JDK9把G1設定成預設的垃圾回收器
一些資料整理：
http://openjdk.java.net/jeps/248
https://docs.oracle.com/javase/9/gctuning/garbage-first-garbage-collector.htm#JSGCT-GUID-ED3AB6D3-FD9B-4447-9EDF-983ED2F7A573
https://docs.oracle.com/javase/9/gctuning/toc.htm
https://www.azul.com/resources/azul-technology/azul-c4-garbage-collector/
https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)
