Here are some key features and enhancements introduced in Java 12:

1. JEP 189: Shenandoah: A Low-Pause-Time Garbage Collector (Experimental): Shenandoah reduces GC pause times by evacuating the collection set concurrently with the running Java threads.

2. JEP 230: Microbenchmark Suite: This JEP adds a basic suite of microbenchmarks to the JDK source code, which helps developers and performance engineers to create high-quality code.

3. JEP 325: Switch Expressions (Preview): This enhancement extended the switch statement so it can be used as either a statement or an expression. It simplified the classic switch statement by allowing a more concise and flexible syntax.

4. JEP 334: JVM Constants API: Introduced a new API to model nominal descriptions of key class-file and run-time artifacts, in particular constants that are loadable from the constant pool.

5. JEP 340: One AArch64 Port, Not Two: Retired the arm64 port while retaining the aarch64 port in the main-line source repository, providing a single port for the 64-bit ARM architecture.

6. JEP 341: Default CDS Archives: Enhanced the JDK build process to generate a class data-sharing (CDS) archive by default, using the default class list, improving start-up time.

7. JEP 344: Abortable Mixed Collections for G1: Enhanced G1’s ability to abort a mixed collection if it might exceed the pause target, making G1 more responsive to specified pause targets.

8. JEP 346: Promptly Return Unused Committed Memory from G1: G1 returns unused Java heap memory to the operating system more promptly, improving the overall system’s performance.