http://docs.oracle.com/javase/7/docs/technotes/guides/vm/G1.... - at the end it says "G1 is planned as the long term replacement for the Concurrent Mark-Sweep Collector (CMS). Comparing G1 with CMS, there are differences that make G1 a better solution." - as G1 shipped with Java 7 I think it's stable, otherwise they probably would have waited until Java 8 imho.