JDK 12 · JSR 386
JDK 12 is Java SE 12, JSR 386, generally available on 19 March 2019. There is no new permanent language feature. Switch expressions are a preview. Shenandoah is an experimental collector. G1 gains two latency-oriented changes.
A switch can be an expression that yields a value. The preview form is disabled unless preview features are enabled.
Preview again in Java 13 (JEP 354). Standard in Java 14 (JEP 361).
OpenJDK JEP 325 (leaves JavaMastery)
Related reading: Control flow
Shenandoah does evacuation work concurrently to keep pauses short. In Java 12 it is experimental.
It becomes a product feature in Java 15 (JEP 379).
OpenJDK JEP 189 (leaves JavaMastery)
Related reading: JVM and memory
G1 can abort a mixed collection that is taking too long, so a pause is more likely to meet its goal.
G1 can return unused Java heap memory to the operating system during idle time.
The JDK ships a default class-data sharing archive, so the feature is available without a separate dump step.
The final switch-expression syntax is in the Java 17 cumulative example, which is not a Java 12 program. Java 12’s own switch work was a preview.
Sources. General availability, JSR 386, and the JEP list: OpenJDK JDK 12. OpenJDK JDK 12 (leaves JavaMastery)