JDK 27 makes G1 the default garbage collector in all environments and stops resizing the heap on the free-ratio flags

Sources: primary, discussion
Summary: A post by a G1 developer describes JDK 27 selecting G1 as the default garbage collector in all environments, including the small containers that previously got Serial GC. It records the MinHeapFreeRatio and MaxHeapFreeRatio defaults moving from 40 and 70 to 0 and 100, so the heap is no longer resized on those flags after a full GC.
Why it matters: Container-sized deployments that ran Serial GC by default change collector on upgrade unless the flag is set, and heap sizing after a full GC changes with them.