Four JEPs have just been proposed for JDK 12 and they are briefly summarized in this post based on the JEPs' own text descriptions.
JEP 189 ["Shenandoah: A Low-Pause-Time Garbage Collector (Experimental)"] is described as "a new garbage collection (GC) algorithm ... [that] reduces GC pause times by doing evacuation work concurrently with the running Java threads." The JEP text adds, "Shenandoah is an appropriate algorithm for applications which value responsiveness and predictable short pauses."
JEP 334 ["JVM Constants API"]'s "Summary" states that this JEP's goal is to "Introduce an API to model nominal descriptions of key class-file and run-time artifacts, in particular constants that are loadable from the constant pool." This JEP is also proposed for Java SE 12.
JEP 344 ["Abortable Mixed Collections for G1"] aims to "make G1 mixed collections abortable if they might exceed the pause target."
JEP 346 ["Promptly Return Unused Committed Memory from G1"] is another JEP related to the G1 garbage collector and it aims to "enhance the G1 garbage collector to automatically return Java heap memory to the operating system when idle."