JDK8默认垃圾回收器

➜  Contents java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=536870912 
-XX:MaxHeapSize=8589934592 -XX:+PrintCommandLineFlags 
-XX:+UseCompressedClassPointers -XX:+UseCompressedOops 
-XX:+UseParallelGC
java version "1.8.0_261" Java(TM) SE Runtime Environment (build 1.8.0_261-b12) Java HotSpot(TM) 64-Bit Server VM (build 25.261-b12, mixed mode) 

UseParallelGC 即 Parallel Scavenge + Parallel Old,再查看详细信息

➜  Contents java -XX:+PrintGCDetails -version
java version "1.8.0_261" Java(TM) SE Runtime Environment (build 1.8.0_261-b12) Java HotSpot(TM) 64-Bit Server VM (build 25.261-b12, mixed mode) Heap
 PSYoungGen      total 153088K, used 5263K [0x0000000715580000, 0x0000000720000000, 0x00000007c0000000) eden space 131584K, 4% used [0x0000000715580000,0x0000000715aa3ef8,0x000000071d600000) from space 21504K, 0% used [0x000000071eb00000,0x000000071eb00000,0x0000000720000000) to   space 21504K, 0% used [0x000000071d600000,0x000000071d600000,0x000000071eb00000) ParOldGen       total 349696K, used 0K [0x00000005c0000000, 0x00000005d5580000, 0x0000000715580000) object space 349696K, 0% used [0x00000005c0000000,0x00000005c0000000,0x00000005d5580000) Metaspace       used 2245K, capacity 4480K, committed 4480K, reserved 1056768K
  class space    used 243K, capacity 384K, committed 384K, reserved 1048576K

从JDK7u4开始，就对 “-XX:+UseParallelGC” 默认的老年代收集器进行了改进，改进使得HotSpot VM在选择使用 “-XX:+UseParallelGC” 时，会默认开启 " -XX:+UseParallelOldGC “，也就是说默认的老年代收集器是 Parallel Old。 综上，JDK8中默认的选择是”-XX:+UseParallelGC"，是 Parallel Scavenge + Parallel Old组合。

参考

• https://bugs.openjdk.java.net/browse/JDK-6679764
• http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/rev/24cae3e4cbaa