an API for low-level memory access as an 
"incubator" module in JDK 14 [6]. As these APIs mature, we hope they 
will become candidates to preview in a Java SE release, thus maximizing 
feedback and improving them still further.

Alex

[1] http://openjdk.java.net/jeps/12
[2] https://mail.openjdk.java.net/pipermail/jdk-dev/2020-March/003991.html
[3] 
https://docs.oracle.com/javase/specs/jls/se14/html/jls-1.html#jls-1.5-300
[4] https://cr.openjdk.java.net/~iris/se/12/latestSpec/#Preview-features
[5] 
https://download.java.net/java/early_access/loom/docs/api/java.base/java/lang/Thread.html
[6] 
https://docs.oracle.com/en/java/javase/14/docs/api/jdk.incubator.foreign/jdk/incubator/foreign/package-summary.html