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jdk1.7 默认垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）
jdk1.8 默认垃圾收集器Parallel Scavenge（新生代）+Parallel Old（老年代）
jdk1.9 默认垃圾收集器G1
在程序启动里加jvm参数可打印相关信息：
-XX:+PrintCommandLineFlags 可查看默认设置收集器类型
-XX:+PrintGCDetails 可通过打印的GC日志的新生代、老年代名称判断
-XX:InitialHeapSize=132871552 -XX:MaxHeapSize=2125944832
-XX:+PrintCommandLineFlags -XX:+PrintGCDetails
-XX:+UseCompressedClassPointers -XX:+UseCompressedOops
-XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC
Heap
PSYoungGen total 38400K, used 3328K [0x00000000d5c00000, 0x00000000d8680000, 0x0000000100000000)
eden space 33280K, 10% used [0x00000000d5c00000,0x00000000d5f40210,0x00000000d7c80000)
from space 5120K, 0% used [0x00000000d8180000,0x00000000d8180000,0x00000000d8680000)
to space 5120K, 0% used [0x00000000d7c80000,0x00000000d7c80000,0x00000000d8180000)
ParOldGen total 87552K, used 0K [0x0000000081400000, 0x0000000086980000, 0x00000000d5c00000)
object space 87552K, 0% used [0x0000000081400000,0x0000000081400000,0x0000000086980000)
Metaspace used 2853K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 305K, capacity 386K, committed 512K, reserved 1048576K